PROGNOST Systems’ failure mode survey of most common reciprocating compressor failure modes shows valve failures among the most frequent root cause of unplanned compressor shutdowns. New valve designs and improved materials have been introduced in the past 10 years and have reduced the percentage significantly. However, for many compressor operators, compressor valve monitoring is the main concern when evaluating condition monitoring systems to reduce unplanned downtime.
While the high percentage of other machine failures calls for further improvement, valve leakages in an early stage are usually not safety-relevant. Undetected suction valve failures might lead to a complete loss of compression causing more dangerous failures, e.g., seizing cross-head wrist pins resulting from missing rod load reversal. In the early days of reciprocating machine monitoring, maintenance strategies were mainly based on temperature measurement. Today, different methods of online condition monitoring can be applied to create precise diagnostic information of the valves and other components. By comparing strengths and weaknesses of temperature monitoring with cylinder acceleration vibration measurement and p-V diagram analyses, this article provides a decision making the guideline to identify the best suitable permanent compressor monitoring technology for a specific compressor.
Description of the compressor monitoring methods
Highlighting these completely different methods, it becomes obvious that valve monitoring can be improved regardless whether it is delivered with a new machine or retrofitted to an existing machine. In any case, it is the main objective to detect a leakage of a valve that is causing a loss in efficiency of the compressor. Other damages such as broken valve springs or cracks in the valve plate or rings are considered as early stages of a leaking valve.
In compressor thermodynamics, it is considered that a gas leakage in either a suction or discharge valve is causing an increase of the gas temperature in the valve pocket. Two installation options are common to monitor the valve temperature:
- Temperature sensor mounted into the valve cover.
- Temperature sensor mounted in a sleeve installed in the valve pocket through a drilled valve cover.
In both cases, typically one temperature sensor is installed on each valve cover. The valve pocket temperature provides a higher quality of the measurement in terms of early detection of the leakage. Valve cover temperatures are subject to bigger influences from environmental conditions such as sunlight or wind. Temperature sensors installed through the valve cover into the valve pocket provide an earlier indication of changing temperatures. The signal coming from, e.g., RTD or thermocouple sensors can be transferred to the distributed control system (DCS), to a PLC or machine monitoring system (MMS). In the DCS or MMS, temperatures can be trended to generate long-term information about the valve condition. Compressor condition monitoring systems provide additional analyses for the signals such as the group deviation analysis to maximize the value of valve temperature monitoring, no matter at which position (cover or pocket) the sensor is mounted.
Compressor monitoring with p-V diagram
p-V diagram analyses require the installation of one pressure sensor for each compression chamber to monitor the compressor condition of suction and discharge valves. The sensors can be installed on indicator taps prepared by the machine manufacturer and may not be mixed up with the suction and discharge cylinder pressure sensor installed in the pulsation dampers or piping. Such taps are required for API 618 machines and typically indicator valves are installed between the sensor and the cylinder to allow easy replacement of sensors without machine shutdown. If indicator taps are not available, e.g., old machines, suction or discharge valves can be modified with a special centre bolt.
p-V diagram analyses are based on dynamic pressure measurement and require sensors that allow sampling rates in the kHz range to allow detect small leakages and high frequency pressure pulsations caused by valve dynamics or stepless unloaders. The p-V diagram can be visualized with suitable software. Intelligent diagnostic systems automatically monitor the p-V diagrams. In addition to the suction/discharge valves, the p-V diagram analysis indicates leakage of various other sealing elements such as piston sealing rings, piston rod packing etc. Furthermore, the dynamic cylinder pressures in conjunction with other parameters, e.g., speed of the compressor, connecting rod ratio and weight of the piston, allows the calculation and monitoring of the dynamic piston rod load and its reversal periods. Piston rod load is amongst the most critical when monitoring the condition and integrity of a compressor and help to identify critical overload conditions.
Compressor Monitoring tasks
- The earliest indication of a valve leakage/damage shall be provided?
- Typically the earliest information is received by cylinder vibration followed by valve temperature measurements. Both methods do not allow a quantitative assessment of the leakage volume. This can only be achieved by p-V diagram monitoring.
- Shall the compressor monitoring solution be scalable and extendable for future expansions?
- Best options for upgrades are provided with dedicated compressor monitoring platforms (CMS – Condition Monitoring System).
- Should other sealing elements e.g. piston rings or stuffing box be monitored?
- Other sealing elements especially piston rings can best be monitored with p-V diagram monitoring.
- Shall the compressor monitoring provide more than valve monitoring?
- Cylinder vibration and p-V diagram analysis are the most versatile compressor monitoring methods that provide further information on mechanical changes, e.g., loose valve cages etc.
- Shall the quantity of the leakage be monitored?
- p-V diagram analysis is the only quantitative compressor valve monitoring methods that allows to determine the best time for a valve change by assessing the quantity of leakage.