CONTENTS

1. GENERAL

2. TYPES OF TANKS

2.1 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

2.2 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 Bolted Production Tanks

Figure 5.2 - Dimension 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 Tanks Supported By Soil

Figure 5.6 - The Floating Roof by Minimizing Vapor Space Eliminates 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

Figure 5.11 - US Standard Gauge For Steel and Iron Sheet and Plate.

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 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, 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 specification for Small Welded 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 low-

temperature 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 over-

pressure/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.

• Temperature circuits, if product temperature must be controlled.

6. 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 Total Capacity = (5-4)

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 Thickness Inches

Deduction

11 10 9 8 7 6 5 4 3 2 1 0 00 000 0000 00000 000000 0000000

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

.0653 .0817 .0983 .1147 .1310 .1473 .1637 .1800 .1963 .2127 .2290 .2454 .2617

7. 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 1) depth, 2)

thickness of tank walls, 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 well-

packed 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 : V = Storage Capacity, barrels

T = Retention Time, hours

Q = Liquid Flow Rate to be Storaged, gpm.

11. RETENTION TIME

An empirical equation for estimating retention time : u T = A -------- Hrs Sw - So

Where : u = Oil viscosities, Cp

Sw = Specific Gravity of Water

So = Specific Gravity of Oil

A = Constant which varies from 0.05 - 1.0

12. 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 vapor-

liquid 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 :

(P) (D)1.8

B = ------------- (Fo)(Fp)

A

Where : Metric English

B = Annual Breathing loss m3 API bbl

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

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 Fo m Ft 0.31 1 0.39 1.53 5 0.55 3.05 10 0.72 4.58 15 0.87 6.10 20 1.00 7.63 25 1.12 9.15 30 1.23 10.68 35 1.33 12.20 40 1.43 13.73 45 1.53 15.25 50 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 = ----- (Kf)

A

Where : Metric English

P = True Vapor Pressure kPa (g) psig

V = Volume of liquid in m3 bbl

F = Filling Loss m3 bbl

A = Conversion constant 22 740 3300

The value for Kf is found from the table below :

Kf Tank Turnovers Fields and Per year Refinery Terminals 0 - 10 1.00 1.00 12 0.91 1.00 15 0.75 1.00 20 0.59 1.00 25 0.50 1.00 30 0.47 1.00 40 0.44 1.00 40 - 60 - 0.80 60 -100 - 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 :

3.8-4.6 m3 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

13. 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

tanks) or lowest (for refrigerated tanks) temperature possible under the worst

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 or 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 pad,

use 0.2 PSIG. This does not include velocity head

loss.

• All vapor and liquid stream densities

• If applicable, note vacuum condition

• If drum any have operating liquid level, note what

lines must enter above this level, both in 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.

3. API 12 F "Small Welded Production Tank", 10 th 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.

6. API Std 2501 "Crude Oil Tank Measurement And Calibration",

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 Height

505 1,010 1,515 1,512 2,020 2,100 3,025 3,020 3,765 4,030 5,040 5,020 5,485 6,040 6,855 6,010 7,160 7,515 8,950 10,100 10,315 11,330 12,100 12,100 12,890 13,595 13,985

15-0 21-3 21-3 26-0 21-3 25-0 26-0 30-0 33-6 30-0 30-0 33-6 35-0 30-0 35-0 36-8 40-0 36-8 40-0 42-6 48-0 45-0 42-6 52-0 48-0 45-0 50-0

16-0 16-0 24-0 16-0 32-0 24-0 32-0 24-0 24-0 32-0 40-0 32-0 32-0 48-0 40-0 32-0 32-0 40-0 40-0 40-0 32-0 40-0 48-0 32-0 40-0 48-0 40-0

15,470 15,130 15,060 16,785 20,140 24,170 25,120 27,415 30,140 30,100 32,905 35,810 40,425 42,970 45,320 44,760 54,390 54,165 55,950 67,140 67,705 81,245 80,580 96,690 100,470 109,700 120,563

48-0 52-0 58-0 50-0 60-0 60-0 67-0 70-0 67-0 73-4 70-0 80-0 85-0 80-0 90-0 100-0 90-0 110-0 100-0 100-0 110-0 110-0 120-0 120-0 134-0 140-0 134-0

48-0 40-0 32-0 48-0 40-0 48-0 40-0 40-0 48-0 40-0 48-0 40-0 40-0 48-0 40-0 32-0 48-0 32-0 40-0 48-0 40-0 48-0 40-0 48-0 40-0 40-0 48-0

Capacity In Barrels Exact

Tank Dimensions Feet and Inches

Capacity In Barrels Exact

Tank Dimensions Feet and Inches

Diameter Height Diameter Height

125,895 231,600 143,200 150,995 171,900 181,300 217,500 223,800 268,600

150-0 140-0 160-0 150-0 160-0 180-0 180-0 200-0 200-0

40-0 48-0 40-0 48-0 48-0 40-0 48-0 40-0 48-0

325,000 387,000 453,500 526,000 604,000 687,500 776,000 789,800

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

CAPACITY

(BBLS)

DIAMETER*

(FT-IN)

PRESSURE

+

(PSI)

ACTUAL

VOLUME

......(FT3)

INSIDE SURFACE

AREA

(FT3)

1000

1500

2000

2500

3000

4000

5000

6000

7500

10000

12000

15000

20000

25000

30000

40000

22-3

25-6

28-0

30-3

32-0

35-3

38-0

40-6

43-6

48-0

51-0

54-9

60-6

65-0

69-0

76-0

380

326

299

274

260

234

215

202

136

167

157

144

123

117

109

96

5770

8680

11490

14490

17160

22930

28730

34780

43100

57910

69460

85930

115950

143790

172010

229850

1555

2043

2463

2875

3217

3904

4536

5153

5945

7238

8171

9417

11500

13270

14960

13150

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

liquid having a product density of 32 lb/FT3. 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

(FT - IN)

INSIDE SURFACE

AREA (FT2)

VOLUME

(FT3)

PRESSURE

(PSI)

FREE GAS

(FT3)

25-6 2043 8680 20

30

40

50

60

75

100

125

150

200

250

336

11800

17700

23600

29500

35400

44300

59000

73800

88600

118100

147700

198400

32-0 3217 17160 20

30

40

50

75

100

125

150

200

266

23300

35000

46700

58300

70000

87500

116700

145900

233400

310500

Table 5.8

BULLET TANK CAPACITIES

CAPACITY I.D. OVERALL LENGTH*

(GALLONS) (IN.) (FT. - IN.)

2000

3000

4000

5000

6000

8000

10000

12000

15000

18000

26000

30000

35000

40000

45000

50000

55000

60000

70000

75000

80000

85000

90000

95000

100000

46

46

65

65

72.4

72.4

93.5

93.5

93.5

108

108

130

130

130

130

130

130

130

130

130

144

144

150

150

150

23 - 10 3/8

35 - 5 3/8

24 - 11 1/2

31 - 4 1/2

30 - 3 1/4

38 - 11 1/4

29 - 8 7/8

37 - 1 1/4

44 - 5

41 - 4 5/8

57 - 7 1/2

47 - 2 3/8

54 - 6 1/8

61 - 9 7/8

69 - 1 5/8

76 - 5 3/8

83 - 9 1/8

91 - 7/8

98 - 4 5/8

105 - 8 3/8

92 - 7 1/4

98 - 6

96 - 10

107 - 8

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

: 0.65 ft3/S

From figure A, liquid speed of tank inlet : 3 fps

Header diameter of tank inlet :

= 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 = 1.95 ft3/S

t0.8x0.525f=ft 3.

0.65.4?

From figure A, liquid speed of tank outlet 4 tps Diameter of tank outlet :

= 9.46" Use 10"? pipe ASTM 106 B STDWT - For header diameter :

Use 18 " ? pipe ASTM 106B STDWT. - Capacity of pump : 14,69 gps = 876 gpm flow rate of pump outlet : 14,6 gps = 1,95 ft3/S from figure A, liquid speed 4 pump discharge 10 fps diameter of pump discharge :

= 5,96" Use 6"? pipe ASTM 106B STDWT.

0.79ft= 4

1,95.4?

1,36ft= 4.

3.1,95.4?

0,49ft= 10

1,95.4?