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Damage Analysis in Refrigeration Compressors - Part I

In a refrigeration system the compressor is rarely the direct cause of mechanical accidents. When a damage of a certain amount occurs in a system equipped with reciprocating compressors, it is therefore important to know the operating conditions thoroughly and analyze the sequence of events that led to the fault.
This study deals with failure cases with a systematic approach, first examining the damaged parts, then analyzing the operating conditions likely to have caused the failure, and finally trying to establish the causes of the failure that occurred. No repair should be undertaken without first ascertaining the origin of the fault and proceeding to eliminate the cause: otherwise the accident could happen again.


PREVENTIVE MAINTENANCE

The best way to prevent faults in a compressor is to establish a preventive maintenance program, which includes a regular record of operating conditions. Daily recording of pressures, temperatures, etc. allows to check the evolution of the performances during its operating cycle. The analysis of these data also allows to detect, in advance, any operating anomalies, before the operating conditions change beyond acceptable limits.
This system also allows, in case of failure, to follow the evolution of the operating conditions before the occurrence of the fault, and very often can be used to trace the origin of the fault itself.


FIRST INVESTIGATIONS

To ascertain the root causes of a damage, every possible source of information must be taken into consideration. For example, when talking to the system personnel, you can try to know if the compressor noise was normal before the fault occurred, if the operation was regular or irregular, at what time the accident happened, etc. The answers to these questions could allow to evaluate the load of the system at the time of the failure. If the operator regularly records the operating data, as we have suggested, the research work can be facilitated.


IDENTIFICATION AND INSPECTION OF PARTS

The parts must be identified and marked as the compressor is dismounted so that they can easily determine their position in the compressor. Preferably, an unalterable mark should also be applied during workpiece maintenance. The numbering of the piston-connecting rod assemblies will indicate from which cylinder they come and whether or not the cylinder is equipped with a load variation device. If, for example, it is found that the damage only affects the cylinders without load variation device, it is likely that the shutdown occurred with the compressor completely discharged. The verification of these facts serves to diagnose the cause of the accident.
In addition to the marking, the conditions of the disassembled parts must be checked, that is to say if they are clean and in good condition, and if so, note the observations made. If, on the other hand, the parts are dirty, it should be ascertained whether there are deposits of soot, residues of paint, calamine, mud, traces of copper, rust, aluminum powder
(or long or short metal chips or scrapes), and link these findings with the zones or individual parts of the compressor. If the valves are broken, ascertain where and how it happened. Even if you have to thoroughly clean the various parts to find traces of damage, the information obtained from it can be extremely useful at the time of the final diagnosis.


MAIN CATEGORIES OF DAMAGES

To study the causes of a shutdown of a compressor, one should first identify the main categories of damages that a maintenance technician can find. Each category will then be studied separately to consider the specific damages caused by the various types of faults and the solutions that can be brought.
Most compressor shutdowns (with the exception of those caused by hidden defects in the material) can fall into one of the following categories:

  • Liquid shots - Damages caused by hydrostatic pressure, resulting from the liquid suction by the compressor (oil, refrigerant, or both)
  • Lubrication problems - Excessive wear resulting from lack of lubricant in the essential areas of the compressor.
  • Contamination of the refrigerant circuit - Excessive wear caused by the presence of foreign particles in the refrigerant circuit, and therefore mechanical failures to the motor and overheating.
  • Electrical damages - These are problems other than those deriving from mechanical faults (categories 1-2-3). In the paragraph dedicated to these problems some electrical faults of mechanical origin will also be dealt with.

To be continued in Part II.

Damage Analysis in Refrigeration Compressors - Part I

Author, Riccardo Tigani  Linkedin Page
General Manager at Linea3C Srl

11th March 2019

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