2008
Design Manual
2008 Design Manual
P.O. Box 528 • Rochester Roc hester,, Indiana 46975 AIRVAC, Inc. : Inc. : 4217 N. Old U.S. 31 • P.O. Phone: 574.223.3980 Phone: Fax: 574.223.5566 574.223.3980 • Fax: 574.223.5566 • www.airvac.com
2008 Design Manual
P.O. Box 528 • Rochester Roc hester,, Indiana 46975 AIRVAC, Inc. : Inc. : 4217 N. Old U.S. 31 • P.O. Phone: 574.223.3980 Phone: Fax: 574.223.5566 574.223.3980 • Fax: 574.223.5566 • www.airvac.com
The World Leader Leader in Vacuu Vacuum m Sewer Sewer Technol ogy
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COPYRIGHT AIRVACr, INC. 1976,77,78,80,86,89,9 1976,77,78,80,86,89,92,96,00,05, 2,96,00,05, 08
THIS DOCUMENT IS COPYRIGHT 2008 AIRVAC r , INC.
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NO PART OF THIS DOCUMENT MAY BE COPIED, OR USED IN ANOTHER DOCUMENT, WITHOUT THE EXPRESS WRITTEN PERMISSION OF AIRVAC, INC.
DISCLAIMER
AIRVAC is not an engineering firm, and cannot and does not provide engineering services. AIRVAC reviews the design, plans, and specifications for a project only for their compatibility with AIRVAC's vacuum products, and accepts no responsibility for the overall project design. Any information provided to project engineers is provided solely to assist the engineer in designing and engineering and maintaining an overall system that can utilize AIRVAC vacuum products.
______________________________________________________________________ TAB LE OF CONTENTS
CHAPTER 1: INTRODUCTION
History of Vacuum Collection Systems General Description Vacuum Transport Process Major System Components Applications
• • • • •
CHAPTER 2: DESIGN FLOWS
Basis of Design Average Daily Flow Peaking Factor Peak Flow Infiltration Flow Entering via Pumps
• • • • • •
CHAPTER 3: VACUUM STATION DESIGN • • • • • • • • • •
Station Sizing-General Collection Tank Sizing Sewage Pump Sizing Vacuum Pump Sizing Electrical Panel Level Controls Other Station Components Noise & Heat Considerations Standby Generator Odor Control
CHAPTER 4: VA CUUM MAIN DESIGN • • • • • • • • • • •
Construction Issues System Layout Guidelines Line Sizing Line Lengths Main Line Design Parameters Service Lateral Design Parameters Line Connections Losses Due to Friction Losses Due to Lifts (Static Loss) Pipe Volume Summary of Vacuum Design Fundamentals
______________________________________________________________________ TAB LE OF CONTENTS
CHAPTER 5: VAL VE PITS • • • • •
Valve Pit Arrangements Components Common to Both Pit Types Valve Pit Components: PE 1-Piece Pits Valve Pit Components: Fiberglass 3-Piece Pits Building Sewer (House to Pit)
CHAPTER 6: BUFFER TANKS
Description When & Where Used Effect on System Hydraulics Buffer Tank Sizing Limitations on Use Situations to Avoid Potential Adverse Impacts
• • • • • • •
CHAPTER 7: PREFAB RICATED VACUUM STATION SKIDS
General Possible Skid Configurations Modifications to the Standard AIRVAC Skid Vacuum Station Equipment Configurations
• • • •
CHAPTER 8: A IRVAC SERVICES • • • • • • • • •
The Team Approach Preliminary System Layout Detailed Design Assistance Construction Field Services System Start-Up Home Hookup & Valve Installation Initial System Operation Operator Training After-Market Services
Design Manual 2008
CHAPTER 1 INTRODUCTION
The design guidelines contained in this manual are intended for buried vacuum sewer systems serving municipalities or land developments that utilize AIRVAC’s 3” vacuum valve. For indoor vacuum piping systems and applications that utilize vacuum valves smaller than 3” and/or vacuum toilets, contact AIRVAC’s Environmental Group for application-specific design manuals.
A.
HISTORY OF VACUUM COLL ECTION SYSTEMS
Vacuum sewers were first used in Europe in 1882. However, it has been only in the last 35 years that vacuum transport has been utilized in the United States. During this time, the use and acceptance of vacuum sewers have expanded greatly. Entering 2008, there were nearly 300 AIRVAC vacuum systems in operation in 27 states and 500-600 more in 27 foreign countries. This technology is also recognized in two industry publications: the 1991 United States Environmental Protection Agency (EPA), publication number EPA/625/1-91/024, The Manual For Alternative Wastewater Collection Systems and the 2008 Water Environment Federation (WEF) Alternative Sewer Systems, 2nd ed.; Manual of Practice No. FD-12. While both publications describe vacuum systems in general, the design criteria presented in both are based on the AIRVAC system.
B.
GENERAL DESCRIPTION
Vacuum sewers are a mechanized system of wastewater transport. Unlike gravity flow, vacuum sewers use differential air pressure to move the sewage. A central source of power to operate vacuum pumps is required to maintain vacuum on the collection system (See Figure 1-1). The vacuum sewer system requires a normally closed vacuum/gravity interface valve at each entry point to seal the lines so that vacuum is maintained. The interface valves; located in a valve pit, open when a predetermined amount of sewage accumulates in the collecting sump. The resulting differential pressure between atmosphere and vacuum becomes the driving force that propels the sewage towards the vacuum station. Vacuum sewer lines are designed to maintain a generally downward slope toward the vacuum station so essentially they are a vacuum assisted, gravity flow piping network.
Introduction 1-1
Design Manual 2008
FIGURE 1-1: TYPICAL LAYOUT
Introduction 1-2
Design Manual 2008 C.
VACUUM TRANSPORT PROCESS
When a sufficient volume of sewage accumulates in the sump, the associated vacuum valve cycles. The differential pressure between the sewer main and atmosphere forces the sump liquid content and additional air into the collection main at maximum velocities. This liquid and air join with the liquid and air that is at rest within the piping network, and is then propelled downstream. If no additional valves are opened during this period, the liquid that has not exited the main to the collection tank once again comes to rest at downstream low points. The magnitude of the propulsive forces starts to decline noticeably when the vacuum valve closes, but remains important as the air continues to expand within the pipe. Eventually friction and gravity bring the sewage to rest at a low spot. Another valve cycle, at any location upstream of the low spot, will cause this sewage to continue its movement toward the vacuum station.
Sawtooth profile The sawtooth profile ensures that an open passage of air between the vacuum station and the interface valves is maintained throughout the piping network. This provides the maximum differential pressure at the interface valves to insure self-cleansing of the valves as well as maximum energy input to the vacuum mains. When the liquid comes to rest at the base of various lifts, it does not come in contact with the crown of the pipe and therefore does not seal the pipe (Figure 1-2). Should the lift be sealed for any reason, liquid would become suspended within the lift and an associated vacuum loss incurred. The AIRVAC design philosophy includes an accounting of all lifts for all flow paths within a system and the associated potential vacuum loss incurred. The static loss limit assures that sufficient vacuum will be available for valve operation in the event of a system upset such as 100% lift saturation.
Air to Liquid ratio (A/L ratio) Vacuum systems are designed to operate on two-phase (air/liquid) flows with the air being admitted for a time period twice that of the liquid. Open time of the AIRVAC valve is adjustable; hence, various air to liquid (A/L) ratios are attainable. Sewage scouring velocities of 15 to 18 feet per second are attained using the standard air/liquid ratio. The ability of the vacuum main to quickly recover to the same level of vacuum that existed prior to the cycle, is commonly referred to as “vacuum response”, and is very important in vacuum sewer design. Vacuum response is a function of line length, pipe diameter, number of connections, and the amount of lift in the system. Introduction 1-3
Design Manual 2008
Back-surge Sewage admitted to a vacuum main through an AIRVAC valve initially moves in two directions. Approximately 80% flows toward the vacuum station and 20% flows in the opposite direction. When the back-surge slows, all of the flow moves toward the vacuum station. (See Figure 1-3). The 80%-20% split occurs when a wye fitting is used at the connection location. The back-surge would become 50% if a tee fitting were used. This would result in a less efficient system. For this reason, a tee is never used in a vacuum system. The vacuum transport process is clearly demonstrated at the AIRVAC Demonstration Rig at the factory.
FLOW OPEN PASSAGE OF AIR
SEWAGE AT REST
FIGURE 1-2: VACUUM MAIN DURING PERIOD OF NO FLOW
INITIAL FLOW (80%)
E T L I N E A G W S E INITIAL BACK-SURGE (20%)
FINAL FLOW (100%)
FIGURE 1-3: TYPICAL BA CKSURGE
Introduction 1-4
Design Manual 2008
D.
MAJ OR SYSTEM COMPONENTS
There are three (3) major system components: the valve pit, the vacuum piping and the vacuum station. These are described below:
Valve Pit The valve pit houses the vacuum valve and provides the interface between the vacuum system and the house. The sump portion of the valve pit is used to accept the wastes from the house. AIRVAC has two styles of valve pits; a single piece molded PE pit and a 3 piece fiberglass pit. In either style, there are two (2) separate internal chambers; an upper chamber that houses the vacuum valve and a bottom chamber that is the sump into which the building sewer is connected. These two chambers are sealed from each other. The valve pit is able to withstand traffic loads. The vacuum valve, which is housed in the upper chamber, operates without the use of electricity. The AIRVAC Three Inch Valves are manufactured of materials suitable for handling sanitary sewage. The valve is entirely pneumatic by design, and has a full 3-inch opening size. Some states have made this a minimum size requirement, as this is slightly larger than the throat diameter of the standard toilet. The vacuum valve provides the interface between the vacuum in the collection piping and the atmospheric air in the building sewer. System vacuum in the collection piping is maintained when the valve is closed. With the valve opened, system vacuum evacuates the contents of the sump. Once the contents of the holding sump have been evacuated, an amount of atmospheric air is admitted through the vacuum valve to provide propulsion to the liquid. The source of this atmospheric air is either a 4” air-intake that is placed on each house gravity line or a 6” dedicated air-terminal that is connected to each valve pit. With the first option, the 4” air-intake is installed downstream of all of the house traps. This could circumvent the potential problem of inadequate house venting which results in trap evacuation. While local codes may dictate exact locations, AIRVAC recommends placing the air-intake on each gravity line connected to the sump and at least 20 feet from the vacuum valve. With the second option, the 6” dedicated air-terminal is located a short distance from the valve pit and is connected to the pit via 6” piping to one of the four (4) openings in the sump. No sewage is permitted to enter this “air-only” line.
Introduction 1-5
Design Manual 2008 Some local codes may require the installation of a backwater valves on the homeowner’s building sewer. With most backwater valves, positioning is critical to ensure proper operation of the AIRVAC valve (see Chapter 5); however by using a normally opened type backwater valve, this concern can be alleviated. AIRVAC should be consulted before such a device is installed.
Vacuum Piping AIRVAC sewers are laid in a saw tooth profile. When AIRVAC sewers are installed at 0.2% fall in flat land, the trench depth increases 12" every 500 feet. Profile changes, also called lifts, may be used to bring the sewer invert back to the commencing level, keeping the trench depth to a minimum. Where the natural ground profile has a fall in excess of 0.2% in the flow direction, the vacuum sewer profiles follow those of the ground with no profile changes. PVC thermoplastic pipe, Schedule 40, SDR 21 or class 200, in sizes 3", 4", 6", 8" and 10" is normally used for vacuum sewers. Fittings used are of the Schedule 40 (pressure) type. Fittings include: wyes (for incoming branches or AIRVAC valve crossovers), 45 degree ells (for making changes in direction and fabricating profile changes), and concentric reducers (for making changes in main size). Tee fittings are not used for vacuum sewers. Pipe and fitting joints are normally solvent welded or rubber ring type joints. Not all rubber ring pipe joints are suitable for vacuum service and engineers should require a guarantee from the manufacturer along with test certification. When temperatures fall below 32 ° F, solvent weld joints cannot be reliably made, so o-ring pipe joints are recommended. AIRVAC maintains a comprehensive recommended specification for sewer piping along with a list of manufacturers who have acceptable products. The 3” pipe used to connect the valve pit to the vacuum main or branch is called a service lateral. Branch sewers are the tributary sewers connected to the main. Installation of vacuum sewer pipes and fittings are depicted in numerous details shown within this manual; additional construction details are available on request. Division or shut off valves are installed in branch and main lines to allow portions of the piping system to be isolated for troubleshooting and maintenance. These valves are typically resilient-wedge gate valves using mechanical joint connections with transition gaskets to PVC pipe that suitable for underground vacuum service.
Introduction 1-6
Design Manual 2008 Vacuum Station The vacuum station is the heart of the vacuum collection system. The machinery installed is similar to that of a conventional sewage pumping station or lift station, except vacuum is applied to the wetwell (collection tank) that is sealed. Major components including the tank, are the sewage pumps, the vacuum pumps, and a control panel. Most modern vacuum systems utilize factory pre-fabricated collection stations mounted on skids for ease of installation and are typically housed in a two story structure with the vacuum pumps and control panel located on the top floor and the collection tank and sewage pumps on the lower floor (See Figure 1-4).
CONTROL PANEL
VACUUM PUMPS COLLECTION TANK
FORCE MAIN
VACUUM SEWER MAIN SEWAGE PUMPS
FIGURE 1-4: TYPICAL VACUUM STATION SCHEMATIC
Introduction 1-7
Design Manual 2008
Collection tanks are normally steel with protective coatings; fiberglass and stainless steel tanks have also been used. The vacuum sewers terminate at this tank where vacuum is maintained automatically and passed to the sewer system. Duplicate sewage discharge pumps are used, with each pump capable of pumping design peak flow. Dry- pit, horizontal, centrifugal sewage pumps are typical, although dry-pit submersibles have been used as well. Multiple sliding-vane type vacuum pumps are also typical. These pumps are capable of an ultimate vacuum range near 29” Hg and offer efficient airdelivery-to-horsepower ratios. Horsepower varies with total flow rate, but is normally 10 - 25 Hp. Check valves for discharge pumps feature soft seats with outside weights and levers; vacuum pump inlet check valves are also fitted with soft seats. Typical electrical controls include: • • • • • • •
Vacuum switches with stainless bellows. Liquid level controls suitable for sanitary sewage Motor starters with overload Automatic alternators for pump cycling Hour run meters A solid state telephone alarm system A seven (7) day circular vacuum chart recorder
Since the systems require only one source of power, many systems utilize existing portable generators for emergency power; others have permanently installed back up generators. For unusually large flow areas, a customized, built-in place vacuum station can be utilized using similar equipment available from AIRVAC. See Chapter 3 for design parameters and details of vacuum stations.
Introduction 1-8
Design Manual 2008
E.
APPLICATIONS
The consulting engineer usually drives the community’s choice of collection system type during the planning stages. This choice is normally based on the results of a cost-effectiveness analysis.
Where used Below are the general conditions that are conducive to the selection of vacuum sewers. Unstable soil Flat terrain • Rolling hills with small elevation changes • • High water table Restricted construction conditions • • Rock • Urban development in rural area Sensitive eco-system • •
Advantages-Construction The advantages of vacuum collections systems may include substantial reductions in water use, material costs, excavation costs and treatment expenses. In short, there is a potential for overall cost-effectiveness. Specifically, the following advantages are evident:
•
Small PVC pipe sizes (3", 4", 6", 8" and 10”) are usually used.
•
No manholes are necessary.
•
Field changes can easily be made as unforeseen underground obstacles can be avoided by going over, under or around them.
•
Installation of smaller diameter pipes at shallow depths eliminates the need for wide, deep trenches reducing excavation costs and the duration and severity of community/environmental impacts.
•
Only one source of power, at the vacuum station, is required. No onlot power demands exist at valves.
Introduction 1-9
Design Manual 2008
TYPICAL DEPTH 3' TO 5'
TYPICAL DEPTH 3' TO 20'
VACUUM SEWERS ARE INSTALLED IN THE RIGHT OF WAYS AND REQUIRE SHALLOW TRENCHES WITH MINIMAL ENVIRONMENTAL IMPACT.
GRAVITY SEWERS ARE INSTALLED ALONG THE ROAD CENTERLINE. THEY REQUIRE DEEP TRENCHES AND CONSIDERABLE PAVEMENT RESTORATION.
VACUUM SEWERS
GRAVITY SEWERS
FIGURE 1-5: VACUUM vs GRAVITY EXCAVATIONS
3' TYP. 20' TYP.
GRAVITY SEWERS
3' TYP. 5' TYP.
VACUUM SEWERS
FIGURE 1-6: VACUUM AND GRAVITY PROFILES
Introduction 1 - 10
Design Manual 2008
Advantages-Operation (general) There are several operating advantages to using vacuum sewers as described below: •
High scouring velocities are attained, reducing the risk of blockages and keeping wastewater aerated and mixed.
•
Elimination of the exposure of maintenance personnel to the risk of H2S gas hazards.
•
The system will not allow major leaks to go unnoticed, resulting in reduced environmental damage from exfiltration of wastewater.
•
The elimination of infiltration permits a reduction of size and cost of the treatment plant.
•
The air/sewage mixture enters sewers at high velocity, and the air provides some pretreatment to the sewage inside the vacuum sewers.
Advantages-Operation (in hurricane prone areas) There are several advantages to using vacuum sewers for hurricane prone areas. System operators of actual systems that have experienced hurricanes report the following advantages:
•
Vacuum systems are sealed and massive amounts of I&I cannot enter the system and overwhelm the treatment plant.
•
All vacuum stations have either a fixed or portable generator, which ensures uninterrupted service to the customer.
•
Vacuum systems eliminate the threat of massive I&I and sewage spills. In coastal areas where 1 vacuum station typically replaces 7 or 8 lift stations, this means less hurricane preparation is required.
•
The maintenance staff is not exposed to the severe weather as the generators automatically start during a power outage.
•
If water level rises to the point where the air-intakes are in danger of flooding, the entire system can be turned off, thus preventing damage to system components.
Introduction 1 - 11
Design Manual 2008
CHAPTER 2 DESIGN FLOWS
A.
BASIS OF DESIGN
All of the major vacuum system components are sized according to peak flow, expressed in gallons per minute (gpm). Peak flow rates are calculated by applying a peaking factor to an average daily flow rate.
B.
AVERAGE DAILY FLOW
Based on the current Ten State Standards, sewage flow rates shall be based on one of the following:
C.
1.
Documented wastewater flow for the area being served. records are typically used for this purpose.
Water use
2.
100 gallons per person per day combined with home population densities specific to the service area. Most approval agencies will accept published U.S. Census Bureau home density for this criterion.
PEAKING FACTOR
The peaking factor suggested by the design firm will be used, with one exception: the minimum peaking factor should never be less than 2.5. If not established by the consulting firm, regulatory agency or other applicable regulations, the peaking factor should be based on the following formula:
18 + 4+
POPULATION / 1000 POPULATION / 1000
Design Flows Page 2 - 1
Design Manual 2008
For example, if the service area has a population density of 1200, the peaking factor would be:
18 +
1.2
= 3.75 4+
1.2
Table 2-1 shows peak factors for various populations. Please note that these are not the exact figures that would be returned by the formula but rather are rounded figures for presentation purposes only.
Table 2-1 Peak Factors Based on Ten State Standards formula Population 100 500 1200 2500 5000 9000
D.
Peak factor 4.25 4.00 3.75 3.50 3.25 3.00
PEAK FLOW
Applying the peak factor to the average daily flow rate and converting to gpm will yield the peak flow to be used as the basis of design. Qa /1440 x PF = Q max where: Qa PF Qmax
= Ave daily flow (gpd) = Peak factor = Peak Flow (gpm)
Design Flows Page 2 - 2
Design Manual 2008
Using local flow rates and a peak factor sug gested by design firm If the design firm provides the average daily flow based on local water records and recommends a peaking factor, AIRVAC will use both as the basis for design.
Example: Average daily flow rate: Persons/house # houses: Peak factor:
75 gpcd 3.5 400 3.50
Qa = PF =
75 gpcd x 3.5 per/hse x 400 hses = 105,000 gpd 3.50
Qmax = =
105,000 gpd/1440 x 3.50 255 gpm
Using Ten State Standards Peak factor and flow rates If the design firm does not suggest average daily flow rates and peaking factors, then the Ten State Standards will be used for both.
Example Average daily flow rate: Population (3.0 x 400): Peak factor
100 gpcd 1200 3.75
Qa = PF =
100 gpcd x 1200 persons = 120,000 gpd 3.75
Qmax = =
120,000 gpd/1440 x 3.75 313 gpm
Design Flows Page 2 - 3
Design Manual 2008
E.
INFILTRATION
The vacuum system is a sealed system that eliminates ground water infiltration from the piping network and the interface valve pits. However, ground water can enter the system as a result of leaking house plumbing or as a result of building roof drains being connected to the plumbing system. It is therefore important for designers to consider methods of eliminating ground water from plumbing systems during the design phase of a project. AIRVAC engineers will be happy to share ideas and methods used successfully in the past.
F.
FLOW ENTERING VIA PUMPS
For purposes of vacuum design, any flow that enters the vacuum system via a pump should be expressed in terms of the actual discharge pump rate rather than the standard peak flow rate from the house.
Design Flows Page 2 - 4
Design Manual 2008
CHAPTER 3 VACUUM STATION
A.
STATION SIZING-GENERAL
Nomenclature used in the station design is given below: Term Qmax Qa Qmin Qdp Qvp Vo Vct t Vp Vt TDH Hs Hf Hv
Definition Station peak flow (gpm) Station average flow (gpm) Station minimum flow (gpm) Discharge pump capacity (gpm) Vacuum pump capacity (cfm) Collection tank operating volume (gal) Collection tank volume (gal) System pump-down time (min) Piping system volume (gal) Total system volume (gal) Total dynamic head (ft) Static head (ft) Friction head (ft) Vacuum head (ft)
The major station components are generally sized as described in Table 3-1. Detailed formulas for each component are shown in this Chapter. A Vacuum Station Calculation Sheet is included in this chapter.
Table 3-1 Vacuum Station Component Sizing Component
How Sized
Collection Tank
To insure adequate operating volume to prevent excessive sewage pumping cycles and to provide emergency storage volume.
Sewage Pumps
Based on total peak flow to the vacuum station or as necessary to maintain 2 ft/sec scouring velocity within the force-main whichever is greater.
Vacuum Pumps
Based on: 1) peak flow & length of line and 2) the total system piping volume. Vacuum Station 3-1
Design Manual 2008
B
COLLECTION TANK SIZING
Materials Mild steel, Type 304 or 316 stainless steel and fiberglass tanks are acceptable. Steel and SS tanks should be of a welded construction and fabricated from not less than 1/4- in. thick steel plates. The tanks should be designed for a working pressure of 20 in. Hg vacuum and tested to 28 in. Hg vacuum. The tank should be furnished with the required number and size of openings, man-ways, and taps, as shown on the plans. In addition, the tank should be supplied complete with sight glass and its associated valves. Steel tanks should be sand-blasted and painted as follows:
• •
Internally: Two coats of epoxy suitable for immersion in sewage. Externally: One coat of epoxy primer and one coat of epoxy finish.
Fiberglass tanks may be substituted using the same specifications. All tanks are to have 150 psi rated flanges. It is recommended that only one vessel be used for most projects. Should the single tank concept result in a tank too large to transport, dual tanks are recommended but this is very rare.
Tank Sizing Collection Tanks are sized to insure adequate operating volume to prevent sewage pump short cycles and emergency storage volume. Also, tanks may be sized in anticipation of any future growth. The tanks are sized based on peak flow to the station. To this peak flow quantity, factors are applied to establish the operation volume for sewage pump cycling. Using these criteria, the sewage pumps will not operate more than 4 times per hour at minimum flow periods (2 starts per pump), nor start more than 7 times per hour at average flow (3.5 starts per pump). This is represented by the following formulas: Vo = 15Q min(Qdp − Q min) ÷ Qdp Vct = 3Vo + 400
Where
= Operating Volume (gal) Vo Q min = Minimum Flow (gpm) = Qa /2 Qdp Vct
= Sewage Pump capacity (gpm) = Collection Tank Size (gal) Vacuum Station 3-2
Design Manual 2008
To the operating volume a safety factor of 3.0 is applied for emergency storage. An additional 400 gallons is added to this subtotal as a reserve volume within the tank for moisture separation and vacuum pump reserve volume. Table 3-2 gives the value of V o for a 15-minute cycle at Q min for different peaking factors. The total volume of the collection tank should be 3 times the operating volume (Vt = 3 x V o) with a minimum recommended size of 1000 gal.
Table 3-2 Values of Vo for a 15-Minute Cycle at Q min for Different Peaking Factors Peaking Factor
Vo
3.0 3.5 4.0
2.08 x Qmax 1.84 x Qmax 1.64 x Qmax
After sizing the operating volume, the designer should check to ensure an excessive number of pump starts per hour will not occur. This check should be performed for a sewage inflow equal to one-half the pump capacity. When designing the collection tank, the sewage pump suction lines should be placed at the lowest point on the tank and as far away as possible from the main line inlets. The main line inlet elbows inside the tank should be turned at an angle away from the pump suction openings. The minimum recommended tank size is 1000 gallons. AIRVAC prefabricated skids are available with nominal tank sizes starting at 1000 gallons and then increasing in nominal increments of 500 gallons. See Chapter 7 for more information on AIRVAC Prefabricated skids.
Vacuum Station 3-3
Design Manual 2008
C.
SEWAGE PUMP SIZING
Materials Duplicate pumps, each capable of delivering the design capacity at the specified TDH should be used. These are typically horizontal, non-clog centrifugal pumps. Because the pump is drawing from a tank under vacuum, the NPSH calculations are especially critical. A certification from the pump manufacturer that the pumps are suitable for use in a vacuum sewerage installation is strongly recommended. Each pump should be equipped with an enclosed, non-clog type, two port, gray iron impeller that is statically and dynamically balanced and capable of passing a 3-in sphere. The impeller should be fastened to a stress-proof steel shaft by a stainless steel lock screw or locknut. Pumps should have an inspection opening in the discharge casing. The sewage pumps are the most susceptible component to submergence so it would be wise to consider dry-pit submersible pumps for flood prone areas. These pumps may operate in a dry pit under normal conditions and if necessary continue to operate while submerged. The obvious disadvantage is a motor coupled to the pump casing that is sealed making maintenance more difficult. Submersible pumps tend to require slightly higher NPSHa than their nonsubmersible counterparts so some special arrangements may be necessary to satisfy this criterion as well.
Pump capacity To size the discharge pumps, use the following formula:
Qdp = Qmax = Qa x Peak Factor
Typical peak factors range from 3.0 to 4.0 (see Chapter 2). The final selected pump capacity should be as calculated above (Qdp) or as necessary to maintain 2 ft/sec scouring velocity within the force-main, whichever is greater.
Vacuum Station 3-4
Design Manual 2008
TDH Calculations The Total Dynamic Head (TDH) is calculated using the following formula:
TDH = Hs + Hf + Hv
TDH is calculated using standard procedures for pumps with the exception that the head attributed to overcoming the vacuum in the collection tank (Hv) must also be considered. This value is usually 23 ft, which is roughly equivalent to 20 in. Hg, which is the typical upper operating value. Since Hv will vary depending on the tank vacuum level, (16-20 in Hg, with possible operation at much lower and higher levels during problem periods) it is prudent to avoid a pump with a flat capacity/head curve. Where possible, horizontal sewage pumps should be used, as they have less suction losses compared to vertical pumps. To reduce suction line friction losses, the pump suction line should be 2 in. larger than the discharge line.
NPSH Calculations To calculate NPSHa, use the following formulas. Nomenclature and typical valves used in the NPSH calculations are given in Table 3-3
NPSHa = havt + hs – hf - hvpa where
havt = ha - Vmax
NPSHa and TDH should be calculated for both the high and low vacuum operating levels and compared to the NPSHr at the corresponding point on the head/capacity curve. NPSHa must be greater than NPSH r . NPSH characteristics differ from manufacturer to manufacturer, with some better suited for use in a vacuum system than others. Depending on the actual pump selected and where it will operate on the pump curve, it would be prudent for the designer to include some margin between NPSHa and NPSHr. This is especially important for pumps that will operate away from the BEP (Best Efficiency Point) of the pump curve.
Vacuum Station 3-5
Design Manual 2008
Historically, NPSH has proven to be the most critical factor regarding sewage pump performance in a vacuum system. While several pumps may meet the head, capacity and NPSH requirements, AIRVA C reco mm end s th at th e designer choose the pump with th e lowest NPSHr .
Table 3-3 Discharge Pump NPSH Calculation/ Nomenclature Term
Definition
Typical Value -
NPSHa
Net positive suction head available, ft
NPSHr
Net positive suction head required by the pump selected, ft
ha
Head available due to atmospheric pressure, ft
havt
Head available due to atmospheric pressure at liquid level less vacuum in collection tank, ft
Vmax
Maximum collection tank vacuum, ft
hs hvpa
hf
Depth of wastewater above pump centerline, ft Absolute vapor pressure of wastewater at its pumping temperature, ft Friction loss in suction pipes, ft
33.9 @ sea level 33.2 @ 500 ft 32.8 @ 1,000 ft 29.4 @ 4,000 ft -
18.1 @ 16-inHg 22.6 @ 20-inHg 1.0 (min) 0.8
2.0 (ver. Pump) 1.0 (hor. Pump)
Figure 3-1 is a diagram for calculation of NPSHa in a vacuum system.
Vacuum Station 3-6
Design Manual 2008
FIGURE 3-1: NPSHa CALCULATION WITH TYPICAL VALUES
Vacuum Station 3-7
Design Manual 2008
D.
VACUUM PUMP SIZING
Materials AIRVAC recommends sliding vane type vacuum pumps, as they appear to be more efficient (air delivered versus electrical energy usage) in the operating range of 16-20” Hg, are more dependable, and generate less noise than do the liquid ring type. Vacuum pumps should be air-cooled and have a minimum (ultimate) vacuum of 29.3 in. Hg at sea level. Even though the pumps operate on short cycles, they should be capable of continuous operation. Lubrication should be provided by an integral, fully re-circulating oil supply. The oil separation system should also be integral. The entire pump, motor, and exhaust should be factory assembled and tested with the unit mounted on vibration isolators, and should not require special mounting or foundation considerations. When the vacuum pump capacity calculations call for 2 pumps, duplicate pumps each capable of delivering 100 percent of the required airflow, should be provided. When more than 2 pumps are required, 100 percent of the required airflow should be provided by the cumulative total capacity of all of the pumps less 1, with the additional pump having the same capacity as the others.
Pump Sizing Vacuum pumps are sized based on 2 factors: 1) peak flow & A/L ratio and 2) system pump down time (‘t’) which is a function of the total system piping volume. Both criteria should be checked and the larger value used. System pump-down time is the controlling factor for the majority of the vacuum pumps used in municipal sewer systems.
Pump Size Based on Flow and Line Length To size the vacuum pumps, the following empirical formula has been used successfully:
Qvp = A x Q max/7.5 gal/ft3
"A" varies empirically with mainline length as shown in Table 3-4. Vacuum Station 3-8
Design Manual 2008
Table 3- 4 “A” Factor for Use in Vacuum Pump Sizing Longest Line Length (ft)
A
0 - 5,000 5,001 - 7,000 7,001 -10,000 10,001 -12,000 >12,000
6 7 8 9 11
Pump Size Based on Pipe Volume (see page 4-30) After the initial vacuum pump size has been selected based on peak flow & A/L ratio, system pump- down time (known as ‘t’) for an operating range of 16-20 in. Hg should be checked. This calculation will show the amount of time it will take the selected vacuum pumps to evacuate (pump-down) the collection piping starting at a level of 16 in. Hg to a final level of 20 in. Hg.
t=
(0.045 cfm-min) (2/3 Vp + (Vct-Vo)) gal _______________ x ____________________________ gal Qvp cfm
where: t Vp Vct Vo Qvp
= = = = =
System pump-down time (min) Volume of collection system piping (gal) Volume of collection tank (gal) Operating volume of collection tank (gal) Vacuum pump capacity (cfm)
In no case should "t" be greater than 3 minutes nor less than 1 minute. If greater than 3 minutes, the capacity of the vacuum pumps should be increased or additional pumps added. If “t” is less than 1 minute, the capacity must be reduced. The minimum recommended pump size is 170 cfm. AIRVAC prefabricated skids use the following 3 vacuum pump sizes (See Chapter 7). 170 cfm 305 cfm 455 cfm
10 hp 15 hp 25 hp
Please contact AIRVAC if the design includes an EAAC (see NOTE on page 427) or system operation at levels higher than 16-20 in. Hg, as this may require an additional vacuum pump or a larger vacuum pump. Vacuum Station 3-9
Design Manual 2008 PROJECT
PROJECT NUMBER
STATION NUMBER
DATE
FLOW CALCULATIONS # connections
x Present
Residential Flow rate (gpcd)
rate
(b)
=
Ave daily flow (residential)
(a)
= growth
Future Per/hse
x
x
(a)
=
(b)
(c)
=
gpd
(e)
Peak factor
Peak flow (Residential)
(d) x (e) 1,440
=
x
=
gpm
(f)
gpm
(g)
=
gpm
Qmax
=
gpm
Qa
=
gpm
Qmin
=
gpm
Qdp
=
ft
=
ft
=
ft
=
ft
=
gal
=
gal
(e)
1,440
Other Peak flow (Comm)
Total Peak Flow
(d)
(c)
(f) + (g)
=
+ (f)
(g)
Average Flow
=
Qmax (e)
=
Minimum Flow
=
Qa 2
= 2
SEWAGE PUMP CAL C’s Pump Capacity
=
TDH @ 16” Hg =
TDH @ 20” Hg =
NPSHa @ 16” Hg =
18.1 Vacuum
+
22.6 Vacuum
+
+ (havt)
NPSHa @ 20” Hg =
Qmax + Static + Static -
+
(hf) -
(hs)
NOTE:
Havt
Friction -
(hs)
(havt)
Friction
(hvpa) -
(hf) =
[ ha
(hvpa) -
Vmax ]
COLLECTION TANK SIZING Operating Volume
=
15 Qmin (Qdp – Qmin) Qdp
Tank Volume required
=
[Vo x 3] + 400
Selected tank Volume
= 15 x
=
(
x(
-
)
x 3 ) + 400
(round up to nearest 500 gal)
FIGURE 3 – 2: STATION CALCULATIONS Vacuum Station 3 - 10
gal
Vo
Vct
Design Manual 2008 PROJECT
PROJECT NUMBER
STATION NUMBER
DATE
VACUUM PUMP CALCULATIONS A. Based on Peak Flo w an d Li ne L eng th Longest Line Length 0 5,001 7,001 10,001 12,001 -
“A” 6 7 8 9 11
5,000 7,000 10,000 12.000 15,000
Longest Line
=
Vacuum Pump capacity required
=
Pump Capacity 170 305 455
Horsepower
cfm cfm cfm
10 15 25
lf
Qvp
=
A x Qmax 7.5 gal/ft3
=
(
x 7.5 gal/ft3
)
=
Minimum 2 vacuum pumps 1 pump used as a standby
cfm
:
cfm
# pumps
Qvp
Round up to common size
B. Based on Pipe Volume & “ t” LINE LENGTHS & PIPE VOLUME Line
3”
4”
6”
8”
10”
A B C D Total Pipe footage
lf x
Pipe Volume
0.0547
=
Vp
3
ft /lf
lf 0.0904
3
ft /lf
ft
3
ft
=
ft
0.1959
3
3
t= “t” should be less than 3 min. If over, increase Qvp or add vacuum pumps
lf
t=
lf
3
ft /lf
0.3321
3
ft /lf
3
0.5095
3
ft
x 7.48
lf
3
ft 3
gal/ft
=
ft
gal
(.045 cfm-min) x [2/3 Vp + (Vct-Vo)] gal gal [# pumps –1] x Qvp cfm .045 x [ (2/3 x )+ ( ( - 1) x (
If “t” is under 1 m in, increase Vct t=
min
FIGURE 3 – 2: STATION CALCULATIONS Vacuum Station 3 - 11
)
3
ft /lf
)]
Design Manual 2008 VACUUM FORMULAS AND CONVERSION FACTOR Absolute Pressure Based On U.S. Std Atmosphere ALTITUDE
AIRVAC SYSTEM PUMP DOWN TIME
PRESSURE
(Feet)
In. Hg.
PSI
0
29.92
14.70
500
29.38
14.43
600
29.28
14.38
700
29.18
14.33
800
29.07
14.28
900
28.97
14.23
1,000
28.86
14.18
1,500
28.33
13.90
2,000
27.82
13.67
2,500
27.31
13.41
3,000
26.81
13.19
3,500
26.32
12.92
4,000
25.84
12.70
4,500
25.36
12.45
5,000
24.89
12.23
5,500
24.43
12.00
6,000
23.98
11.77
6,500
23.53
11.56
7,000
23.09
11.34
7,500
22.65
11.12
8,000
22.22
10.90
8,500
21.80
10.70
9,000
21.38
10.50
9,500
20.98
10.30
10,000
20.58
10.10
The following formulas will give pump down time for an empty vacuum system. SYSTEM PUMP DOWN TIME
t -
t V S P1 P2
2.3V S
log
The following examples illustrate two (2) typical operating conditions For pumping 0 to 20” Hg
⎛ c fm - m in ⎞ Vp + Vc t + Vrt ⎜ ⎟ t (m i n ) - 0 .1 47 ⎜ ⎟ gal Qv p ⎝ ⎠ For pumping 16” Hg to 20” Hg
⎛
t (m i n ) - ⎜ 0.0 45
⎝
g al
⎟ ⎠
Qv p
FREE AIR (SCFM) Free Air is air at normal atmospheric pressure. Because the altitude, barometer and temperature vary at diff erent localities and at different times, it follows that this term does not mean air under identical conditions. DELIVERED AIR (ACFM) Vacuum Pump: Delivered air is the actual volume of expanded air under the vacuum condition expressed in CFM at the vacuum pump intake.
To Obtain
lb./square inch (psi)
2.036
Inches mercury
lb./square inch (psi)
27.684
Inches water
lb./square inch (psi)
5.17
Cm. mercury
lb./square inch (psi)
70.317
Cm. water
lb./square inch (psi)
703.09
Kg/m
Inches water
0.0735
In. mercury
30"
Inches water
0.036
Lb./sq. inch
30" − V (in.Hg)
Inches water
2.54
Cm. water
Inches mercury
0.4912
Lb./sq. inch
Inches mercury
13.60
In. water
Inches mercury
2.54
Cm. mercury
Gal. Water
8.337
Lb.
Gal.
0.1337
Cu. Ft.
Cu. Ft.
7.48
Gal.
Cu. Ft.
0.0283
Cu. Meters
Horsepower
746.
Watts
Kilowatts
1.341
Horsepower
M /min.
35.31
Cfm
cfm
1.6992
m /hr.
3
c fm - m in ⎞ Vp + Vc t + Vrt
DEFINITIONS
By
3
P2
= time in minutes = volume of system in cubic feet = average pump speed in CFM from P1 to P2 = initial pressure - psia = final pressure - psia
Multiply
2
P1
SCFM Standard Cubic Feet per Minute. Delivered, free air at 14.7 PSIA and 70°F
x SCFM = ACFM
EXPANDED AIR Expanded Air is air under a partial vacuum or below atmospheric pressure. A true vacuum is a space without any air or gas and represents zero absolute pressure. ABSOL UTE PRESSURE Absolute Pressure is the total pressure above true zero. When working below atmospheric pressure it is less than 14.7 pounds per square inch. When working above atmospheric pressure, the absolute pressure is the sum of the atmospheric pressure and the gauge pressure. ATMOSPHERIC PRESSURE Atmospheric Pressure at sea level is 14.7 pounds per square inch above zero absolute pressure or 29.93 inches mercury.
Vacuum Station 3 - 12
Design Manual 2008
E.
ELECTRICAL PANEL
Motor Control panel Rather than using a Motor Control Center (MCC), most vacuum stations use a smaller control panel that is attached directly to the equipment skid. These control panels are specifically designed for each station. The control panel enclosure is to be NEMA Type 12 and is generally mounted on the equipment skid. A main disconnect switch, sized to handle the current draw for all related vacuum station equipment is to be provided. The panel includes motor starters for each motor. Typically, IEC type motor starters are used. To reduce in-rush current, “soft-starts” can be also used. The control panel also incorporates control relays, which then are connected to the various functions of the control panel system design. Discharge pump level control relays are mounted inside the panel. Conductance type level control rods are mounted through the tank wall and set to specific heights to maintain sewage pump operation and to provide alarm functions. The panel should include pilot lights and hand-off-auto (HOA) switches. Hourrun meters are included for the operator to track daily run times of the vacuum and discharge pumps. To monitor system performance, a 7-day chart recorder is installed in the enclosure. The control panel should also include a telephone alarm dialer, which monitors (3) three alarm functions: low vacuum levels, high sewage conditions and power outages. In small, simple vacuum station designs, the control panel is wired to each motor and control device by AIRVAC. Larger, more complex designs require the contractor to wire the control panel to the related junction boxes provided on the equipment.
Motor Control Center In larger vacuum stations, the controls are typically housed in a Motor Control Center (MCC). The Motor Control Center (MCC) is to be manufactured, assembled, wired, and tested in accordance with the latest issue of NEMA Publication ISC2-322, for Industrial Controls and Systems. The vertical section and the individual units should bear a UL label, where applicable, as evidence of compliance with UL Standard 845.
Vacuum Station 3 - 13
Design Manual 2008 Wiring inside the MCC is to be NEMA Class II, Type B. Where Type B wiring is indicated, the terminal blocks should be located in each section of the MCC. The enclosure should be NEMA Type 12-with-Gasketed Doors. Vertical sections shall be constructed with steel divider side sheet assemblies formed or otherwise fabricated to eliminate open framework between adjacent sections or full-length bolted-on side sheet assemblies at ends of the MCC. The MCC should be assembled in such a manner that it is not necessary to have rear accessibility to remove any internal devices or components. All future spaces and wire-ways are to be covered by blank doors.
Relay logic vs. PLC Control panels can use either relay-logic or PLC logic. Some prefer the simplicity of relay-logic and may not want the sophistication of using a PLC. Others wish to have more control over the system by changing the ladder logic through laptop computers or by phone. Many of the larger municipalities prefer this type of logic as it matches other controls they may have with lift stations and/or their treatment plant.
F. LEVEL CONTROLS Seven (7) probes inside the collection tank control the discharge pumps and alarms. These probes are ¼ in. stainless steel with a PVC coating. The seven positions are as follows: 1. 2. 3. 4. 5. 6. 7.
Ground probe Both discharge pumps stop Lead discharge pump start Lag discharge pump start High level alarm Reset for probe #7 High level cut-off: stops all discharge pumps (auto position only) and vacuum pumps (auto and manual positions)
Figure 3-3 gives approximate elevations of these probes in the collection tank relative to the discharge pumps and incoming vacuum mains. An acceptable alternative to the seven probes is a single capacitance-inductive type probe capable of monitoring all seven set points. This type of probe requires a transmitter/transducer to send a 4-20 mA signal to the MCC.
Vacuum Station 3 - 14
Design Manual 2008
FIGURE 3-3 LEVEL CONTROL PROBE DIAGRAM Vacuum Station 3 - 15
Design Manual 2008
G.
OTHER STATION COMPONENTS
Equalizing Lines A 1” NPT equalizing line is installed on each sewage pump. Its purpose is to equalize the liquid level on both sides of the impeller so that air is removed. This ensures that the impeller is filled with liquid when the pump starts which allows the sewage pump to start without having to pump against the vacuum in the collection tank. Clear PVC pipe is recommended for the equalizing lines because small air leaks or blockages will then be clearly visible to the system operator. For discharge pumps with small flow, the equalizing lines should be fitted with motorized full port valves that close after the pump is started.
Vacuum Gauges On the upstream side of each side of each vacuum sewer isolation valve, a vacuum gauge of not less than 4.5-in. diameter should be installed. Gauges should be positioned so that they are easily viewed when the isolation valves are operated. Diaphragm seals should not be used with compound gauges. All vacuum gauges should be specified to have a stainless steel bourdon tube and socket and to be provided with ½ in. bottom outlets. Polypropylene or stainless steel ball valves should be used as gauge cocks. The connection from the incoming main lines to the vacuum gauges should be made of PVC or CPVC pipe. Copper pipe is not to be used for this purpose.
Vacuum Chart Recorder The Motor Control Center of control panel should contain a 7-day circular chart recorder. The recording range is to be 0-30 in. Hg vacuum, with the 0 position at the center of the chart. The chart recorder should have stainless steel bellows.
Station Piping & valves Station piping includes all piping within the station, connecting piping to the collection tank, vacuum sewer lines, and force mains. This item includes piping, valves, fittings, pipe supports, fixtures, drains, and other work involved in providing a complete installation. Vacuum Station 3 - 16
Design Manual 2008
Wastewater, vacuum, and drain lines 4-in. and larger should be ductile iron, using AWWA/ANSI C110/A21.10 as standard. Pipe and fittings with the vacuum station should be flanged with EPDM gaskets. Exposed vacuum lines and other piping smaller than 4-in. can be Sch 80 PVC, 304 Stainless Steel or galvanized. Building sanitary drains may be constructed of Sch 40 PVC with DWV fittings. The vacuum station piping should be adequately supported to prevent sagging and vibration. It also should be installed in a manner to permit expansion, venting, and drainage. For fiberglass tanks, all piping must be supported so that the tank flanges support no weight. Flange bolts should only be tightened to the manufacturer's recommendations. Provisions must be allowed for inaccurate opening alignment. All shut-off valves fitted within the vacuum station should be flanged, resilient coated plug valves with circular ports. Check valves fitted to the vacuum piping are to be of the 125 lb. bolted bonnet, rubber flapper, and horizontal swing variety. Check valves are to be fitted with Buna-N soft seats. Check valves fitted to the sewage discharge piping are to be supplied with an external lever and weight to ensure positive closing. They also should be fitted with soft rubber seats.
Station Sump Valve The basement of the vacuum station should be provided with a 15 in. x 15 in. x 12 in. deep sump to collect wash-down water. A vacuum valve that is connected by piping to the collection tank empties this sump. A check valve and eccentric plug valve should be fitted between the sump valve and the collection tank.
Fault Monitoring System A voice communication-type automatic telephone dialing alarm system is typically used to alert the operator of system abnormalities and station emergencies. The system should be self-contained and capable of automatically monitoring up to four independent alarm conditions. The telephone dialer is usually within the Motor Control Panel. When doing so, provisions must be made to isolate the system from interference. The monitoring system should be provided with continuously charged batteries for 24 hours standby operation in the event of a power outage. Vacuum Station 3 - 17
Design Manual 2008
H.
NOISE & HEAT CONSIDERATIONS
Noise Noise generated by a vacuum station is typically associated with the vacuum pumps. If desired, soundproofing features can be added to the vacuum station. Of the 250+ vacuum stations in the U.S., only a handful has utilized this added protection. In these cases, there were 4 or 5 vacuum pumps, a large generator, and the station was located in close proximity to a house. A study of 6 operating vacuum stations, which employed masonry construction, resulted in measured decibels range of 50 to 60 dBA at the door of the vacuum station while vacuum pumps were in operation. Ambient readings indicated 50 to 55 dBA while no pumps were operating. No difference between ambient and operating pumps could be detected at the property line. Heat The vacuum station generates heat primarily from the sewage pump motors and the vacuum pumps. Obviously this heat is added to ambient conditions and can add up during hottest periods of the day. To prevent heat buildup, vacuum stations should be designed with adequate air exchange rates provided through ventilation equipment. Allowing natural heat rising through roof-mounted vents with low air intakes is recommended. Maximum recommended operating temperature for station equipment is 104 degrees (F). Contact AIRVAC for recommended minimum ventilation standards.
I.
STANDBY GENERATOR
A generator is used to provide standby power for duty discharge and vacuum pump operation. It can be located either inside or outside the vacuum station. A portable generator unit is sometimes used which may provide power for several vacuum stations. This is more common where power interruptions are rare and allows the unit to be taken in for service when required.
J.
ODOR CONTROL
Odor control for vacuum systems is normally associated with airborne H2S within the vacuum pump exhaust. If odor is a concern, various methods can be used to successfully eliminate H2S. The use of a bio-mass compost bed Vacuum Station 3 - 18
Design Manual 2008 designed in accordance with EPA Manual for "Odor and Corrosion in Sanitary Sewerage Systems" have been used for many years in various locals in the U.S. and in foreign countries. Other methods employed include chemical neutralization, activated carbon absorption systems and absorption by manufactured bio-mass filters. The bio-mass filter bed is recommended, assuming space is available at the site. An Excel spreadsheet for bio-filter sizing is available from AIRVAC upon request. An example follows for reference only.
1
Number of vacuum pumps
2
Pump capacity (e.g. – 455) Maximum vertical velocity thru filter bed (suggested 4.0 ft/min or less) Minimum area required at maximum air velocity (pump cap x # pumps/max velocity) (455 x 4/4.0) Assumed Filter bed area (Length x Width) Example: 30 ft x 12 ft = 360 Ft 2 Actual filter bed loading (pump cap x # pumps/filter bed area) (455 x 4)/360
3 4
5 6
7 8 9 10 11 12 13
14
15
16
Assumed filter bed depth (suggest 3.0 ft) Total filter bed volume (area x depth) (360 x 3.0) Compost material density (suggest 50 lb/Ft3) Compost total weight (density x volume) (50 x 1080) Bulk weight of compost (assuming 75% void space) (54,000 x .25) Bulk weight of compost in Kilograms (13,500 / 2.2) Possible hydrogen sulfide gas removal (assuming 2.2 mg/Kg-min concentration) (6,124 x 2.2) Total possible weight of exhaust gas (in Kg) (assume weight of exhaust gas = .075 lb/ft3) (455 x 4 x 0.075)/2.2 Total possible H2S in gas (assuming max 60 ppm concentration (60 mg/kg) (60 x 62.05) Factor of safety (should be minimum of 3.0) (Possible H2S removal/Total Possible H 2S) (13,472 / 3,723)
Vacuum Station 3 - 19
4 455
ACFM
4.0 Ft/min
455
Ft2
360
Ft2
5.06 Ft/min 3.0 1080 50
Ft Ft3 Lb/ft3
54,000
Lb.
13,500
Lb.
6,124
Kg
13,472
mg/min
62.05
Kg
3,723
mg
3.6
Design Manual 2008
K.
DO’S AND DON’TS
Table 3 – 5 Do’s and Don’ts: Vacuum Station Design
DON’T DO THIS:
DO THIS:
Forget to include vacuum head in the TDH calculations
Include an additional 23 ft of vacuum head to overcome tank vacuum
Size vacuum pumps solely on peak flow and A/L ratio
Perform secondary check based on pipe volume (“t” calculation)
Manifold exhaust lines from vacuum pumps
Use individual exhaust lines for each vacuum pump
Undersize the collection tank
Use a minimum collection size of 1000 gal Always include 400 gallon reserve
Select a discharge pump with poor NPSH characteristics
Make sure NPSHa exceeds NPSHr with some margin depending on the actual pump selected
Use vertical discharge pumps without first consulting AIRVAC
Use horizontal discharge pumps when possible
Use odd tank sizes and station configurations that require custom design
Use standard AIRVAC skid arrangements as shown in Chapter 7
Forget to consider heat gain inside the vacuum station
Provide adequate air exchange rates
Vacuum Station 3 - 20
Design Manual 2008
CHAPTER 4 VACUUM MAINS
A.
CONSTRUCTION ISSUES
Piping materials The piping network connects the individual valve pits to the collection tank at the vacuum station. Typical sizes include 4-in, 6-in, 8-in and 10-in. pipe. PVC thermoplastic pipe is normally used for vacuum sewers. Schedule 40 or SDR 21 PVC pipe have been used, with SDR 21 being recommended. In some European installations, HDPE, MDPE and ABS have been successfully used. In certain cases, DCIP has also been used, assuming the joints have been tested and found suitable for vacuum service. To reduce expansion- and contraction- induced stresses, flexible elastomeric joint ("rubber ring" joint) pipe is recommended. Where rubber ring joint pipe is used, a certificate is to be provided by the manufacturer stating that the pipe has been tested at 22” Hg vacuum in accordance with ASTM D-3139 and is guaranteed for use under vacuum conditions. AIRVAC recommends a doublelipped “Reiber style” gasket be used. If solvent-welded joint pipe is used, the pipe manufacturer’s recommendations for installation regarding temperature considerations should be followed. The Uni-Bell Handbook of PVC Pipe provides guidance as to proper practices.
Fittings PVC pressure rated fittings are needed for directional change as well as for the crossover connections from the service line to the main line. Fittings for sewers and service laterals shall be schedule 40 pressure rated with solvent weld connections in accordance with ASTM D-1784 and ASTM D-2466. Rubber ring gasket joints on fittings have been used successfully on some projects. The shape of gasket on smaller pipe sizes is of critical importance. For all pipe sizes, the engineer shall ensure that suitable fittings are available to suit the requirements of the vacuum system. Tee fittings shall not be used for vacuum service.
Vacuum Mains 4-1
Design Manual 2008
Profile changes (lifts) Lifts or vertical profile changes are used to maintain shallow trench depths as well as for uphill liquid transport (Figure 4.1). These lifts are made in a sawtooth fashion.
SCHEDULE 40 OR SDR 21 PVC PIPE
45° PVC SOLVENT WELD SCHEDULE 40 PRESSURE FITTING (TYP.)
FIGURE 4-1: LIFT DETAIL
A single lift consists of two (2) 45-degree fittings connected with a short length of pipe. AIRVAC recommends that lifts be made in either 1.0 ft. or 1.5 ft. increments depending on pipe size (see Table 4-13).
Location tape An inert polyethylene polyethylene tape having a metallic core should be laid in the sewer trench at a depth not exceeding 18" below ground surface.
Vacuum Mains 4-2
Design Manual 2008
Isolation Valves AIRVAC recommends recommends an isolation isolation valve valve at the beginning beginning of of each branch branch and on on the mainline near these branch connections. Should branch spacing exceed 1500 feet, AIRVAC recommends an additional isolation valve for that section. The purpose of these valves is to isolate sections of the vacuum system for trouble shooting purposes and the above listed standards have proven to be sufficient in past years. While both plug and resilient-wedge gate valves have been used, AIRVAC recommends the resilient-wedge gate valves.
Gauge taps A gauge tap, installed just downstream of the isolation valve, makes it possible for one person to troubleshoot without having to check vacuum at the station. This reduces emergency maintenance expenses, both from a time and manpower standpoint. AIRVAC only recommends recommends the use of gauge taps for entities with limited operating resources (ex- only 1 operator) where areas of the collection system are not easily reached by vehicle within a few minutes time. If the system is fairly contained where the farthest reaches of the system are only minutes from the vacuum station and the operating entity has 2 or more operators, trucks, radios, etc., then gauge taps are of little value and are not recommended.
Cleanouts/Access Points AIRVAC does not recommend the use of cleanouts or inspection inspection ports on vacuum lines. There are two very good reasons for this. First, cleanouts/access points are unnecessary as access to the vacuum main can be gained at any valve pit by removing the valve. A second, and perhaps more important reason, is the potential they pose for vacuum leaks. Early systems that used them experienced as many, if not more, leaks due to cleanouts than were associated with the actual vacuum main itself. While not required, cleanouts/access points may be used where line size changes occur and at the end of the vacuum main.
Installation Tolerances-Open Cut Tolerances for open-cut trenching is typically ± 0.05’ per 100 ft, or 0.05%, for all pipe sizes. With a target slope of 0.20 %, this means the slope could vary from 0.15% to 0.25%. There should be no sags or bellies in the line. Trench water should not be allowed to enter the pipe during construction. Vacuum Mains 4-3
Design Manual 2008
Installation Tolerances –Horizontal Directional Drilling Tolerances for directional drilling are the same as those required for open trenching. The “minimum 50’ at 0.20% prior to a lift” rule applies to directional drilling just as it does to open trenching. Making a directional drill at a slope greater than 0.20% and then immediately using a lift to make up the difference is not acceptable. At the time this Manual went to print, AIRVAC still had not endorsed endorsed the use of horizontal directional drilling, primarily due to grade control issues. This is likely to change as some HDD companies are taking additional steps to ensure that proper grade is attained. Table 4-1 shows AIRVAC’s requirements for HDD.
Table 4-1 AIRVAC requirements requirements for Horizontal Directional Drilling
Item Slope Sags (bellies) & summits Quality control System integrity Pipe materials
AIRVAC Requirement Requirement Installed pipe must meet AIRVAC’s AIRVAC’ s slope tolerances (± 0.05’ per 100 ft) Are not acceptable acceptable HDD firm must be able to electronically verify installed slopes at the time of the installation Installed pipe must be capable of passing AIRVAC’s daily & final vacuum vacuum tests Must mate up to AIRVAC products and other system components
On non-critical lines, it may be possible to actually design for slopes greater than 0.20% in areas where directional drilling is desired. This would not be a construction tolerance but rather a design decision that allows for a more attainable slope (say 0.50%). Incorrect slopes or sag and summits have larger negative affect when a vacuum main is involved rather than when a branch or service line is involved. An incorrect slope on a service line would affect the operation of just that particular service line, whereas an incorrect slope on a vacuum main would affect not only that main, but all connected branch and service lines as well.
Vacuum Mains 4-4
Design Manual 2008
Testing – Daily Test At the completion of each day's work, all sewer mains and lateral connections laid that day are to undergo a 2-hour vacuum test. A vacuum of 22" Hg is applied to the pipes and a maximum loss of vacuum of 1% per hour for a two (2) hour test period is permissible. As pipe is laid the new section will be tested in addition to the previously laid pipe on that main.
Testing – Final Test At the end of the construction period, and prior to the installation of the AIRVAC valve, the complete vacuum sewer system is to be vacuum tested. The testing procedure is the same as the daily test, except the final test is a 4-hour test. Again, the maximum permissible loss is 1% per hour over the four (4) hour test period (approximately 1” Hg loss).
B.
SYSTEM LAYOUT GUIDELINES
There are four (4) major items to consider when laying out a vacuum system: Multiple servic e zones: By locating the vacuum station centrally, it is possible for multiple vacuum mains to enter the station. This allows the service area to effectively be divided into zones. The net result is smaller pipe sizes and less overall vacuum loss. More importantly, this results in operational flexibility as well as service reliability. With multiple service zones, the system operator can respond to system problems, i.e., low station vacuum, by analyzing the collection system zone by zone to see which zone has the problem. The problem zone can then be isolated from the rest of the system so that normal service is possible in the unaffected zones while the problem is identified and solved. Minimize pipe sizes: By dividing the service area into zones, the total peak flow to the station is also spread out among the various zones. This makes it possible to minimize the pipe sizes. For example, a total peak flow of 350 gpm in system with only 1 service zone would require a 10-inch vacuum main to the station with progressively smaller pipe toward the system extremities. By spreading this peak flow equally over four vacuum service zones; only 6-inch and 4-inch pipe would be required. Minimize vacuum loss: Vacuum loss is generally limited to 13 feet. Items that can result in vacuum loss are increased line length and elevation differences, utility conflicts and the relationship of the valve pit location to the vacuum main. The system layout should take these factors into consideration.
Vacuum Mains 4-5
Design Manual 2008 Valve pit spacing: Another important consideration is the location of the valve pits along the vacuum main. To the designer, the valve pit is more than just the connection point for the customer; it is an “energy input” location. Movement of liquid within the vacuum main depends on differential pressure. The differential is created when a vacuum valve opens and allows atmospheric air to enter a vacuum main that is under a negative pressure. The only place this can happen is where a vacuum valve exists. The fewer the energy inputs that exist, the poorer the transport characteristics become. As a result, designers should avoid layouts that result in long stretches of vacuum main with no house connections. C.
LINE SIZING
Based on hydraulic testing, AIRVAC has established both absolute maximum flows as well as recommended maximum flows for the various main pipe sizes. Table 4-2 shows these recommendations.
Table 4-2 Maximum Flow for Various Pipe Sizes (Assuming SDR 21 pipe) Pipe Diameter (in)
Absolute Maximum Flow (gpm)
Recommended Maximum Flow (gpm)
4 6 8 10
55 152 305 544
38 105 210 374
The minimum diameter for a vacuum main is 4 inches. Three (3) inch vacuum lines are only used for the service lateral (pit to main). Only one (1) valve pit may be connected to a 3” service lateral. The values in Table 4-2 should be used for planning purposes or as a starting point for the detailed design. In the latter case, estimated site-specific flow inputs along with the friction tables should be used in the hydraulic calculations. A correctly sized line will yield a relatively small friction loss. If the next larger pipe size significantly reduces friction loss, the line was originally undersized. Vacuum Mains 4-6
Design Manual 2008
D.
LINE LENGTHS
The length of vacuum mains is governed by two factors. These are static lift and friction losses. As discussed later in this chapter, the static loss generally should not exceed 13 ft and friction losses should not exceed 5 ft. Due to restraints placed upon each design by topography and sewage flows, it is impossible to give a definite maximum line length. In perfectly flat terrain with no unusual subsurface obstacles present, a length of 10,000 ft can typically be achieved. Should there be some elevation difference to overcome, this length could be shorter. On the other hand, with positive elevation toward the vacuum station, this length could be longer. As an example, one operating system has a single main line branch exceeding 16,500 feet in length. Since most projects have a combination of uphill, downhill, and level sections, AIRVAC recommends that the 10,000 feet figure be used as a starting point by the designer when doing the preliminary line layout. Any length in excess of this should only be considered if static and friction losses calculated during the final design allow for such additional length. Guidelines for line lengths and sizes are shown in Table 4-3.
Table 4-3 Maximum Line Length for Various Pipe Sizes Pipe Diameter (in)
Where used
Maximum recommended length (ft)
3
Service lateral
300
4
Branch Line/end of main line
2,000
6, 8 & 10
Main Lines
Limited by static & friction losses
AIRVAC engineers will be pleased to advise and assist clients with the design of the vacuum mains once topography and flow rates are available.
Vacuum Mains 4-7
Design Manual 2008
E.
MAIN LINE DESIGN PARAMETERS
Vacuum sewer design rules have been developed largely as a result of studying operating systems. Important design parameters are shown in Tables 4-4.
Table 4-4 Main Line Design Parameters Minimum distance between lifts, ft
20
Minimum distance of 0.20% slope prior to a lift or series of lifts, ft
50
Minimum distance between top of lift and any service lateral, ft Minimum slope
0.20%
Maximum # of lifts in a series (see page 4-29)
5
FLOW
SERIES OF LIFTS
6
50 FT M INIMUM SLOPE AT 0.20%
DOWNHILL SLOPE GREATER THAN 0.2 %
FIGURE 4-2: PROFILING BEFORE A SERIES OF LIFTS
Vacuum Mains 4-8
Design Manual 2008
Slope on Vacuum Mains Table 4-5 provides general guidelines for vacuum main slopes.
Table 4- 5 Guidelines for Determining Line Slopes Level
0.20% slope until depth is unacceptable; then use lifts
Uphill
Use lifts spaced close together (see Table 4.6)
Downhill
Follow ground profile when ground >0.20% slope
LAY SEWER TO A SLOPE OF NOT LESS THAN 0.2% UNIFORMLY WITHOUT POCKETS
VACUUM MAIN FLOW
FIGURE 4-3: DOWNHILL TRANSPORT
Vacuum Mains 4-9
Design Manual 2008
0.2% SLOPE 0.25 FT FALL (MIN) 0.25 FT FALL (MIN)
45 DEG ELLS
VACUUM MAIN
0.2% SLOPE
FLOW 20 FT MIN. FALL BETWEEN LIFTS IS GREATER OF TWO VALUES: 0.25 FEET OR 0.2% TIMES DISTANCE
FIGURE 4-4: UPHILL TRANSPORT
500 FT TYPICAL 0.2% SLOPE
0.2% SLOPE VACUUM MAIN
FALL BETWEEN LIFTS IS GREATER OF TWO VALUES: 0.25 FEET OR 0.2% TIMES DISTANCE
FIGURE 4-5: L EVEL TRANSPORT
Vacuum Mains 4 - 10
FLOW
Design Manual 2008
Slope between lifts For slope between lifts, Table 4-6 shows the distance at which the 0.20% slope, rather than the minimum fall, will govern the design for a given pipe diameter.
Table 4- 6 Slopes Between Lifts Pipe Diameter (in) 3 4 6 8 10
Minimum fall between lifts* Use greater value of (A) or (B) (A) 0.20 ft 0.25 ft 0.25 ft 0.25 ft 0.25 ft
Distance at which (B) governs
(B) 0.20% x distance 0.20% x distance 0.20% x distance 0.20% x distance 0.20% x distance
> 100 ft > 125 ft > 125 ft > 125 ft > 125 ft
* When not between lifts, use 0.20% slope; distance measured in feet
F.
SERVICE LATERAL (Pit to Main) DESIGN PARAMETERS
Vacuum sewer service laterals connect the vacuum main to the valve pit. Service laterals are 3 inches in diameter and are limited to one valve pit only. The type of pipe and fittings are the same as those used on the vacuum mains.
When lifts are required in the service lateral If it is necessary to locate an AIRVAC valve with its invert lower than the main, the procedure shown in Figure 4-6 should be followed. Where this method is used, the maximum lift must not exceed the vacuum available in the main at the point of connection (less the five inches of mercury required for vacuum valve operation). Lifts used must be included when calculating the system line losses.
Table 4- 7 Service Lateral Design Parameters Maximum number of 1’- lifts in service lateral st Minimum distance from valve pit to 1 lift Minimum distance from last lift to main Minimum slope between lifts
5 ea 5 ft 5 ft 0.20 ft or [0.2% x distance (ft)] (whichever is larger)
Vacuum Mains 4 - 11
Design Manual 2008
FIGURE 4-6: DIAGRAM OF 3” SERVICE LATERAL WITH LIFTS
Vacuum Mains 4 - 12
Design Manual 2008
G.
LINE CONNECTIONS
Two (2) types of connections are made to a vacuum sewer: •
Connections from a branch to a main
•
Connections from a service lateral (valve pit) to either a branch or main.
“Tee” fittings are not acceptable for use in vacuum sewers, including both of these types of connections.
Connecting a branch to a main Several configurations involving wyes and fittings have been used to connect a branch to a main. See AIRVAC’s standard details for the various methods that exist for connecting branch vacuum sewers to vacuum mains. In one method, connections to the main are made "over-the-top," by using a wye fitting in the vertical position. Due to the restraints placed upon the depth of sewers by the connections entering "over the top", engineers should consider the ground cover required on service laterals. In another method, the invert to invert spacing can be reduced by rolling the wye fitting to a 45 degree angle. This method is acceptable, provided the invert of the connecting pipe is at least 2 inches above the crown of the main. Note that 90 ° ells are not allowed on branch connections. Also, where a lift or profile change is required in a branch sewer prior to entering the main, it should be made 20 or more feet from the main.
Connecting a Service lateral to a branch or main Several configurations involving wyes and fittings have been used to connect a valve pit to a branch or main. See AIRVAC’s standard details which show the various methods that are acceptable. A long turn 90° ell is allowed on a service lateral connection.
Vacuum Mains 4 - 13
Design Manual 2008
H.
LOSSES DUE TO FRICTION
Friction losses are only calculated for sewers that are laid on a downward slope between 0.20% and 2.0% and are cumulative for each flow path from the furthest valve on the line to the vacuum station. Friction losses in sewers installed at greater than 2.0% percent are ignored. Friction loss charts for SDR 21 PVC pipe and a 2:1 air/liquid ratio have been developed by AIRVAC. These charts were based on the following formula: F = 2.75 x 0.2083 x (100/C) 1.85 x Q1.85 d4.8655 Where: F= C= Q= d=
friction loss (ft/100 ft) 150 for PVC flow (gpm) inside pipe diameter (in.)
Table 4 – 8
Pipe Type
PVC Pipe Sizes and Related AIRVAC Design Information C = 150 Rec’d Design Flow Pipe Diameter I.D. (F=0.25) (inches) (inches) (gpm)
Absolute Max Flow (F=0.50) (gpm)
SDR 21
4 6 8 10
4.05 5.96 7.76 9.67
38 105 210 374
55 152 305 544
SCH 40
4 6* 8* 10 *
4.00 6.03 7.94 9.98
37 108 223 406
53 157 324 591
* These sizes not recommended in Schedule 40 AIRVAC recommends SDR 21 and maximum flow based on F = 0.25
Vacuum Mains 4 - 14
Design Manual 2008
Table 4-9 FRICTION LOSS TABLE FOR 4 INCH PIPE FLOW (gpm) 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
HEAD LOSS (ft/100 ft) .0011 .0023 .0039 .0059 .0083 .0110 .0141 .0175 .0213 .0254 .0299 .0346 .0397 .0451 .0509 .0569 .0632 .0699
FLOW (gpm) 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
HEAD LOSS (ft/100 ft) .0769 .0841 .0917 .0995 .1077 .1161 .1249 .1339 .1432 .1582 .1627 .1729 .1834 .1941 .2051 .2164 .2280 .2399
FLOW (gpm) 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55
*SHADED AREAS NOT RECOMMENDED
Vacuum Mains 4 - 15
HEAD LOSS (ft/100 ft) .2520 .2644 .2771 .2900 .3032 .3167 .3305 .3445 .3588 .3734 .3882 .4033 .4187 .4343 .4502 .4663 .4827 .4994
Design Manual 2008
Table 4-10 FRICTION LOSS TABLE FOR 6 INCH PIPE FLOW (gpm) 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
HEAD LOSS (ft/100 ft) .0004 .0006 .0009 .0013 .0017 .0022 .0027 .0033 .0039 .0046 .0053 .0061 .0069 .0078 .0087 .0096 .0107 .0117 .0128 .0140 .0152 .0164 .0177 .0190 .0204 .0218 .0233 .0248
FLOW (gpm) 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58
HEAD LOSS (ft/100 ft) .0264 .0280 .0296 .0313 .0330 .0348 .0366 .0384 .0403 .0423 .0442 .0462 .0483 .0504 .0525 .0547 .0569 .0592 .0615 .0638 .0662 .0687 .0711 .0736 .0762 .0787 .0814 .0840
Vacuum Mains 4 - 16
FLOW (gpm) 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86
HEAD LOSS (ft/100 ft) .0867 .0895 .0922 .0951 .0979 .1008 .1037 .1067 .1097 .1128 .1159 .1190 .1222 .1254 .1286 .1319 .1352 .1385 .1419 .1454 .1488 .1523 .1559 .1594 .1631 .1667 .1704 .1741
Design Manual 2008
Table 4-10 (cont) FRICTION LOSS TABLE FOR 6 INCH PIPE FLOW (gpm) 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111
HEAD LOSS (ft/100 ft) .1779 .1817 .1855 .1894 .1933 .1973 .2013 .2053 .2093 .2134 .2176 .2217 .2259 .2302 .2345 .2388 .2431 .2475 .2519 .2564 .2609 .2654 .2700 .2746 .2792
FLOW (gpm) 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136
HEAD LOSS (ft/100 ft) .2839 .2886 .2933 .2981 .3029 .3078 .3126 .3176 .3225 .3275 .3325 .3376 .3427 .3478 .3530 .3582 .3634 .3687 .3740 .3793 .3847 .3901 .3956 .4010 .4065
FLOW (gpm) 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154
*SHADED AREAS NOT RECOMMENDED
Vacuum Mains 4 - 17
HEAD LOSS (ft/100 ft) .4121 .4177 .4233 .4289 .4346 .4404 .4461 .4519 .4577 .4636 .4695 .4754 .4813 .4873 .4934 .4994 .5055 .5117
Design Manual 2008
Table 4-11 FRICTION LOSS TABLE FOR 8 INCH PIPE FLOW (gpm) 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112
HEAD LOSS (ft/100 ft) .0471 .0481 .0492 .0502 .0513 .0524 .0534 .0545 .0556 .0568 .0579 .0590 .0602 .0613 .0625 .0636 .0648 .0660 .0672 .0684 .0696 .0709 .0721 .0734 .0746 .0759 .0772 .0785
FLOW (gpm) 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140
HEAD LOSS (ft/100 ft) .0798 .0811 .0824 .0837 .0851 .0864 .0878 .0892 .0905 .0919 .0933 .0947 .0962 .0967 .0990 .1005 .1019 .1034 .1049 .1064 .1079 .1094 .1109 .1124 .1139 .1155 .1170 .1186
Vacuum Mains 4 - 18
FLOW (gpm) 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168
HEAD LOSS (ft/100 ft) .1202 .1217 .1233 .1249 .1265 .1282 .1298 .1314 .1331 .1347 .1364 .1381 .1398 .1415 .1432 .1449 .1466 .1483 .1501 .1518 .1536 .1554 .1571 .1589 .1607 .1625 .1643 .1662
Design Manual 2008
Table 4-11 (cont) FRICTION LOSS TABLE FOR 8 INCH PIPE FLOW (gpm) 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196
HEAD LOSS (ft/100 ft) .1680 .1698 .1717 .1736 .1754 .1773 .1792 .1811 .1830 .1849 .1869 .1888 .1907 .1927 .1947 .1966 .1986 .2006 .2026 .2046 .2066 .2086 .2107 .2127 .2148 .2168 .2189 .2210
FLOW (gpm) 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224
HEAD LOSS (ft/100 ft) .2231 .2252 .2273 .2294 .2315 .2337 .2358 .2380 .2401 .2423 .2445 .2467 .2489 .2511 .2533 .2555 .2578 .2600 .2623 .2645 .2668 .2691 .2714 .2737 .2760 .2783 .2806 .2829
FLOW (gpm) 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252
*SHADED AREAS NOT RECOMMENDED
Vacuum Mains 4 - 19
HEAD LOSS (ft/100 ft) .2853 .2876 .2900 .2923 .2947 .2971 .3995 .3019 .3043 .3067 .3092 .3116 .3141 .3165 .3190 .3214 .3239 .3264 .3289 .3314 .3339 .3365 .3390 .3416 .3441 .3467 .3492 .3518
Design Manual 2008
Table 4-12 FRICTION LOSS TABLE FOR 10 INCH PIPE FLOW (gpm) 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235
HEAD LOSS (ft/100 ft) .0786 .0793 .0800 .0808 .0815 .0822 .0830 .0837 .0845 .0852 .0860 .0867 .0875 .0883 .0890 .0898 .0906 .0914 .0921 .0929 .0937 .0945 .0953 .0961 .0969 .0977 .0985 .0993 .1001 .1009 .1017 .1026 .1034 .1042 .1050 .1059
FLOW (gpm) 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271
HEAD LOSS (ft/100 ft) .1067 .1075 .1084 .1092 .1101 .1109 .1118 .1126 .1135 .1144 .1152 .1161 .1170 .1178 .1187 .1196 .1205 .1214 .1223 .1231 .1240 .1249 .1258 .1267 .1276 .1286 .1295 .1304 .1313 .1322 .1332 .1341 .1350 .1359 .1369 .1378
Vacuum Mains 4 - 20
FLOW (gpm) 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307
HEAD LOSS (ft/100 ft) .1388 .1397 .1407 .1416 .1426 .1435 .1445 .1454 .1464 .1474 .1483 .1493 .1503 .1513 .1523 .1532 .1542 .1552 .1562 .1572 .1582 .1592 .1602 .1612 .1623 .1633 .1643 .1653 .1663 .1674 .1684 .1694 .1705 .1715 .1725 .1736
Design Manual 2008
Table 4-12 (cont) FRICTION LOSS TABLE FOR 10 INCH PIPE FLOW (gpm) 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343
HEAD LOSS (ft/100 ft) .1746 .1757 .1767 .1778 .1789 .1799 .1810 .1821 .1831 .1842 .1853 .1863 .1874 .1885 .1896 .1907 .1918 .1929 .1940 .1951 .1962 .1973 .1984 .1995 .2006 .2018 .2029 .2040 .2051 .2063 .2074 .2085 .2097 .2108 .2120 .2131
FLOW (gpm) 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379
HEAD LOSS (ft/100 ft) .2143 .2154 .2166 .2177 .2189 .2201 .2212 .2224 .2236 .2247 .2259 .2271 .2283 .2295 .2307 .2319 .2331 .2343 .2355 .2367 .2379 .2391 .2403 .2415 .2427 .2440 .2452 .2464 .2476 .2489 .2501 .2513 .2526 .2538 .2551 .2563
FLOW (gpm) 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 410 415 420 425 430 435 440 445 450 455
*SHADED AREAS NOT RECOMMENDED
Vacuum Mains 4 -21
HEAD LOSS (ft/100 ft) .2576 .2588 .2601 .2614 .2626 .2639 .2652 .2664 .2677 .2690 .2703 .2715 .2728 .2741 .2754 .2767 .2780 .2793 .2806 .2819 .2832 .2845 .2858 .2872 .2885 .2898 .2965 .3032 .3100 .3172 .3238 .3308 .3378 .3450 .3522 .3594
PROJECT LINE
AIR/LIQUID
STATION
FRICT
TO
LINE
STATION
LENGTH
Q
Q
MEAN ACC'M
FRICT
STATIC
PROJECT #
STATION
MAX FLOW
DATE
STATIC
FRICT
LOSS
LOSS
LOSS
LOSS
100'
LINE
ACCUM
LINE
ACCUM
PIPE SIZE
NUMBER AIRVAC
4"
6"
8"
10"
VALVES
V a c u 4 u -2 m 2 M a i n s
FIGURE 4-7: FRICTION LOSS CALCULATION SHEET
Design Manual 2008
I.
LOSSES DUE TO LIFTS (STATIC LOSS)
Operating vacuum ranges of 16 to 20 inches of mercury are typical for most modern vacuum systems. While deeper vacuum ranges are attainable, this range is generally considered the most practical in terms of equipment operating cost and dependability. When considering the limit of water lifted by a vacuum, it is common to think in terms of a water manometer in which the liquid is lifted in a vertical column. The height of this column is in direct proportion to the difference between available atmospheric pressure and the sub-atmospheric pressure within the vacuum.
Basis of Design In an AIRVAC vacuum system, the lower operating level of 16-in mercury vacuum pressure is used as the basis of system hydraulic design. In this case,
HOMES
Design Manual 2008
I.
LOSSES DUE TO LIFTS (STATIC LOSS)
Operating vacuum ranges of 16 to 20 inches of mercury are typical for most modern vacuum systems. While deeper vacuum ranges are attainable, this range is generally considered the most practical in terms of equipment operating cost and dependability. When considering the limit of water lifted by a vacuum, it is common to think in terms of a water manometer in which the liquid is lifted in a vertical column. The height of this column is in direct proportion to the difference between available atmospheric pressure and the sub-atmospheric pressure within the vacuum.
Basis of Design In an AIRVAC vacuum system, the lower operating level of 16-in mercury vacuum pressure is used as the basis of system hydraulic design. In this case, a vertical column of liquid could be suspended 18 feet at sea level (1" Hg = 1.13’ water). Five (5) feet of this liquid column must be reserved for the vacuum valve operation and sump liquid evacuation. This results in 13 feet of liquid available for transportation within a vacuum system (Figure 4-8).
VACUUM PUMP 30" HG = 34' H 2O
20" HG = 23' H2O NORMAL OPERATING RANGE
16" HG = 18' H 2O 18' TOTAL AVAILABLE LIFT (AT LOW END) -5' REQ'D FOR VALVE OPERATION 13' AVAIL. FOR SEWAGE TRANSPORT
FIGURE 4-8: VACUUM LIFT CAPABILITY
Vacuum Mains 4 - 23
Design Manual 2008
Recommended lift heights Static losses are those incurred by using lifts, or vertical profile changes. For efficient use of the energy available, lifts should be as small as possible. Numerous smaller lifts are recommended over one large lift. Table 4-13 shows the recommended lift height for various pipe sizes.
Table 4-13 Recommended Lift Height Pipe Diameter (in)
Lift Height (ft)
3 4 6 8 10
1.0 1.0 1.5 1.5 1.5
In no case should a lift exceeding 3 feet be made without consulting the AIRVAC technical staff.
Static loss calculations Static losses are calculated by subtracting the pipe diameter from the lift height (Figure 4-9). Static Loss = Lift Height - Pipe Diameter STATIC LOSS = LIFT HEIGHT - PIPE DIA EX: 1.5 FT LIFT ON A 6" MAIN STATIC LOSS = 1.5' - 0.5' = 1.0'
STATIC LOSS LIFT HEIGHT
PIPE DIA
FIGURE 4-9: STATIC LOSS DETERMINATION Vacuum Mains 4 - 24
Design Manual 2008
Observations from in-house hydraulic studies The idea behind the AIRVAC saw-tooth profile is to maintain an open passageway throughout the top portion of the piping network. If this is the case, the same level of vacuum will be present at the end of the system as exists at the vacuum station. The AIRVAC system operates on the concept of moving a mixture of air and liquid only part of its intended travel distance. As propulsive forces are diminished, the liquid and air will separate and the liquid will come to rest at low points within the system. Subsequent AIRVAC valve cycles will then boost this liquid downstream to a new low point and the process repeats itself until the liquid enters the vacuum station. In reality, the only time a vacuum loss occurs is when liquid is suspended within a vertical lift or when it is in motion; rarely will these events occur simultaneously. Whether or not liquid is suspended in lifts is a function of many factors: pipe diameter, pipe slope, air-to-liquid ratio, number of valves opening at a given moment, and so on. The distance liquid travels, is also a function of many factors. Due to the uncertainty of predicting the exact occurrence of so many variables, previous design limits have been based on the worst-case scenario, that is, that all lifts are completely filled or saturated. Substantial data has been gathered from operating vacuum systems in an effort to more accurately predict exactly where and when these variables will occur. While these studies remain on-going, it has become obvious that the occurrence of lift saturation is rare and normally associated with a temporary air to liquid imbalance. The ability of a vacuum system to automatically recover from such an imbalance (thereby restoring normal vacuum to line extremities) is the actual limit of vacuum line design. During these brief periods, it has been shown that all losses within the system are associated with suspended liquid with very little friction loss occurring. This is also supported by the fact that little or no flow takes place.
Vacuum Mains 4 - 25
Design Manual 2008 Recommended static loss limits Empirical studies currently indicate that system performance may be grouped as follows:
Table 4-14 Static & Friction Losses: Design Guidelines
Group
Static Loss (ft)
Friction Loss (ft)
Operation during Normal conditions
Recovery from temporary A/L imbalance
A
0.0 -13.0
5
Dependable
Automatic recovery without disruption of service
B
13.1- 16.0
5
Dependable if
Automatic recovery with momentary low vacuum at line extremities
a) high static loss is limited to 1 or 2 branches & b) optimum placement of valves and lifts are considered
C
> 16.0
5
Reasonably dependable if : a) high static loss is limited to 1 or 2 branches &
Recovery relies on the injection of atmospheric air at selected points to restore adequate vacuum to line extremities
b) optimum placement of valves and lifts are considered
AIRVAC recommends that systems be designed with the guidelines stated in group A: 13 Feet of Vacuum Loss due to Li fts 5 Feet of Vacuum Los s due to Fricti on The static loss and friction losses are separate calculations and are a summation of losses for each flow path. Simply stated, a flow path is the piping through which liquid will travel from the last AIRVAC valve to the vacuum station. AIRVAC s hould be consul ted fo r any design fo ll owin g the gui delin es of group B or C. Vacuum Mains 4 - 26
Design Manual 2008 Widespread use of deep valve pit For any single flow path where more than 25% of the valve pits are of the deep variety (8’ and/or 10’ deep pits), the allowable static loss of that vacuum main may need to be reduced from 13 ft. to a lower amount to account for the additional lift within the valve pit. Consult AIRVAC’s Engineering Department for details.
Electronic Air Admission Control (EAAC) The Electronic Air Admission Control (EAAC) is an accessory for a typical 3” AIRVAC vacuum valve. This device monitors line vacuum and will open if vacuum falls below a pre-set limit for an extended period of time. The opening of this valve injects large quantities of atmospheric air to boost liquid through various lifts and downstream towards the vacuum station. The end result is an increase of available vacuum for valve operation. The primary purpose of the EAAC is to improve the transport characteristics of a system that is already in operation. Occasionally, this device can be included as a design feature when static losses are slightly in excess of the recommended 13 feet. In these cases, the EAAC device may be needed to insure adequate vacuum is available for those particular sections of vacuum lines where the 13 ft limit has been exceeded. NOTE: Minimum vacuum pump capacity w hen utili zing EAAC units is 305 acfm. Since there are limits to the use of these devices, AIRVAC should be involved in the placement and parameters for their use.
J.
PIPE VOLUME
It is necessary to calculate the total connected pipe volume, expressed in gallons, for use in the vacuum pump sizing process (see Chapter 3). Table 4-15 shows the pipe volume per lineal foot for the various pipe sizes based on the ID of SDR 21 pipe. Multiply these factors by the length of pipe and then by 7.48 to convert to pipe volume in gallons.
Table 4-15 Pipe Volume per Linear Foot For Various Pipe Sizes Pipe Diameter Volume per foot (ft 3/ft) 3” 0.0547 4” 0.0904 6” 0.1959 8” 0.3321 10” 0.5095 Vacuum Mains 4 - 27
Design Manual 2008 PROJECT
PROJECT NUMBER
STATION NUMBER
DATE
LINE
4" PIPE
6" PIPE
8" PIPE
10" PIPE
NUMBER SERVICE LATERALS
PEAK
NUMBER AIRVAC VALVES
TOTALS AVERAGE SERVICE LATERAL LENGTH TOTAL 3” PIPE
(VOLUME OF PIPEWORK: B ASED ON SDR-21 PVC PIPE)
Vp = (.0547 x Length 3") + (.0904 x Length 4") + (.1959 x Length 6") + (.3321 x Length 8") + (.5095 x Length 10") ft3 Vp = (
+
+
Vp = 7.48 gal/ft3 x ( 2/ Vp = 3
+ ) ft3
+ =
) ft 3 gal
gal
FIGURE 4-10: PIPE VOLUME CALCULATION SHEET Vacuum Mains 4 - 28
HOMES
Design Manual 2008
K.
SUMMARY OF VACUUM MAIN DESIGN FUNDAMENTALS
1.
SLOPES: a. b. c. d.
2.
FALL BETWEEN LIFTS: Use larger of two (2) values a. b. c.
3.
0.20% x Length 0.20 ft. Fall for 3" Service Lines if lifts are less than 100 lf apart. 0.25 ft. Fall for ALL Vacuum Mains if lifts are less than 125 lf apart.
LIFTS: a. b. c. d. e.
f.
4.
Use natural ground slope if greater than 0.2% Use 0.2% slope for flat terrain Use saw tooth profile for uphill transport Use 0.2% slope at 50’ minimum prior to first lift in any series
Use 1’-0” for 3” or 4” pipe Use 1’-6” for 6” or larger pipe Static loss = Lift Height - Pipe Diameter Maximum vacuum loss due to lifts from any AIRVAC valve to the vacuum station = (13 ft Static Loss + 5 ft Friction Loss) Maximum series of lifts = 5. Separate one series of 5 lifts from the next lift or series of lifts by at least 100 ft of vacuum main. In addition, there must be at least 1 energy input in the zone of separation. First lift on a branch minimum 20 lf from connection to main.
CONNECTIONS: a. b. c. d.
Use wye connectors for all branch and lateral connectors, wye may be vertical or at 45 ° angle Use long sweep 90 ° ell for 3” service connectors only Use 45 ° ells for 4” and larger connectors and any directional change Recommended minimum Invert to Invert elevation difference for connections: 4 x 3 = 0.66 ft 4 x 4 = 0.71 ft
6 x 3 = 0.84 ft 6 x 4 = 0.85 ft Vacuum Mains 4 - 29
8 x 3 = 1.40 ft 8 x 4 = 1.40 ft
Design Manual 2008
5.
FLOW LIMITS: (Maximum Friction Loss not to exceed 5 ft) 3” = 3 gpm 4” = 38 gpm 6” = 106 gpm 8” = 210 gpm 10” = 375 gpm
6.
MAXIMUM LINE LENGTHS: 3” = 300 lf 4” = 2,000 lf 6” & Larger determined by static limits
7.
PIPE VOLUMES
Pipe Size
Volume per foot (vf) ft /ft
3 inches
.0547
4 inches
.0904
6 inches
.1959
8 inches
.3321
10 inches
.5095
Volume in a specific size of pipe is determined by multiplying the length of pipe by the appropriate vf factor above: L x vf = ft 3 The volume of 300 lf of 6” pipe is therefore: 300 x .1959 = 58.77 ft 3.
8.
ISOLATION VALVES a. Place a line isolation or division valve for each branch vacuum sewer near its connection to main line. b. Place isolation or division valves for main lines at 1500-foot centers or near branch connection. Vacuum Mains 4 - 30
Design Manual 2008
L.
DO’S AND DON’TS
Table 4–16 Do’s and Don’ts: Vacuum Main Design
DON’T DO THIS:
DO THIS:
Exceed 13.0 feet static loss on any flow path
Contact AIRVAC if static loss of 13.0 ft or less is not attainable
Exceed 5.0 friction loss on any flow path
Increase pipe size if necessary
Design more than (5) lifts in any series
Separate a series of (5) lifts from the next lift or series of lifts by at least 100 ft of vacuum main and insure that at least 1 energy input is in the zone of separation
Design a lift greater than 3 ft without first contacting AIRVAC
Use multiple, smaller lifts
Use 0.20% slope between lifts when distance between lifts is < 125 ft.
Use 0.25 ft of fall between lifts when distance between lifts is < 125 ft.
Slope a main greater than 0.20% immediately prior to a series of lifts
Use a minimum of 50 lf of main sloped at 0.20% prior to a series of lifts
Use more than 2000 lf of 4” vacuum sewer at the end of any vacuum line
Increase pipe size to 6”
Design a vacuum system with all flow in a single vacuum main
Divide the system into zones by using a multiple vacuum main concept
Design long stretches of vacuum mains without valve pit connections
Install a valve pit or other means of energy input at regular intervals
Use a vacuum sewer as an interceptor sewer by connecting a considerable amount of gravity sewers via buffer tanks
Limit flow admitted via buffer tanks as discussed in Chapter 6
Vacuum Mains 4 - 31
Design Manual 2008
CHAPTER 5 VALVE PITS A.
VALVE PIT ARRANGEMENTS
Pit types Table 5-1 shows the various AIRVAC valve pit arrangements that are available. AIRVAC recommends the use of the 1-piece PE pits for most projects. In situations where deeper pits are required, the 3-piece fiberglass pits are used.
Table 5-1 AIRVAC Valve Pit Types Pit Type
AIRVAC Model No
Pit Depth
Pit Description
Recommended Maximum # Conn
Shallow
VP3030WT
5.0 ft
PE 1-piece
2
Shallow
VP3030
5.0 ft
Fiberglass 3-piece
2
Standard
VP4830WT
6.5 ft
PE 1-piece
4
Standard
VP3042T
6.0 ft
Fiberglass 3-piece
4
Deep
VP5442
8.0 ft
Fiberglass 3-piece
4
Extended
VP5466
10.0 ft
Fiberglass 3-piece
4
1.
Maximum combined peak flow to these valve pit types is limited to 3 gpm.
2.
The sumps to these valve pit types can physically accept up to 4 incoming lines, subject to the 3 gpm maximum (see Note 3).
3.
If a Dedicated Air Terminal (DAT) is used, one of the 4 sump openings must be reserved for that purpose, reducing the maximum possible # of connections from 4 to 3. The manifolding of a building sewer to the DAT outside the sump is not permitted (See page 5-17).
4.
One or more 1 ft extension pieces can be added to any valve pit. Extending in this manner will result in the valve residing lower in the upper chamber.
For larger water users with peak flows in excess of 3 gpm, see Chapter 6 (Buffer Tanks). Valve Pits 5-1
Design Manual 2008 Pit sharing Up to four separate building sewers can be connected to one sump, each at 90 degrees to one another. However, this is rarely done as property lines considerations and other factors may render this impractical. By far, the most common valve pit sharing arrangement is for two adjacent houses to share a single valve pit. In certain cases, such as a cul-de-sac or when small lots exists, it may be possible to serve 3 or even 4 houses with a single valve pit; however, all other design factors, such as limiting peak flow to 3 gpm must be considered.
FIGURE 5-1 6.5’ AIRVAC VALVE PIT PE 1-PIECE PIT
Valve Pits 5-2
Design Manual 2008
FIGURE 5-2 6’ AIRVAC VALVE PIT FIBERGLASS 3-PIECE PIT
Valve Pits 5-3
Design Manual 2008
B.
COMPONENTS COMMON TO BOTH PIT TYPES
3” Valve The AIRVAC valve is vacuum operated on opening and spring assisted on closing. System vacuum ensures positive valve seating. The valve has a 3inch full-port opening, is made of glass filled polypropylene, and has a stainless steel shaft, delrin bearing and elastomer seals. The valve is equipped with a rolling diaphragm-type vacuum operator and is capable of overcoming all sealing forces; and of opening using vacuum from the downstream side of the valve. All materials of the valve are chemically resistant to normal domestic sewage constituents and gases. The AIRVAC 3” valve and controller combination requires 5 in. Hg vacuum for operation and to avoid low air-to-liquid ratios. The lower the vacuum level, the less differential (atmospheric pressure to line vacuum) exists. This equates to less air entering the system resulting in lower line velocities and sluggish flow characteristics.
Valve capacity: Chapter 6 contains a detailed discussion of the 30 gpm rated capacity of the AIRVAC 3" valve and when that capacity is possible. It is very important to note that the rated capacity of the valve is not to be confused with the recommended design capacity of t he valve pits. To ensure proper air-to-liquid ratios, AIRVAC recommends a maximum peak flow of 3 gpm be used for all valve pits serving residential customers. Buffer tanks may be sized using higher design capacities. Table 5-1 shows the recommended maximum # of connections for the various pit types.
Size: Many states do not have vacuum sewer regulations. Some of those that do, apply concepts of the Ten State Standards that advocate the ability to pass a 3” solid through any part of a sewage collection and treatment system. In addition, some plumbing codes have provisions that to prohibit restrictions less than 3” downstream of any toilet. To be consistent with industry standards, AIRVAC’s vacuum valve was designed to be a full-port 3” valve capable of passing a 3” solid while matching the outside diameter of 3” PVC SDR pipe.
Valve Pits 5-4
Design Manual 2008 Pressure loss through the interface valve: The driving force in a vacuum system is the pressure differential that exists between atmosphere and vacuum in the system. This differential occurs when the valve opens. As a result, the only place to impart energy in a vacuum system is at the valve itself. Any loss through the valve further depletes this energy resulting in less available for transport within the pipeline. This is especially critical considering that this loss occurs at every valve and during each valve cycle. The flow coefficient (Cv factor) is that flow rate in gpm which would yield a head loss of 1 psi. The AIRVAC valve is produced with an internal geometry that keeps friction loss to an absolute minimum, having a C v factor of 268.
Controller The controller/sensor is the key component of the valve. The device relies on three forces for its operation: pressure, vacuum, and atmosphere. As the sewage level rises in the sump, it compresses air in the sensor tube. This pressure initiates the opening of the valve by overcoming spring tension in the controller and activates a three-way valve. Once opened, the three-way valve allows the controller/sensor to take vacuum from the downstream side of the valve and apply it to the actuator chamber to fully open the valve. The controller/sensor is capable of maintaining the valve fully open for a fixed period of time, which is adjustable over a range of 3 to 10 seconds. After the preset time period has elapsed, atmospheric air is admitted to the actuator chamber permitting spring assisted closing of the valve. The AIRVAC vacuum valve controller is chemically resistant to sewage and sewage gases and is capable of operating when submerged in water. It was designed to give consistent timing as set by the system operator. The AIRVAC valve pit was also designed so that a very repeatable, specific amount of liquid is withdrawn on each cycle (10 gallons). With a consistent amount of air and a specific amount of liquid, the air-to-liquid ratio is kept consistent. In-sump breather The controller requires a source of atmospheric air to the actuator chamber permitting spring assisted closing of the vacuum valve. Without this air, the valve would remain in the open position. The in-sump breather uses atmospheric air from the sump and its associated 4” gravity building sewer and 4” air intake (or the optional 6” dedicated air-terminal). The in-sump breather is designed to protect the controller from unwanted liquid during system shutdowns and restarts. It also prevents sump pressure from forcing the valve to open during low vacuum conditions and provides positive sump venting regardless of traps in the gravity service line. Note: Conceptually the in-sump breather provides the same function for both styles of valve pits; however, in the 1-piece pit the in-sump breather is combined with the 2” sensor line as a single unit. Valve Pits 5-5
Design Manual 2008 Flexible Service Lateral The contractor must be careful when making the connection between the valve pit and the vacuum main. The difficulty arises when the contractor must connect two fixed points at different locations/elevations with rigid, solvent-welded pipe. Many times this requires multiple fittings, some of which may be deflected beyond an acceptable range. In certain cases, this can result in either a vacuum leak, or worse, a line break caused by overstressing of the joint. The AIRVAC Flexible Service Lateral, which uses 3" flexible PVC hose, eliminates this problem. Connections at both ends of the flexible service lateral are the same as with PVC pipe. The use of a flexible service lateral virtually eliminates stress-related leaks caused by poor workmanship or ground settlement.
FIGURE 5-3: AIRVAC FLEXIBLE SERVICE LATERAL
Valve Pits 5-6
Design Manual 2008
Stub-Out To minimize the risk of damage to the fiberglass valve pit during homeowner connection to the system, a stub-out pipe of sufficient length, typically 6 ft. from the valve pit, is recommended. The orientation of the valve pit as it relates to the house and the wye connection varies according to the number of connections to the pit (see Figure 5-4). If the building sewer piping network does not have a cleanout within it, one should be placed outside and close to the home. Some agencies prefer having a cleanout at the dividing line to establish the point at which the agency’s maintenance begins. This typically would be at the end of the stub-out pipe.
6' MIN. LENGTH GRAVITY SERVICE LATERAL WITH GLUED CAP PRIVATE YARD RIGHT-OF-WAY
45°
45°
5 ' M I N . VALVE PIT ASSEMBLY
5' MIN.
VACUUM MAIN FLOW SERVING 2 HOUSES
SERVING 1 HOUSE
FIGURE 5-4: TYPICAL CONFIGURATIONS FOR GRAVITY CONNECTIONS
Valve Pits 5-7
Design Manual 2008
Valve pit covers: traffic rated Cast iron covers and frames, designed for H-20 traffic loading, are typically used for all valve pit installations. For traffic-rated covers, the frame weight is generally 90 lbs. and the lid weight about 100 lbs. AIRVAC recommends that concrete collars be used for all AIRVAC valve pits located in traffic areas. The actual design of the concrete collar is the responsibility of the Design Engineer. In addition, the Design Engineer’s must clearly define “traffic areas” on their construction drawings and/or specifications. Unless otherwise specified, the words “AIRVAC SEWER” in 1” tall lettering will be used on the cover. Contact AIRVAC’s engineering department for a list of vendors that supply traffic-rated frames and covers that match AIRVAC’s valve pits.
Valve pit covers: non-traffic rated When a lighter lid is desired, as in non-traffic situations, a lightweight aluminum or cast iron lid may be used. These lids should clearly be marked "Non-Traffic". For consistency reasons, AIRVAC does not recommend the use of non-traffic covers if there are only a few isolated cases in the project where a non-traffic lid can used. Contact AIRVAC’s engineering department for a list of vendors that supply nontraffic rated frames and covers that match AIRVAC’s valve pits.
Valve pit covers-pick holes & seals The type of lid used varies according to which AIRVAC valve pit (1-piece or 3piece) is used: •
•
Covers for the 1-piece pit have a concealed pick hole and elastomer seals. Covers for the 3-piece pit have an open pick hole and no elastomer seals.
Valve Pits 5-8
Design Manual 2008
C.
VALVE PIT COMPONENTS: PE 1-PIECE PITS
Figure 5-5 shows the AIRVAC PE 1-piece valve pit assembly.
1-PIECE PIT 3" VALVE
INTEGRAL ANTIBUOYANCY COLLAR
IN-SUMP BREATHER/ SENSOR PIPE SUCTION PIPE
FIGURE 5-5 VALVE PIT COMPONENTS PE 1-PIECE PIT
Valve Pits 5-9
Design Manual 2008
1-Piece Pit The 1-piece pit is manufactured by the rotational molding process using PE with an integral upper valve chamber, lower collection sump and anti-buoyancy collar. The wall thickness of the entire unit is ½” in order to be suitable for H-20 traffic loading. The upper chamber houses the vacuum valve, controller and in-sump breather with a 36” inside diameter at the bottom and is conically shaped to allow fitting a 26 ¾” frame with a 23 ½” diameter clear opening cast iron cover. The upper chamber includes a 3” vacuum service lateral pipe support with rubber o-ring seal to insure proper pipe alignment with the valve/suction pipe. Both the 5’ and 6.5’ deep pit have an upper chamber depth of 30”. The lower chamber contains 4 stabilizing embosses to support the valve pit and is designed to allow up to (4) houses to be connected with either 4” or 6” pressure rated Sch 40, SDR 21 or SDR 26 PVC pipe. Elastomer connections are used for the entry of the building sewer. Holes for the building sewers are field cut at the position directed by the engineer. The 5’ deep pit has a lower collection sump depth of 30” and a capacity of 57 gallons, while the 6.5’ deep pit has a lower collection sump depth of 48” and a capacity of 115 gal.
In-Sump Breather/Sensor Pipe The 2” sensor pipe is incorporated into the sump-breather and the unit has a twist lock mechanism to mate with the hole that connects the upper and lower chambers.
Suction Pipe The 3” suction pipe is PE and also has a twist lock mechanism to mate with the hole that connects the upper and lower chambers.
Integral Anti-Buoyancy Collars The AIRVAC 1-piece valve pit has a factory installed integral PE anti-buoyancy collar.
Valve Pits 5 - 10
Design Manual 2008
D.
VALVE PIT COMPONENTS: FIBERGLASS 3-PIECE PITS
Figure 5-6 shows the AIRVAC fiberglass 3-piece valve pit assembly. The 3 major pieces are the valve pit cone, the pit bottom and the sump.
VALVE PIT CONE
3" VALVE
IN-SUMP BREATHER
ANTI-BUOYANCY COLLAR
PIT BOTTOM
SUCTON PIPE
STUB-OUT
SUMP
SENSOR PIPE
FIGURE 5-6 VALVE PIT COMPONENTS FIBERGLASS 3-PIECE PIT
Valve Pits 5 - 11
Design Manual 2008 Valve Pit Cone The valve pit cone houses the vacuum valve, controller and in-sump breather. It is fabricated with filament-wound fiberglass with a wall thickness of 3/16" in order to be suitable for H-20 traffic loading. The valve pit is 36" in diameter at the bottom and is conically shaped to allow the fitting of a 23 ½" diameter clear opening cast iron frame and cover at the top. Depths are normally 42". One 3" diameter opening, with an elastomer seal, is pre-cut to accept the 3" vacuum service line.
Pit Bottom The pit bottom is made from the reaction injection molding process (RIM) using heavy duty liquid molding resin polymer. The pit bottom has a nominal thickness of .320”, with .500” reinforcing ribs. The pit bottom is provided with holes pre-cut for the 3" suction line, 4" cleanout/sensor line (later sealed by grommets), the in-sump breather and 16 stainless steel sump-securing bolts. A factory supplied elastomer o-ring provides a watertight seal between the pit bottom and the sump. The pit bottom has a lip that allows the valve pit to fit perfectly centered on top of the sump/pit bottom. While groundwater can enter the valve pit cone area, the seal between the pit bottom and sump prevents any extraneous water from entering the sump.
Sump The sump has a wall thickness of 3/16" and is designed for H-20 traffic loading with 2 ft. of cover. Elastomer connections are used for the entry of the building sewer. Holes for the building sewers are field cut at the position directed by the engineer. The sumps employed thus far have two different heights: 30" and 54". Both are 18" in diameter at the bottom and 36" in diameter at the top, with the smaller size having a capacity of 55 gallons and the larger one 100 gallons.
In-Sump Breather The controller requires a source of atmospheric air to the actuator chamber permitting spring assisted closing of the vacuum valve. Without this air, the valve would remain in the open position. The in-sump breather uses atmospheric air from the sump and its associated 4” gravity building sewer and 4” air intake (or the optional 6” dedicated airterminal). The in-sump breather is designed to protect the controller from unwanted liquid during system shut-downs and restarts. It also prevents sump pressure from forcing the valve to open during low vacuum conditions and provides positive sump venting regardless of traps in the gravity service line.
Valve Pits 5 - 12
Design Manual 2008
Suction & Sensor Pipes Both the 3” suction pipe and the 2” sensor pipe are Sch 40 PVC and are installed through grommets in the pit bottom.
Anti-Buoyancy Collars AIRVAC fiberglass anti-buoyancy collars are sometimes used on the fiberglass valve pit settings (see Fig 5-7). The designer should perform buoyancy calculations to see if they are necessary. These collars fit around the tapered valve pit and rely on soil burden to keep the pit from floating.
53" NOMINAL VALVE PIT
36" DIAMETER (+/- 1/8")
COMPACTED SELECT FILL
1/2" THICK FRP RESIN L A N I M O N " 3 5
COMPACTED GRANULAR FILL 5" RADIUS 4 CORNERS TOP VIEW
ELEVATION VIEW
FIGURE 5-7: AIRVAC ANTI-BUOYANCY COLLAR
Anti-buoyancy collars consisted of mass concrete rings are not recommended. These concrete rings required care be taken during the valve pit installation as poor bedding and backfill could lead to settlement problems. Settlement of the concrete ring most likely will coincide with damage to the building sewer and/or the pit itself.
Valve Pits 5 - 13
Design Manual 2008
E.
BUILDING SEWER (House to Pit)
Building Sewer The term building sewer refers to the gravity flow pipe extending from the home to the valve pit setting. This includes the pipe in private property as well as in the public right-of-way (i.e. – the stub-out). In many cases, state or local authorities regulate installations of building sewers. For residential service, the building sewer should be 4" and slope continuously downward at a rate of not less than 0.25-in/ft (2-percent grade). Line size for commercial users will depend on the amount of flow and local code requirements. AIRVAC recommends that pressure rated pipe be used for the building sewer. There are two reasons for this. First, the pipe OD must match the valve pit inlet grommet to prevent the entrance of I/I in to the valve pit sump. Second, the building sewer may be exposed to high vacuum at times. Under normal conditions, these vacuum levels would be low (5-10 in. Hg) and would occur for just a short duration (the 3-5 second valve cycle). However, should the airintake be blocked and the vacuum valve fail in the open position at the same time, the building sewer could see the full system vacuum of 20 in. of Hg. AIRVAC also recommends that pressure rated fittings be used for the stub-out pipe portion of the building sewer that is located in the public right-of-way. Table 5-2 shows AIRVAC’s recommendations. These recommendations assume that the pipe is also in compliance with all applicable codes.
4” Air Intake An air intake, consisting of 4" PVC pipe, fittings and a screen (Figure 5-8), is required for each building sewer. Its purpose is to provide a sufficient amount of air to enter into the service line and vacuum main to act as the driving force behind the liquid that is evacuated from the sump. The house vent may not be used in place of an air intake. While it may provide the necessary air, its location could result in the house plumbing traps being evacuated during a valve cycle. For this reason, the air intake must be located downstream of the last house plumbing trap. The use of a check valve within the air intake mechanism itself is not recommended as this will result in problems with the traps inside the house. Most plumbing-code enforcement entities require the air intake to be located against a permanent structure, such as the house or a wall. Valve Pits 5 - 14
Design Manual 2008
Table 5-2 Recommended Pipe Types for Stub-Outs and Building Sewers
Type
Rated Press. (psi)
O.D (in).
I.D. (in)
Wall thickness (in.)
Weight per 100 ft (lbs)
Sch 40
DWV
220
4.500
4.026
0.237
210.2
D1784 D1785 D2466
SDR 21
DWV
200
4.500
4.046
0.214
188.6
D2241 D2464
SDR 26
DWV
160
4.500
4.145
0.173
154.0
D2241 D2464
Sch 40 (solid wall)
DWV
0
4.500
4.026
0.237
200.0
D1784 D1785 D2665
Sch 40 (foam core)
DWV
0
4 .500
4.026
0.237
146.0
F 891 D1784 D4396 D2665
SDR 26
S&D
0
4.215
3.891
0.162
140.0
D4396 D3034
SDR 35 (solid wall)
S&D
0
4.215
3.965
0.125
113.0
D2729 D3034 F 679
SDR 35 (foam core)
S&D
0
4.215
3.965
0.125
ASTM
Pressure
RECOMMENDED For Stub-out and Building Sewer
Non-Pressure DO NOT USE For Stub-out NOT PREFERRED For Building Sewer (But OK if Stub-out uses recommended pipe)
Non-Pressure
DO NOT USE For Stub-out or Building Sewers
SDR: S&D: DWV: Stub-out: Bld’g sewer:
F 891
Standard Dimension Ratio = OD/wall thickness Sewer and Drain Drain, Waste & Vent Pipe from the valve pit to property line (installed by system contractor) Pipe from stub-out to air-intake (installed by homeowner pl umbing contractor)
NOTE: AIRVAC recommends that pressure rated fittings be used for the stub-out pipe portion of the building sewer that is located in the public right-of-way.
Valve Pits 5 - 15
Design Manual 2008
FIGURE 5-8: 4” AIR INTAKE
FIGURE 5-9: OPTIONAL 6” DEDICATED AIR-TERMINAL Valve Pits 5 - 16
Design Manual 2008
Optional 6” Dedicated Air-Terminal As an alternative to requiring each house to have a 4” air-intake on their building sewer, the AIRVAC’s molded 6” Dedicated Air Terminal (DAT) may be used at each valve pit instead. The DAT was designed to look like other utility boxes/structures typically seen in rights-of-way. Testing indicates that using the 6” DAT results in a much more effective method of air induction than using the combined air/sewage method associated with the 4” air intakes. Other advantages include: 1) The DAT’s use eliminates the need for the unsightly 4” air-intake in the homeowner’s gravity piping that is located either next to the house or in the middle of the yard. 2) Eliminates the possibility of vacuum emptying home plumbing traps. 3) Allows the valve pit to be put into service before any home gravity laterals are installed (see NOTE). This also relieves the fear of possible pit implosion that could occur when a valve is installed without the homeowner’s 4” air-intake in place. 4) Provides a mounting point for future valve pit options such as valve Cycle Counter, Alarm Monitoring Systems, or v alve breather. 5) Allows for the use of a normally-opened backwater valve prevention device on home gravity laterals without the design or installation restrictions associated with normally closed backwater valves used on vacuum systems. 6) Eliminates any adverse effects on the valve operation associated with improper gravity lateral installations (improper fall, belly in piping). 7) Prevents municipalities from becoming involved with problem diagnosis relating to improper gravity lateral installations. As long as the valve pit functions properly, any other problem is the homeowner’s responsibility.
If a Dedicated Air Terminal (DAT) is used, one of the 4 sump openings must be reserved for that purpose. The manifolding of a building sewer to the DAT outside the sump is not permitted. This reduces the possible number of connections to the valve pit from 4 to 3. After the vacuum system has been installed and passed the final test the AIRVAC Valves may be installed and put into service. The system vacuum must be maintained in service once the valves are installed. This is the only approved method of valve installation before home gravity connection installation that retains the AIRVAC product warranty. Valve Pits 5 - 17
Design Manual 2008
Proper Time to Install the Vacuum Valve Unless the optional 6” Dedicated Air Terminal is used, the AIRVAC vacuum valve should be installed only after the homeowner has connected the building sewer to the stub-out at the valve pit. (See Figures 5-10 & 5-11) and the 4” air intake is in place. The normal sequence of events is as follows: • • • • • •
Contractor installs lines and pits Contractor conducts final 4 hour test on lines & station Contractor flushes vacuum mains The system is accepted by the operating entity Homeowner’s plumber installs building sewer and the 4” air intake Operating entity installs the vacuum valve
Potential Problems if Valve is installed During Construction The desire to have a complete system has led some design engineers to require the system installation Contractor to actually install the AIRVAC vacuum valve during the construction period. AIRVAC does not recommend this procedure as this can result in some very serious problems as described below:
Pit collapse/implosion: Cycling the AIRVAC valve without the homeowner’s lateral and 4” air intake installed can cause bottom sump to implode. This would require the pit to be re-excavated and replaced. This not only is costly, but finger-pointing on who is to blame will surely ensue. Homeowner may illegally hook-up early: Knowing that a complete system is available, the homeowner could connect to the valve pit without the Owner’s knowledge. This action would preclude the Owner from doing the normal inspection of the homeowner’s gravity lateral, air intake, etc. Again, this could lead to some serious problems such as I&I, water in the controller, etc. Valve may be improperly installed: It is conceivable that the Contractor may connect hoses to the wrong port, fail to tighten all hose clamps, fail to adjust the controller timing properly, etc. Any of these could severely affect the operation of the system. Valve timing may be improper (timing is site specific): The timing of the valves is a major concern. Each valve needs to be adjusted differently according to its location within the system. AIRVAC believes that it is expecting too much for the Contractor to know how to balance the system by properly timing each valve. The Contractor cannot be expected to be a vacuum expert. The valve should be properly installed and timed by someone who has been trained to do so…either the system operator or AIRVAC personnel. Valve Pits 5 - 18
Design Manual 2008
FIGURE 5-10: VALVE PIT INSTALL ATION PRIOR TO HOME HOOK-UP
FIGURE 5-11: VALVE PIT INSTALLATION AFTER HOME HOOK-UP
Valve Pits 5 - 19
Design Manual 2008
Backwater Valves Some entities prefer the use of backwater valves on the homeowner's building sewer in order to provide sewage backup protection for houses that share a valve pit. This is especially true when the houses are at different elevations. For the typical “normally closed” type backwater valve, positioning is critical. If not installed between the home and the gravity air intake pipe, the AIRVAC valve will not operate. AIRVAC recommends backwater valves that are designed to be “normally opened”. Positioning is not an issue for this type of valve and they have proven to have superior performance when used in a vacuum system. In any case, AIRVAC should be consulted before such a device is installed.
Grease Traps State and local codes typically require grease traps for restaurants, hotels or any other establishment that serves meals. These are simple devices, usually placed in the kitchen floor that are designed to provide retention time for grease to cool and solidify so that it can be removed before it enters the sewer system. Grease that is allowed to enter and solidify in a valve pit can cause problems with the operation of the valve. The most common problem would be grease that enters the sensor pipe and later accumulates between the surge suppressor and controller. Grease in this area would not allow the free flow of air necessary for valve closure. The result would be a valve that does not close. In general, grease is rarely a problem with the standard AIRVAC valve pit. It is more likely to be a problem in a buffer tanks as these are used to serve the type of commercial establishments where grease can be a problem. AIRVAC recommends that the sewer authority require all customers to follow codes regarding the installation of grease traps.
Connection via a Pump AIRVAC does not recommend connecting a grinder pump or lift station directly to an AIRVAC valve pit. Instead, a transitional manhole should be used as an interface between pressure and vacuum. As an option, a buffer tank can be used for this transition. If there is no other practical way to serve a customer other than connecting a pump directly to an AIRVAC valve pit, please contact AIRVAC’s Engineering Department for guidance. Valve Pits 5 - 20
Design Manual 2008
D.
DO’S AND DON’TS
Table 5-3 Do’s and Don’ts: Valve Pits
DON’T DO THIS:
DO THIS:
Connect more than 4 homes to a single valve pit
Use a 2:1 ratio as a standard and carefully analyze where service to 3 or 4 homes is feasible
Exceed a peak flow rate of 3 gpm for a single valve pit
Limit peak flow to 3 gpm for a single valve pit Use buffer tanks for higher flows
Allow infiltration in the homeowner’s building sewer
Ensure building sewer meets specification and is inspected by the local authority
Connect any home or other establishments with storm systems attached
Ensure no downspouts, sump pumps, cisterns or other storm water receptacle are connected
Use non-pressure pipe & fittings for stub-outs or building sewers
Use pressure rated pipe & fittings for stub-outs and building sewers as shown on Table 5-2
Allow bellies in the building sewer which could fill with liquid and block off air
Inspect the building sewer prior to pipe burial to ensure no bellies exist
Combine sewage and air by manifolding a building sewer to the 6” Dedicated AirTerminal
Use the 6” Dedicated Air-Terminal as an “aironly” device
Allow installation of the 3” interface valve until the 4” air intake is in place
Install 4” air intake prior to the valve installation
Connect a grinder pump directly to an AIRVAC valve pit
Use a transitional manhole as an interface between pressure & vacuum
Valve Pits 5 - 21
Design Manual 2008
CHAPTER 6 BUFFER TANKS
A.
DESCRIPTION
Buffer tanks are designed with a small operating sump in the lower portion. Additional emergency sewage storage is provided above the sump. The operating sump is to be configured 18” in diameter with a depth of 12”. In no case should a buffer tank be constructed without one such sump per valve. It is very important that all joints and connections be watertight to eliminate ground-water infiltration. Equally important is the need for a well-designed pipe support system, since these tanks are open from top to bottom. The support hardware should be of stainless steel and/or plastic. For all buffer tanks, above ground venting of the AIRVAC valve must be installed to insure proper venting in the event that the buffer tank becomes filled with sewage. AIRVAC recommends the use of its 6” dedicated air-terminal for this purpose. In addition, AIRVAC recommends the use of an external, flexible breather rather than the in-sump breather that is used for other valve pits. Figure 6-1 shows a typical single buffer tank arrangement, which may be used to accept high sewage flows from schools, apartments, nursing homes, or conventional sanitary pump stations. Dual valve buffer tanks may also be used for higher flows (Figure 6-2). A prefabricated buffer tank made of a composite material in lieu of concrete is also available (contact AIRVAC for details).
B.
WHEN & WHERE USED
Buffer tanks are typically used for schools, apartments, nursing homes, and other large-volume users. Their use should be limited to users where service at only one or two locations is possible (e.g.: a commercial user) and when no other option is available. Stated another way, buffer tanks should not be used where individual valve pits could otherwise be utilized. A system with nothing but high-flow inputs is not recommended, although it is possible to have a limited amount of high-flow inputs (buffer tanks) into a vacuum system. Much depends on the location of the buffer tank itself, the length of travel to the vacuum station, the amount of lift in the system, and how many other buffer tanks there are in close proximity. In general, the closer to the station, the fewer lifts to overcome, and the fewer other buffer tanks that exist, the less likely it is that the buffer tank will have an adverse effect on the system operation. Buffer tanks 6-1
Design Manual 2008
FIGURE 6-1: SINGLE BUFFER TANK
Buffer Tanks 6-2
Design Manual 2008
FIGURE 6-2: DUAL BUFFER TANK
Buffer Tanks 6-3
Design Manual 2008
C.
EFFECT ON SYSTEM HYDRAULICS
Efficient flow within a vacuum system relies on partial liquid and partial air inputs at controlled rates. This is normally accomplished by uniform distribution of valve pits through a service area with each valve pit attached to no more than four (4) homes. This allows relatively small liquid flow followed by a volume of air for propulsion of liquid. In this manner, a uniform system air-to-liquid ratio is maintained thereby maximizing available vacuum throughout the system. By contrast, buffer tanks with valves that cycle frequently can sometimes contribute larger volumes of the liquid, but with the same amount of air. This can lead to irregular air to liquid ratios, inefficient vacuum pump performance and sluggish system operation. For these reasons, buffer tanks should only be utilized under the following conditions: •
•
•
•
•
D.
Buffer tanks should be limited to users where service at only one or two locations is possible (e.g.,: a commercial user). Buffer tanks should not be used where individual valve pits could otherwise be utilized. Buffer tanks should not be placed at line extremities without contacting AIRVAC. Single buffer tanks are to be connected to a 6” or larger vacuum main. Dual buffer tanks are to be connected to an 8” or larger vacuum main.
BUFFER TANK SIZING
The AIRVAC 3-inch valve is capable of admitting 30 gpm of liquid, assuming sufficient vacuum levels exist at the valve location . This capacity is achieved when the valve cycles 3 times in a minute, with each cycle discharging 10 gallons as described in the following paragraph. The length of one complete valve cycle is about 6-8 seconds, consisting of 2-3 seconds for the liquid, followed by 4-5 seconds of air. During this time, vacuum levels at the valve location temporarily drop as energy is used to admit the sewage into the main line. The valve rests before the next cycle begins, which occurs when another 10 gallons accumulates in the sump. Vacuum recovery occurs during the valve-closed time as the vacuum level in the main is restored. Typically a rest period of 10-15 seconds is needed to allow vacuum to be restored to normal levels. Buffer Tanks 6-4
Design Manual 2008 When vacuum levels decrease there is a corresponding decrease in the valve capacity. This is due to the lower pressure differential that exists which results in less available energy and hence slower evacuation times. A low vacuum condition condition can occur when a larger than usual amount of flow enters the main, followed by a relatively small amount of air, resulting in a low air-to-liquid air-to-liqui d ratio. This could occur, for example, at a buffer tank where high discharge rates are common and where repeated firing of the valve over short periods of time may not allow sufficient vacuum recovery. The net result would be progressively decreasing pressure differentials and lower evacuation rates. The net result is that large flows and high valve discharge capacity are typically not compatible. Buffer tanks require high discharge rates; however, this higher rate may result in water-logging which would result in a lower evacuation capacity of the next valve cycle. Considering the previous discussion, a lower evacuation rate of 15 gpm is recommended for buffer tank sizing. This lower rate assists proper vacuum recovery and provides a safety factor of sorts. Table 6-1 shows the recommended design capacities as well as the maximum allowable design flow rates to use for buffer tanks.
TABLE 6-1 Recommended Design Flow Rates For Buffer Tanks
*
Buffer Tank Type
Recommended Design Peak Flow (gpm) (as (as a general r ule)
Absolute Maximum Maximum Peak Flow (gpm) (case by c ase) *
Single Buffer tank
3.1 - 15.0 gpm
30 gpm
Dual Buffer tank
15.1 - 30.0 gpm
60 gpm
Consult AIRVAC
> 30.0 gpm
> 60 gpm
Depending on on static and friction friction loss, the overall amount of peak flow entering the system through buffer tanks and the exact location of the buffer tank, it may be possible to size a particular buffer tank with the upper limits shown in this column. Consult AIRVAC’s Engineering Department for guidance and approval.
For flow inputs greater than 30 gpm, please consult AIRVAC. Assuming system hydraulics allow, it may be possible to use a splitter manhole, which will evenly split and divert the flow to two or more dual valve buffer tanks (see Figure 6-3). Buffer Tanks 6-5
Design Manual 2008
FIGURE 6-3: TWO DUAL BUFFER TANKS TANK S WITH SPLITTER MANHOLE
Buffer Tanks 6-6
Design Manual 2008
E.
LIMITATIONS ON USE
Maximum flow contributed by buffer tanks To minimize the possibility of system water-logging, AIRVAC recommends the use of buffer tanks be limited as follows: •
•
•
25% rul e: No more than 25% of the total peak flow of the entire system should enter through buffer tanks. 50% rul e: No more than 50% of the total peak flow of a single vacuum main (i.e. – single flow path) should enter through buffer tanks. On a case by case basis: Depending on static and friction loss and the exact location of the buffer tank, it may be possible to exceed the 25% and 50% limits shown above. Consult AIRVAC’s AIRVAC ’s Engineering Department for guidance and approval.
Maximum flow at a single location The positioning of a buffer tank(s) within the collection system has an impact on system hydraulics. In general, the greater the distance from the vacuum station the buffer tank is positioned and the higher the static loss that must be overcome, the larger the negative effect becomes on the overall transport capabilities of the system. There are no hard and fast rules regarding this issue, however please consult AIRVAC’s Engineering Engineering Department for guidance on the placement placement of buffer tanks.
Buffer tanks fed by a pump When a lift station or grinder pump discharges to a buffer tank, the conventional peak flow figures for the customers served by the pump should not be used. Rather, the rated discharge capacity of the pump should be used to size the buffer tank. Consult AIRVAC for guidance on input values for friction loss.
Buffer Tanks 6-7
Design Manual 2008 F.
SITUATIONS TO AVOID
Accepting flow from an existing gravity sewer Perhaps no one single factor has resulted in more problems with vacuum systems than excessive flow from an existing gravity system entering a vacuum system via buffer tank(s). There have been several isolated cases of severe system problems caused by excessive flow that entered from existing gravity systems. In the most extreme case, a system could be totally out of operation resulting in house backups. Obviously this is a situation AIRVAC wants to avoid. One problem is that it is difficult to accurately predict the flow that can be expected from existing gravity systems. With new construction, one can accurately predict average and peak flow rates and design the vacuum mains and vacuum station accordingly. However, with an existing system, another element is introduced into the equation: I&I. In these cases, AIRVAC’s experience is that the consultant usually does one of the following during design:
•
•
I&I is either not considered or is understated. If it actually occurs when the system is on-line, the system does not function properly because it is undersized. Result: During normal flow periods the system works fine, but during rain events it may experience problems. In an effort to anticipate I&I, the designer overstates its magnitude. Larger vacuum mains, a larger collection tank and larger and possibly more vacuum pumps are used. If I&I is not of the magnitude anticipated, the system may not function properly during normal operation because it is oversized (flow transport is sluggish, problems occur at the vacuum station, etc.). Result: System may work fine during rain events, but system operation may be inefficient during normal periods. Additionally, there are significant additional costs, both capital and O&M, associated with the larger system components.
Obviously, neither of the above situations is acceptable. Should it be possible to accurately predict I&I, it may be that this flow can be considered in a vacuum system. However, before AIRVAC will consider this, an analysis of the existing gravity system must be done. As a minimum, this would include having flow records that identify the magnitude of flow that can be expected during normal periods as well as rain events (minimum 1 year of flow data). Even then, should there be a large difference between normal daily flow and flow during rain events, AIRVAC would recommend against accepting this flow. Buffer Tanks 6-8
Design Manual 2008
Overuse of Buffer tanks Some of the earliest systems tried using larger flow inputs (300 gallons at a time). Hydraulic studies of these systems showed very poor vacuum transport characteristics due to low air/liquid ratio. This phenomenon is called “waterlogging”. The vacuum main ends up with too much liquid and not enough air, which is the driving force, and vacuum levels are choked-off at the extreme ends. For these reasons, the AIRVAC design calls for small flow inputs (10 gallons) with a sufficient amount of air following at the end of each valve cycle. There is no problem in using buffer tanks on a limited basis. However, AIRVAC does not recommend using buffer tanks on a widespread basis. The earlier discussions of this chapter provide guidelines on the limited use of buffer tanks. (See “Limitations on Use”).
Allowing more than 30 gpm to a buffer tank The AIRVAC valve typically will not cycle more than 3 times in 1 minute, with each cycle being 10 gallons. It is possible for one cycle to be greater than 10 gallons, as long as the incoming flow rate is sufficient to keep pressure on the controller’s sensor, which will allow the valve to stay open. So, at 40 gpm, 3 cycles will still occur, but each one at 13.3 gallons instead of 10. Since the amount of air that follows each cycle does not change, a lower air to liquid ratio will result (same air/more liquid) and eventual waterlogging is likely.
G.
POTENTIAL ADVERSE IMPACTS
There are three negative aspects associated with the use of buffer tanks that the designer should consider: •
•
•
Unlike the regular AIRVAC valve pit where the lower sewage sump is completely enclosed and separated from the top portion where the valve resides, a buffer tank simply has an AIRVAC valve mounted in a manhole. This means the operator is exposed to raw sewage should maintenance on the valve be required. Rather than the system being completely sealed, a system with buffer tanks will have points open to the atmosphere, which can result in possible odor problems. Just like manholes in a gravity system, buffer tanks may be the source of unwanted infiltration/inflow. Buffer Tanks 6-9
Design Manual 2008
H.
DO’S AND DON’TS
Table 6 –2 Do’s and Don’ts: Buffer Tanks
DON’T DO THIS:
DO THIS:
Use an in-sump breather for a buffer tank
Use an external breather and a 6” dedicated air-terminal for all buffer tanks
Allow more than 25% of peak flow to enter the system via buffer tanks w /out AIRVAC’s approval
Limit peak flow entering system via buffer tanks to 25%, Consult AIRVAC to request higher limit on a case by case basis.
Allow more than 50% of peak flow to enter any single flow path via buffer tanks w /out AIRVAC’s approval
Limit peak flow entering any flow path via buffer tanks to 50%. Consult AIRVAC to request higher limit on a case by case basis
Use buffer tanks when individual valve pits could otherwise be used
Use buffer tanks for larger flows only when service is possible at just one or two locations
Use buffer tanks at line extremities
Contact AIRVAC if service at line extremities is necessary
Exceed recommended capacity of single and dual buffer tanks w/out AIRVAC’s approval
Use SBT for peak flows up to 15 gpm and DBT for peak flows up to 30 gpm Consult AIRVAC to request higher capacities on a case by case basis
Size a buffer tank using conventional peak flow figures when the buffer tank is connected to a lift station or grinder pump
Use the rated discharge capacity of the pump to size the buffer tank.
Connect a buffer tank to an undersized vacuum main
Use a minimum 6” main for a SBT and a minimum 8” main for a DBT
Design a system that accepts flow from an existing gravity sewer via buffer tanks without first consulting AIRVAC
Serve by other means unless gravity flow and I&I can be accurately predicted
Buffer Tanks 6 - 10
Design Manual 2008
CHAPTER 7 PREFABRICATED VACUUM STATION SKIDS
A.
GENERAL
AIRVAC provides complete vacuum station skid packages. The skid-mounted mechanical and electrical plant is assembled, tested and painted at our AIRVAC production facility in Rochester, IN prior to shipment to the job site. At the job site, the skid is lifted into the building and connected to the incoming vacuum mains and the outgoing force main. The AIRVAC prefabricated vacuum station is a result of our nearly 30 years of experience in the vacuum sewer industry. The equipment brands used, the materials and the various configurations are all a product of this experience. Various fail-safe features are incorporated into the skid design making the complete package operator friendly and reliable. Because of these features, the vast majority of vacuum stations in operation in the U.S. use AIRVAC prefabricated skids. Should a custom-designed vacuum station be desired, contact the AIRVAC Engineering Department for guidance.
B.
POSSIBL E SKID CONFIGURATIONS
Using the formulas in Chapter 3, the design engineer can calculate the size of the vacuum tank, the quantity and size of the vacuum pumps and the capacity and head requirements of the sewage pumps. With this information, the skid model that most closely meets the sizing requirements can be selected. AIRVAC skid models are identified using the following designations.
SKID MODEL DESIGNATIONS
Model 3 D - 45
Vacuum Pump Type (capacity) Collection tank size/100 Vacuum Pump Type (size) # of Vacuum Pumps
A B C D
117 cfm * 170 cfm 305 cfm 455 cfm
* Used for special applications only
Tables 7-1 and 7-2 that follow can be used to make a preliminary selection of the skid model that best suits the application. To use these tables, select the Peak flow and the appropriate ‘A” factor to determine the number and capacity Prefabricated Vacuum Station Skids 7-1
Design Manual 2008 of the vacuum pumps required. Then, a check must be made to insure that the maximum pipe volume has not been exceeded. If it has, either larger vacuum pumps or additional vacuum pumps of the same capacity are required. The collection tank volume and the maximum pipe volume figures shown in Tables 7-1 and 7-2 are based on a peak factor of 3.50. If a lower peak factor is used, a slightly larger collection tank volume is required and slightly less pipe volume can be tolerated. Conversely if a higher peak factor is used, a slightly smaller collection tank can be used and slightly more pipe volume can be tolerated. In any case, detailed vacuum station calculations as described in Chapter 3 should be performed before making the final skid model determination.
Table 7 - 1 Skid model - Preliminary selection When 2 vacuum pumps are used
Peak
Collection Tank Vol
Vacuum Pump Capacity Required (cfm)
based on 3.50 Peak
based on Peak Q & longest line (Lmax)
Vacuum
Q
Calculated
Rounded
Pumps
(gpm)
(gal)
(gal)
L max
Max Pipe
7000'
10000'
12000'
>12000'
Vol
7
8
9
11
(gal)
A
50
676
1,000
47
53
60
74
75
813
1,000
70
80
90
110
100
951
1,000
94
107
120
147
2 -170 cfm
125
1,089
1,000
117
134
150
184
2B
150
1,227
1,500
140
160
180
221
175
1,364
1,500
164
187
211
257
200
1,502
1,500
187
214
241
294
225
1,640
2,000
211
241
271
331
2 - 305 cfm
250
1,778
2,000
234
267
301
368
2C
275
1,915
2,000
257
294
331
404
300
2,053
2,000
281
321
361
441
325
2,191
2,500
304
348
391
350
2,329
2,500
328
374
421
2 - 455 cfm
375
2,466
2,500
351
401
451
2D
400
2,604
3,000
374
428
425
2,742
3,000
398
455
450
2,880
3,000
421
AIRVAC standard skids; others by special request Prefabricated Vacuum Station Skids 7-2
15,200
28,300
42,200
Design Manual 2008
Table 7 - 2 Skid model – Preliminary selection When 3 or 4 vacuum pumps are used
Peak
Collection Tank Volume
Vacuum Pump capacity req'd
based on 3.50 Peak
based on Peak Q & longest line (Lmax) L max
Max Pipe
7000'
10000'
12000'
>12000'
Volume
7
8
9
11
(gal)
Q
Calculated
Rounded
(gpm)
(gal)
(gal)
200
1,502
1,500
187
214
241
294
225
1,640
2,000
211
241
271
331
250
1,778
2,000
234
267
301
368
3 - 170 cfm
275
1,915
2,000
257
294
331
404
3B
300
2,053
2,000
281
321
361
441
325
2,191
2,500
304
348
391
478
350
2,329
2,500
328
374
421
515
375
2,466
2,500
351
401
451
551
400
2,604
3,000
374
428
481
588
425
2,742
3,000
398
455
511
625
3 - 305 cfm
450
2,880
3,000
421
481
541
662
3C
500
3,155
3,500
468
535
602
735
550
3,431
3,500
515
588
662
809
600
3,706
4,000
561
642
722
882
650
3,982
4,000
608
695
782
956
700
4,257
4,500
655
749
842
1029
750
4,533
4,500
702
802
902
1103
3 - 455 cfm
800
4,808
5,000
749
856
963
1176
3D
850
5,084
5,000
795
909
1023
1250
900
5,359
5,500
842
963
1083
1324
950
5,635
5,500
889
1016
1143
1000
5,910
6,000
936
1070
1203
1050
6,186
6,000
983
1123
1263
4 - 455 cfm
1100
6,461
6,500
1029
1176
1324
4D
1150
6,737
7,000
1076
1230
1200
7,012
7,000
1123
1283
A
AIRVAC standard standard skid; others by by special request
Prefabricated Vacuum Station Skids 7-3
31,200
56,800
86,300
130,500
Design Manual 2008
TANK SIZE (gal)
Table 7 - 3 Available AIRVAC AIRVAC skids 2 VACUUM 3 VACUUM PUMPS PUMPS
1000 gal
2B-10
1500 gal
2B-15 2C-15
3B-15 3C-15
2000 gal
2B-20* 2C-20
3B-20 3C-20
2B-25* 2C-25* 2D-25
3B-25 3C-25
3000 gal
2C-30* 2D-30
3C-30
3500 gal
2C-35*
3C-35
4000 gal
2C-40*
3C-40
4500 gal
2D-45*
3D-45
5000 gal
2D-50*
3D-50
5500 gal
2D-55*
3D-55
2500 gal
6000 gal
3D-60*
4 VACUUM PUMPS
4D-60
6500 gal
4D-65
7000 gal
4D-70
AIRVAC standard standard skid; others by special request request *
Normally not available but can be if additional vacuum pumps are to be added in the future Prefabricated Vacuum Station Skids 7-4
Design Manual 2008
C.
MODIFICATIONS MODIFICATIONS TO THE STANDARD AIRVAC SKID
Minor modifications Each project has requirements that may require slight modifications to the standard skid arrangement. Some examples are: •
•
•
Skids can be oriented to face either right or left (i.e. – mirror images are available). The collection tank nozzles will vary according to the number of vacuum mains and their associated diameters. Sewage pump capacity and head conditions may result in spacing and configuration changes.
Major modifications Standard equipment used in an AIRVAC skid is shown on Table 7-4. Optional equipment is available; however, these are major modifications that will require additional design and fabrication time by AIRVAC. Also, it is not unusual for optional equipment to add 25-50% to the cost of the vacuum station skid.
Table 7-4
Standard Equipment used on an AIRVAC skid EQUIPMENT
STANDARD
OPTIONAL
Vacuum Pumps
Rotary vane (Busch)
n/a
Sewage Pumps
Horizontal, non-clog centrifugal (various mfg’s)
Vertical pumps (various mfg’s)
Collection Tank
Carbon Steel w/epoxy coatings
Stainless Steel or Fiberglass
Control logic
Relay logic
PLC (Allen Bradley)
Contact AIRVAC’s Engineering Department for a complete, detailed list of standard skid components and available upgrades.
Prefabricated Vacuum Station Skids 7-5
Design Manual 2008 D.
VACUUM STATION EQUIPMENT CONFIGURATIONS
The vacuum station equipment can be configured in many different ways. Some common configurations are shown on Table 7-5.
Table 7-5
Typical skid configurations Configuration
Description
#1
Vacuum Pumps, sewage pumps, control panel & collection tank all on a single skid
#2
Vacuum Pumps, sewage pumps & control panel on a single skid; collection tank separate (floor mounted)
#3
Vacuum pumps and control panel on skid #1; sewage pumps on skid #2; collection tank separate (floor mounted)
Contact AIRVAC’s Engineering Department for sample drawings of these skid arrangements.
Configuration 1: all equipment on a single skid (smaller skid models) In this configuration, all of the equipment is located on a single skid. control panel is typically mounted next to the collection tank.
The
This is typically used only for the smaller sized skids (i.e. - only 2 vacuum pumps and when the collection tank is 2000 gallons or smaller) and when protection from flood is not an issue. Typical skid sizes are 8’ wide with the length varying between 14’ and 26’ long. The building footprint is usually the skid dimensions plus 4 feet on all sides.
Prefabricated Vacuum Station Skids 7-6
Design Manual 2008
Configuration 2: all equipment on 1 floor (larger skid models) This configuration also has all equipment located on the same floor, but does not have the collection tank located on the skid.
This is typically used for the larger stations (>2000 gallon collection tank) that have more than 2 vacuum pumps. The control panel is mounted on the mechanical equipment skid. Typical skid sizes are 7’ wide with the length varying between 17’ and 27’ long. The building footprint is usually the skid dimensions plus 4 feet on all sides.
Configuration 3: split-level design (all skid model sizes) This is a split-level design, with either a full or partial basement. This is used when protection from flood is an issue.
In this configuration, the vacuum pumps and controls are located on the top floor above flood level and the collection tank and sewage pumps are located in the basement. The vacuum pump and control panel are located on a single skid, the sewage pumps are on another skid and the collection tank is mounted on the floor. The vacuum pump skid is usually 6’-6” wide and the length can run between 16’ and 24’ long. The sewage pump skid is usually 7’ wide and the length can run between 3’ and 8’. The building footprint is usually the skid dimensions plus 4 feet on all sides. Space for steps and step landings must also be considered.
Prefabricated Vacuum Station Skids 7-7
Design Manual 2008
CHAPTER 8 AIRVA C SERVICES
A.
THE TEAM A PPROACH
AIRVAC believes in the team approach where we assist the engineer, contractor and owner in all aspects of the project. Below are examples of the level of support AIRVAC provides during all phases of a project.
Table 8-1
AIRVAC Services PROJECT PHASE
AIRVAC SERVICE
Planning
Preliminary system layout
Design
Detailed design assistance
Construction
Field Services/daily testing
Start-up
System start up – lines & station
Home hook-up
Plumbers class/valve installation
Initial operation
Initial system operation
Operator training
Operator training at factory/on-site
After-market
Technical support/operation services
These services are described in more detail on the following pages.
AIRVAC Services 8-1
Design Manual 2008
B.
PRELIMINARY SYSTEM LA YOUT
Upon receipt of some basic project information, AIRVAC engineers will assist consulting engineers with the preparation of a preliminary system layout as well as a budget estimate.
System layout/preliminary design/Technical information A preliminary system layout will be provided, including vacuum main sizing. Preliminary sizing of the vacuum pumps, sewage pumps and the collection tank will be made. The estimated quantities of the various system components will be identified. A Technical Report will be provided that summarizes the design assumptions, line and station sizing, the valve pit quantities required and other the project specific information.
Cost information A budget estimate will be provided that includes an estimate of both capital costs as well as the annual Operation & Maintenance costs.
C.
DETAILED DESIGN ASSISTANCE
When a consulting engineer commences the detailed system design, an AIRVAC engineer will be available for design assistance ranging from engineering training to hands on design work. An AIRVAC engineer will also review the completed design to verify the aspects of the design that relate to the AIRVAC system.
Line profiling assistance AIRVAC can provide assistance with vacuum main profiling. Typically this includes assistance with profiling the most critical vacuum main. AIRVAC will review the line profiles when completed and do a hydraulic analysis of both static loss and friction loss. Plan and Profile sheets Each design firm has their own style and ideas on how much detail to include on the plan and profile drawings. Information that typically is included in other utility design work should be included in vacuum sewer plans as well. In addition, there are certain vacuum-specific items that need to be shown as well. Table 8-2 shows some of the critical vacuum-specific items that AIRVAC encourages designers to include when preparing the P&P sheets.
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Table 8-2
Key features to include on the Plan & Profile sheets
ITEM
WHY IS THIS NEEDED?
GENERAL
Horizontal & vertical scale (Ex: 1” = 50’; 1” = 5’) Bar scale
Design review, bidding & construction purposes Plans are sometimes reduced for printing
PLAN VIEW
Flow arrow
To clearly indicate direction of flow
Length, size & type of pipe (Ex: 320 lf- 8” SDR 21 PVC)
For review, bidding & construction purposes
Pipe termination
For bidding & construction purposes
Branch line connection
To clearly indicate the use of wye fittings
Division valves
For bidding & construction purposes
Houses/lots to be served
To indicate which houses/lots share a pit
Slab or basement elevation (Ex: El 110.25)
For pit depth determination
Valve pit location and callouts Valve pit designation (Ex: VP – A23)
For construction purposes & maintenance records
Valve pit type & depth (Ex: 3030WT)
To establish product quantities
Valve pit rim elevation (Ex: El 103.50)
Combined with slab elevation & pit type this will be used to determine if the pit is deep enough to serve the intended house(s)
Valve pit stub-outs
To determine pit orientation & stub-out lengths
PROFILE VIEW
Flow arrow
To clearly indicate direction of flow
Valve pit locations
To insure they are not too close to the top of a lift
Pipe length, size, type & slope (Ex: 320 lf-8” SDR 21 PVC @0.20%)
For review, bidding & construction purposes
Pipe termination
For bidding & construction purposes
Division valves
For bidding & construction purposes
Inverts - top & bottom of lifts
To indicate lift heights
Inverts - change in pipe slope
For construction purposes
Inverts – branch to main connection
To insure sufficient ‘over the top’ spacing
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There are many different ways to graphically present the items shown in Table 8-2. For example, some firms like to repeat plan view information on the profile view to act as a cross-check. Others prefer not to do this as changes made in one view during the design may inadvertently be missed in the other view resulting in conflicting information. Each method has its pros and cons. AIRVAC has no preference on how the information is presented, but rather is more concerned on what information is actually included on the plan & profile sheets. As a general guideline, the designer should consider the following when deciding on the format used for the plan & profile sheets: •
•
•
•
Bid-ability and constructability. Is there sufficient detail for bidders to fairly price the job and for contractors to properly construct the system? AIRVAC review. Is there sufficient information to allow AIRVAC engineers to review for the design for compliance with the guidelines contained in this manual, and to allow for a hydraulic review to be completed (static & friction loss)? Regulatory agency review. Is there sufficient information to allow a regulatory agency to review for compliance with rules and regulations? Owner and operator use. Is there sufficient information to serve as basis for the as-built drawings and for future maintenance records?
Station skid drawings Detailed drawings of the various skid arrangements are available from AIRVAC. Typical drawings include: • • • • • • •
Plan & section: skid assembly Control panel layout Power distribution Vacuum/sewage pump controls Electrical wiring Alarm/level controls Point to point wiring
Contact AIRVAC’s Engineering Department for assistance with skid drawings.
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Standard details AIRVAC maintains an excellent inventory of standard detail drawings. These details are periodically updated to include new products, product changes, etc. Contact AIRVAC’s Engineering Department for a copy of the latest standard details.
Sample Specifications AIRVAC can provide detailed specifications using the Construction Specifications Institute (CSI) format for the vacuum-related aspects of a project, including: •
Valve & valve pit equipment Specifications
•
Pre-Fabricated Vacuum Station Specification with Electrical
•
Custom Constructed Vacuum Station Specification with Electrical
Contact AIRVAC’s Engineering Department for format and availability.
Use of AIRVAC-supplied drawings & documents Any AIRVAC drawing or other documents provided by AIRVAC during the design phase are done so with the understanding that they can be used in the final construction documents only if AIRVAC is chosen as the vacuum system supplier.
DISCLAIMER: AIRVAC is not an engineering firm, and cannot and does not provide engineering services. AIRVAC reviews the design, plans, and specifications for a project only for their compatibility with AIRVAC's vacuum products, and accepts no responsibility for the overall project design. Any information provided to project engineers is provided solely to assist the engineer in designing and engineering and maintaining an overall system that can utilize AIRVAC vacuum products.
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D.
CONSTRUCTION FIELD SERVICES
A correct installation is vital to the ultimate success of a vacuum system. AIRVAC can provide skilled field representatives to advise and assist contractors and consulting engineers with the construction of AIRVAC systems. In most first-time vacuum projects, AIRVAC provides a Field Representative to be present during the entire construction period. For repeat clients, or for projects where a consulting firm with vacuum experience is involved, AIRVAC can also provide part time field services.
E.
SYSTEM START-UP
AIRVAC technicians can assist with the critical stage of system start-up. Startup is usually conducted in two phases: the vacuum station and the collection system. The AIRVAC Technician conducts all of the tasks related to the vacuum station start-up. The Contractor, with AIRVAC’s help, does the collection system startup task. Table 8-3
Start-up Services
VACUUM STATION
COLLECTION SYSTEM
Vacuum & sewage pump tests
Review As-built drawings
Electrical review
Final 4-hr vacuum test
Level control tests
Flush ends of lines
Final Vacuum test
Open & clean collection tank
Place system in automatic mode
Place system in automatic mode
Prepare Start-up Report
Prepare Start-up Report
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F.
HOME HOOKUP & VAL VE INSTALL ATION
Along with system start-up, this is one of the most critical stages of the entire project. Even a well-designed and constructed system can experience operational difficulties early on if the following activities are not carried out.
Public Education To ease homeowner fears, AIRVAC can participate in various public education endeavors, including participation in public meetings, conducting tours of existing systems and participation in Owner sponsored workshops.
Plumbers Class AIRVAC has found it to be very advantageous to conduct training classes with local plumbers who will be doing the home hook-ups. In some cases, the Owner will require the local plumbers to attend the AIRVAC class to become certified to install the homeowner’s gravity line and connect it to the vacuum valve pit.
Home hook-up inspection & valve installation AIRVAC can provide the Owner with inspection assistance regarding the homeowner gravity line connections to the valve pits as well as actual installation of the AIRVAC 3” valve. Specifically, AIRVAC will: •
•
G.
Provide inspection of the homeowner gravity line in conjunction with County plumbing inspectors. AIRVAC will help to insure that all air intakes are in the proper locations, that sufficient slope exists on all piping and that the homeowner makes a proper connection to the Owner supplied gravity stub leading to the valve pit. Install the 3” AIRVAC interface valve in each valve pit after the homeowner plumbing inspections are completed and approved by the local plumbing inspectors and the Owner.
INITIAL SYSTEM OPERATION
When a system first goes on line, the system hydraulics will change with every valve that is installed. To help the Owner during this critical time, AIRVAC can assist them during the early stages of system operation.
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H.
OPERATOR TRAINING
Training of the system operators is an important part of any sewage scheme. AIRVAC provides excellent operator training to ensure optimal system operation.
Factory training - Operator’s school AIRVAC offers a 1-week operator training school conducted at the AIRVAC factory. Training is provided on the following topics: •
•
AIRVAC valve operation and overhaul. Troubleshooting procedures; faults are set up in the AIRVAC rig for the trainees to locate and rectify.
•
Collection station maintenance.
•
Record keeping.
•
Installation of AIRVAC valves, valve pits, holding tanks, service laterals and sewers (most owners make minor additions to the system).
On-site training AIRVAC can also perform on-site training. This hands-on training would include troubleshooting tips, assistance with setting valves and the controller timing, and making adjustments to the vacuum station and vacuum valves to mirror the changing system hydraulics. Arrangements for other such training sessions can be made with the AIRVAC Service Department.
AIRVAC Test Rig The AIRVAC glass pipe test and demonstration rig is located in Rochester, Indiana. Engineers and clients are welcome to view the rig but are requested to telephone AIRVAC for an appointment. All aspects of the AIRVAC system, including an operating clear valve, are demonstrated.
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I.
AFTER-MARKET SERVICES
Technical support from the AIRVAC Engineering and Service departments provide any necessary support to assist the owner in providing timely information and solutions to problems. 24/7 Technical support AIRVAC provides on-site technical support to the owner during system operation and in emergency situations as required. With our toll free 800#, service assistance is available 24 hours a day, 7 days a week.
Problem Simulation AIRVAC maintains a complete vacuum system at our factory that can be used to simulate field conditions and onsite problems to help solve owner problems or suggest solutions.
Annual System Evaluation & Tune-up An AIRVAC technician will cycle and re-time all AIRVAC valves, conduct exterior leak tests on each controller and visually inspect each valve pit. The technician will visit each vacuum station to perform various equipment tests.
Contract Maintenance In some cases, AIRVAC can provide the operation and maintenance functions for the owner for an annual fee. This service would include normal day-to-day maintenance, preventive maintenance, service calls and emergency callouts.
Response to natural disasters and emergencies AIRVAC has been involved several times with the Federal Emergency Management Agency (FEMA) reacting to damage from floods, tornadoes and hurricanes. We understand and work within the FEMA guidelines to restore service as quickly as possible. AIRVAC will contact the system operator prior to offer standby assistance. Field personnel can be dispatched to help restore the system, if requested. Spare parts AIRVAC maintains an inventory of spare parts at our Rochester, IN facility. This includes every single part and piece that makes up our products. With this inventory, the owner can expect spare parts within 24 hours of their request.
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Design Manual 2008 Af ter-market Servic es 24/7 Technical support Annual System Evaluation & Tune-up Contract Maintenance Problem simulation Response to natural disasters & emergencies Spare parts
8-9 8-9 8-9 8-9 8-9 8-9
Backwater valves
5-20
Buffer Tanks Description Dos’ & Don’ts Effect on system hydraulics Sizing When & where used Limitations on use Potential adverse impacts Situations to avoid
6-1 6-10 6-4 6-4 6-1 6-7 6-9 6-8
Building sewer Building sewer 4” air-intake 6” dedicated air-terminal
5-14 5-14 5-16
Collection Tank Materials Sizing
3-2 3-2
Controller Description
5-5
Construct ion Field Servic es
8-6
Conversion factors
3-12
Electron ic Air Ad missi on Control (EAAC)
4-27
Electrical Panel Motor control center Motor control panel Relay logic vs. PLC
3-13 3-13 3-14
Fault Monito ring System Description
3-17
Index 1
Design Manual 2008 Figures Fig 1-1: Fig 1-2: Fig 1-3: Fig 1-4: Fig 1-5: Fig 1-6: Fig 3-1: Fig 3-2: Fig 3-3: Fig 4-1: Fig 4-2: Fig 4-3: Fig 4-4: Fig 4-5: Fig 4-6: Fig 4-7: Fig 4-8: Fig 4-9: Fig 4-10: Fig 5-1: Fig 5-2: Fig 5-3: Fig 5-4: Fig 5-5: Fig 5-6: Fig 5-7: Fig 5-8: Fig 5-9: Fig 5-10: Fig 5-11: Fig 6-1: Fig 6-2: Fig 6-3:
Typical layout Vacuum main during period of no flow Typical back-surge Typical vacuum station schematic Vacuum vs. gravity: excavation Vacuum vs. gravity: profiles NPSHa diagram Station calculation sheet Level control probe diagram Lift detail Profiling before a series of lifts Downhill transport Uphill transport Level transport Service lateral with lifts Friction loss calculation sheet Vacuum lift capability Static loss determination Pipe volume calculation sheet 6.5’ PE 1-piece pit 6’ fiberglass 3-piece pit AIRVAC flexible service lateral Typical configuration for gravity connections Valve pit components PE 1-piece pit Valve pit components fiberglass 3-piece pit AIRVAC anti-buoyancy collar 4” Air-intake diagram Optional 6” Dedicated air terminal Valve pit prior to home hookup Valve pit after home hookup Single buffer tank Dual buffer tank 2 Dual buffer tanks with splitter manhole
1-2 1-4 1-4 1-7 1-10 1-10 3-7 3-10 3-14 4-2 4-8 4-9 4-10 4-10 4-12 4-22 4-23 4-24 4-28 5-2 5-3 5-6 5-7 5-9 5-11 5-13 5-16 5-16 5-19 5-19 6-2 6-3 6-6
Flexible service l ateral
5-6
Flow determinatio n Average daily flow Peak flow Peaking factors Infiltration
2-1 2-2 2-1 2-3
Index 2
Design Manual 2008 Friction Charts Loss calculations Loss limits
4-15 4-14 4-26
Gauge taps
4-3
Grease traps
5-20
Homeowner Issues Air-intake Backwater valves Building sewer Home Hookup & valve installation Grease traps Optional 6” dedicated air terminal Proper time to install valve
5-14 5-20 5-14 8-7 5-20 5-17 5-18
Horizontal Direction al Drill ing (HDD)
4-4
In-sump breather Description
5-5
Level controls 3-14 Lifts Lift detail Loss calculations Loss limits Recommended lift heights Slope between lifts When lifts are required in the service lateral
4-2 4-24 4-26 4-24 4-11 4-11
Location t ape
4-2
Losses Due to friction Due to lifts (static loss)
4-14 4-23
Prefabricated vacuum s tation ski ds AIRVAC standard skid models Modifications to the standard AIRVAC skid Skid model designations Vacuum station equipment configurations
7-4 7-5 7-1 7-6
Index 3
Design Manual 2008
Prelimi nary System Layout Cost information System layout/preliminary design Technical information
8-2 8-2 8-2
Operator Training AIRVAC test rig Factory training-Operator’s school On-site training
8-8 8-8 8-8
Sewage pumps Materials NPSH calculations Pump capacity TDH calculation
3-4 3-5 3-4 3-5
Static lo sses Observations from in-house hydraulic studies Recommended static loss limits Static loss calculations
4-25 4-26 4-24
System Start-up Initial system operation Start-up procedures When to install the vacuum valve
8-7 8-6 5-18
Index 4
Design Manual 2008
Tables Table 2-1: Peak factors Table 3-1: Vacuum station component sizing Table 3-2: Values of Vo for different peak factors Table 3-3: Discharge pump NSPH calculations/nomenclature Table 3-4: A” factor Table 3-5: Do’s and Don’ts: Vacuum Station Design Table 4-1: Directional drill guidelines Table 4-2: Maximum flow for various pipe sizes Table 4-3: Maximum line length for various pipe sizes Table 4-4: Main line design parameters Table 4-5: Guidelines for determining line slopes Table 4-6: Slope between lifts Table 4-7: Service lateral design parameters Table 4-8: PVC pipe sizes & related AIRVAC design information Table 4-9: Friction tables: 4” Table 4-10: Friction tables: 6” Table 4-11: Friction tables: 8” Table 4-12: Friction tables: 10” Table 4-13: Recommended lift heights Table 4-14: Static & friction loss design guidelines Table 4-15: Pipe volume per linear foot for various pipe sizes Table 4-16: Do’s and Don’ts: Vacuum Piping Design Table 5-1; AIRVAC Valve pit types Table 5-2: Recm’d pipe types for stub-outs & building sewers Table 5-3: Do’s and Don’ts: Valve Pits Table 6-1: Recm’d max design flow rates for buffer tanks Table 6-2: Do’s and Don’ts: Buffer tanks Table 7-1: Skid model- preliminary selection (2 VP’s) Table 7-2: Skid model – preliminary selection (3 or 4 VP’s) Table 7-3: Available AIRVAC skids Table 7-4: Standard equipment used on an AIRVAC skid Table 7-5: Typical skid configurations Table 8-1: AIRVAC services Table 8-2: Key features on Plan & Profile sheets Table 8-3: Start-up services
2-2 3-1 3-3 3-6 3-9 3-20 4-4 4-6 4-7 4-8 4-9 4-11 4-11 4-14 4-15 4-16 4-18 4-20 4-24 4-26 4-27 4-31 5-1 5-15 5-21 6-5 6-10 7-2 7-3 7-4 7-5 7-6 8-1 8-3 8-6
Vacuum Chart Recorder Description
3-16
Vacuum Gauges Description
3-16
Index 5
Design Manual 2008
Vacuum Mains Cleanouts/access points Construction issues Do’s & Don’ts Fittings Gaskets Installation Tolerances – Directional drilling Installation Tolerances – Open cut Isolation valves Line connections Line lengths Line sizing Location tape Piping materials Pipe volume Profile changes (lifts) Slope between lifts Slope on vacuum mains System layout guidelines
4-3 4-1 4-31 4-1 4-3 4-4 4-3 4-3 4-13 4-7 4-6 4-2 4-1 4-27 4-2 4-11 4-9 4-5
Vacuum Main tests Daily test Final test
4-4 4-5
Vacuum Pump Materials Pump-down time (‘t’) Sizing: based on flow & A/L ratio Sizing: based on pipe volume
3-8 3-9 3-8 3-9
Vacuum Stations Bio-mass sizing Component sizing Do’s and Don’ts Equalizing lines Heat Level controls Noise Odor control Standby generator Station piping Station sump valve
3-19 3-1 3-20 3-16 3-18 3-14 3-18 3-18 3-18 3-16 3-17
Index 6
Design Manual 2008
Vacuum Station Calculation s Calculation sheets Hs (static head) Hf (friction head) Hv (vacuum head) NPSHa (Net Positive Suction Head available) NPSHr (Net Positive Suction Head required) Qmax (peak flow) Qa (average flow) Qmin (minimum flow) Qdp (discharge pump capacity) Qvp (vacuum pump capacity) “t” (system pump-down time TDH (Total Dynamic Head) Vo (collection tank operating volume) Vct (collection tank volume) Vp (piping system volume)
3-10 3-6 3-6 3-6 3-5 3-6 2-2 2-2 2-2 3-4 3-8 3-9 3-5 3-2 3-2 4-28
Vacuum sy stems Advantages-Construction Advantages-Operation (general) Advantages-Operation (hurricane prone areas) Applications General description History Major system components System layout guidelines Vacuum piping Valve pit Vacuum station Where used
1-9 1-11 1-11 1-9 1-1 1-1 1-5 4-5 1-6 1-5 1-7 1-9
Index 7
Design Manual 2008
Vacuum System Hydraulics Air to Liquid ratio Back-surge Saw-tooth profile Summary of vacuum main design fundamentals
1-3 1-4 1-3 4-29
Vacuu m Valve Description Valve capacity
5-4 5-4
Valve Pits Do’s and Don’ts – valve pits Flexible service lateral In-sump breather Pit covers Pit sharing Pit types Shipping & handling Stub-outs
5-21 5-6 5-5 5-8 5-2 5-1 5-8 5-7
Valve Pits – PE, 1-piece pit s 1-piece pits In-sump breather/sensor pipe Suction pipe Integral anti-buoyancy collar
5-9 5-10 5-10 5-10
Valve Pits – fiberglass, 3-piece pits 3-piece pits Valve pit cone Pit bottom Sump In-sump breather Suction & sensor pipes Anti-buoyancy collar
5-11 5-12 5-12 5-12 5-12 5-13 5-13
Index 8
