Best practices for compressor vibration monitoring: Where sensors need to be positioned for reciprocating compressor monitoring

Trigger sensors, also known as speed sensors, are used to monitor the current rotation speed of the piston compressor. Among other things, the speed serves as a reference value for signal analysis.
For signal acquisition, we recommend installing a trigger CAM on the flywheel of the machine. The sensor is attached to the machine with an angle, for example, so that the Trigger CAM passes the sensor once with each revolution. In this way, the speed sensor provides the reference point for calculating the top and bottom dead center. The speed signal is a decisive factor for the signal analysis of the reciprocating compressor.

Two proximity sensors are installed to determine the piston rod position.
The sensors measure the distance of the piston rod to the sensor tip. The measured values are used by the condition monitoring system for various analyses, firstly as machine protection parameters for the piston rod and secondly as a reference for the rider ring wear. The use of two sensors for the X and Y direction is mandatory, since the piston rod does not move only in the horizontal direction due to its mechanical connections. Thus, in addition to the horizontal change, the vertical change per revolution can also be monitored, which is crucial for assessing the condition of the machine.

Dynamic pressure sensors are installed at each compression chamber to measure the performance of the compressor
Access to the compression chamber is possible via a so-called indicator port. API compressors usually have these indicator ports. If this possibility is not given, there is still the option, for example, to modify the suction valves with a indicator kit. This involves replacing the center plate of the valve through a modified plate that has an opening in the center bolt area. This opening allows the pressure sensors access to the pressure chamber or compression chamber and thus measures the present pressure.

No matter whether imbalances on the entire compressor, broken support rings, loose mechanical connections of the crankcase to the foundation or even cracks in the foundation: properly installed vibration velocity and vibration acceleration sensors detect these vibrations immediately. Therefore, vibration sensors are a key element in compressor vibration monitoring.
The vibration sensors are mounted on the crankcase in a horizontal orientation both once on the drive side and once on the non-drive side. For further monitoring of compressor vibrations, we also recommend mounting acceleration sensors on the crosshead. The exact position depends on the direction in which the crankshaft is turning. Here, it is important from where the force of the connecting rod acts on the crosshead slide. If the force is directed upwards, the sensor must be installed above the crosshead slide. If the force is directed downward, the sensor must be installed below the crosshead slide. In the next step, we will concentrate on the valves. Statistically, suction and discharge valves in particular are the most frequent cause of faults on reciprocating compressors. Therefore, to monitor the valves per cylinder, we recommend installing a vibration acceleration sensor midway between the valves on the head end side and on the crank end side near the suction nozzle. These sensors allow the online condition monitoring system to detect all vibrations and anomalies caused by the valves. These are, for example, error patterns such as sticking valves, valves that do not close, or broken valve plates.