Sensors for compressor monitoring

For permanent condition monitoring and reliable machine protection of rotating machinery, the most important thing is properly recorded sensor signals. Therefore, it is important for reciprocating compressor monitoring that sensors are installed in the right position on the machine.

Learn what the best sensor positions are for acquisition of speed, vibration, position and pressure signals on reciprocating compressors.

Where do sensors need to be installed to best monitor a reciprocating compressor?

Speed sensor

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.

Proximity sensor

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.

Pressure sensor

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.

Vibration sensor

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 condition 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.

Checklist: Component monitoring with different sensors for reciprocating compressors

Trigger sensors or speed sensors on the flywheel of the reciprocating machine

Vibration acceleration sensors or vibration velocity sensors for machine protection or early fault detection:

  • Cylinder housing
  • Piston
  • Piston rod
  • Plunger mounting
  • Crosshead
  • Connecting rod
  • Crankshaft
  • Bearing
  • Valve

Proximity sensors for machine protection, early fault detection or wear monitoring:

  • Piston rod
  • Plunger
  • Crosshead
  • Rider rings

Pressure sensors for machine protection, wear monitoring or efficiency optimization:

  • Cylinder housing
  • Piston
  • Piston rod
  • Connecting rod
  • Crankshaft
  • Valve
  • Packing
  • Piston rings
  • Valve lift-off
  • Clearance pockets

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