7 replies to this topic

[Wheristo]

Hello all-
I'm new to process engineering & need expert advise and key steps on how to perform a "Blowdown Study". Situation is, my client wants to install a new flare. Several new blowdown valves shall be installed at several identified location and routed to this new flare.

Also, their existing PSV (which used to discharge to atm) will be rerouted to this new flare too. Any special considerations need to take into account due to this?

Many thanks!

[kkala]

I understand you have to determined diameter of the flare header along with its branches. Useful links to previous topics are: http://www.cheresour...e-system-design and http://www.cheresour...discharge-lines. In general software (like TGnet) is used, Mach Number in the header should not exceed 0.7.
The critical question is which PSVs will discharge to flare simultaneously. Several senaria have to be developed and header / subheaders be sized for the worst cases. Look in http://www.cheresour...ultiple-plants/, post 8. Generally not all PSVs can discharge to flare simultaneously, unless in very simple cases. I have seen this specialized study (with scenaria, etc) to be undertaken by Licensors or Contractors, colleagues just sized the lines based on the specified max flows.

[Wheristo]

Thanks, info provided is informative to me. But can you/anyone advise a step by step method on how to perform the blowdown study?

[Butterfly]

You can use API 520, API 521 and API RP 14E as guides in designing blowdown systems.

First of all, you have to think that mainly, relief sources to flare will be originated by:

-Flow from BDVs during emergency depressurization of your system
-Flow from PSVs due to overpressure (here you have to take into account all possible process failures, you can find info in API RP 521)

Your flare system shall be capable of handling the worst scenario, that means, the higher rate that is possible to have. This means, that you have to define your “design scenario” by defining “design flowrates to flare”.

When defining blowdown flowrates, is important to make clear which philosophy you want to follow. Do you want to depressurize everything at the same time? This may lead to high relief rates to flare and therefore to a huge flare system (this happens mostly offshore, where you have little space and radiation constraints need to be taken into account. A whole blowdown in an offshore installation could lead to a extremely high flare to avoid radiation problems). The alternative here would be to design a staggered blowdown system: you define different blowdown sections and depressurize sequentially, allowing for smaller flowrates.

Also, in order to determine blowdown flowrates you have to take into account that you need to reduce the pressure to 6,9 barg or 50% of initial pressure in 15 min (as per API)


Hope this helps you somehow.

Regards

Edited by Butterfly, 12 January 2012 - 11:14 AM.

[aransari85]

Hi,

You can also use HYSYS depressurization utility to find out Peak flows at blowdown.
Some senarios to be considered are depressurization at design pressure, depressurization at PAHH, and at firecase at design condition. Which ever flow is maximum shall be taken as a Peak flow.

Regards,
ARA

[Wheristo]

Butterfly & ARA-
Your input really helps. Thanks!

[fallah]

Hello all-
I'm new to process engineering & need expert advise and key steps on how to perform a "Blowdown Study". Situation is, my client wants to install a new flare. Several new blowdown valves shall be installed at several identified location and routed to this new flare.

Also, their existing PSV (which used to discharge to atm) will be rerouted to this new flare too. Any special considerations need to take into account due to this?

Many thanks!

Wheristo,

I would add some points as follows:

1) Blowdown study limited to Depressurization and relevant relief loads through BDV's, while as you mentioned you are taking PSV's relief loads into account for design of new flare network. Hence, You are studying flare network design including blowdown study.

2) Other than relief loads from BDV's and PSV's you may have relief loads from some pressure control valves (PCV) relieving to flare. If so, you should consider them as "Continuous Flare Loads".

3) You should consider major scenarios, could generate a sizing load, as follows:
Fire or gas detection in one fire zone,
Loss of utilities (Instrument air, Fuel gas, Cooling water,...)
HHLL in flare KOD's
Major upset due to instrument failure,.....

Fallah

[Wheristo]

Fallah-
Sorry for my late reply. Your explanation help improve my understanding. THANKS!