January 20, 1998 Rev: 4 (03/10/08)

Storage Tank Relief Calculation Philosophy for Chemical Processing Plant Storage Facilities

Relief Cases

When determining the relief requirements for atmospheric storage tanks there are seven independent cases. They are:

1. Inflow2. Outflow3. External Fire4. Steam Coil Rupture5. Liquid Overfill6. Chemical Reaction7. Steam Out

The following are contributions that are used in combination to determine the loads required for each case:

1. Liquid Movement In2. Liquid Movement Out3. Thermal Out Breathing4. Thermal In Breathing5. Fire Exposure6. Pressure Transfer Blow-through7. Fail Open the Nitrogen Padding System8. Failure or loss of Control of Heat Transfer Devices; loss of heating/cooling utility or instrument air.9. Failure of Internal/External Heating/Cooling Coils10. Failure of Vent Vapor Collection System11. Chemical Reaction12. Steam Out

Table 1 shows which contributions need to be considered when calculating each Case.

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January 20, 1998 Rev: 4 (03/10/08)

Relief Devices

1. Conservation Vents - These are Pressure Vacuum breather valves. They may be used as final pressure and/or vacuum relief or for normal pressure control on atmospheric tanks.

2. Emergency Vents - These devices are like conservation vents except that they only have the pressure side. They may be used to supplement the conservation vent(s) when additional capacity is needed to meet a large pressure relief load while additional vacuum relief is not needed. They can take the inexpensive form of simple, weighed blind flanges on top of a tank roof nozzle; the blind cover flange is often hinged or chained in order to avoid a hazard with the cover being expelled.

3. Fire Hatches - These devices may be used in the same cases as Emergency Vents. These devices are usually weighed down and swing open when over-pressured. They will not re-seal after the relief venting is complete. Because of this feature, their relief capacity is restricted to mitigate the Fire, Coil Failure, and Chemical Reaction contributions. Other devices must be available to provide relief for all of the other contributions.

4. Pipe Away Vent Valves - These are like Emergency or Conservation Vents with a piping flange on the outlet. They are used to connect tanks to environmental control devices where the pressure in the tank is the driving force pushing the vapors through the piping and environmental control device. These devices are usually not considered as relief devices; however, their capacity can be considered as contributing to a relief scenario if there are no block valves or control valves between the Pipe Away Vent Valve and the atmosphere or all block valves are locked open and there are no control valves. If their capacity is considered, calculations will need to be made to de-rate the capacity due to back pressure effects of the down stream piping and other equipment.

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January 20, 1998 Rev: 4 (03/10/08)

Table 1Pressure Relief Cases for Atmospheric Storage Tanks

Case:1 2 3 4 5 6 7

Inflow(Pressure)

Outflow(Vacuum)

Fire Coil Rupture

Liquid Overfill

ChemicalReaction

Steam Out (Vacuum)

Contributions:1. Liquid / Vapor In Yes Yes Yes Yes2. Liquid Out Yes3. Out Breathing Yes Yes Yes Yes4. In Breathing Yes5. Fire Exposure Yes6. Pressure Transfer Yes Yes Note 37. N2 Regulator

FailureYes Yes

8. Loss Heat Transfer

8.b) & c) 8. a) & d) Note 1

9. Coil Failure 9.a) 9.b)10. Vent System

Failure10.b) 10.a) Note 2

11. Chemical Reaction

11.a) 11.b) 11.b) 11.c)

12. Steam Out Yes

Notes:1. Fail to full heating or no cooling.2. Fail Closed any vent valves going to a vent collection system.3. Rate as Liquid.

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January 20, 1998 Rev: 4 (03/10/08)Calculating Contribution Loads

1. Liquid Movement In

The liquid movement into a tank is the lesser of the following:

a) Determine the Maximum flow, at the horse power limit, for each pump that can be lined up to the tank. Consider the motor’s service factor (usually 1.15) when determining the horse power limit.

For sources that are not pump driven, calculate the maximum flow for the flow limiting element. For control valves use the largest trim for the valve body and add the capacity of any bypass valves. An alternate to using the largest trim for a control valve, is to use the actual trim size and affix a metal tag to the valve indicating that a safety device rating is dependent on the trim size. The total inflow is the sum from all of these sources. The total inflow can be reduced if engineering judgment indicates that it is unlikely that all of the sources will be feeding into the tank simultaneously.

b) Determine the maximum flow that the piping between the inlet manifold and tank can handle at the highest available pressure drop. The highest available pressure drop is the highest-pressure source’s deadhead gauge pressure.

For tanks that have multiple liquid inlets, each inlet can be considered independently using the flow calculated by method (a) or (b). Sum the flows for each inlet line to get the total for the tank.

Once the total liquid inflow is determined, multiply the flow by the appropriate factor as per API-2000 based on the flash point and normal boiling point of the material stored. API-2000 has a logic error in the sections covering the selection of the factor (2.4.2.2.1 and 2.4.2.3.1). The selection criteria as presented in API-2000 is flash point below 100 oF or boiling point below 300 oF for the more conservative factor and flash point at or above 100 oF or boiling point at or above 300 oF for the less stringent factor. A fluid with a flashpoint of 113 oF and a normal boiling point of 271 oF will test true for both criteria. In cases such as this fluid, use the more conservative factor. In other words, replace the “or” in “flash point at or above 100 oF or boiling point at or above 300 oF” with an “and”.

Table 2

Test Factor, SCFH/GPMFlash Point below 100 oF or Boiling Point below 300 oF 17.14Flash Point at or above 100 oF and Boiling Point at or above 300 oF

8.57

2. Liquid Movement Out

Determine the Maximum flow, at the horse power limit, for each pump that can take suction from the tank. The total liquid outflow is the sum of all of these, including installed spare(s). Consider the motor’s service factor (usually 1.15) when determining the horse power limit.

Once the total liquid outflow is calculated, multiply it by the API-2000 factor of 8 SCFH/GPM to determine the vent rate.

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January 20, 1998 Rev: 4 (03/10/08)

3. Thermal Out Breathing and In Breathing

The thermal out and in breathing can be determined using Table 2 in API-2000 based on the capacity of the tank. This table is presented below:

Table 3

Tank Capacity In Breathing (Vacuum)

Out Breathing

Barrels Gallons SCFH AirFlash Point >

100°F or Normal Boiling Point >

300°F

Flash Point < 100°F or Normal Boiling Point <

300°F60 2,500 60 40 60100 4,200 100 60 100500 21,000 500 300 500

1,000 42,000 1,000 600 1,0002,000 84,000 2,000 1,200 2,0003,000 126,000 3,000 1,800 3,0004,000 168,000 4,000 2,400 4,0005,000 210,000 5,000 3,000 5,00010,000 420,000 10,000 6,000 10,00015,000 630,000 15,000 9,000 15,00020,000 840,000 20,000 12,000 20,00025,000 1,050,000 24,000 15,000 24,00030,000 1,260,000 28,000 17,000 28,00035,000 1,470,000 31,000 19,000 31,00040,000 1,680,000 34,000 21,000 34,00045,000 1,890,000 37,000 23,000 37,00050,000 2,100,000 40,000 24,000 40,00060,000 2,520,000 44,000 27,000 44,00070,000 2,940,000 48,000 29,000 48,00080,000 3,360,000 52,000 31,000 52,00090,000 3,780,000 56,000 34,000 56,000100,000 4,200,000 60,000 36,000 60,000120,000 5,040,000 68,000 41,000 68,000140,000 5,880,000 75,000 45,000 75,000160,000 6,720,000 82,000 50,000 82,000180,000 7,560,000 90,000 54,000 90,000

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January 20, 1998 Rev: 4 (03/10/08)

4. Fire Exposure

Roof tank with a weak (frangible) roof-to-shell attachment as described in API Standard 650, the roof-to-shell connection will fail preferentially to any other joint and the excess pressure will be safely relieved if the normal venting capacity should prove inadequate. For a tank built to these specifications, consideration need not be given to any additional requirements for emergency venting; however, additional emergency vents may be used to avoid failure of the joint. Care should be taken to ensure that the requirements for a frangible roof-to-shell attachment are met, particularly on a smaller tank.

When a tank is not specified with a weak roof-to-shell attachment the equation below should be used to calculate the venting requirement due to fire.

Where: SCFH = Venting Requirement

Q = Heat Input from Fire Exposure (BTU/h). Heat input is provided in the table listed below:

Table 4

Wetted Surface Area(square feet)

Design Pressure(psig)

Heat Input(Btu/hr)

< 200 < 15 Q = 20,000A> 200 and <1000 < 15 Q = 199,300A0.566

> 1000 and < 2800 < 15 Q = 963,400A0.338

> 2800 Between 1 psig and 15 Q = 21,000A0.82

> 2800 < 1 Q = 14,090,000

A = Wetted Surface Area of the Tank (ft2) Sphere and Spheroids - The wetted area is equal to 55 percent of the total surface area or

the surface area to a height of 30 feet (9.14 meters) above grade, whichever is greater. Horizontal Tanks - The wetted area is equal to 75 percent of the total surface area or the

surface area to a height of 30 feet (9.14 meters) above grade, whichever is greater. Vertical Tanks - The wetted area is equal to the total surface area of the vertical shell to a

height of 30 feet (9.14 meters) above grade. For a vertical tank setting on the ground, the area of the ground plates is not to be included as wetted area. For a vertical tank supported above grade, a portion of the area of the bottom is to be included as additional wetted surface. The portion of the bottom area exposed to a fire depends on the diameter and elevation of the tank above grade. Engineering judgment is to be used in evaluating the portion of the area exposed to fire.

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January 20, 1998 Rev: 4 (03/10/08)F = Environmental Factor from the table listed below. Credit may be taken for only one environmental

factor.

Table 5

Tank Design/Configuration

Insulation Conductance (BTU/h – ft2 - °F)

Insulation Thickness(inches)

F Factor

Bare Metal Tank - 0 1.0Insulated Tank 4.0 1 0.3Insulated Tank 2.0 2 0.15Insulated Tank 1.0 4 0.075Insulated Tank 0.67 6 0.05Insulated Tank 0.5 8 0.0375Insulated Tank 0.4 10 0.03Insulated Tank 0.33 12 0.025

Concrete Tank or Fireproofing

- - Equivalent Conductance Value of Insulation

Water-Application Facilities

- - 1.0

Depressurizing and Emptying Facilities

- - 1.0

Underground Storage - - 0Earth-Covered Storage

Above Grade- - 0.03

Impoundment Away from Tank

- - 0.5

L = Latent Heat of Vaporization of the Stored Liquid at the Relieving Pressure and Temperature (BTU/lb)

T = Temperature of the Relieving Vapor (R). It is normally assumed that the temperature of the relieving vapor corresponds to the boiling point of the stored fluid at the relieving pressure.

M = Molecular Weight of the Vapor being relieved

5. Pressure Transfer Blow-through

a) For tanks that are not expected to be unloaded into, the vapor rate is the flow capacity of two (2) 1” by 20 foot utility hoses with a pressure drop of the maximum nitrogen header gauge pressure. One 1” (3/4” ID) by 20’ utility hose can pass 18,825 SCFH of nitrogen.

b) For tanks where unloading is expected, use the greater of (a) above or the sum of one (1) 1” by 20 foot long utility hoses with a pressure drop of the maximum nitrogen header gauge pressure and the unloading line size with the pressure drop as the maximum gauge pressure rating of the shipping container to be used.

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January 20, 1998 Rev: 4 (03/10/08)6. The Nitrogen Padding System Fails Open

Calculate the flow capacity of the flow limiting device using the maximum nitrogen header gauge pressure for the pressure drop. For padding systems that have a step down regulator, the step down regulator will almost always be the flow limiting device.

7. Failure or loss of Control Heat Transfer Devices; loss of heating/cooling utility or instrument air.

This contribution covers loss of heating, excessive heating, loss of cooling, or excessive cooling and must be determined on a tank by tank basis. These failures can be caused by failures in the control systems, measurement instruments, control valves, instrument air, and the loss of the heating or cooling utility.

a) Loss of heating - determine the in breathing required if heating were lost. This will probably be a transient inflow until equilibrium is reached.

b) Excessive heating - determine the out breathing required at full heating. The steam control valve usually limits this. This will probably be a continuous condition.

c) Loss of Cooling - determine the out breathing required if cooling were lost. This could be a transient and continuous condition.

d) Excessive Cooling - determine the required in breathing if the cooling were to fail to maximum. This will probably be a transient inflow until equilibrium is reached.

8. Failure of Internal/External Heating/Cooling Coils

This contribution covers a mechanical failure of a heat transfer device.

a) For tanks with heating coils, determine the flow capacity of the flow-limiting device fully open. If the flow-limiting device is a control valve, use the largest trim available for the valve body. Also, add the capacity of any bypass valves. An alternate to using the largest trim for a control valve is to use the actual trim size and affix a metal tag to the valve indicating that a safety device rating is dependent on the trim size. Determine the vapor generated by vaporization of the product or by the steam itself - whichever is greater.

b) For tanks with cooling coils, determine the flow capacity of any flow-limiting devices. If there are no flow-limiting devices perform a reasonable pipe pressure drop calculation, taking into consideration that the return line can potentially contribute to the inflow.

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January 20, 1998 Rev: 4 (03/10/08)9. Failure of Vent Vapor Collection System

a) For vacuum relief, this pertains to any failure in a vapor collection system that can cause the system to pull excessive vapors from the tank. Calculate the flow capacity of the flow-limiting device. For systems that vent through a vacuum device that can pull down below the vacuum rating of the tank, calculate the vapor flow at the pressure where the capacity of the vacuum device equals the full open capacity of the vent valve. Use Atmospheric pressure for the upstream pressure of the vent valve. Do not take credit for pressure losses in the piping or any nitrogen from a padding system.

b) For pressure relief, if the vacuum-producing device discharges into a header that can have a pressure higher than the pressure rating of the tank, the back flow case must be considered in the event of the failure of the vacuum-producing device. For this case calculate the capacity of the vent valve using the maximum anticipated vacuum producing device discharge header gauge pressure as the pressure drop. Do not take credit for check valves or piping losses.

10. Chemical Reaction

A chemical reaction in a tank can be caused by exposure to heat, incomplete reaction in a reactor, or by reaction of the tank contents with some contaminant.

a) Thermal runaway reaction. This issue must be considered on a tank by tank basis. For tanks where the product decomposes before it reaches a normal boiling point, the runaway case is not considered since it would result in a relief device as large as the tank.

b) A reaction can take place if two materials are contacted unintentionally. This issue must be considered on a tank by tank basis. The primary cause is due to water contamination as a result of the failure of a heat transfer device. Heat of mixing should also to be considered.

c) This contribution covers the possibility of incomplete reaction in a reaction system. This issue must be considered on a tank by tank basis and will rarely need to be calculated.

11. Steam Out

This case covers the steaming out of a storage tank for tanks that are steamed out during normal product switchovers or for maintenance and inspection reasons. OSHA mandates, for good human safety reasons, that all vessels should be cleaned and purged free of any chemicals or substances hazardous to subsequent entry by humans. Steaming achieves a lot of the gross residual chemicals removal in a tank. However, in the process of steaming a tank out, a sudden rain shower may develop a dangerous situation where the internal steam is quickly condensed and the tank collapses due to a vacuum produced internally by the condensed steam. Proper nozzle sizes should be ensured for atmospheric air to enter the tank in that event such that sonic velocity is not achieved prior to breaking the vacuum. For tanks that are designed for a steam out case, a placard is to be affixed to the tank stating “Vacuum Breakers are Sized for Steam Out”.

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January 20, 1998 Rev: 4 (03/10/08)

a) For tanks up to 20,000 gal the steam inflow rate is assumed to be one 3/4” ID hose by 20 ft long with a differential pressure is 40 psi gauge to atmospheric pressure. A material and energy balance must be done to determine the vacuum relief due to a hard sudden 40 oF rain.

b) For 20,000 gal tanks up to and including 70,000 gal the steam inflow rate is assumed to be three parallel 3/4” ID hose by 20 ft long with a differential pressure is 40 psi gauge to atmospheric pressure. A material and energy balance must be done to determine the vacuum relief due to a hard, sudden 40 oF rain.

c) For tanks larger than 70,000 gal, the steam inflow rate is assumed to be one 3/4” pipe by 50 ft long. The differential pressure is 250 psi gauge to atmospheric pressure. A material and energy balance must be done to determine the vacuum relief due to a hard, sudden 40 oF rain.

Art’s Note: This tank collapse (A.K.A. “suck-in” in the Texas Gulf Coast) has been personally witnessed by me and the results have also been seen out in the field. Some of the steam-out vacuum failures have resulted because of a proven lack of flow capacity in the tank’s venting nozzle – i.e., the nozzle reached a maximum sonic velocity capacity which was not enough for the scenario and the tank immediately “sucked-in”.

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January 20, 1998 Rev: 4 (03/10/08)

Sample Calculations

Out Breathing CalculationsLiquid Movement In = 700gpmFrom Table 2 – Flash Point > 100 °F, Boiling Point > 300 °F = 8.57 SCFH of Air/gpmTherefore, 700 gpm * 8.57 SCFH of Air/gpm = 5999 SCFH of Air

Thermal Out BreathingFrom Table 3 - Flash Point > 100 °F, Boiling Point > 300 °F, 1500 bbl TankInterpolate between two points 1000 bbl = 600 SCFH of Air and 2000 bbl = 1200 SCFH of AirTherefore, 1500 BBL = 900 SCFH of Air

Total Out Breathing = 5999 SCFH of Air + 900 SCFH of Air = 6899 SCFH of Air

In Breathing CalculationsLiquid Movement Out = 70 gpm per pump, 4 pumps in total = 280 gpmFrom Table 2 – Flash Point > 100 °F, Boiling Point > 300 °F = 8 SCFH of Air/gpmTherefore, 280 gpm * 8 SCFH of Air/gpm = 2240 SCFH of Air

Thermal In BreathingFrom Table 3 - Flash Point > 100 °F, Boiling Point > 300 °F1500 BBL = 1500 SCFH of Air

Total In Breathing = 2240 SCFH of Air + 1500 SCFH of Air = 3740 SCFH of Air

Emergency Venting Due to Fire

A = л * d * h = л * 21 ft * 24.3 ft = 1603.16 ft2 (Horizontal Tank)From Table 4, A > 1000 ft2 and < 2800 ft2, Design Pressure < 15 psiTherefore, Q = 963,400 * A0.338 = 963,400 * (1603.16)0.338 = 11670482.82 Btu/hFrom Table 5, F = 1.0 (Bare Metal)L = 144 Btu/lbT = Boiling Point = 360 °F + 459.67 = 819.67 RMW = 274

Therefore, = SCFH = 3.09 * = 433140.76 SCFH of Air

Note: For the pressure requirements it is important to note that most tanks made will have a maximum pressure of 16 oz/inch. Anything higher will require more money to manufacture. So for most cases the EPRV can be set at roughly 14 oz/inch, while the PVRV (or Thief Hatch) can be set at 8 oz/inch.

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