API 520 PRV Reaction Force Calculation

[tjpengineer]

For the API 520 All Vapor Flow case reaction force calculation (API 520, PArt II, 4.4.1.1), a component P is referenced as the static pressure in discharge pipe at point of discharge. What value is this? Is this 0 psig for all discharges to atmosphere? Is it the built up back pressure due to the discharge at the time of discharge minus the pressure drop due to friction along the discharge pipe?

 

[kkimic]

Hi,

This is the static pressure at the point of discharge, so for instance if the PSV is discharging to atmosphere this value will be 0, if on the other hand it is discharging to a vessel operating at 14 psig, this value will then be 14 psig.

Or at least it is what I would have done.

Regards.

 

[EGT01]

There is an exception to the two conditions that kkimic has indicated and that is when flow is choked at the end of the outlet pipe. In that situation, you need to use the choked flow pressure.

 

[RJB32482]

EGT01 or Others,
Can you give me an example of a situation that you are talking about? Maybe a sample calculation on where the static pressure is not 0 PSIG for an atmospheric discharge.

Thanks.

 

[garfio]

If you have sonic flow at the end of the discharge pipe and you are discharging to the atmosphere, the pressure at the end of the pipe would be above atmospheric pressure (the difference will be lost through shock waves).

API 521 (or 520?) shows how to calculate the critical pressure (Pc) at which the fluid reaches Mach 1. That is the pressure that I would use.