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Plant Operations/Maintenance/Reciprocating Compressor Rod Run out


QUESTION: Dear Expert Robert,I would like to ask you bellow questions.
Q1)This is regarding rod runout indicator and for my better understanding.
Please assume the gap between the piston rod and the rod runout indicator is originally 500micrones when the compressor is just started.After some while we see the indicator shows the gap is 1100micrones.This means the machine is misaligned by 600micrones.Please confirm if I am correct?
Q2)I am of the opinion that my understanding about first question is wrong.The reason is, after shutdown the compressor we measured the rod vertical run out which was 200micrones.We are confused why the indicator show 600micrones while we measured 200micrones.
Appreciating your assistance in advance.

ANSWER: When was the last time this runout indicator was checked and recalibrated? What are the set tolerance range for it?

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QUESTION: Dear My Good Friend,
This is Dresser Rand Hyper Compressor with four stages and the final discharge pressure is 1300bar.
The indicator calibration seemed to be okay as just one week ago it was re-calibrated.
Dear Expert if there shall be significant differences between hot and cold run out?
I think as the piston is not Aluminum.there shall NOT be big differences.Do you agree,please?If so,why we measured differences values?

ANSWER: Alignment can be an issue or seal rings but oscillation test would pin the issues down. Ultra sound testing is indicated.

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QUESTION: Unfortunately,I seem unable to understand your kind reply.Please be so kind and reply in more details.
I strongly believe hot and cold alignment shall be the same and if indicator shows 600micrones,then of course the dial gauge in stop condition shall show 600micrones NOT 200micrones.

Stop thinking in absolutes and thinking in terms of range and conditions over time. Sure your readings may or may not be consistent over time but that has to be verified by other tests and checks and conditions have to be factored in. New parts wear in, old parts wear out. It's a process and few machines or parts meet a exact standard. But specs allow for an acceptable range of operation.

You just can't take one anomaly and project a cause and effect because there is likely many contributing factors involved. I mean without being there its hard to say if a 200-600 micron reading difference is a big deal or not. I would certainly have to get a bigger picture from other factors to have a complete and accurate picture of the conditions that lead to the reading I was getting.

Without checking bushings, bearing and seats, run out indicator readings are inconclusive just by themselves, and may be leading you to the wrong conclusions. You make no mention of having a log to determine performance trends so diagnosing if you do indeed have a specific problem is near impossible. You cannot assume one reading at one point of any machine is  specific problem without evaluating all the parts of the process.

Sensory instruments work in ranges, and I cannot know what your instrument ranges are. Or how they are calibrated or where, or by whom.  

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Robert Crawford


Just about anything regarding the maintenace or product production in a manufacturing (Steel)indusry or heavy industry


30 years in the steelmaking industry

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