7/27/2019 Volume Tank Sizing for Shutdown Valve actuator.docx

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Volume Tank Sizing for Shutdown Valve actuator

Volume tank in the shutdown valve application is used to provide a back up in case loss of instrument airsupply. The main function of this volume tank is to reserve the instrument air pressure during the first operationof actuator and use it later. The operating principle of this volume tank is similar with an accumulator onhydraulic actuator except it just a tank without any diaphragm or bladder to separate different fluid like inaccumulator. The only fluids that fill the volume tank is the instrument air itself.

Usually the volume tank is sized for 3 times stroke without any instrument air supply available from theinstrument air header. Who will do the sizing of this volume tank? Of course it will be the responsibility of theshutdown valve vendor. But its better if we know how to size it and check the vendor calculation.First of all, lets take a look the typical schematic diagram of the shutdown valve actuator with a volume tankbelow.

From this typical schematic diagram, we can use the following assumption and simplification:

1. The expansion process (while stroking) of thevolume tank + volume of actuator can beconsidered as adiabatic process (athermodynamic process in which no heat istransferred to or from the working fluid).

2. The pressure regulator in the downstream ofthe volume tank will regulate the pressure atthe actuator to its minimum required pressure.It means, the final pressure of the volume tank

after the actuator does a several strokes(usually 3 times stroke) shall be equal orgreater than the pressure regulator set point.Otherwise the pressure regulator wouldntdeliver the minimum required pressure to theactuator. For simplicity purpose the finalpressure of the volume tank will be the samewith the set pressure of the pressureregulator.

From the above assumption and the adiabatic equation we can get the following equation:

7/27/2019 Volume Tank Sizing for Shutdown Valve actuator.docx

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Example,Known variables:

- V2 = 280 in3- P1 = 95.3 psia

- P2 = 65.3 psia- Adiabatic index = 1.4 (for instrument air)- x = 3 times

V1 = (3*280/((95.3/65.3)^(1/1.4)-1))

V1= 2709.6 in3 = 44 literAvailable volume tank size is 50L, 100L. Then we could select the 50L volume tank capacity with 6 Liter spare.

Notes:Usually the vendor is sizing the volume tank based on maximum pressure of instrument air header and notbased on minimum or normal pressure. Because once the instrument air header reach its maximum pressure,the instrument air is trapped inside the volume tank and couldnt back again to the instrument air header.