Have you ever wondered how ‘set pressure’ and ‘relieving pressure’ differ from one another? And why does it matter? Let’s break it down in simpler terms.

Say for example you have a 2-phase separator with MAWP of 100 barg. It has a single PSV and non-fire case, so as per Table 1 of API 520 Part 1, one can set that PSV at max. 100 barg. It means when vessel pressure reaches 100 barg, then PSV starts to open.

Also, its overpressure or accumulation shall be a maximum of 10% of MAWP. It means in this case PSV sizing should be such that the vessel pressure should never exceed 110 barg. This maximum allowable PSV upstream pressure is called relieving pressure.

I hope this was useful to you. Please do share this with your friends.
Take care.
Ajay S. Satpute