Back pressure 3 %/ outlet loss 10% guage or absolute

[jamesbanda]

We had a question recently on engineers using differnet methods to check the inlet losses on PSV's as a percent of set pressure. So.. what is the API measure

DP/ set pressure *100 = % loss
so is set pressure in absolute or guage ?

and the same for outlet losses..

 

[psafety]

Assuming steam flow protecting a Section VIII vessel, no fire-case:

3% inlet loss is a guide to prevent chattering and beating the seat/nozzle to premature death.

3% of set pressure with a valve set at 150-pisg, max to design is 4.5 psig loss on inlet for the stamped capacity. To me this is P1= 154.5 to P2= 150 (set) for loss determination

In my book this case now allows 5.5 psi on the discharge for "meeting the interpretation" of the Code Appendixes, not required, recommended.

Remember the delta P is across the nozzle the calculation now becomes P1= 5.5 psi and P2=0, the specific volume increases dramatically, as does the tail-pipe diameter. This assures if I use the minimum diameter & maximum length I prevent the vessel from seeing more than a 10% OP as defined by Code.

 

[CJKruger]

Gauge pressure (not absolute).

 

[zdas04]

For historical (not engineering or scientific) reasons, all psv calcs are in gauge pressure. At really high pressures, this is a pretty small thing. For low-pressure psv's it isn't trivial, but that's the way it is done.

David