thePetroStreet

Aoa Friends,

My question is about the PSV sizing criteria for liquid NH3 service for following two scenarios;

1. Error of operation - over pressurization
2. Fire scenario

In case of over pressurization of liquid NH3 we can size a PSV with a specific orifice size. Now for the second scenario wih the same PSV, the external heat due to fire can increase the temperature of NH3, and if it is increase beyond its critical temperature at defined pressure [132.4°C at 112.8 bar], the NH3 will vaporize. The PSV sized for the first scenario may not handle the flow for over pressurization due to phase change. And if i use the latter scenario for sizing of PSV then it will be ovesized for the first scenario.

Can anyone explain me the required criteria for such scenarios?

In such case API recommends the usage of two PSVs with two different set-points. Set one of the PSV with lower relief load to pop first (at MAWP) and the second one at higher setpoint (at 105% MAWP). This will give the freedom to avoid chattering along full protection.

the way I have understood your problem that you want to use the same PSV for the release of liquid Ammonia as well as gaseous Ammonia resulted in case of external fire in the vicinity.
If my understanding is correct then remember that any PSV is designed to cater only one scenario because the basis of both the scenarios (Operational error / instrument malfunction or external fire) are entirely different & API deals these scenarios separately.
Usually over pressurization of any vessel caused by external fire takes into account the following assumptions.
a) Vessel volume
b) rate of vapor generation as a result of external fire
c) Liquid in and out flow
For liquid flowing through the line the basis of PSV are always
a) Maximum possible flow through that line in any given scenario.
as it is obvious from the basis of both the scenarios that they are not meant to be the alternate for each other unless you are lucky enough.
My process evaluation suggests that it is not needed to provide line protection (against fire)where highly volatile liquid (like Ammonia) is flowing continuously and there is no accumulation of liquid to get exposed to fire for so long that its temperature exceeds its saturation temperature and whole liquid travelling through line turns into vapors /gas. Such protection is provided either on downstream vessel where the line is delivering or in its upstream vessel form where it is drawing liquid
The only possible way to come across vapor transformation in a liquid Ammonia line is that it has been isolated from both sides and than exposed to external fire. In that case trapped Ammonia may turn into vapor which is why it is never recommended to trap liquid Ammonia between two isolations unless provided with a drain valve. whenever there is such scenario, a thermal relief PSV is also installed to prevent over pressurization. Even thermal PSV is not intended to prevent over pressurization in case of external fire. At least I haven't come across any PSV that is installed on a liquid line to release vapors in case of fire.

Bottom line, Over pressurization protection in case of fire is only provided on storage vessels and not on liquid lines
Please share your experience of fire protection PSV on liquid Ammonia line if any