4 replies to this topic

[Beergson]

Hi,
I'm working on plant revamping project and during relief analysis I found that superimposed backpressure generated inside relief header during relief scenario will exceed design pressure of relief header which is equal to 3,5 barG.

The question is - should relief header design pressure be increased to maximum superimposed backpressure or not?

[ankur2061]

Beergson,

Yes, the design pressure of your relief header should be greater than the total backpressure that the header can see. For providing design pressure data the highest backpressure in the relief system components should be utilized but this is purely an academic exercise specifically when you are talking about piping. Piping is generally given in pound rating and if you and your piping engineer agree for a 150 # rating then specifying the design pressure of the piping as 3.5 barg or even 4.5 barg becomes an academic exercise because the MAWP of a 150 # rating pipe is much higher than 3.5 barg.

The vessel such as the Flare KOD is a different story. Here, although 3.5 barg as DP is a very well known rule of thumb or recommendation as per API STD 521 it is possible to have the Flare KOD design pressure to be higher. API STD 521 in fact mentions it as the minimum value for the DP. Shell mandates the DP to be 7 barg if there is no water seal vessel in the flare system.

Hope this helps.

Regards,
Ankur.

[Beergson]

Dear Ankur,
Thanks for Your prompt reply.

But what about builtup backpressure? If static (superimposed) is 20 kPaG and builtup backpressure is 400 kPaG, what should be minimum design pressure of relief header - 20 kPaG or 420 kPaG?

[fallah]

Dear Ankur,
Thanks for Your prompt reply.

But what about builtup backpressure? If static (superimposed) is 20 kPaG and builtup backpressure is 400 kPaG, what should be minimum design pressure of relief header - 20 kPaG or 420 kPaG?

Beergson,

It should be 400+20=420 kpag.

Fallah

[fallah]

the MAWP of a 150 # rating pipe is much higher than 3.5 barg.

Ankur,

It really depends on piping material and piping design temperature and therefore couldn't be generalized.

Fallah