Spring-operated Relief Valve with greater than 10% backpressure

[msharptx]

I am tying in a spring-operated pressure relief valve into a vent piping system, which imposes a backpressure greater than 10% (of the set pressure) on the relief valve. From my experience, when the backpressure is greater than 10%, you are required to run the calculations of the specific relief valve to see if it is suitable for the amount of backpressure. I ran the calculations, and it is suitable.

Having said that, does anyone know if any API guideline or ASME code requires you to switch to a pilot-operated relief valve in lieu of a spring-operated relief valve when the backpressure is greater than 10%?

Thanks.

 

[Chance17]

To answer the question, need to know if the back pressure is static or due to frictional flow.
Static back pressure reduces the valve capacity but is not normally a code issue.
Frictional back pressure can lead to "chattering" and is the basis for 10% limitaion in the code.

 

[msharptx]

Frictional back pressure.

Thanks.

 

[CMA010]

API Guideline: API STD 520.

For a conventional valve the limit is 10% due to the concern of chattering. For balanced bellows valves it is 50% (valve capacity is reduced when the back pressure is higher than 30%).

In the case of "static" back pressure you'll might need to adjust the cold differential test pressure (CDTP), depending on the type of valve. If your static back pressure is variable it could influence the choice of type of valve (conventional/balanced/pilot) to be used.

 

[Chance17]

Frictional backpressure in ASME code lingo is built up back pressure "BUBP".
It may be true the relief device and greater than 10% BUBP will adequately protect the equipment from overpressure.
But the Code rules also address valve chatter and preventing internal damage.
One always wants the safety valve to be avalable and correctly operating.
For the system you described, faciity modifications should be made to eliminate the greater than 10% BUBP.

 

[sheiko]

msharptx,

Using a conventionnal PSV, built-up backpressure is allowed to reach up to the allowable accumulation (10% for non-fire case or 21% for fire case) times the MAWP, as per ASME code.

Thus, if you set the PSV opening pressure (Pset) below the MAWP, then the maximum allowable built-up backpressure is 1.1*MAWP - Pset (or 1.21*MAWP - Pset if fire case is the sizing case).

Example 1:
allowable overpressure = 10% (sizing case is not fire case)
MAWP = 10 barg
Pset = 10 barg
maximum allowable built-up backpressure = 1.1*MAWP - Pset = 1 bar (10% of set pressure).
In this case accumulation = overpressure as Pset = MAWP.

Example 2:
allowable overpressure = 10% (sizing case is not fire case)
MAWP = 10 barg
Pset = 9 barg
maximum allowable built-up backpressure = 1.1*MAWP - Pset = 2 bar (22% of set pressure!)

TO finish, please note that some companies consider the total backpressure instead of the buil-up backpressure in the above (which is more conservative).

And sorry for my english, i am french

"We don't believe things because they are true, things are true because we believe them."

 

[msharptx]

Thanks for everyone's help.

Sheiko, your english is good.

 

[LSThill]

January 2010 Hydrocarbon Processing

Desinging for pressure safety valves in supercritical services:

http://www.nxtbook.com/nxtbooks/gulfpub/hp_201001/#/0

 

[sheiko]

Corrections:

In the examples, please read "allowable accumulation" instead of "allowable overpressure".

Sorry.

"We don't believe things because they are true, things are true because we believe them."