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Five simple methods to check reciprocating compressor performance

How to spot-check the performance of a reciprocating compressor

By Dr. Bryan Long, principal consultant – vibration dynamics and noise

There are often occasions when it is desirable to spot-check the performance of a reciprocating compressor; just after a rebuild, as part of commissioning or when something seems different.

Here are a few things you can do:

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Method, pros and cons

Rating
    Quantification of difference in throughput Diagnostic insight Ease of use
1 Compare measured flow with that predicted by OEM sizing (or other) software. This is easy to do once a model has been created. Creating the model is not difficult but requires accurate load step numbers (clearances).

pro  Can identify if the throughput is below capacity
con  Does not help identify the cause, requires a dedicated meter and loading curves or a software model
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2 Compare inter-stage pressures with software model predictions

pro  Points to which stage is at fault
con  Only applies to multi stage, needs a software model
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3

For cylinders on the same stage, compare discharge temperatures.
This method must consider that single-acting cylinders normally have somewhat higher discharge temperatures.
pro  Can indicate which cylinder is at fault
con  Only applies with 2+ cylinders on stage

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4

Check for hot valves by comparing equivalent valve cap temperatures

pro  Capable of pinpointing which valve is leaking
con  May not be elevated if the valve leak is big

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5

Calculate the capacity of each stage separately based on observed pressures and suction temperatures and look for significant discrepancy. A model is not required but best to set up an Excel calculation (which requires gas properties). If there is a significant difference, the lower value is the more accurate result, and the other stage has a problem.

pro  No model required
con  Gas properties required

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6

Detailed analysis of pressure-volume curves along with ultrasonic patterns. Well, this is not so simple; included here for comparison.
 

pro  The most detailed analysis
con  Time-consuming to conduct, requires specialized equipment

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Examples
Cylinder valve leak - compressor

 

The discharge temperature of a cylinder with a valve leak (black curve) increased, compared to another cylinder on the same stage (red), until a repair was made.

 

Suction valve cap temperatures indicating a leak

Suction valve cap temperatures show one significantly higher reading, indicating a leak.

 

Leak verified by rising discharge temperature deviation

 

Calculated capacity of stage 1 increases relative to stage 2 as a leak develops. The leak is verified by the rising discharge temperature deviation (black curve).
 

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