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Non-standard PSV area and outlet piping size

Non-standard PSV area and outlet piping size

Non-standard PSV area and outlet piping size

Hello everyone,

I have run into a couple issues while trying to size a PSV in FluidFlow. The data for my problem and my questions are given below:


- FluidFlow sizing of PSV
- Working fluid is steam
- Design pressure of pipe is 1379 kPag
- PSV is protecting this pipe
- PSV design flow is 55167 kg/h
- Relief to atmosphere


1. In order to avoid choked flow and sonic fluid velocity, we had to calculate using non-standard outlet piping sizes. The largest diameter standard size we used was 12 in Schedule 40S, but still resulted in sonic velocity. Using outlet piping that was 18 in Schedule 10 reduced our fluid velocity to below sonic, but it still remained high (274 m/s) and decreased with increasing pipe diameter, as expected. How should we tackle this problem to avoid using non-standard piping but simultaneously keep our fluid below sonic velocities?

2. Using the same parameters for question 1, our PSV area was calculated to be a non-standard size of 9801.6 mm2. The closest actual standard PSV area size is 11471 mm2 (Farris Engg PRVs). I am worried that the manufacturer’s size is too big, resulting in issues with our system. How can I check if the manufacturer’s size will still work for my system? I am unable to enter area values for my PSV in FluidFlow.

Thank you all for your help.

RE: Non-standard PSV area and outlet piping size

Have you considered using more than one PSV?

RE: Non-standard PSV area and outlet piping size

We prefer to avoid chokes in the tailpipe, because a choke can constrain our ability to get more capacity from the pipe, if that's ever needed. A choke will also cause a higher reaction force, which may require additional pipe support. But a choke isn't a show-stopper of a problem. A choke results in a higher backpressure on the PSV. That means we need to ensure that the PSV can tolerate the resulting backpressure caused by the choke. Calculate the resulting backpressure on the PSV caused by the choke, and use a bellows PSV, as long as the backpressure is within the limits of the bellows. In other services a pilot operated valve can be used for cases where the backpressure is too high for a bellows, but in your case (200 psi steam) that's not an option because the temperature is too high for a pilot valve.

I'm guessing that you're designing for 10% accumulation. If so, the fact that you're protecting piping rather than a vessel means that you can gain additional capacity (if it's ever needed) by allowing the accumulation to rise as high as 33% for a limited amount of time (ref: ASME B31.3, 322.6.3(c)). When you have a choke, the upstream pressure must increase in order to get more mass flow. A pressure increase from 10% accumulation to 33% accumulation will significantly increase the flow.

RE: Non-standard PSV area and outlet piping size

Have you considered moving the PSV to a location you don't need a tailpipe?

Good luck,

To a ChE, the glass is always full - 1/2 air and 1/2 water.

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