PSV set pressure vs vessel design pressure 2

[phoenixmoca]

Dear all,

One question occuring recently is when the design pressure of vessel is 5.5 MPaG, is it reasonable to determine its PSV set pressure as 4.5 MPaG with the operating pressure of 4MPaG?

Any comments about this item are greatly appreciated. Thank you.

 

[georgeverghese]

Nothing wrong with setting the PSV lower than the mechanical design pressure of the vessel, but there must be a reason. Can you explain?

If the MAX operating pressure is 4MPag, with a PSV set at 4.5MPag, ratio would be 89%, which is just within the max permissible of 90% for a conventional PSV. You havent said if the MAX operating pressure is indeed 4MPag. This should cover all operating modes for this vessel, and should include reasonable process control margins for pressure excursions beyond the normal range due to process upsets.

 

[georgeverghese]

Additional margins are also required for the PSHH setting, and there should also be adequate process safety time for the related ESDV to isolate the source of overpressure before reaching PSV set pressure.

 

[mk3223]

On the other hand, it is okay to have PSV set at 5.5 MPaG as permitted by the Code. So, you can run the process to at the pressure up to the maximum limit of the vessel without a change needed for the PSV setting.

 

[phoenixmoca]

thanks a lot for all the answers.

But if the operating pressure is 3.5 MPaG is it also OK for such PSV setting?

 

[mk3223]

Yes, you may set the PSV based on the operation pressure, and plus a safety margin, of course, but it can't be higher than the MAWP.

 

[LittleInch]

I don't understand where you're coming from. Most people try and set the PSV as high as they can so it never goes off so long as it protects all the equipment it needs to.

Most pressure vessels have a MAWP which can be higher than design pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Well, with normal operating pressure at 35barg rather than 40barg, that gives you some more room for all these pressure excursions, but we cannot tell you if it is adequate since we have no information about your plant and the associated process controls and operating modes. To be sure, draw out a pressure map and make room for each of the pressure levels discussed earlier.

a) Normal control operating pressure for all operating modes
b) Max normal control operating pressure for all operating modes
c) A reasonable gap for small pressure control oscillations/ fluctuations
d) Any overpressure to flare control setting
e) High pressure alarm PAH
f) High pressure trip PAHH
g) PSV setting

This is only what I guess you need, and there may less or more of these levels depending on how this vessel is operated etc. The process safety time is the time allocated for the safety ESDV to close, initiated at PAHH, and the associated worst case build up in pressure. This pressure build up should be less than the pressure gap between PSV setpoint and PAHH, else you'll have the PSV lifting on shutdown.