Recip Compressor MAWP 1

[TomX]

Hi,

Does anyone have any experience or idea of the alternative of increase the discharge pressure of a recip compressor, to levels near the MAWP.
Nobody in the Dresser has answered my questions (they say they have to do a study of 6 weeks and I need some idea now).
I know that there is a possibility of change the cilynder of the compressor because of the change in the discharge pressure, but if I keep the DP at a level just below the MWAP stated in the compressor, I think is feasible, and acceptable even by codes like API 618.

The other question is, increasing the discharge pressure shall mean a reduction in volumetric efficiency and actual inlet volume ACFM. But regarding the power, I expect not an important increase, because of the therotetical performance of this kind of compressors when there is a change in disch pressure is a little increase at the beggining, but beyond certain value of compression ratio, there is a reduction because the reduction in de actual inlet volume becomes more important than the increase in the compression ratio.
Anybody has experience with this?, I can do this without to think in a change of motor?

Thanks.

 

[25362]

I'm not sure I understood your post. The implications of higher pressures are complex.

I'm not sure but reading the following threads may be of help:

Thread135-82819
Thread384-11938
Thread404-43767
Thread483-77086
Thread483-88947
Thread798-72876
Thread1036-98536

 

[Montemayor]

Tom:

I'm presently out of my home office so I don't have access to my compressor files. I've increased the discharge pressures on a variety of reciprocating compressors - all the way to 5-stage machines; but like 25362, I'm having a lot of trouble understanding your post due to misspellings and composition. I believe I understand your basic dilemma in question #1: you want to increase the discharge presssure of your reciprocating compressor to the Maximum Allowable Working Pressure (MAWP) rating of your machine. I presume you are dealing with an Ingersoll-Rand (I-R) machine since you mention Dresser. I also presume you mean a single-stage machine since you fail to detail out the specific characteristics of your machine.

My response to question #1 is: YES, you can operate your reciprocating machine at the maximum discharge pressure rating given to you by the manufacturer (I-R, or Dresser Industries). If that is veritably the MAWP that I-R stamped on the machine, then that is what the machine will perform - continuously and 24-hrs/day for as many days as you dare run without maintenance. However, note that I say "stamped on the machine" - not stamped on the cylinder. Cylinders are produced for a variety of applications and services, and so are compressor frames. Your compressor frame may be rated for a smaller HP than the HP coresponding to your cylinders MAWP. You fail to make any distinction between the two. Only the manufacturer is technically capable of knowing the compatibility of your machine and the different ratings on each of its components. I would not challenge the manufacturer's recommendations.

Why do you want to consider changing the cylinder(s) ? You haven't mentioned anything about desiring more capacity. Don't forget, you can speed up a reciprocating machine within the manufacturer's design criteria - if you want more capacity or if you want to increase the discharge pressure and compensate for the loss in volumetric efficiency.

If your machine is a multi-stage one, then you have a more sensitive and complex problem. You will upset (or change) the mechanical balance due to the varying of the compression ratio(s). The manufacturer knows this and has to make sure the operation is mechanically safe and can be warrented. You are not in a condition to predict or critique pin loadings, compression ratios, valve velocities, piston ring leakage and wear, frame loadings, wrist pin loads, main bearing performance changes, etc., etc., etc.... My firm advise (based on over 40 years) is: don't try to out-engineer Dresser. If they made your machine, then they are the experts in how to design and manufacturer it for the service you want. You are only the expert in how it operates in the field. Don't try to second-guess them on design. They are way ahead of you and me on this point.

Question #2 has to do -I think - with the additional HP required when you take the machine to a higher discharge pressure. Again, you fail to give specific basic data: how many stages, what discharge pressures, what HP, what capacities, etc., etc. . I have done this many times - but with machines that were HP-rated to sustain the indicated discharge pressures and resulting capacities. I always specified my recips with 25% oversized motors where possible, so I didn't have any problems in running out of HP. The reason for extra-HP motors was because my motors were expected to run 330 days/yr, continuously. So, you see, it makes a big difference when you consider the application and the operation of the reciprocating machine. You just can't generalize, using only generalities for basic data in arriving at a decision on whether you can do what you propose or not.

You asked for experience in what you are proposing and I hope mine is of some help.


Art Montemayor
Spring, TX

 

[121202]

TomX,

If I understand you correctly, you want to increase the normal discharge pressure of a reciprocating compressor so that it operates closer to the MAWP.

Although the compressor casing may be rated for the increased pressure there is no guarantee that it is capable of performing the revised duty, unless of course that operation at different suction and discharge pressures was foreseen when the machine was originally specified. There are a number of possible things to check including

- Electric motor load
- Compressor rod loading
- Compressor valve limitations
- Motor coupling torque capacity
- Compressor discharge temperature
- Compressor loop settle out pressure
- Margin on discharge PSV setpoint (op. pressure should be 95% of SP maximum)
- Settings on alarms and trips

I could go on and on. The bottom line is that the situation needs to be thoroughly and professionally studied. The first step is to establish any process system limitations and to fully define the revised performance requirements. When this is done you need to then involve machinery experts to evaluate the compressor.

There are really no shortcuts I'm afraid.