2 replies to this topic

[jacosher]

I have a 5 stage recip compressor used to compress CO2 from 6 psig to 3800 psig. I'm trying to quantify the affect of adding restricting orifice plates into the interstage lines as suggested from a pulsation control study. The pressure drop is expected to increase by ~25 psi between the 3rd and 4th stage. The 4th stage currently has a suction pressure of ~900 psig. Here's what I'm thinking so far:

Since it is recip, volumetric flow rate (Q) is constant (correct?). So 4th stage is going to continue to remove same Q irregardless of dP. But higher dP results in lower suction pressure, thus lower density, and lower mass flow (w). 3rd stage discharge will need to increase to maintain same suction pressure for 4th stage. But in this case we have a relief valve that limits the interstage pressure so I don't think we can increase our 3rd stage discharge pressure any more. From GSPA: Q = 10.73/MW * (w T1 Z1 / P1 / ZL). Assuming that temp, MW, and compressibility is constant, then w2 = w1 (P2/P1). P1 = 914.7 psia and P2 = 889.7 psia. P2/P1 = .972. So mass flow will have to decrease ~2.8% to ensure we don't exceed RV set pressure? I realize that T and Z will probably be different given intercooler duty but I'm just looking for ball park effect and I assume that it won't be that significant?

What if I didn't have the interstage RV and could increase 3rd stage discharge pressure enough to keep 4th stage suction pressure the same. How will this affect the flow rate? I figure that increasing 3rd stage pressure ratio should hinder the mass flow rate through it but I'm not sure how to quantify it. I'd really appreciate any suggestions or resources that you can provide.

By the way, I have 3 years experience in a chemical plant. I have very little experience with recip compressors.

Thanks!

[breizh]

Hi ,

This resource may support your query.
http://books.google....epage&q&f=false

Hope this helps
Breizh

[Art Montemayor]

Jaco:

I just got back from a 2-week absence in the Forums.

You cannot do what you are proposing with CO₂ unless you have a VERY SPECIAL compressor.

Basically, after the approximate 3rd Stage of compression (around 1,200 psig), you will be trying to compress a SUPER CRITICAL FLUID (SCF) – a unit operation that doesn’t follow the normal laws and techniques of gas compression. You don’t have gaseous CO₂ after 1,200 psig unless you keep it well above 88-90 ^oF – which defeats the purpose of employing interstage coolers like you are proposing.

Before discussing the need and methods for compressing CO₂, please tell us WHY you want to compress it to 3,800 psig. You must have a very good reason, so please share that with us in order to discuss your topic seriously and accurately. I suspect you want to get into the SCF state. If you tell us the total scope of your proposed project, we may be of assistance to you.

Another thing that is confusing is that you are adding orifice plates to the interstages. I strongly advise you not to do this. Orifice plates are ineffectual and never used for this purpose. Use pulsation drums or a combination of inlet separator and pulsation drum design. That will get you out of the dilemma that you have created.

I spent a lot of years compressing CO₂ in reciprocating compressors and could write/talk about the subject until your ears fall off.

Await your response.