Monitoring Compressor Conditions

The operation and maintenance of centrifugal compressors is a key issue facing natural gas pipeline operators. Of course, centrifugal compressors and turbocompressors are well known, and they are widely used because they are reliable due to their robust and relatively simple design. Their main representative is the inline type, in gas transmission frequently known as the “pipeliner,” typically designed to industry standards such as API 617.

A prominent but dated source cites 13% of all failures of turbocompressors as being due to errors or omissions in condition monitoring and maintenance. With the advance in monitoring technology and modern operating and maintenance practices, one would assume that this general number might not be nearly as high today. What then are good monitoring and maintenance practices around turbocompressors that would help lower the above quoted statistic?

Compressor condition monitoring has the following components: • Proper and timely response to supervisory instrumentation such as alarms and trips. • Periodic observation and evaluation of operating parameters such as the compressor physical condition and its performance efficiency. This would include measuring and judging the rate of deterioration of mechanical and performance conditions for input into maintenance plans. Vibration analysis and aerodynamic performance calculations come to mind. Daily compressor operator rounds should be structured following the principles of operator-driven reliability. • Evaluation of operating trends. This should include auxiliary systems such as lubrication and seal oil consoles, compressor on-line washing facilities and dry gas seal support systems. • Periodic testing of lubrication and seal oils. Six basic analyses are required: appearance tests, testing for dissolved water, flash point tests, viscosity tests, the determination of the Total Acid Number (TAN), and ascertaining the additive content. • Periodic testing of emergency safety and shutdown devices (ESD) and other fail-to-danger components, such as exercising the compressor’s surge control valve (kick-back) loop and the trip and throttle (T&T) valve on steam turbine-driven compressor trains. • Data logging and automated record keeping such as the number of unplanned trips per train per year as a basic indication of compressor reliability. • Diagnosis of problems, appraising their severity and deciding what action to take. • Remedial action and execution planning. • Corrective measures should be preferably applied on-stream to reduce the impact on compressor availability. On-line washing would be a good example.

Generally, turbocompressors receive maintenance inspections, overhauls and repairs (MIO&R), also referred to as turnarounds, in intervals ranging from two to ten years, depending upon the type of service. Maintenance intervals of six to ten years in clean hydrocarbon industry services are not uncommon. The extent of MIO&R efforts ranges from simple bearing inspections to opening the compressor, cleaning the internal passages, judging their condition and often replacing the rotor with a spare drawn from specialized spare parts storage facilities. Used radial compressor rotors are examined for rubs at labyrinth seal locations and for fissures and cracks around their impeller eyes. On axial compressors, moving and stationary blades receive thorough attention. In all cases, non-destructive test (NDT) procedures should be applied.

As the opportunity for a compressor turnaround approaches – after an appropriate running period – one should review the machine’s operating and maintenance history. If and when there are defects noted, it would be well to ask the following questions: 

• Are any of these defects repeaters? 
• If so, can they be expected at this turnaround? 
• What steps can be taken to eliminate them? 
• What action should be taken at this opportunity?

A thorough pre-turnaround review should be undertaken in order to plan the work required. It should consist of: 

• An assessment of the compressor’s mechanical condition. 
• A performance check. 
• Review of the machine’s past history.