CONTENTS
1.
GENERAL
2.
TYPES OF TANKS 2.1
2.2
Atmospheric Tanks 2.1.1
Floating Roofs
2.1.2
Cone Roofs
2.1.3
Dome Roof
2.1.4
Double Wall
2.1.5
Bolted Tanks
2.1.6
Small Welded Tanks
2.1.7
Large Welded Production Tanks
Pressure Storage
3.
TABLES OF DIMENSIONS
4.
TANK ACCESSORIES
5.
INSTRUMENTATION
6.
SIZE AND CAPACITIES
7.
TANK STRAPPING
8.
PUMPING AND SIZING OF TANK
9.
TANK GRADES AND FOUNDATION
10.
STORAGE CAPACITY
11.
RETENTION TIME
12.
LOSSES
13.
VESSEL SKETCH
13.1
Illustration
13.2
Table of Process Connections
13.3
Process Data
13.4
Notes
14.
LOAD SHEET
15.
VESSEL CONNECTION SUMMARY
16.
TABLE OF CONNECTIONS
17.
PROCESS ENGINEERING FOLLOW UP
18.
REFERENCES
19.
APPENDIX I -
FIGURE AND TABLES
Figure 5.1 - Cone Bottoms of API API Bolted Production Tanks Figure 5.2 - Dimension Dimen sion of an API Small Welded Tank Figure 5.3 - Cone Bottom Types of API Small Welded Production Tank Figure 5.4 - Typical Tank Grade Figure 5.5 - Recommended Foundation For Large Large Tanks Tanks Supported Supported By By Soil Figure 5.6 - The Floating Floating Roof by Minimizing Minimizing Vapor Space Eliminates Elim inates Filling Filling Loss Figure 5.7 - Both Types of Hortan Floating Roofs Meet Requirements In API Standard 650 Appendix C Figure 5.8 - Roof Support Figure 5.9 - Automatic Float Gauges Figure 5.10 - Floating Roof Accessories
11.
RETENTION TIME
12.
LOSSES
13.
VESSEL SKETCH
13.1
Illustration
13.2
Table of Process Connections
13.3
Process Data
13.4
Notes
14.
LOAD SHEET
15.
VESSEL CONNECTION SUMMARY
16.
TABLE OF CONNECTIONS
17.
PROCESS ENGINEERING FOLLOW UP
18.
REFERENCES
19.
APPENDIX I -
FIGURE AND TABLES
Figure 5.1 - Cone Bottoms of API API Bolted Production Tanks Figure 5.2 - Dimension Dimen sion of an API Small Welded Tank Figure 5.3 - Cone Bottom Types of API Small Welded Production Tank Figure 5.4 - Typical Tank Grade Figure 5.5 - Recommended Foundation For Large Large Tanks Tanks Supported Supported By By Soil Figure 5.6 - The Floating Floating Roof by Minimizing Minimizing Vapor Space Eliminates Elim inates Filling Filling Loss Figure 5.7 - Both Types of Hortan Floating Roofs Meet Requirements In API Standard 650 Appendix C Figure 5.8 - Roof Support Figure 5.9 - Automatic Float Gauges Figure 5.10 - Floating Roof Accessories
Table 5.1
- Sizes And General Dimensions of API Bolted Prodution Tanks
Table 5.2
- Details of Bottoms, Shells, and Docks of API Bolted Production Tanks
Table 5.3
- Dimensions of an API Small Welded Tanks
Table 5.4A - Dimensions of an API Welded Production Tank Table 5.4B - Dimensions of an API Welded Production Tank Table 5.4C - Dimensions Di mensions of an API Welded Production Tank Table 5.4D - Dimensions of an API Welded Production Tank Table 5.5
- Flat Bottom Storage Tank Capacities
Table 5.6
- Spherical Tank Liquid Capacities
Table 5.7
- Spherical Tank Gas Capacities
Table 5.8
- Bullet Tank Capacities
20.
APPENDIX II -
EXAMPLE CALCULATION
21.
APPENDIX III -
DATA SHEET
1.
GENERAL
In the processing of petroleum, sizeable inventories of crude, semi-finished and finished hydrocarbons are required. Both atmospheric and pressure storage vessels are used. A major refinery offsite cost is represented by storage facilities and related piping, access roads, dikes, and fire and safety equipment.
The major portion of oil, water and other liquids stored in refineries is contained in atmospheric storage tanks. These are normally steel vessels which operate at or only slightly above atmospheric pressure. The capacity of various storage tanks is set by processing, blending, blending, shipping and marketing requirements, shipment/transportation periods.
The design of storage facilities for feedstock, intermediate, and final product liquids is one of the responsibilities of Offsites Systems. A brief description of the types of storage tanks normally used is given in this subject, divided into atmospheric and pressure storage. A listing of tankage accessories and instrumentation commonly required follows, along with a brief decription. Finally, the subject outlines the procedure for completing a vessel sketch and corresponding vessel connection summary and a vessel load sheet, along with an example of each.
2.
TYPES OF TANKS
Tanks for storing liquids as atmospheric pressure or low pressure are built in two basic styles, floating roof design where the roof floats on top of the liquid, rising and falling with level, and the cone roof design the roof is fixed.
The most frequently used storage tanks are welded steel thanks fabricated in accordance with the API 650 Specification. Small tanks generally conform to API 12F specifica specification tion for Small Small Welded Welded Production Production Tanks. Other API tank specifications are for tanks which
are considered portable and are employed in producing fields. These include bolted tanks (API 12B) and prefabricated welded production tanks (API 12D). Tanks for water storage should be fabricated in accordance with the AWWA specification which takes account of the greater weight of water and the need for a corrosion allowance.
The main distinguishing feature of atmospheric oil storage tanks is the type of roof employed. The two basic types of roofs are fixed and floating.
The choice between types of roof should be preicated on 1) evaporation loss, 2) fire risk, 3) product contamination from atmosphere and
4)
maintenance cost resulting from corrosion.
Evaporation
losses vary
with the type of material stored and the tank
operating cycle. The two causes of evaporation losses are tank filling and breathing. Filling losses are influenced by the throughput of the plant and methods and frequency of shipping. Breathing losses are caused by variations of ambient atmospheric conditions and depend on the vapor pressure of the material and the volume of vapor space in the tank.
When the stored material is subject to ready ignition, a floating roof is desirable to reduce the risk of fire. Such materials, for which the tank vapor space is usually in the explosive range, include crude petroleum, gasoline components, jet fuels, heavy naphtha and kerosines with flash points below about 80? F.
Some water soluble solvents, lubricating oils and materials adversely affected by air are occasionally stored in floating-roof tanks. Alternatively, fixed-roof tanks with inert gas blankets are also used to protect air-sensitive products.
Materials that can evolve corrosive vapors, such as crude petroleum and some gasoline components, often require special types of floating-roof tanks to reduce the effects of such corrosive vapors.
Fluids that are vapors at ambient temperatures also can be stored in atmosperic tanks as liquids at low temperatures. These tanks normally operate at low pressure (measured in inches of water) and are therefore constructed in accordance with API Standard 620. Proper insulation of lowtemperature atmospheric storage tanks is important.
2.1
Atmospheric Tanks
So called because they operate at or slightly above atmospheric pressure (1 ~ 2 PSIG), these tanks can be sub-divided into three smaller groups; floating roofs, cone roofs,and dome roofs, according to their characteristic design. A fourth category will examine the various types of "double wall" design.
2.1.1
Floating Roofs
As the name implies, these tanks have a roof that literaly floats on the surface of the liquid. The roof is fitted with a seal to close the gap between the roof and shell and pantograph hangers (or similar mechanisms; to accomodate variations of the rim space and to center the roof in the tank. The tank is equipped with "stops" to held the roof off the bottom when the tank is emptied. There are three basic types of floating roofs:
•
Pan roofs - Unstable and dangerous, there are rarely used.
•
Pontoon roof - These consist of a deck plate supported by one or more pontoons. Referred to as high-or low-deck pontoon roof depending upon whether the deck is above the liquid or in contact with it, respectively. The number of pontoons required is determined by vendor.
•
Double deck roofs - These consist of two deck plates with suitable structural support between them, resting on one or more pontoons. The space between the plates serves as insulation.
A fourth type, the covered floating roof tank, is just the addition of a truss supported roof over any of the above types of floating roofs. Floating roofs are used in situations where, for economic or safety reasons, vapor generation must be minimized or atmospheric contamination avoided. Water soluble solvents and naptha are two examples of products often stored in floating roof tanks.
Floating roofs for storage tanks have long been justified largely on rounds vapor losses. As more stress in placed on environmental protection, there is increased interest in floating roofs to reduce hydrocarbon emissions. Most floating roof give long service and perform their function with difficulty and with minimum attention. However, when problems do occur, they may be annoying and costly.
Development of early floating roofs involved many empirical relationships and confirming tests, experience pointed out basic essentials, but with demand for larger roofs, more refined methods of analysis were needed to justify extrapolation of design.
Normally, pontoom and double-deck roofs meet the requirements of Appendix C of API Std 650, Covered floating roofs are designed to meet Appendix H of API Std 650 for pontoom roofs, the governing design condition from 50 to 150 feet diameter is the rainfall condition again governs.
When either the ruptured-deck condition or sag-full condition governs, pontoom roofs have reserve strength. This strength enables them to carry a load somewhat greater than that equivalent to 10 in of rainfall over the tank area.
2.1.2
Cone Roofs
These roofs are made of a series of columns and supporting beams,onto which the roof plates are placed and lap welded to each other (but not to the support beams). Obviously, these tanks cannot take any internal pressure,
and are therefore limited to low vapor pressure liquids. In addition, great care must be taken to adequately size breathing vents to handle all input and drainage rates that the tank may see.
The cone bottom in either the bolted or the welded tank offers a means of draining and removing water, or water-cut oil, from only the bottom of the tank, leaving the merchantable oil above. With a flat-bottom tank some of the merchantable oil must be removed if all of the water is removed from the tank. Corosion on the tank bottom is kept to a minimum by keeping all water removed.
The cone-bottom tank can be cleaned without a man entering the tank. A water hose, handled just outside the cleanout opening, is used to flush the solids to the center of the cone and drain connection.
Welded tanks offer cone bottoms in two basic patterns; (1) the bottom of the tank is cone-shapped and must set on a cone-shaped grade; (2) the cone bottom is placed up in the shell of the tank, leaving a base ring or flat bottom to rest on a flat tank grade. In the latter pattern the producer may select a standard-height tank which will have less capacity than a flat-bottom tank but of necessity of slightly greater height.
The cone bottom adds approximately 12 percent to the cost of a welded tank, depending on which pattern is selected. It adds approximately 3 to 4 percent to the cost of most popular sizes of bolted tanks.
Proper grade preparation can also have an important bearing on bottom corrosion. Tanks erected on poorly drained grades, directly contacting corrosive soils or on heterogeneous mitures of different types of soils, are all subject to electrolytic attack on the bottom side. Typical tank grade is shown on Fig. 5.4.
In selecting the proper type of foundation, the bearing power of the soil is the primary factor. Where no previous experience in the same area is available, soil booring to determine existing conditions are usually cheap insurance
against future trouble. We have seen a number on instances where tank sites were judged solely from surface conditions only to have the empty tank settle so seriously during construction that the water test could not be performed until the foundation was rectified. With the tanks already erected, this could only be accomplished at great expense.
While this are extremes, they serve to illustrate the importance of first knowing the nature of the foundation base. Knowledge of gelogical formation or experience with other heavy structures in the same vicinity will often suffice, but if such knowledge is absent, soil borings are the safest means of investigation.
The grade for the tank should preferably be elevated slightly above the surrounding terrain to insure drainage. Sufficient berm should be provided to prevent washing and weathering under the tank shall. The berm width should be at least 5 feet. Wastering can be minimized if the berm is subsequently protected with trap rock, gravel, or an apshaltic flashing.
The sand pad should be at least 4 in. deep. The sand should be clean and free from corrosive elements. Care should be taken to exclude clay or lumps of earth from coming into contact with the bottom. Frequently the difference in potential between two types of earth will set up an electrolytic cell with resultant pitting.
Drainage is important both from the standpoint of soil stability and bottom corrosion. Good drainage should be provided not only under the tank itself, but the general area should preferably be well drained. Where the terrain does not afford atural drainage, proper ditching around a group of tanks may help to correct the deficiency. Where suitable bearing soil is not available at the surface, but is available a reasonable distance below the surface, a ring wall foundation is indicated. The purpose of the ring is to confine the soil and prevent lateral movement.
2.1.3
Dome Roof
Similar to cone roofs, except in this case trusses extend from the shell to support the roof beams, and the roof plates are welded to the shell. These tanks can operate at slight positive pressures, approximately 1.0 to 2.0 PSIG and are therefore used extensively for storage of high vapor pressure liquids at below-ambient temperatures, since the variance in internal pressure necessary to operate most refrigeration packages is necessary.
2.1.4
Double Wall
A double wall tank is actually a tank-within-a-tank. The inner tank contains the liquid product, while the outer tank maintains pressure and serves as protection for the insulation placed between the inner and outer tank walls. The outer tank has a dome roof with an insulated suspended deck that fits just inside the inner tank walls. This deck is not vapor tight, allowing the inner tank to be designed only for hydrostatic loads. Initially inert gas (nitrogen) will fill the void between the inner and outer tank but during operation product vapor will flow to the space over the suspended deck and will mix with the inert gas. Continuous inert gas purging is not required all lines pressure vacuum protection blanker gas, and initial purge connection should be extended through the suspended deck.
A further refinement of the double wall tank is the "double integrity" tank. Since in a standard double wall tank the outer wall is always at ambient temperature and only maintains vapor pressure, it is made of standard carbon steel and is not designed for the liquid temperature. Therefore, should the inner tank evelop a leak, the outer tank would likewise fail. In a double integrity tank, however, the outer tank wall is made of the appropriate materials of construction for the liquid contained, and is designed to hold the maximum level, along with any thermal shock. Also, relieving devices must be provided to protect the tank from overpressure upon failure of the inner tank due to the vapor generated with the cooldown of the outer tank.
2.1.5
Bolted Tanks
Table 5.1 gives sizes and geneal dimensions and Table 5.2 gives details of bottoms, shells and decks as given in API Std. 12 B. Eleventh Edition, May, 1958, " API Specification for Bolted Production Tanks."
For the flat-bottom elements, Standard 12 B gives detailed specifications to assure interchangeability between different makes of tanks. This is indicated by references in Table 5.2 to certain figures in Standard 11 B. The requirements for cone bottoms, for tanks 29 ft. 8 5/8 in. in diameter or smaller, are shown in Figure 5.1.
In regard to practice for relief valves for bolted tanks the Standard gives the following recommendations.
B-1
For tanks 21 ft. in diameter and smaller, the maximum setting of pressure-relief valves should be 3 oz. per sq.in; relief valves should be of such a size that the pressure in the tank will not exceed 4 1/2 oz.per sq.in.
B-2
For tanks larger than 21 ft. 6 in. but not larger than 29 ft. 8 5/8 in. in diameter, the maximum setting of pressure-relief valves should be 2 oz. per sq.in; relief valves should be of such a size that the pressure in the tank will not exceed 3 oz.per sq.in.
B-3
For tanks larger than 29 ft. 8 5/8 in. in diameter, the maximum setting of pressure-relief valves should be 1 oz. per sq.in; relief valves should be of such a size that the pressure in the tank will not exceed 1 1/2 oz.per sq.in.
B-4
The venting capacity of vacuum relief valves should be such that the internal vacuum will not exceed 3/4 oz. per sq.in. at the maximum setting of the valve opening.
2.1.6
Small Welded Tanks
Table 5.3 and Figure 5.2 show dimensions for small welded production tanks as given in API Std. 12 F, fifth edition, March 1961, "API Specification for Small Welded Production Tanks".
The bottom of the tank is to be flat or of Type A (unskirted) or Type B (skirted) design (Figure 5.3).
The thickness of bottom plates is to be 1/4" (10.20 lbs. per sq. ft.) nominal except that the thickness of the sump of the Type A cone bottom is to be 3/8" (15.30 lbs. per sq.ft.) nominal.
The thickness of the shell plates can be either 3/16" (7.65 lbs. per sq.ft.) nominal, or 1/4" (10.20 lbs. per sq. ft.) nominal, as specified. The standard gives detail welding requirements.
Regarding relief valves the Standard recommends the following :
B-1
The maximum setting of pressure relief valves should be 16 oz. per sq. in; relief valves should be of such a size that the pressure in the tank will not exceed 24 oz. per sq. in.
B-2
The venting capacity of vacuumrelief valves should be such that the internal vacuum will not exceed 3/4 oz. per sq. in. at the maximum setting of the valve opening.
2.1.7
Large Welded Production Tanks
Table 5.4. shows dimensions of the tanks, as given in API Std. 12 D/650.
The plate thickness of these tanks are the same as those of small welded production tanks. The bottom can be flat or the Type A cone design (Figure 5.3).
Regarding the relief valves, the standard recommends the following:
B-1
For tanks 15 ft. 6 in. in nominal diameter, the maximum setting of pressure-relief valves should be of such a size that the pressure in the tank will not exceed 18 oz. per sq.in.
B-2
For tanks 21 ft. 6 in. and 29 ft. 9 in. in nominal diameter, the maximum setting of pressure-relief valves should be 8 oz. per sq.in.; relief valves should be of such a size that the pressure in the tank will not exceed 12 oz.per sq.in.
B-3
The venting capacity of vacuum relief valves should be such that the internal vacuum will not exceed 3/4 oz. per sq. in. at the maximum setting of the valve opening.
2.2
Pressure Storage
Spherical vessels are used to store liquids at high pressure; common uses include the storage of butane, ethylene and refinery stocks of similar volatility. A "bullet" storage tank is a horizontial, saddle supported cylindrical vessel with the hemispherical or elliptical heads. Bullets are often used at higher pressures than spheres.
3.
TABLES OF DIMENSIONS
Tables 1, 2, and 3 at the end of this manual subject list some approximate dimensions for flat bottom atmospheric tanks, spheres, and builets, respectively. These values are to be used only during the preliminary stages of a job; the final dimensions are determined by the tank vendor. This is especially important for the atmospheric storage tanks.
4.
TANK ACCESSORIES
The following are items that Offisites Systems might require and that can be specified as accessories to be provided by the tank vendor:
a.
In-tank pumps - These pumps, along with their motor drivers, sit on the bottom of the tank, their discharge lines extending up through the roof. Their primary advantage is one of safety, since their use can eliminate bottom and side penetrations of the tanks. The necessary pump data sheets should be sent to Vessel mechanical along with all necessary vessel data, so that the tank vendor can design the tank to accommodate the pump(s).
b.
Relief valves/vacuum breakers - These are used to prevent overpressure/vacuum in tanks that cannot be allowed to vent freely. Sizing of these valves is critical, so care must be taken to accurately determine the combination of situations that results in the maximum flow for both occurances. For relief valves, some of the sources that should be considered are maximum-boil-off, blocked vapor outlet, maximum rundown, barometric pressure drop, flashing of equilibrium fluid, heat gain of liquid rundown and recirculating lines, and roll-over due to stratification. For vacuum breakers, some considerations are: minimum boil-off, blocked vapor inlet, maximum liquid withdrawl rate, and barometric pressure rise. Inlet piping pressure losses to relief valves on atmospheric tanks should be limited to 3% of set pressure at design flowrate. This may be very difficult with large, low pressure tanks. If the pressure drop exceeds 3%, remote pilot operators should be added and the following capacity through the valve must be reduced accordingly. A minimum of one spare relief valve and vacuum breaker shall be installed. The relief valves shall have inlet block valves such that a relief valve can be serviced without jeopordizing the tank. Appropriate locks or seals should be added. In addition, some clients may require interlocks to ensure that if one relief valve block is closed, the other is open.
c.
Floor valves (also called internal tank valves) - For bottom nozzles in low pressure tanks, these serve as emergency manual shut-off valves
in the event of a line breaking. In pressure storage tanks, these valves are called excees flow valves, and close automotically on high flow.
d.
Roof drains - For floating roof tanks. Require pipe with swivel joints or flexible hose drains.
e.
Automatic bleeder vents - On pontoon roofs, allow air to be vented during filling and emptying when roof is resting on "stops".
f.
Weather caps - All open vents require weather caps or goosenecks with bug screens. Flame arrestors may also be necessary.
g.
Rim vents - These are used on floating roofs equipped with metallic seals to allow release of excess pressure due to expansion of vapor in the rim space.
h.
Cooldown system - Storage tanks taht will contain products at a temperature significantly below ambient (ammonia, LPG, etc.) should be brought into service by having their internal temperature lowered in a controlled manner. Care must be taken to ensure that the cooling of the tank is uniform; the formation of cold and hot spots in the tank material could lead to excessive thermal stresses. For this reason, the vendor should supply a spray ring near the top of the tank with a line extending outside the tank to allow connecting a product source for the tank cooldown. The high pressure drop across the spray ring nozzles results in even distributions of small diameter droplets, yielding uniform temperature distribution. The vendor should also supply a recommended "cooldown" procedure, including amount, pressure, and temperature of cooldown liquid necessary and the time required to reach a tank temperature at which the product rundown can begin. (For information on cooldown of LNG storage tanks, refer to LNG Systems Manual Subject.
Vessel Mechanical will specify those items that are normally required on tanks, regardless of service.
5.
INSTRUMENTATION
The Instrumentation Division determines the type and quantity of instrumentation required throughout the plant. Storage tank instrumentation is usually purchased as an accessory from the vendor. Rules of thumb are as follows:
•
Two separate, and preferably different, level detection circuits with low and high level alarms (usually two stages for each) and both local and remote indicators.
•
Pressure detection circuit, with high and low alarms (again, will probably require more than one stage) and indicators, local and remote. Not required on vented cone roof tanks and floating roof tanks.
•
6.
Temperature circuits, if product temperature must be controlled.
SIZE AND CAPACITIES
Following formulas are seful in estimating tank capacities when exact accuracy is not required:
Capacity of cylindrical tanks in barrels of 42 gallons is : Per inch of depth
= A² x .00118115
(5-1)
Per 1/4 inch of depth
= A² x .00029529
(5-2)
Per foot of depth
= A² x .01416
(5-3)
Where
A =
Inside circumference in feet B² x D
7.15307
Where :
B=
Inside diameter in feet
D=
Depth in feet
The inside circumference is found by making deduction for the thickness of wall from the measured outside circumference. These deductions for different thicknesses of steel plates will be as follows: STEEL TANKS Gauge 11 10 9 8 7 6 5 4 3 2 1 0 00 000 0000 00000 000000 0000000
7.
Thickness Inches 1/8 9/64 5/32 11/64 3/16 13/64 7/32 15/64 1/4 17/64 9/32 5/16 11/32 3/8 13/32 7/16 15/32 1/2
Deduction .0653 .0817 .0983 .1147 .1310 .1473 .1637 .1800 .1963 .2127 .2290 .2454 .2617
TANK STRAPPING
Strapping is a procedure for measurment of tanks to provide dimensions
necessary for computing the gage tables to show the quantity of oil in a tank at any given depth. Tank strapping involves measurment of thickness of tank walls,
1) depth, 2)
3) circumference and 4) deadwood. The "working"
steel tapes, used in strapping have to be calibrated against "standard" tapes. Field production tanks may be measured any time after they have once been filled.
API Std. 2501, Second Edition, July 1961, "Crude Oil Tank Measurement and Calibration", covers, among others, the tank strapping requirements.
Table 5.3 Dimensions of an API Small Welded Tank (see Figure 2) Table 5.4A Dimensions of an API Welded Production Tank. Table 5.4B Dimensions of an API Welded Production Tank. Table 5.4C Dimensions of an API Welded Production Tank. Table 5.4D Dimensions of an API Welded Production Tank.
8.
PUMPING AND SIZING OF TANK
The average size of storage tanks has been increasing steadily, With the advent of the supertanker, there has been a dramatic jump in the size of tanks being built. And there is a even sharper increase in pumping rates.
API RP 2003 indicates that it is common practice to limit velocity of incoming liquid initially to 3 fps until a floating roof becomes buoyant. Velocity may be limited to 3 fps until the roof is floating of the lower ends of the pipe supports about 1 feet above the tank bottom. The automatic bleeder vent will then be closed. Pipeline velocities in large diameter tanks, can likely be increased to about 20 fps. This could be done without causing enough turbulence beneath the roof to be of concern. Initially inlet pipe velocities
higher than 3 fps may be used. If so, the designer must consider slotted inlet pipe extensions or flared low-type inlets to limit the velocity.
Pumping rates should be reduced as the floating roof nears the top of the tank. This is important in the case of the covered floater. There the roof could
be sunk if pumping is continued after the tank is full. Consider a pumping rate of 10.000 bbl/hr in a 150 feet diameter tank equipped with a covered floating roof. Only about 10 second would be required to fill the rim space if the floating roof contacted the fixed roof. Product would then be forced past the seal and through deck openings, sinking the roof.
To determine size of the tanks, designer needs to know :
- Liquid speed of tank suction ..................fps - Liquid speed of pump suction ..................fps - Liquid speed of pump discharge ................fps - Capacity of ship/road tanker/barge.............BBLS - Periods of ship ...............................Hrs - Loading time
Normally, API 650 or BS 2654 is used as a reference in calculating the welded steel tanks for oil storage at atmospheric pressure
(+ 38 mm
Aqua)< API Std 620 for low pressure (between 38 mm aqua to 15 psig), and so ASME Code Section VIII Division I is used as a reference in calculating the pressure vessel, where the pressure of tank above 15 psig. Figure B gives typical Standard is used as a reference in calculating the low pressure tanks, with variable pressure of tank and flash point. Tangent value of cone roof between 1/6 to 1/3.
9.
TANK GRADES AND FOUNDATION
Selection of the proper location on the lease for storage tanks is of prime importance. The location should provide good drainage and be on wellpacked soil, not a fill, if possible. The tank foundation or grade should be slightly elevated, level, and some-what larger in diameter than the tank itself. For steel tanks, either bolted or welded, the best grade is one made of small gravel, crushed rock, etc., held in place by steel bands 8 in high. This type of grade allows no water to stand undernearth the tank and provides air circulation. If the tank is to be set directly on the ground, felt tar paper should
be applied to the grade first and the tank set on this. If concrete is used for the grade, it should be sligthly larger in diameter than the tank and have shallow grooves on the surface to provide air circulation.
If the grade is not level in the beginning, or if it later settles unevenly, the tank will inevitably have a distorted shell. Often the tank builder is blamed for a poor shell that should properly be charged to a poor foundation. In order to obtain good tanks, good foundations must be provided.
10.
STORAGE CAPACITY
Q x 60 x T V = -------------
Barrels
42
Where :
11.
V=
Storage Capacity, barrels
T=
Retention Time,
Q=
Liquid Flow Rate to be Storaged, gpm.
hours
RETENTION TIME An empirical equation for estimating retention time :
T=A
u -------Sw - So
Where :
u =
Hrs
Oil viscosities, Cp
So = Specific Gravity of Oil A =
12.
Constant which varies from 0.05 - 1.0
LOSSES
Oil may be stored in a fixed roof tank of constant volume or a floating head (variable volume) tank. The latter is used to minimize breathing losses and those losses which occur by virtue of the filling method. If the fixed volume tank is filled from the bottom some stripping of the liquid already there accurs as gas "breaks out" of the entering oil. If the tank is filled at the top some splashing or agitation may occur to cause excess liquid entrainment.
A type of breathing also occurs when the tank is being emptied. Air or gas must be admitted to keep the tank from collapsing. Some of the oil must be vaporize to maintain an equilbrium mixture.
If this loss is too great some alternative to a simple fixed volume tank is indicated. One modification uses a layer of small spheres which float on the surface of the oil to from a barrier between the oil and gas. Another alternative is a vapor recovery system. This usually is a refrigeration system operating on the very rich effluent tank vapors. A floating head tank is used for most large storage volumes.
The process of loss involves several mechanisms and thus use of vaporliquid theory is limited to predict said loss. An API study committee has developed some empirical correlations for predicting oil tank losses from fixed volume tanks.
The actual loss will depend on prior conditioning of the oil, the method and rate of filling and the ratio of liquid surface area to liquid volume. The calculation of losses involves many factors but the two equations which follow are useful approximations of the breathing loss and filling loss for fixed, cone roof tanks.
Breathing Loss The basic equation is :
1.8
(P) (D)
B = ------------- (F o)(F p) A
Where :
Metric English
B=
Annual Breathing loss
m
D=
Tank Diameter
Meters
Feet
P=
TVP at avg. Liquid Temperature
KPa (g)
psig
A =
Unit Factor
74
14.5
Fp =
Paint factor = 1.0 fro aluminium; 0.75 for chalking white; 1.1 for light
3
API bbl
gray; 1.25 for black, no paint and tank needing repainting. Fo =
Outgate factor based on the average distance to the top flange of the tank found from the table below.
Outgate m
Fo Ft
0.31 1.53 3.05 4.58 6.10 7.63 9.15 10.68 12.20 13.73 15.25
1 5 10 15 20 25 30 35 40 45 50
0.39 0.55 0.72 0.87 1.00 1.12 1.23 1.33 1.43 1.53 1.62
Above equation is based on a tank being about half full on the average, when storing a 65 KPa (9.5 psia) RVP product. Unfortunately, predicting breathing losses may show a 25% variation because of the many factors that cannot be accounted for in a quantitative manner. (RVP : Reid Vapor Pressure)
Filling Losses.
The filling loss prediction is more reliable than that for
breathing loss. The basic cause of loss is the displacement of the air-vapor mixture by the incoming liquid. Once again, the experience varies with the company and the location. The recommended equation is : PV F = ----- (K f ) A
Where :
Metric
English
P=
True Vapor Pressure
kPa (g)
V=
Volume of liquid in
m
3
bbl
F=
Filling Loss
m
3
bbl
A =
Conversion constant
22 740
psig
3300
The value for K f is found from the table below : Kf Tank Turnovers Per year 0 - 10 12 15 20 25 30 40 40 - 60 60 -100
Refinery
Fields and Terminals
1.00 0.91 0.75 0.59 0.50 0.47 0.44 -
1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.80 0.50
Conservation type (floating head) tanks are used to reduce losses. The Pan Type floating head tank is primarily of historical interest. The two other common types, pontoom floating roof and double deck, each has its own parricular advantages.
Filling losses are usually negligible. To estimate total losses the following rule-of-thumb may be used : 3
3.8-4.6 m per meter of diameter per bar of TVP per year. 0.5-0.6 API bbl per foot of diameter per psi of TVP year.
Accurate value of the proposed pumping rates in and out of a tank should be specified so that bleeder vents can be proearly sizes. Normally, API Std. 2000 is used as a reference in calculating the required vent capacity. For filling, the vent capacity is based on flow of a mixture of hydrocarbon vapour and air and a pressure differential equal to the weight of the roof. For emptying, the capacity is based on the flow of air and a pressure differential equal to the specified live load. Allowable vacuum on the roof is assumed to be equal to the specified live load. So, floating roof should not be landed on their supporting legs while carrying any live load.
Filling an emptying venting
a.
Out breathing at maximum filling rate if flash point below
100 ?F ------- Q = 1200 SCFH for each
100 BBLS/Hr Flash point above
100 ?F ------- Q = 600 SCFH for each
100 BBLS/Hr
b.
In breathing at maximum emtying rate Q = 600 SCFH for each 100 BBLS/Hr
VESSEL SKETCH
All offsite storage tanks that do not involved liquid-gas separation or a similar process will have a vessel sketch prepared by Offsite System, to be sent to Vessel Mechanical.
13.1
Illustration
The blank upper half of the form is used to illustrate the type of storage tank and the approximate locations of the various nozzles. The nozzle connection should be flagged and assigned a letter symbol; care should be taken to show the connections as accurately as possible (bottom penetrations should be from bottom of tank sketch, relief valve inlet extending through suspended deck, etc.). Height and diameter should be indicated as being determined by vendor.
13.2
Table of Process Connections
Under this heading, list all the nozzle connections flagged out on the tank sketch above, indicating both the letter symbol used (A,B,etc.) and the service for that particular connection (top liquid inlet, steam inlet for heating coil, RV connection, etc.). The third column, REMARKS, should be used to point out any pertinent information concerning that particular connection. Some examples would be "w/floor valves" for bottom penetration liquid inlets/outlets, "w/splash plates" and/or "slug flow" for top loading of liquid and "emergency blankes" for inlet gas line.
13.3
Process Data
This portion of the vessel sketch gives two very important pieces of information: the working (or normal) temperature and pressure of the tank. For refrigerated storage, working pressures will usually range between 0.5 and 1.5 PSIG. Cone and floating roof tanks should be listed as "ATM". Bullets and spheres are determined by process requirements. The temperature listed should correspon to the highes (for ambient and heated
operating conditions. A note of caution: clients will sometimes request that a tank be capable of handling more than one product (not simultaneously). The working temperature shown should be that corresponding to the product with the highest or lowest temperature. In the case of double wall tanks, with suspended decks, the working pressure applies to the outer tank, the working temperature applies only to the inner tank. For double integrity tanks, the working temperature applies to both inner and outer tanks. The outer tank must also be capable of handling the thermal shock due to inner tank rupture. The line "PRESSURE DROP THROUGH INTERNALS" is left blank.
13.4
Notes
This section is used to supply additional information about the storage tank and its contents and operation. Certain items should be considered as mandatory:
•
Tank type (cone roof, double wall, etc.)
•
Tank capacity (cubic meters and/or barrels)
•
Product (s)
•
Maximum specific gravity
•
Number of tanks required
Additional pieces of information that should be included if applicable are:
•
Maximum liquid rundown to tank and maximum liquid withdrawl from tank
•
Maximum heat leak or gain allowable (heat leak is sometimes phrased as a percentage of tank capacity allowed to boil-off).
•
When the liquid rundown is superheated and it will flash when it reaches the tank, then the flashing conditions should be specified.
•
In complex cryogenic storage tanks, i.e LNG, LPG, a battery limit summary will be prepared by Offsite Systems.
•
All vendor supplied accessories All special features unique to the job
service involved
•
Operating temperature range of other product(s) stored
•
Simultaneous operation of top inlet and vapor outlet (there is a danger of excessive liquid carryover)
14.
LOAD SHEET
Vessels involving liquid-gas separation, such as knock-out drums, will have minimum dimensions and selected nozzle locations determined by Vessel Analytical. Offsites System will prepare a load sheet with the following format and information:
1.
At the top of the page, the drum name and equipment number.
2.
The upper half of the page should contain a rough sketch of the drum with lines indicating incoming and exiting flowrates, along with the sources and destinations of all the streams. The sketch should also show all major internal stuctures (demister pad, spray rings, etc.)
3.
A section of notes should follow, with a minimum of the following information:
• Products contained (composition(s) if available) • Operating pressure • Any special operating procedures • Operating temperature(s) • Maximum allowable pressure drop. For compressor knockout drums with demister use 0.2 PSIG. This does not include
pad,
velocity head
loss. • All vapor and liquid stream densities • If applicable, note vacuum condition • If drum any have operating liquid level, note lines must enter above this level, both in
what
the notes
and on the sketch.
Offsites system will also prepare a Vessel Connection Summary based on the vessel sketch received from Vessel Analytical.
15.
VESSEL CONNECTION SUMMARY
This from gives detailed information about the nozzle connections indicated on the corresponding vessel sketch. In the upper left hand corner, there are four lines of information necessary.
a.
Vessel type - Cone roof, double integrity, etc. Usually specified by Project Plan or Client.
b.
Design pressure - Normally ranges from atmospheric to 2.0 PSIG for atmospheric tanks. Spheres shall have a minimum design pressure of 110% of the maxium normal operating pressure or 10 lbs above maximum normal operating pressure, whichever is greater. Bullets shall have a minimum design pressure of 100 PSIG or 100% of the maximum normal operating pressure, whichever is greater. If not specified by Project Plan or Client, consuit with Process before deciding upon a design pressure.
c.
Max. operating temperature - Important. This should correspond to the highest temperature that the product is expected to attain. For refrigerated storage, the word "maximum" should be scratched out, and two temperatures shown in the space provided separated by a "/". One should correspond to the warmest temperature the tank will see when pressurized; this would normally be during dryout and purge. The outer should be the lowest temperature the product will reach.
d.
Minimum flange rating - This is determined by consulting the Class "M" specifications for the particular job, and checking the flange
ratings for all pipe specs that will flange up to the tank.
16.
TABLE OF CONNECTIONS
Each connection shall be listed in the table, the symbol designating the letter used to flag out that particular nozzle on the vessel sketch. If the connection is flanged, note the rating and facing as dictated for that line spec in the Class "M" specifications. If the connection is welded, note the schedule and style (Ref. Kellogg Standard 4-63). remarks should correspond to those given on the appropriate vessel sketch. If drum may be placed in vacuum condition, this should be noted also.
17.
PROCESS ENGINEERING "FOLLOW-UP"
The responsibility of the Process Engineer with regards to the design of storage tanks does not end with the issue of the vessel sketches and connection summaries. He should work closely with vessel mechanical to confirm the accuracy and adequacy of vendor calculations of heat leaks and gain, relief valves, and vacuum breakers. He should also check relative nozzle locations for possible operating conflicts (e.g. vapor outlet adjacent to top liquid fill) and effect on previous hydraulic calculations (pump calcs, battery limit summaries, etc.)
18.
REFERENCES
1.
API 12 B
"Bolted Tanks For Storage of Production Liquids" 12
th
edition, 1977. 2.
API 12 D
"Field Welded Tanks For Storage of production Liquids"' 8
th
edition, 1977. th
3.
API 12 F
"Small Welded Production Tank", 10
edition, 1988.
4.
API 650
"Welded Steel Tanks For Oil Storage. 8
th
edition,
1988. 5.
API 620
"Recommended Rules For Design And Construction of Large, 7
th
edition, 1985.
Second Edition, Juli 1961. 7.
API RP 2003 "Protection Against Ignitions Arising Out Of Static, Lightning, and Stray Currents, 4
th
edition, 1982.
APPENDIX I FIGURES AND TABLES
Figure 5.1. Cone Bottoms of API bolted production tanks ( API Fig.I)
Figure 5.2. Dimension of an API small welded tank. (From API Fig.1)
Figure 5.3. Cone Bottom types of API small welded production tank (API Fig.2 and 3)
Figure 5.4. Typical Tank Grade
Figure 5.5. Recommended Foundation For Large Tanks Supported By Soil
Table 5.5 FLAT BOTTOM STORAGE TANK CAPACITIES Capacity In Barrels Exact
Tank Dimensions In Feet and Inches
Capacity In Barrels Exact
Tank Dimensions In Feet and Inches
Diameter
Height
Diameter
505 1,010 1,515
15-0 21-3 21-3
16-0 16-0 24-0
15,470 15,130 15,060
48-0 52-0 58-0
48-0 40-0 32-0
1,512 2,020 2,100
26-0 21-3 25-0
16-0 32-0 24-0
16,785 20,140 24,170
50-0 60-0 60-0
48-0 40-0 48-0
3,025 3,020 3,765
26-0 30-0 33-6
32-0 24-0 24-0
25,120 27,415 30,140
67-0 70-0 67-0
40-0 40-0 48-0
4,030 5,040 5,020
30-0 30-0 33-6
32-0 40-0 32-0
30,100 32,905 35,810
73-4 70-0 80-0
40-0 48-0 40-0
5,485 6,040 6,855
35-0 30-0 35-0
32-0 48-0 40-0
40,425 42,970 45,320
85-0 80-0 90-0
40-0 48-0 40-0
6,010 7,160 7,515
36-8 40-0 36-8
32-0 32-0 40-0
44,760 54,390 54,165
100-0 90-0 110-0
32-0 48-0 32-0
8,950 10,100 10,315
40-0 42-6 48-0
40-0 40-0 32-0
55,950 67,140 67,705
100-0 100-0 110-0
40-0 48-0 40-0
11,330 12,100 12,100
45-0 42-6 52-0
40-0 48-0 32-0
81,245 80,580 96,690
110-0 120-0 120-0
48-0 40-0 48-0
12,890 13,595 13,985
48-0 45-0 50-0
40-0 48-0 40-0
134-0 140-0 134-0
40-0 40-0 48-0
100,470 109,700 120,563
Height
Capacity In Barrels Exact
Tank Dimensions Feet and Inches
Diameter
Height
125,895 231,600 143,200
150-0 140-0 160-0
40-0 48-0 40-0
150,995 171,900 181,300
150-0 160-0 180-0
48-0 48-0 40-0
217,500 223,800 268,600
180-0 200-0 200-0
48-0 40-0 48-0
Capacity In Barrels Exact
325,000 387,000 453,500 526,000 604,000 687,500 776,000 789,800
Tank Dimensions Feet and Inches
Diameter
Height
270-0 240-0 260-0 280-0 300-0 320-0 340-0 343-0
48-0 48-0 48-0 48-0 48-0 48-0 48-0 48-0
Table 5.6 SPHERICAL TANK LIQUID CAPACITIES
NOMINAL
DIAMETER*
CAPACITY
(FT-IN)
(BBLS)
PRESSURE
ACTUAL
INSIDE SURFACE
+
VOLUME
AREA
3
3
(PSI)
......(FT )
(FT )
1000
22-3
380
5770
1555
1500
25-6
326
8680
2043
2000
28-0
299
11490
2463
2500
30-3
274
14490
2875
3000
32-0
260
17160
3217
4000
35-3
234
22930
3904
5000
38-0
215
28730
4536
6000
40-6
202
34780
5153
7500
43-6
136
43100
5945
10000
48-0
167
57910
7238
12000
51-0
157
69460
8171
15000
54-9
144
85930
9417
20000
60-6
123
115950
11500
25000
65-0
117
143790
13270
30000
69-0
109
172010
14960
40000
76-0
229850
13150
96
SPHERICAL TANK LIQUID CAPACITIES
*
Provides at least two percent vapor space above top liquid capacity line.
+
Approximate maximum pressures based on maximum shell thickness of 1½ inches, an allowable stress of 17,500 psi. A steel having a shell tensile strength of 70,000 psi, 100% radiography of welded shell seams for a joint efficiency of 1.0, and for a 3
liquid having a product density of 32 lb/FT . Higher presures may be obtained by using higher strength steels, or using codes and specifications that allow a higher allowable design stress for design. Field postweld heat treating the completed vessel will permit greater shell thickness and, consequently, higher pressures.
Table 5.7 SPHERICAL TANK GAS CAPACITIES
DIAMETER
INSIDE SURFACE
(FT - IN)
AREA (FT )
25-6
2043
32-0
2
3217
VOLUME
PRESSURE
FREE GAS
(FT )
(PSI)
(FT )
8680
20
11800
30
17700
40
23600
50
29500
60
35400
75
44300
3
17160
100
59000
125
73800
150
88600
200
118100
250
147700
336
198400
20
23300
30
35000
40
46700
50
58300
75
70000
100
Table 5.8 BULLET TANK CAPACITIES
3
87500
125
116700
150
145900
200
233400
266
310500
(GALLONS)
(IN.)
(FT. - IN.)
2000
46
23 - 10 3/8
3000
46
35 - 5 3/8
4000
65
24 - 11 1/2
5000
65
31 - 4 1/2
6000
72.4
30 - 3 1/4
8000
72.4
38 - 11 1/4
10000
93.5
29 - 8 7/8
12000
93.5
37 - 1 1/4
15000
93.5
44 - 5
18000
108
41 - 4 5/8
26000
108
57 - 7 1/2
30000
130
47 - 2 3/8
35000
130
54 - 6 1/8
40000
130
61 - 9 7/8
45000
130
69 - 1 5/8
50000
130
76 - 5 3/8
55000
130
83 - 9 1/8
60000
130
91 - 7/8
70000
130
98 - 4 5/8
75000
130
105 - 8 3/8
80000
144
92 - 7 1/4
85000
144
98 - 6
90000
150
96 - 10
95000
150
107 - 8
100000
150
113 - 2
APPENDIX II EXAMPLE CALCULATION
Field Production
6,500 BOPD (Real)
Production field
10,000 BOPD (Assumed)
Capacity of tanker
30,000 BBLS (Contact to shipping agency)
Loading time
8 Hrs (Based on pump rate type)
Period of loading
once/3 day (Contact to shipping agency)
Capacity of storage/terminal tank form 31,500 BBLS Number of tank
3 Pcs
Tank volume
31,500/3 = 10500 BBLS
According API 650 welded oil storage tanks Capacity of tank 11,330 BBLS : Diamater of tank ( ? ) 45 ft Height of tank (H)
40 ft
Flow rate of tank inlet
: 10,000 BOPD : 0.116 BBLS/S : 4.86 gps 3
: 0.65 ft /S
From figure A, liquid speed of tank inlet : 3 fps Header diameter of tank inlet :
0.65.4 3.?
ft = 0.8x0.525f t
= 6.3 "
Header use 8" ? pipe ASTM 106 TS STD WT 3 Tanks ? each tank inlet diameter (6.3")
2/3
assumed ? 4"
Flow rate of tank outlet : 30,000 BBLS ----------- = 1250 bbls/hr 3 x 8 Hrs 3
= 1.95 ft /S
From figure A, liquid speed of tank outlet 4 tps
Diameter of tank outlet :
1,95.4
= 0.79ft
4?
= 9.46" Use 10"? pipe ASTM 106 B STDWT
- For header diameter : 3.1,95.4 4.?
= 1,36ft
Use 18 " ? pipe ASTM 106B STDWT.
- Capacity of pump : 14,69 gps = 876 gpm 3
flow rate of pump outlet : 14,6 gps = 1,95 ft /S from figure A, liquid speed 4 pump discharge 10 fps diameter of pump discharge :
1,95.4 10?
= 0,49ft
= 5,96" Use 6"? pipe ASTM 106B STDWT.
