Gas Mixing And Separator Backpressure 1

[zamakaze]

If in a gas plant, we have HP sep. at 1200 psig and we compress the gas from Inter-Separator at 500 psig to 1150 psig to mix with the HP gas as per the sketch. In case we change the setting of control valve downstream of the compressor to 1175 psig, I assume the compressor will build up pr. Accordingly. But what happens with the HP gas, will it be able to maintain 1175 psig before mixing with compressed gas? Will there be an effect on operating pr. Of HP separator?

Sketch:

https://files.engineering.com/getfi...9c1-4dc6-9ea4-4fc007d8972f&file=Document2.pdf

 

[georgeverghese]

At low flow from the HP sep, there may be no effect - the HP sep gas side backpressure CV may be okay. But at high flow from the HP sep, there may be a capacity constraint at this HP sep gas exit backpressure CV, since operating dp at this CV has dropped from (1200-1150)= 50psi to (1200-1175)= 25 psi. So run a capacity calc at this revised dp of 25 psi to see if this CV can handle max flow and this lower dp. Would recommend using no more than 80-85% of the max Cv for this valve in the debottlenecking rate check. Also add some surge margin to the HP gas flow from this sep to account for uneven flows.

 

[zamakaze]

hi georgeverghese,

thanks for your explanation and insight.

i have one more ques. in case we send the gas from the plant after mixing and compressing into a pipeline to another plant to mix with gas there and there is pr. drop across pipeline.
would having a backpressure control valve on the new plant at 1140-1150 psig serve any purpose? since there will be around 40 psi pr. drop in pipeline ...would we able to mix gases at new plant.
please see process flow sketch attached.

https://files.engineering.com/getfi...617-4e6d-afe8-937053676ae7&file=Document3.pdf

 

[georgeverghese]

I cannot tell: what is the purpose of this downstream backpressure PIC -PCV? The mixing with gas from the compressor at the downstream plant may work okay with this new PCV provided you have the compressor at your plant running at the higher pressure of 1175psig , rather than the settings you have shown on this second diagram??

 

[zamakaze]

dear georgeverghese,

for the 2nd downstream plant, the purpose of PIC-PCV is to make sure that gas coming from pipeline and gas from compressor @ downstream plant mix together. however, since in current scenario i get gas from pipeline at 1100 psig only and the 2nd downstream plant compressor will maintain 1140 psig due to the PIC_PCV arrangement, i believe gas mixing would not be possible.
would there be any solution other than gas entering pipeline at higher pressure? meaning we increase compressor discharge pressure at first plant

 

[georgeverghese]

How about gas from the first plant joining the gas from 2nd plant downstream of the PCV, rather than upstream of the PCV?