Underground Piping (U/G) and Types of Underground System- Part 1 June 13, 2013 Pipe fittings http://www.piping-engineering.com/underground-piping-ug-piping-part-2.html
Underground Piping (U/G)
The piping system is taken underground generally for the utility services like cooling water supply to various units and cooling water return to cooling tower for line sizes normally 18 inch NB and above, other water services with big pipeline sizes, big oil supply lines and various sewer systems in the process units of the chemical, p etrochemical and refinery type of plants. The term “underground” applies to the piping – both – both buried or in trenches. The underground system consist of gravity flow drainage system carrying process waste, spillage, reclaimable hydrocarbons, sanitary and storm water, firewater and drinking water system. Good engineering practice, local code / regulations, specific client requirements shall govern the design of the underground piping p iping system in the plant. The following are the common underground services in a chemical / petrochemical / refinery plants. -
Cooling water (line size normally ≥18″ NB)
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Fire Water
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Contaminated Rain Water Sewer from process catchment area.(CRWS)
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Oily Water Sewer (OWS)
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Liquid Effluent to the Effluent Treatment Plant.
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Closed Blow Down system (CBD)
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Sanitary system
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Storm Water
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Equipment drainage to slop tank
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Electrical cables
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Instrument cables
Types of Underground system
Depending on the service and the material that each system handles there are different system defined in the Underground system.Various underground systems can be described in the following way both for Utility system and sewer system. 1.
Cooling Water System (CWS & CWR)
This is generally a buried system with protective wrapping and coating or with cathodic protection or both. Any valve for isolation of a part of the th e cooling water system shall be enclosed in a valve pit. The normal compacted earth cover shall be 1200 mm over the top of the the pipeline. Theearth cover over pipe (back-fill) shall be compacted to at a t least 95% Proctor compactionIndex to protect p rotect the pipe from aboveground loadings as per ASTM D-698. 2.
Oily Water Sewer (OWS)
This system collects waste, drips and leaks from equipment and piping in areas that contain process equipment in non-corrosive services. The layout engineer should consult the process engineer to fully identify all such equipment and provide a drain hub at each item. The piping engineer should locate the oily water drain hubs using the above ground piping studies, setting each invert elevation and routing th e line with relevant components / fittings. Oily water sewer shall collect oily waste / drain from pump, equipment through funnel points and shall run with separate headers and manholes in in the units. units. The regular oil contamination areas shall also be segregated and discharge from these areas shall be collected in catch basins to be joined in oily water sewer. Oily water sewer shall consist of carbon steel sewer, sewer, funnel points, clean outs, RCC catch basins, RCC manholes, vent pipes, flame arrestor etc. 3.
Contaminated Rainwater Sewer (CRWS)
The areas which are contaminated due to floor wash drains etc. inside unit boundaries shall be demarcated. Contaminated areas collected in catch basin shall be drained through CRWS while while un-contaminated areas, normally at periphery of the units shall be drained through ditches covered with grating. CRWS shall consist of underground carbon steel sewer with corrosion protection, funnel points, clean outs, RCC catch basins, RCC RC C manholes, vent pipes, flame arrestor etc. Open ditches of units should have a bypass either to the CRWS or to storm water, drains of offsite.
This system collects surface drainage from areas containing h ydrocarbon – bearing equipment. This water must pass through a treatment facility before being discharged into an uncontaminated system or natural body of water e.g. river or a stream. 4.
Closed Blow Down (CBD) sewer
This system picks up drains around boilers and steam d rums and is run as a separate system preferably to the battery limit. The system shall be designed as per P &ID and the effluent collected from equipment through funnel points and underground piping system shall be connected to the underground CBD drum. 5.
Amine Blow Down (ABD) sewer
The amine blow down system (ABD) shall be designed as per P&ID. The effluent shall be collected from equipment through above ground points into close funnels connected to underground system. The main header shall be connected to the underground Amine sump / drum. 6.
Fire Water System
This system consists of a fire hydrant network around a process unit or equipment, with branches as required for hydrants or monitors to protect the unit in case of fire. This is a close loop system starting from Firewater storage and pump to the specific location of hydrants and monitors. This is always kept under a predetermined working pressure level. 7.
Potable Water System
This water is used for drinking, emergency e ye washes and safety shower facilities. 8.
Sanitary Sewer System
Sanitary sewer system collects waste from all toilet facilities provided in various plants and non plant buildings and shall be discharged to WWTP (Waste Water Treatment Plant) Acids, caustics, hydrocarbons, rainwater or other chemical waste shall not b e discharged to this system. This system is routed to normally to a septic tank. 9.
Underground Electrical and Instrument ducts
In the beginning of a project, the decision to route the major electrical and instrument conduits – above ground in the piperack or buried below grade shall be taken.
In case underground route is selected, electrical and instrument engineers shall be consulted for the optimum layout of ducts by the plant layout engineer. Where conduits enter the unit through a pull box and cables come above ground for routing upto the terminal points, the space shall be kept free of piping, equipment or associated maintenance access.
Underground Piping (U/G) Piping – Part 2 June 17, 2013 Piping layout While developing &preparing the underground piping layout there are different components and terms that would be used and referred in the drawings. Some of the common terms used are explained below: 1.
Catch Basin
This device is used to collect surface draina ge with an outlet liquid seal and sediment trap. The sketch Fig 1 illustrates a typical catch basin feature and Fig 2 illustrates a typicallocation of catch basins. The maximum area coverage of a catch basin is approx 150 sqm. ie. 12m x 12m or15mx10m. The area covered by a catch basin should be of square configuration, as far as possible. The catch basin should be located in the middle of the area as far as possible. Slope of pavement 1 in 100, e.g. HPP (Ridge of catch basin area = 100.00) and LPP (at catch basin peripheral drain = 99.85) No catch basins or manholes should be located within 15m radius of heaters. No vent pipes should be located within 15m of the heaters. Area drainage around heater areas shall be done by pavement sloping towards open ditches. Each catch basin shall be connected to manhole and shall be provided with fire seal.
Fig 1 Catch basin
Fig 2. Plot subdivided into drainage areas 2.
Sealed Manholes
Sealed manholes shall be provided at a. Unit battery limit and b. Junction of sewers and at change in size of main header. These devices are provided so that the unit area is isolated from any fire in offsite area/sewer. Within the unit area, sealed manholes in main headers should be provided in such locations so that each sub-unit within the unit is isolated from the other areas. In case, this demarcation is difficult, one sealed manhole for every 30m length of main sewer in the unit shall be provided.
Sealed manhole with bent pipes seal type shall be used for carbon steel pipes upto size16″ NB and for greater than dia 16″ NB and for all diameters of RCC pipes double compartment type manholes should be used. 3.
Invert elevation
This term, usually associated with any underground line, refers to the elevation of the inside bottom of the line. Because of the wide range of materials used in the underground piping system / drainage system with varying wall thickness, it is the constant that is used to set the elevation on construction drawings. The starting invert level of CRWS shall be normally 750mm below HPP (High Point ofPaving) The invert level at outlet point of CRWS and OWS shall be normally 1500mm below FGL. 4.
Cleanout
A cleanout is a piping connection in a sewer system that is located at grade level for inspections or for cleaning the system. 5.
Vent Pipes
Vent pipes shall be located along piperack columns or building columns and should be taken 2m above the building parapet or last layer of pipes on a piperack. 6.
Valve Pit / Maintenance pit for flanges and instruments.
When the underground system needs valves for isolation and instruments for control, the normal practice is to enclose these valves and instruments in a RCC pit with cover. These valves and instruments in a pit can be operated as well as maintenance work can be done with ease. Piping arrangement-Underground
Under piping are generally arranged based on the location of the consumers and the also depends upon the depth for the soil surface. Some of the guidelines for routing of the underground piping are stated below: 1. The overall Plot Plan allocates the space for the major underground services in the beginning of the project.
The cooling water supply from cooling water pum p discharge to the various units as well as the cooling water return from the various units to the top of cooling tower is routed in a simple, straight orientation at a suitable depth avoiding an y major road crossings. A typical cooling water and potable water system is illustrated in sketch Fig 3. Cooling water cross over piping is illustrated in sketch Fig 4.
Cooling water lines to heat exchangers are t ypically illustrated in sketch Fig 5. Cooling water lines to pumps for various cooling requirements is illustrated in sketchFig 6. 2. The potable water system supplies to various units and a branch is taken to the emergency eyewash and safety shower station as illustrated in sketch Fig 7. 3. Fire water system protects each piece of equipment by providing water through hydrants, monitors or deluge spray systems.
Each process unit will have its own underground firewater piping loop system. 4. A typical hydrant and monitor installations are illustrated in sketch Fig 9 and a typical firemonitor is shown in sketch Fig 10. 5. Normally, chemical process units will have multiple drain systems designed to collect all corrosive or toxic chemical waste as well as surface drainage around the equipment.
Drain / sewer system in a plant can be categorized as: -
Uncontaminated storm water
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Contaminated storm water
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Oily water sewer
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Chemical and process sewer
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Sanitary sewer
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Blowdown system
Uncontaminated storm water system generally collects all service water from equipment areas, access ways, roadways to equipment. This collection is done through area drains, catch basins, roof rain water downcomers. Contaminated storm water system collects surface drainage from areas containing hydrocarbon processing equipment. This system water must pass through a treatment facility before being discharged into an uncontaminated system or natural body of water viz. river or a stream connected to a river. Oily water sewer system collects waste, drips, leaks from equipment and piping in noncorrosive process equipment area. The designer should identify all the specific drain points in consultation with the process engineer.
Chemical and process sewer system recovers acid or chemical drains from equipment / piping as well as surface drainage by providing curbing and drain sump around such equipment. -
Sketch UGP19 depicts a typical process drains – closed system.
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Sketch UGP21 illustrates a typical cross-section of a closed or chemical drain system.
Sanitary sewer system collects raw waste from lavatories and is either connected to the municipal battery limit or routed to a septic tank. Blowdown system picks up drains around boilers and steam drums and is run as a separate system. It is permissible to connect the blowdown system to a sewer box in oily water sewer system downstream of drainage from a furnace. 6. Trench Piping
Occasionally, drain piping or process piping should be run below grade but not buried. Sketch Fig 8 illustrates two insulated lines A and B runnin g below grade to a drain tank. The top of the trench is covered with grating but could be covered with RCC slab depending on the traffic load estimated in the area. The width of trench should allow adequate clearance to valves and drains as required. 7. A typical catch basin is illustrated in sketch Fig 11.
A typical sewer box or manhole is illustrated in sketch Fig 12. A typical Dyked area drain sump is illustrated in sketch Fig 13. A typical lift station by vertical pump is illustrated in sketch Fig 14. 8. A process area for the purpose underground drainage is subdivided into block areas with h igh point ridge and low point catch basins / pits.
The low point catch pits are connected to manholes. The manholes are interconnected by sloping piping and led to the battery limit valve pit and finally discharged into the treatment pond. Sketch Fig 15-16, Fig 17-18, Fig 19 illustrates the details of the subdivision of the process area into blocks with explanatory notes. Sketch Fig 20 illustrates how to handle oily water and storm water system. 9.Closed drain system is illustrated by various sketches Fig 21-22, Fig 23, Fig 24.
10.The underground electrical and instrument cables passing under a road or paved area is takenthrough ducts embedded in lean concrete. Sketch Fig 25 shows a typical arrangement of such underground cabling details.
Fig 3 Process cooling water and potable water system
Fig 4. Cooling water crossover piping
Fig 5 Cooling water to exchangers
Fig 6. Cooling water to pumps
Fig 7 Emergency eyewash and safety shower
Fig 8. Trench piping
Fig 9 Hydrant and monitor installations
Fig 10 Typical grade-mounted fire monitor
Fig 11 Catch basin
Fig 12 Typical sewer box or manhole
Fig 13 Dyked area drain
Fig 14. Lift station
Fig 15. Indicative PFD for oily waste sewer & contaminated rain water sewer & process line
Fig 16. Notes: Indicative PFD for oily waste sewer & contaminated rain water sewer & process line
Fig 17.G.A.for underground piping
Fig 18.Notes: G.A.for underground piping
Fig 19. Plot subdivided into drainage areas
Fig 20. Oily water and storm water system
Fig 21.Process drains: Closed system
Fig 22. Plan for a closed drain system
Fig 24 Closed drain system Sump
Fig 23.Closed drain system : Cross section
Fig 25 Underground cable duct
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