Apr-2014

A straightforward guide to monitoring & diagnostics of reciprocating compressors

Experts worldwide agree — the only way to ensure safe, reliable, efficient, economical operation of reciprocating compressors is to monitor them continuously and take appropriate action based on the information the monitoring systems provide.

Jost-A Anderhub
PROGNOST Systems GmbH

Article Summary

That simple objective, however, is difficult to put into practice. The variety of instruments, systems, and methodologies for machine monitoring and diagnostics can be perplexing. Descriptions and claims are sometimes similar, sometimes conflicting, and almost always confusing. This plain-language guide to system selection is to help provide clarity for your decision process. It lists the essential capabilities a system should have and explains why those capabilities are needed.

Systems should be scalable in two ways. First, they should be functionally scalable, allowing new or additional capabilities, such as measuring loops, to be added to an installed system without inordinate cost or difficulty. Second, systems should be scalable in magnitude. In other words, a system should offer a means to expand monitoring to additional machines. Scalability allows you to take advantage of new developments and keep your system state-of-the-art. As your needs change and your experience with a system grows, you will likely wish to extend coverage to other machines. A truly scalable system provides a simple, cost-efficient pathway to realise a “Think big – Start small” approach.

Detecting the presence of an anomaly is one thing. Defining and pinpointing it is another. Your monitoring system should not only warn you about problems, but also provide an accurate diagnosis with specific component identification, location, and indication of the extent of damage. Armed with this information, you can make well-founded decisions about the maintenance procedures you need to take and when you need to take them. There are no shortcuts in developing a system for automated diagnoses. An accurate, detailed understanding of a problem is acquired only through extensive experience in machinery monitoring. Equally valuable is the message your system communicates to you. In the event of a positive failure pattern detection, alarm, or shutdown, you need answers. Modern systems should provide clear communication about the cause for the alarm. For example, the message “Discharge valve leakage, Cylinder 2, 89% Match” gives the cause of the problem, location of the problem, and confidence in the diagnosis.

The days of time-based maintenance are over. State-of-the-art monitoring technology gives you the ability to apply condition-based maintenance, reliably detecting developing failures and intervening before breakdowns have an opportunity to occur. Early failure detection prevents machine damage, enhances safety, avoids unplanned machinery shutdowns, and reduces costs of operation. Success depends on the ability of the monitoring system to accurately identify mechanical defects at an early stage – regardless of operating conditions – without issuing false alarms. Accurate anomaly detection is accomplished by capturing a complex array of signals and analysing them in a way that allows even minor changes in incoming signals to be recognised. By detecting slight changes and understanding their consequences, early failures are detected – and false alarms are avoided.

The most important and well established technique for machine monitoring is vibration analysis. However, not all vibration analyses are the same. Seemingly minor differences in data acquisition and evaluation strategies can have a dramatic impact on the quality of signal diagnoses. Choosing the proper mathematical evaluation method is the key to reliable early failure detection and safety protection. The Best Approach: First, monitoring systems should continuously acquire and diagnose machine vibrations for each crank revolution and then segment signals into crank-angle-related portions. This allows harmless, but sometimes erratic, machine behaviour to be rightly identified as a “good condition” – thereby avoiding false alarms. The best approach is to subdivide the 360° of one revolution into 36 segments of 10° crank angle each. This is the most accurate proportion of an average impact width related to one revolution. Second, vibration signals must be evaluated using the most accurate mathematical analysis. For reciprocating machinery, only RMS (Root Mean Square) analysis is proven reliable. RMS analysis is superior because it considers not only amplitude, but also the energy content of an impact.

The benefits of optimal compressor performance are clear: improved operating efficiency, reduced energy consumption and increased productivity. Tracking performance can also provide other benefits, such as early warning of impending gas leakages. Unfortunately, losses in efficiency often go undetected by many monitoring systems that focus on vibration, piston position and temperature only. Machine efficiency – like other key parameters – should be monitored continuously and the monitoring system should provide comprehensive analyses that identify how you can restore optimum operation. They can begin by detecting any changes in dynamic pressure during operation and performing specialised p-V analyses to identify components causing reductions in efficiency. They should also incorporate influences resulting from today’s compressor regulations. Finally, they must analyse other key values, such as compression cycle, piston rod load, and piston rod reversal.

When it comes to machine protection, the abbreviation “SIL” is very prominent in sales literature. It is important to understand what SIL certification means – and what it does not mean. SIL (Safety Integrity Level) ratings were established to define a metric for evaluating a system’s level of operational reliability with regard to safety, as defined by IEC 61508. As it applies to machine monitoring systems, a SIL rating refers to the probability of failure on demand (“PFD value”) of the protection system. A key point to remember is that SIL ratings have nothing to do with monitoring precision, which is represented by false trips and missed detects. Keep in mind that, before a SIL-rated safety system comes into play, operators have to determine the appropriate SIL rating for the machinery that has to be safety protected. In other words, IEC 61508 is a risk-based standard and, in order to apply it, criteria for the tolerability of risks must be established for the machine, e.g., a HAZOP study must be carried out (Hazard and Operability). Some marketing phrasing, such as “SIL ready” or “Equates to SIL,” can be puzzling. However, there are two ways to clarify the confusion. First, look for a monitoring system whose vendors provide genuine SIL certificates issued by recognised certification institutions. Keep in mind that the SIL rating must cover not only the safety system itself, but also the inherent components in the safety loop from sensor to the ESD (Emergency Shutdown Device). Second, be aware that SIL ratings should not only be high, but relevant to your application. For instance, SIL certification for monitoring over-speed protection is of no significance to a reciprocating compressor user, but a rating for a safety system that performs segmented RMS vibration analyses may be significant for your machine.