Max. backpressure conventional PSV for fire case 6

[Guidoo]

API RP520 part I, section 3.3.3.1.3 (page 36) reads:
"In a conventional pressure relief valve application, built-up backpressure should not exceed 10% of the set pressure at 10% allowable overpressure. A higher maximum allowable built-up backpressure may be used for allowable overpressures greater than 10% provided the built-up backpressure does not exceed the allowable overpressure"

It is my interpretation that when we have a PSV for fire case only, I can use a conventional relief valve provided that the built-up backpressure stays below 21% of the set pressure. However, the wording "may be used" in the API 520 sounds quite weak to me.

In this particular case we have a relief valve that reliefs directly to atmosphere.

What is your opinion here?

 

[Guidoo]

I see your point, MarkkraM.

There are two phenomena that help to keep the RV open:

1) disc area > nozzle area
As the disc begins to lift, fluid enters the control chamber exposing a larger area of the disc to system pressure. This causes an incremental change in force which overcompensates for the increase in spring force and causes the valve to open at a rapid rate.

2) momentum effect
As the disc begins to lift, the direction of the fluid flow is reversed and the momentum effect resulting from the change in flow direction further enhances lift.

Basically, the relief valve manufacturer must design RV such to ensure that the relief valve opens fully at setpressure + 10% even if built-up backpressure is 10% of setpressure. Ofcourse, replace 10% by 21% for fire case, see my original question...

 

[hacksaw]

I may have missed something in this, built-up back pressure refers to the pressure build up down stream of the relief valve. It only occurs when the valve in question goes into relief.

Assuming that your set pressure is above 15 psig, as long the built up pressure is less than ~40 % of the absolute relief pressure (critical flow conditions in the valve), there is little impact on the discharge flow or the accumulated relief pressure.

If you are discharging into a relief header and you have a constant back-pressure due to other valves relieving, then you need bellows so that the pop pressure of your valve is not modified.

The API requirements are very clear. I agree with pleckner and several others in this.

 

[EGT01]

Okay, I was wrong!

In my last post of 2/24, I incorrectly added the comment (fully open) to the inequalities that I presented. I believe I've seen these inequalities used to argue the point for saying it's okay for builtup backpressure < overpressure but they obviously won't work as argument that the valve remains fully open. Two different issues that I should not have tied together. By simple substitution into the inequalities of my last post, one would conclude that the spring force at full lift was equal to the set pressure which is not correct.

As for full lift, I believe what is missing in the inequalities is (as Guidoo points out) the momentum or rather kinetic energy effect. More appropriately then

set pressure + overpressure + KE >= spring force (full lift) + builtup back pressure

This would be needed to keep the valve fully open and from closing. Ironically, I was just recently involved in another thread where the KE effect was part of the discussion and I totally overlooked it here.

This may be deviating from the original thread somewhat, but now this has got me wondering about vendor's certified capacities. For a conventional type valve, vendor's typical give certified capacities at 10% overpressure. Generally, you can use their equations with no correction for back pressure as long as the back pressure does not cause subsonic flow. I believe relief valves carrying the ASME UV stamp are certified to ASME PTC 25. But PTC 25 outlines test requirements for pressure relief valves either when discharging to atmosphere (I assume this means little or no back pressure) or when discharging with builtup backpressure.

I'm assuming then that vendor certified capacities must be established per the tests when discharging with builtup backpressure for them to say their equations require no correction for backpressure as long as sonic flow exists.

 

[CHD01]

Without getting into truths of earlier discussions, let me try to restate in another way.

The force for the spring is equal to spring constant x compression of spring (in this case). When the valve is closed, the compression of the spring (reduction in original spring length) results in a force exactly equal to the set pressure for the spring at which the valve will simmer as operating pressure increases - and then pop open due the large disk area (huddling chamber). This assumes no superimposed back pressure; which if present increases the opening pressure proportionally because it operates on the back side of the huddling chanber.

Now, since the spring force increases with further compression of the spring as it begins to lift, the huddling chamber must be present to assist in opening the valve; and the 10% overpressure is specified to obtain full opening of the valve and to guarantee accurate calculation of the required orifice size (ie - basis for equations used).

So much for balancing forces, valve stays open so long as relieving pressure on huddling chamber results in a force greater than spring force (Lets leave backpressure out of argument until a bit later). Note that the valve will be less than full lift as relieving pressure decreases until it closes at the reseating pressure the valve is set at. Also note that when the valve is at full lift, there is no further change in spring force since there is no further compression of the spring - there is only an increase in contact force of piston on valve body. If upstream pressure increases, contact pressure increases and flow increases.

Now lets talk about backpressure or should we say variable backpressure since all have agreed constant backpressure translates directly and is additive to the spring set pressure.

Next, for sonic flow occuring at the orifice. The combined total backpressure (superimposed and built-up) will determine if the capacity of the valve is affected and the vendor publishes charts for their valves which desribe this relationship (Kb). Its not as simple as if pressure drops to less than 1/2 the inlet pressure, further pressure reduction will not increase flow. While total backpressure reduction will not increase flow, increases in backpressure will reduce flow to less than certified capacity (specified at 10% normally). But allowable backpressure can be quite a bit higher than 10% without a derate in capacity and is related to how much is variable versus constant and the type of relief valve (balanced, conventional or pilot-operated). In general, however, we usually try to design for 10% builtup backpressure max with a conventional valve, although this can be exceeded, before we go to a balanced bellows valve. At the other extreme, backpressure might be as high as 30% for a bellows valve; or even 60% constant backpressure for a conventional valve relieving gas or vapor without a capacity reduction being required by the vendor. Use the vendors CHARTS! THey were developed to account for backpressure on a orifice at sonic conditions. Remember even though the capacity may be decreased, the orifice is STILL AT SONIC CONDITIONS. Why make it any more complicated? Use the vendors CHARTS!





The more you learn, the less you are certain of.

 

[MarkkraM]

OK,
I realise my misconception now. Once popped the pressure in the PSV chamber equalises. I had some pressure regulator concept going on in my head.
I hope my mistake has help clarification though. I was not trying to make things more complicated, just understand the reasoning behind certain rules. A better engineer is one who understands the rules they are following.
Regards,
MarkkraM
P.S. We do not have vendor charts for many of our PSVs

 

[CHD01]

MarkkraM:

Actually there are some so-called relief valves that do tend to operate like pressure regulators; many of the tubing type end relief valves made by NUPRO and CIRCLESEAL are examples. If you use valves like these - BEWARE! - because they are very much different than standard relief valves because most of them are not sized for a certified overpressure. If you set one of these valves at the MAWP its possible for the actual relieving pressure to be much greater than 10% or 21% for the required flow. So don't completely discard your previous opinions.

IF you have Crosby, or Farris or Consolidated-Dresser relief valve; contact a representative and they will be happy to give you a catalog with these charts; and also they have a software program available to assist in sizing calculations for free as well.

By the way, when we speak of standard relief valves we're talking about &quot;D&quot; (0.11 in2 orifice) orifice valves and larger.

The more you learn, the less you are certain of.