Difference in compression ratio between two stage of recip compressor 1

[weldsv1]

Hi all!
Can anyone tell me which difference in compression ratio between two stage of recip compressor is normal. We have two stage recip compressor, the first stage has 4,75 compression ratio, the second has 3,7. Is it normal? And how I can reduce this difference.
Best regards.
Weld

 

[zdas04]

It isn't real bad, I've seen a lot greater difference between stages. 4.75 ratios is a little high for most recips, but not terrible.

When you spec out a compressor, the designer looks for standard size cylinders that will give you an approximately balanced load between the stages at a very specific suction pressure, discharge pressure, and flow rate. As long as the field conditions are pretty close to the design conditions the stages will be acceptably balanced. The critical value is the suction pressure. It needs to be within +/- 5% of the design value--if the design pressure is 30 psig at sea level then the acceptable range of suctions is 27.76 - 32.24 psig. If you drop to 25 psig then the first stage will take a disproportionate share of the load. If you drift to 35 psig then the second stage will take extra load.

For your small difference, I'd guess that you have fallen just outside the design suction range, if the first stage discharge temperature is too high (or you are approaching rod load) then you can fix it by raising suction pressure slightly. If your suction design pressure was something like 65 psig for a discharge of 1,000 psig then you are probabley close to 55 psig today. In that scenario you would need to raise the suction pressure about 10 psig to balance the cylinders.

If the first stage discharge pressure is in the normal range then I'd do nothing.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[hazel]

dear Simpson,
There are a point in your article I could not understand. Would you mind describing more clearly for me? thank you.

“If you drift to 35 psig then the second stage will take extra load.”

In my experiences, most process reciprocating compressor discharge pressure are varied with process back pressure. If the process does not fluctuate, the discharge pressure should remain about constant. From this view point, increasing the 1st stage suction pressure more and 1st stage outlet pressure will increase more, then, the second stage should load less, because its compression ratio reduced.

 

[zdas04]

Hazel,
There is no absolute right answer to that question. Any change to skid-inlet pressure will cause changes throughout the machine. It has been my experience that (with a constant skid-outlet pressure), an increase in the skid inlet pressure will tend to shift the load from the first stage to the second (i.e., the first stage will take a smaller portion of the new total skid ratios).

This is what I usually see, but I have seen exactly what you describe--the first stage keeps doing the same number of ratios and the second stage ratios drop with an increase in skid-inlet presure. This usually happens when the second stage cylinders are significantly undersized and can't do any more work.

David

 

[weldsv1]

Thank David, for your complete answer. It helps me.
Best regards.

 

[TD2K]

Essentiallly, the interstage pressure in a recip will adjust itself to keep the mass flow through the upstream cylinder equal to mass flow through the downstream cylinder.

Let's say you have a recip running along and you raise the suction pressure. The LP cylinder moves a certain volume and because you have raised the inlet pressure, the mass flow increases due to the increased density. However, the HP cylinder is initially moving the previous volume/gas density which is less than what the LP cylinder is now moving. To increase the mass flow through the second cylinder, the interstage pressure increases until the mass moved by the LP equals the mass moved by the HP cylinder. You can go through the same thought process for a decrease in suction pressure, temperature, etc.

Now, that's a very simplied explanation since as you change pressure to a cylinder the temperature out of the cylinder, gas compressibilities, etc change. Also, cylinders have a volumetric efficiency that I've neglected but the general response is as I've described it. Similarly, if you have a bad valve in say the LP cylinder, the interstage pressure will be lower than it should otherwise be as the LP cylinder isn't moving as much gas as design. Therefore, the suction pressure to the HP cylinder decreases since that reduces the mass flow through the HP cylinder. There's no controls that is doing this, it's essentially just an application of the old law of conservation of mass.

Hope I haven't confused everyone ;-)