3 replies to this topic

[chemperson]

Dear all



Need ur preview / perceptions/ solution on this issue



Following queries related to flare systems :


1) In one of our requirements , PSV set pressure is 230 kg/cm2_g and load is 600TPH . We are choosing conventional PSV for that.

How much back pressure I will consider while designing the OSBL header of Flare so that my size of header should be lesser and safer .As per API 520 we can go up to 10% of set pressure, then will it be possible to maintain backpressure of 24 kg /cm2_g ,since mach no. & noise criteria are not achieved.
2) If at outlet of PSV my back pressure is coming 5 .5 bar _abs then how much I reduce the back pressure when header is entering in the Flare KOD. In our case when line is entering in the KOD then it has back pressure of 1.7 bar _abs .

3) Also let us know what would be the best general practice followed, While running the design mode in flare net to optimize size by taking consideration of mach no , back pressure and noise.  flare problem.bmp   1.22MB   86 downloads  pressure profile.bmp   1.03MB   58 downloads  mach no.bmp   1.03MB   55 downloads

[rxnarang]

Dear Alok,

a) You need to be a lot clearer than this in describing what you want. The rest of us are not as familiar as you are with the proble.

The scans are illegible.

I will attempt to reply to you query best as I understood your proble.

a) The STATIC back pressure for conventional PSV can be as much as 10%. If it less than that it is FINE.

Typically restrict your tailpipe velocity to 0.7 mach. Sometimes this is not possible due to back pressure considerations or for a very high set pressure. That is FINE too.

c) Typically the velocity will increase as you travel the length of pipe, and thend rop as you swage up to the header size. The velocity wil then increase again as you travel down the header. This is FINE. Try to restrict the header velocities to 0.5 mach. Keep an eye on rho v2 criteria as you do all this.

Hope this helps

Rajiv

[chemperson]

Thanks Rajiv.. but wat I am saying is how can one maintain high backpressure by achieving mach no and noise criteria both,, because here when I am doing that keeping eye on both criteria then I am getting only 4.5 bar_abs at PSV outlet. And one another thing is that is it really safe to design for just 4.5 bar_abs back pressure when my PSV set pressure is of 230 kg/cm2_g and load is of 600 tph.

Dear Alok,

a) You need to be a lot clearer than this in describing what you want. The rest of us are not as familiar as you are with the proble.

The scans are illegible.

I will attempt to reply to you query best as I understood your proble.

a) The STATIC back pressure for conventional PSV can be as much as 10%. If it less than that it is FINE.

Typically restrict your tailpipe velocity to 0.7 mach. Sometimes this is not possible due to back pressure considerations or for a very high set pressure. That is FINE too.

c) Typically the velocity will increase as you travel the length of pipe, and thend rop as you swage up to the header size. The velocity wil then increase again as you travel down the header. This is FINE. Try to restrict the header velocities to 0.5 mach. Keep an eye on rho v2 criteria as you do all this.

Hope this helps

Rajiv

[rxnarang]

Alok,

Lesser back pressure than that allowable is NO PROBLEM!!

Obviously with lower pressures the velcoities are higher and hence noise. But if they are within limits then again NO PROBLEMS.

Even if they are not e.g. the velocity in the tailpipe is reaches 1 mach, then again NO PROBLEMS.

I suggest you read my article on high speed compressible flow posted at Che website for a theoritical basis.

So, relax.

Regards