The difference of superimposed back pressure on flares

Question

The difference of superimposed back pressure on flares

anthony9999 (Civil/Environmental)

(OP)

20 Mar 10 22:38

I would like to apologize in advance for my very simplistic, layman comprehension of engineering.  I have to explain the basics of our refinery's high- and low-pressure flare systems to a group of fellow non-engineers in order to approach difficulties we are having with both of our systems, as far as the environmental compliance department is concerned.

What I know of low/high pressure flare systems is that "low" and "high" in the term refers to the portion of the facility's grid that relieves into the system.  It follows that a low pressure flare system services the portion of the grid that contains fluids of lower vapor pressure, and vessels that operate closer to atmospheric pressure.  The high pressure system services the opposite.

My question:
Unfortunately in my search for the basics of flare operations, I've come across the information that even though a high-pressure flare system has sonic fluid speeds at the flare tip itself, there is somehow greater superimposed back pressure felt on the PSVs that relieve into the system than a low-pressure system (which typically have sub-sonic speeds at the flare tip).

How is this?  If the fluid in a high-pressure flare system is moving at a greater speed, wouldn't that mean that the pressure in the system is lower than that of a low-pressure system?

I would greatly appreciate any illumination here, and once again I apologize for my ignorance in this matter.

whammett (Industrial) · Answer 1 · 2010-03-22 20:26:26

It is my understanding that as long as there is combustion/ignition, there will be significant backpressure. I have never heard of it being greater than the output pressure of a flare though. Can you cite your sources?

Will Chevron Corp.

RogerH (Petroleum) · Answer 2 · 2010-03-23 03:16:00

If understand the question correct it is: "why HP flares have higher backpressure than LP flares". If you have soic speed at the flare tip you cannot get more flow through. The it is a matter of backing u pressure through headers, subheaders back to the source, the PSV and that gives a flow based on the delta P and the system will be in balance until the upstream pressure in the PSV drops.
Havent checked but I am somehat surprised that you reach sonic velocities at the flare tip.

Would expect a HP or LP flare to be designed in accordance to a certain maximum backpressure and the remainder of the flare system is an outcome from these numbers
Is there something I miss in the question here?

RH

anthony9999 (Civil/Environmental) · Answer 3 · 2010-03-23 16:45:14

Ah a thank-you to both of you.  What I didn't think about was simply that you have a big continuous combustion activity and of course that creates backpressure.  When I read that HP system have near sonic speeds at the flare tips, I was thinking "well wouldn't that clear out pressure in the header better?"  Not understanding that that speed represents a maximum flow of sorts and would create a sort of bottleneck, with the resulting higher backpressure.

The issue we have is that PSVs servicing the HP system are having trouble, at times, popping under their set pressure, and this is leading to VOC leaks at their flanges, which prompted the attention of my particular department.  This probably has something to do with the blowdown that many of these PSVs have set.

Fear not, my non-engineering section of course has no say in anything regarding the way our facility is run, it's simply my boss wants any of us who has to explain a situation to him and the rest of our section to preface an environmental issue with a basic "engineering lesson" if necessary.

So far, I have been by far the worst at doing this, and this time I'm determined to do it right so maybe he can get off my back, lol.

Thank you so much for your help and information.

Come Join Us!

Posting Guidelines

The difference of superimposed back pressure on flares