Tank Relief Philosophy.doc

7/27/2019 Tank Relief Philosophy.doc

1/6


2/6

Art Montemayor January 20, 1998Rev: 5 (03/12/02)

Coil Failure, and Chemical Reaction contributions. Other devices must be available to provide relieffor all of the other contributions.

4. Pipe Away Vent Valves - These are like Emergency or Conservation Vents with a piping flange onthe outlet. They are used to connect tanks to environmental control devices where the pressure in thetank 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 consideredas contributing to a relief scenario if there are no block valves or control valves between the PipeAway Vent Valve and the atmosphere or all block valves are locked open and there are no controlvalves. If their capacity is considered, calculations will need to be made to de-rate the capacity due toback pressure effects of the down stream piping and other equipment.

Table 1Pressure Relief Cases for Atmospheric Storage Tanks

Case:

1 2 3 4 5 6 7

Inflow(Pressure)

Outflow(Vacuum)

Fire CoilRupture

LiquidOverfill

ChemicalReaction

Steam Out(Vacuum)

Contributions:

1. Liquid / Vapor In Yes Yes Yes Yes

2. Liquid Out Yes

3. Out Breathing Yes Yes Yes Yes

4. In Breathing Yes

5. Fire Exposure Yes

6. Pressure Transfer Yes Yes Note 3

7. N2 Regulator Failure Yes Yes

8. Loss Heat Transfer 8.b) & c) 8. a) & d) Note 1

9. Coil Failure 9.a) 9.b)

10. Vent System Failure 10.b) 10.a) Note 2

11. Chemical Reaction 11.a) 11.b) 11.b) 11.c)

12. Steam Out Yes

Notes:1. Controls Fail to maintain full heating and results in no cooling.2. Vent valve Fails in Closed Position and prevents any vent going to a vent collection system.3. Rate as Liquid.

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 thetank. Consider the motors 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. Forcontrol valves use the largest trim for the valve body and add the capacity of any bypass valves. Analternate to using the largest trim for a control valve, is to use the actual trim size and affix a metal tag to

Page 2 of 6


3/6


the valve indicating that a safety device rating is dependent on the trim size. The total inflow is the sumfrom all of these sources. The total inflow can be reduced if engineering judgment indicates that it isunlikely 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 thehighest available pressure drop. The highest available pressure drop is the highest-pressure sourcesdeadhead gauge pressure.

For tanks that have multiple liquid inlets, each inlet can be considered independently using the flowcalculated 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-2000based on the flash point and normal boiling point of the material stored. API-2000 has a logic error in thesections covering the selection of the factor (2.4.2.2.1 and 2.4.2.3.1). The selection criteria as presentedin API-2000 is flash point below 100 oF or boiling point below 300 oF for the more conservative factorand flash point at or above 100 oF or boiling point at or above 300 oF for the less stringent factor. A fluidwith a flashpoint of 113 oF and a normal boiling point of 271 oF will test true for both criteria. In casessuch as this fluid, use the more conservative factor. In other words, replace the or in flash point at orabove 100 oF or boiling point at or above 300 oF with an and.

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 orabove 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 thetank. The total liquid outflow is the sum of all of these, including installed spare(s). Consider themotors 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 todetermine the vent rate.

3. Thermal Out Breathing

Table 2A in API Standard 2000 (5th Edition, April 1998) is used to determine the thermal out breathingbased on the capacity of the tank. For tanks less than 840,000 gal capacity, a factor can be used that isbased on the flashpoint and normal boiling point of the material stored. API-2000 has the same logicerror in Table 2 as discussed in Liquid Movement In above. The factors for tanks with a capacity up to

840,000 gal are presented below.

Test Factor, SCFH/MGALFlash Point below 100 oF or Boiling Point below 300 oF 23.81Flash Point at or above 100 oF and Boiling Point at or above 300 oF 14.29

For tanks larger than 840,000 gal, refer to Table 2 in API-2000.

4. Thermal In-Breathing

Page 3 of 6


4/6


Per API 2000. Table 2 in API-2000 is used to determine the thermal in breathing based on the capacityof the tank. For tanks less than 840,000 gal capacity, the factor is 23.81 SCFH / MGAL. For tankslarger than 840,000 gal, refer to Table 2 in API-2000.

5. Fire Exposure

Per API 2000

6. 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 gaugepressure. 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) 1by 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 pressurerating of the shipping container to be used.

7. The Nitrogen Padding System Feed Valve Fails Open

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

8. Failure or loss of Control Heat Transfer Devices; loss of heating/cooling utility or instrumentair.

This contribution covers loss of heating, excessive heating, loss of cooling, or excessive cooling and mustbe 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 probablybe a transient inflow until equilibrium is reached.

b) Excessive heating - determine the out breathing required at full heating. The steam controlvalve 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 atransient and continuous condition.

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

9. Failure of Internal/External Heating/Cooling Coils

Page 4 of 6


5/6


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 fullyopen. If the flow-limiting device is a control valve, use the largest trim available for the valvebody. Also, add the capacity of any bypass valves. An alternate to using the largest trim for acontrol valve is to use the actual trim size and affix a metal tag to the valve indicating that asafety device rating is dependent on the trim size. Determine the vapor generated byvaporization 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 thereare no flow-limiting devices perform a reasonable pipe pressure drop calculation, taking intoconsideration that the return line can potentially contribute to the inflow.

10. Failure of Vent Vapor Collection System

a) For vacuum relief, this pertains to any failure in a vapor collection system that can cause thesystem 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 vacuumrating of the tank, calculate the vapor flow at the pressure where the capacity of the vacuumdevice equals the full open capacity of the vent valve. Use Atmospheric pressure for theupstream pressure of the vent valve. Do not take credit for pressure losses in the piping orany nitrogen from a padding system.

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

11. Chemical Reaction

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

a) Thermal runaway reaction. This issue must be considered on a tank by tank basis. For tankswhere the product decomposes before it reaches a normal boiling point, the runaway case isnot 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 beconsidered on a tank by tank basis. The primary cause is due to water contamination as aresult 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 issuemust be considered on a tank by tank basis and will rarely need to be calculated.

12. Steam Out

Page 5 of 6


6/6


This case covers the steaming out of a storage tank for tanks that are steamed out during normal productswitchovers or for maintenance and inspection reasons. OSHA mandates, for good human safetyreasons, that all vessels should be cleaned and purged free of any chemicals or substances hazardous tosubsequent 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 situationwhere the internal steam is quickly condensed and the tank collapses due to a vacuum produced internallyby the condensed steam. Proper nozzle sizes should be ensured for atmospheric air to enter the tank inthat event such that sonic velocity is not achieved prior to breaking the vacuum. For tanks that aredesigned for a steam out case, a placard is to be affixed to the tank stating Vacuum Breakers are Sizedfor Steam Out.

a) For tanks up to 20,000 gal the steam inflow rate is assumed to be one 3/4 ID hose by 20 ftlong with a differential pressure is 40 psi gauge to atmospheric pressure. A material andenergy 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 bethree parallel 3/4 ID hose by 20 ft long with a differential pressure is 40 psi gauge toatmospheric pressure. A material and energy balance must be done to determine the vacuumrelief 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 ftlong. The differential pressure is 250 psi gauge to atmospheric pressure. A material andenergy balance must be done to determine the vacuum relief due to a hard, sudden 40 oF rain.

Arts Note: This tank collapse (A.K.A. suck-in in the Texas Gulf Coast) has been personally witnessedby me and the results have also been seen out in the field. Some of the steam-out vacuum failures haveresulted because of a proven lack of flow capacity in the tanks venting nozzle i.e., the nozzle reached amaximum sonic velocity capacity which was not enough for the scenario and the tank immediatelysucked-in.

Page 6 of 6

Documents

Tank Relief Philosophy.doc