Dear All:
I read in an engineering practice about conventional PSV setting:
"For conventional valves, the spring shall be set at the differential pressure between set pressure and constant back pressure"
Is there any benefit or concept behind of this statement that not setting PSV on set(design) pressure?Is this type of setting convenient?
There is also another statement that recommends using balanced PSV for:
-When two phase flow is expected
-the fluid is corrosive or toxic and psv discharging to sour gas flare
Again I can not undertand the benefit or logic(if any) behind ot this recommendation.
Your valuable comments are appreciated.
Cheers.

First and foremost, and to others asking questions, when referencing a source, please provide us the name of that source. We can't address specific information provided unless we have an idea where the question is coming from.

Now to try to answer your particular question:

A conventional style PSV is a differential device. It works on the principal of Pin - Pout. I can have it set for MAWP (or design pressure) but I must tell the manufacturer exactly what the expected constant backpressure is going to be. They will set the mechanics of the spring so that it will allow the PSV to open at Pin - Pout; Pin being your set pressure. If you tell them that the set pressure is 50 psig and the constant backpressure is 0 psig, then the spring is set for a delta of 50 psi (Pin = 50; Pout = 0). If at any time the Pout is more than 0 psig, the spring will still want to allow the PSV to open at a delta of 50 psi so the PSV will not open until Delta + Pout.

Two-phase usually causes large variable backpressures against a PSV so the conventional PSV is basically out of the quesiton.

It is a construction issue. The balanced bellows valve is more isolated from the nasty environment (this is why backpressure has a much less effect on this type of valve; see the sketches of this valve in API RP520 or any vendor catalogue).

Dear Phil,
Thanks a lot for your valuable comments.
The reason that I usually speak generally about my reference(s) is that most of them are not books or standards or materials on internet that can be accessed by the others so that enable them to read them and discuss.
But about these statements that I mentioned in my post I should add that my reference is a document called "Depressuring and minimum temperature study" that is generated by Foster Wheeler company for one of previous projects in my company.
If I set the PSV on MAWP(or Design)-Constant back pressure,should the relief load calculation be revised?For example the overpressure remains at 10% for non-fire cases?
And about the term "constant backpressure" I remember from your rupture disc series that flare KOD operating pressure can be a convenient component of constant back pressure in flare network.What about the pressure exerted by purge gas stream?I think we should include it.
Finally about the application of balanced PSVs for sour services:
In our NGL project the endorsement ENGINEER recommend us to change the destination of a psv from HP sour flare to LP dry flare because there could be some contamination by sour gases from the HP flare header.The psv set pressure is 18 barg and LP flare should be designed for maximum pressure of 10 barg.The change in destination may cause excessive back pressure in LP flare network.Now do you think that using balanced type for this psv would be a better solution instead of routing it to LP flare?
Thanks in advance.
Warm Regards.

The back pressure has nothing to do with the relief load calculation.



The constant back pressure is just that, constant. You have to decide what that is and at what point in your system to take it from. If purge gas will add pressure to downstream processing, then of course it must be taken into account. If it isn't constant than it will become part of the "variable" portion of the equation.

If your are afraid of contamination or corrosion of the PSV due to sour gases, then why not just put in a rupture disk downstream of the PSV? The RD can be made up of a higher quality material. In general, I would not want to use conventional style relief valves in any header system. There is too much potential for superimposed back pressure issues in this type of arrangement.

Dear Phil,
Is there any benefit of setting conventional PSV set pressure at design pressure-constant back pressure instead of design pressure?
And about the concern of contamination by flare sour gases:
Do you recommned the combination of a conventional psv and a RD downstream of it,instead of using balanced psvs?
Thanks in advance.

It is not necessary. If the back pressure is indeed constant, all you need to do is tell the vendor what the back pressure is. They take care of the mechanical aspects of the spring setting.



Well yes and no. IF I were to use a conventional PSV in this service, I would put the RD in. However, and I know I'm repeating myself, I would never use a conventional PSV in a header system because of the potential back pressure problems. A balanced bellows PSV does not cost all that much more than a conventional style PSV.

AND, putting in a balanced bellows PSV may not necessarily solve the materials of construction issue either. You still have to worry about the valve body and other valve internals. It may still be cheaper to buy a carbon steel (at the low end) body with a stainless steel RD rather than an all stainless steel PSV. Check with the vendor.