Hi everybody.

I have the next doubt. I am checking a engineering and I found the next specification for a Relief Valve :

Type: Conventional Bonnet not vented to Atmosphere.
Set: 10 Kg/cm2g
Flow: 1 MMSMCD natural gas (PM 19)

When I checked the Built Pressure of the vent piping system i get the next result:

Constant Pressure: Atmosphere
Built Pressure: 1.7 Kg/Cm2g

The total back pressure is clearly more than 10% of set pressure that is usually recommended for Conventional Relief Valve. So…

From Reference 1 (see below) only the set of the valve will be affected but not the Capacity. But what will be the real set point? Could the valve chatter?

On the other hand from reference 2 I read that the capacity could be affected to? That´s correct? Which one is right?

Thanks in advance.
Ariel

Reference [1] Surface Production Operation Volume 2 Page 369 – Ken Arnold & Maurice Steward-.
Reference [2] – GPSA Engineering Data Book – Figure 5.10

Please check-out API RP 520 Part 1, 2000 edition, section 3.3.

Your reference 1 is wrong. In case of excessive built-up backpressure the opening pressure of the relief valve is not affected, but the capacity is affected and it can result in unstable operation of the relief valve (flutter or chatter).

When the pressure reaches the set pressure of the relief valve there is no flow yet in the tail pipe and therefore no build-up backpressure. So how could it affect opening pressure of the relief valve?

My advise is to either reduce tail pipe pressure drop (e.g. by reducing equivalent line length and/or by increasing linesize) so that backpressure will be less than 10% of set pressure or to select a balanced bellows type relief valve instead.

Good point. I re-read reference [1] and maybe he wanted to say that, like usually back pressure is not constant because of other PSV´s discharging at the same manifold, the set will be affected. I was misunderstanding the text.



Yes. The first one is very expensive. I prefer the second one.

Gvdlands, Thanks for the quick answer

My pleasure. Good luck with the balanced type relief valve.

Ariel,

In my point of view... Reference [1] is correct...

First, you have to assess if there is a possibility of constant back pressure or superimposed backpressure due to other sources e.g. pressure dump valve or potential of simultaneous /prior r the PSV release...

If your answer is yes. Then your PSV (Conventional Bonnet not vented to Atmosphere) set point will increases with superimposed backpressure.

However, built-up backpressure due to the relieve of PSV only will not affect the PSV set pressure.

Read "Several Impact of Backpressure on Conventional PRV". Hope this answer your questions.

By the way, this post should be moved to RELIEF VALVE FORUM...

OK with that (the text is a bit confuse but your are right). And the answer is no. There´s no constant back pressure and the built up pressure by others elements are no simultaneous or important.



Thanks for the link!!!!!

Prior the PSV release there is constant back pressure (Atmospheric), in the special case of Ariel,therefore in my opinion, gvdlans is right.

Regards