Compressor Problem 1

[HardyPatel]

Process: We have 30+ years old compressor and we are in a process to install a new 2 stage receprocating compressor.

Existing condition: Inlet suction pressure of the old compressor was 15.5 psia @ 120 F. The new compressor is design at this suction pressure & temperature.

Problem:
During recent maintenance work, the process conditions has been changed. The new suction pressure is now 14.2 psia @ 120 F. Now if i want to install new compressor designed at 15.5 psia and will be running at 14.2 psia, what would the effect on other parameters. Do i have to make any changes to my new compressor in order to run under vacuum. Will this compressor work efficiently in my new process condition.

I would appriciate any help on this compressor problem.

Hardy

 

[zdas04]

How big a problem the pressure reduction will be really depends on your machine. Since it is a two-stage you may need to adjust clearance in one or both of the stages.

You didn't say what the process fluid is, but I'll assume it isn't an air compressor. For vacuum operations you need to be pretty careful about air incursions. Typically you do this by checking every threaded or flanged connection very carefully under pressure and get rid of all leaks (open-ended blowdowns and drains are a big problem). Other than that the action of the 2-stage recip is the same under a slight positive pressure as under a slight vacuum.

David

 

[dcasto]

When ever you lower the suction pressure AND the ratios are less than 2, you will lower the amount of work because the volume (or mass) is lower and the increased ratios (work/mass) increases, but not as much as the decrease in throughput, SO your work load will be OK. To vercome the capcity, either bigger cylinders or cool the inlet.

I doubt that a 1 psi change in pressure will increase the rod load enough to make any difference, so mechanical loads should be OK.

Capacity will decrease by 8.4% PLUS another amount say 5% or so for a net decrease in throughput 13%. The additional decrease is because the compressor will have higher ratios and therfore less volumetric effiency and the suction valve velocity will increase and therfore, more drop and less capacity. You might switch to a low pressure drop "poppet" valve that could put the whole compressor back to "normal". You see, you might already have a net suction pressure of 14 psia todat because of valve loss. The net pressure with your cahnge maybe 13 psia. With new valves the new net could be 13 psia again.

The packing on the rod will have to be "reversed" most packing has a prefeered pressure direction. Now on the compression stroke, its always positive in the cylinder, so a solution we came up with is 2/3 packing in normal and 1/3 in backwards on the thread side. OR you could pressureize the dog house (distance piece) at .25 psig or add a tandum distance piece (to much work).