8 replies to this topic

[chemperson]

I am calculating sizes for my OSBL (Outside Battery Limits?) flare header for the governing case (blocked discharge). When I am going for 0.4 mach in main header my size is coming to be telescopic, near about 42". But the main issue is the backpressure that I am getting at PSV (set pressure is 150 barg) outlet is 25 barg. Does anyone of you have any experience of such high back pressure at PSV outlet? Is it safe to do that, or should I increase the size of the header to reduce the back pressure?

[rxnarang]

Do you have balanced bellows valves - with correct back pressure correction factors? If so, stop worrying and go ahead. Don'y worry about the 42" main header size.

Can you go lower than that and use a 36" header size? What back pressure and velocities do you get in the header and tailpipe then?

Rajiv

[Lowflo]

Your question needs clarification. You say the backpressure at the PSV outlet is 25 barg. How much of this backpressure is caused by the tailpipe and how much by the flare header? I'm guessing that most of this backpressure is being caused by the tailpipe rather than the flare header.

Check your basis for performing the flare header sizing calculations. I suggest you follow the guidance in API 521. I've seen some localized high pressure values of about 10 barg in flare headers.

Even if you meet all the concerns such as mach no. , back pressur and etc you should think to modify and minimize the maximum back pressure whithin the flare network because this value will set the design pressure for whole of the flare network pipes.

[JoeWong]

I am calculating sizes for my OSBL (Outside Battery Limits?) flare header for the governing case (blocked discharge). When I am going for 0.4 mach in main header my size is coming to be telescopic, near about 42". But the main issue is the backpressure that I am getting at PSV (set pressure is 150 barg) outlet is 25 barg. Does anyone of you have any experience of such high back pressure at PSV outlet? Is it safe to do that, or should I increase the size of the header to reduce the back pressure?

I have done similar kind of studies for many projects. First time experience such a high backpressure issue. I don't say absolutely not a correct design. However, i reserve to say it is a good design.

The backpressure is 25 barg for this particular PSV. This backpressure will superimpose to other PSV, PCV, etc which are connect to this same flare header. Have you analysed the consequence to other PSV, PCV, etc ?

Your backpressure is 25 barg. Then the flare header design pressure will be even higher. Do you have any system design pressure lower than 25 barg connected to the flare header ?

I am a bit surprise with such a high backpressure system. Probably your inputs e.g. friction factor, temperature, etc are too conservative. This lead to excessive backpressure.

Normally the pressure drop along the flare system should be well distribute among all the elements. You may check you model where is the main contributor of such large pressure drop. Tackle on this particular element.

You have not submitted any data for evaluation. There is no way can identify your problem. Are you using Flarenet to conduct the study ? If so, do you mind to upload the model so that forum member can check your model and advise the problem accordingly ?

[chemperson]

Do you have balanced bellows valves - with correct back pressure correction factors? If so, stop worrying and go ahead. Don'y worry about the 42" main header size.

Can you go lower than that and use a 36" header size? What back pressure and velocities do you get in the header and tailpipe then?

Rajiv

Dear rajiv,

I have maintained mach no in stack 0.5 and in header all through 0.4. If I reduce size further then it will increase back pressure .

[chemperson]

I am calculating sizes for my OSBL (Outside Battery Limits?) flare header for the governing case (blocked discharge). When I am going for 0.4 mach in main header my size is coming to be telescopic, near about 42". But the main issue is the backpressure that I am getting at PSV (set pressure is 150 barg) outlet is 25 barg. Does anyone of you have any experience of such high back pressure at PSV outlet? Is it safe to do that, or should I increase the size of the header to reduce the back pressure?

I have done similar kind of studies for many projects. First time experience such a high backpressure issue. I don't say absolutely not a correct design. However, i reserve to say it is a good design.

The backpressure is 25 barg for this particular PSV. This backpressure will superimpose to other PSV, PCV, etc which are connect to this same flare header. Have you analysed the consequence to other PSV, PCV, etc ?

Your backpressure is 25 barg. Then the flare header design pressure will be even higher. Do you have any system design pressure lower than 25 barg connected to the flare header ?

I am a bit surprise with such a high backpressure system. Probably your inputs e.g. friction factor, temperature, etc are too conservative. This lead to excessive backpressure.

Normally the pressure drop along the flare system should be well distribute among all the elements. You may check you model where is the main contributor of such large pressure drop. Tackle on this particular element.

You have not submitted any data for evaluation. There is no way can identify your problem. Are you using Flarenet to conduct the study ? If so, do you mind to upload the model so that forum member can check your model and advise the problem accordingly ?

Dear Joe,

I understand what you are suggesting. But here case is different .....there is single PSV of 600 tons load at set pressure of 150barg. when this PSV pops no other PSV popping occurs, well i am telling you wat I have done till now... I have sized for that PSV case ...and after that I am checking for mach no for other cases while keeping the header size fixed and so in that case I am getting 5 barg at PSV outlet and mach 0.2 . So is that the right way of thinking that two different back pressures at different PSV for full independent case. I am considering that 150barg PSV will remain close in other cases.

[chemperson]

Even if you meet all the concerns such as mach no. , back pressur and etc you should think to modify and minimize the maximum back pressure whithin the flare network because this value will set the design pressure for whole of the flare network pipes.

Dear friend,

If I have to size for much simpler way then surely i wont consider for HP flare and give a bigger size of header ... but its the benefit of HP flare that we can reduce the size of header.. And as per API I cant find anything contradicting in my design , i only worried about that i dont hav any previour reference project. your all kind of suggestions are welcome dear

[chemperson]

Your question needs clarification. You say the backpressure at the PSV outlet is 25 barg. How much of this backpressure is caused by the tailpipe and how much by the flare header? I'm guessing that most of this backpressure is being caused by the tailpipe rather than the flare header.

Check your basis for performing the flare header sizing calculations. I suggest you follow the guidance in API 521. I've seen some localized high pressure values of about 10 barg in flare headers.

Dear friend ,

you tell me what input you want I will give it to you. But you know flare is always discussed for economy vs safety issues.