2 replies to this topic

[Poet]

Hello,

I'm process engineer working for flare system.

So far, I've known back pressures as follows.

 

1) built-up back pressure: same to pressure loss.

2) superimposed back pressure: consists of variable and constant.

3) total back pressure: sum of built-up and superimposed back pressure

4) flarenet (Aspen flare analyzer) shows total back pressure and static pressure

4-1) total back pressure in flarenet: kinetic portion of pressure (dynamic pressure) + built-up back pressure + superimposed back pressure

4-2) static back pressure in flarenet: built-up back pressure + superimposed back pressure

 

Am I right?

If so, how can I find "variable" superimposed back pressure value in flarenet when several PSV open such as external fire or power failure?

 

Thanks.

Edited by Poet, 20 February 2022 - 07:12 PM.

[Hafiz Abubakar]

Hi everyone,

Can someone please respond to above query?

Thanks.

[latexman]

Hello,
I'm process engineer working for flare system.
So far, I've known back pressures as follows.

1) built-up back pressure: same to pressure loss.
2) superimposed back pressure: consists of variable and constant.
3) total back pressure: sum of built-up and superimposed back pressure
4) flarenet (Aspen flare analyzer) shows total back pressure and static pressure
4-1) total back pressure in flarenet: kinetic portion of pressure (dynamic pressure) + built-up back pressure + superimposed back pressure
4-2) static back pressure in flarenet: built-up back pressure + superimposed back pressure

Am I right?
If so, how can I find "variable" superimposed back pressure value in flarenet when several PSV open such as external fire or power failure?

I've never used Flarenet, so I don't have that experience to answer your question directly. Sorry.

Discussion of back pressure often is confusing. Definitions become extremely important. That's why I am replying; I have some doubts.

The pressure present at a relief device discharge flange before the device opens is called superimposed back pressure. Device performance, and therefore system design, depends on whether superimposed back pressure is constant or not, and if variable, to what extent. Superimposed back pressure is sometimes called constant back pressure or variable back pressure. The pressure increase at the device discharge flange due to relief flow is termed built-up back pressure. Its magnitude also influences device performance and therefore system design. Built-up back pressure is sometimes called flowing back pressure. The total back pressure is the sum of the two independent ones; built-up and superimposed.

So, it seems to me that Flarenet's static back pressure at the discharge flange of each CLOSED PSV is that closed PSV's superimposed back pressure for that specific scenario of "when several PSV open such as external fire or power failure". Other scenarios with different PSVs open may be different. Again, I do not know how to make/manipulate Flarenet to separate this PSV's superimposed back pressure into the constant and variable components.

Also, it seems to me that Flarenet's static back pressure at the discharge flange of each OPEN PSV is that open PSV's built-up + superimposed static back pressure for that specific scenario of "when several PSV open such as external fire or power failure". Other scenarios with different PSVs open may be different. Once again, I do not know how to make/manipulate Flarenet to separate this PSV's built-up + superimposed back pressure into the separate built-up back pressure and the separate superimposed back pressure, much less determine the constant and variable part of just the superimposed back pressure component. This latter part is feeling impossible. Maybe someone with Flarenet experience will chime in.

I hope this has helped.