Compressors, either dynamic or positive displacement, can experience a variety of problems, but there are relatively few diagnostic tools to identify them.
Except in cases where vibration is the issue, a thermodynamic analysis of a compressor is the most straightforward way to determine its health. However, this requires that specific instrumentation be installed on the unit.
Fortunately many problems can be avoided before they become costly. Most of these problems can be avoided or minimized with proper and timely preventive maintenance.
When a problem becomes evident, regardless of the compressor’s age or run time, the following basic questions should be addressed as soon as possible:
Is the problem mechanical, electrical or performance?
- Mechanical problems are associated with vibration, sound or leaks (air, gas, water, oil). As such, mechanical problems are self-evident. Although the cause might not be obvious, the symptoms are.
- Electrical problems are associated with high voltage to the electric components (motors, motor
starters and switchgear) or low voltage to the instruments and control systems. Diagnosing electrical problems is not as easy as diagnosing mechanical problems. However, once the problem is identified, the fix is relatively inexpensive.Advertisement - Performance problems usually result from mechanical deficiencies within the compressor (engine or turbine driver and electrical problems with a motor driver). These problems might or might not be immediately evident during operation, but they will lead to equipment failure. This type of failure is usually costly as far as replacement parts, labor and downtime. Sometimes downtime is the most expensive result of the failure.
Regardless of the type of failure, in many cases catastrophic failure can be avoided by applying the following guidelines.
Upon installation, run the unit to design speed.
- Record the unit’s vibration signature at several points throughout the expected operating range.
- Perform a gas path analysis at several points throughout the expected operating range.
- Record temperatures on the compressor not monitored by control instrumentation (such as suction and discharge valve cover temperatures, cooling water temperatures, etc.)
- Make note of the different sounds emanating from the compressor during startup, shutdown and at normal operating speed.
After the unit has run successfully at a stable temperature for at least 4 hours, shut the unit down and inspect it with minimal disassembly (do not remove any close tolerance or close fit components).
In axial compressors surge is the single most severe and expensive failure, resulting in damage to the bearings and seals, and sometimes to the impellers, diaphragms, blades and vanes. Signs of surge are vibration, gas flow instability, gas pressure and temperature fluctuation and audible “huffing” or “whooshing” sounds. Surge occurs very rapidly and it can only be avoided if proper control techniques are applied.
Compressor fouling
Indications and corrective action
A compressor efficiency drop of 2 percent is indicative of compressor fouling. This can be calculated as shown in Equation 1.
For the field engineer or operator, compressor fouling is best indicated by a 2 percent drop in compressor discharge pressure at constant speed and load or throughput. Another indication of fouling is a 3-5 percent reduction in load or throughput capacity at constant compressor inlet temperature or ambient air temperature.
Corrective action
Dynamic compressor equipment manufacturers normally specify cleaning agents. Typically, a liquid-wash is specified. In extreme cases, complete disassembly might be necessary to clean the compressor.
