This article is part of the whitepaper: “monitoring mechanical behaviour of reciprocating machinery”. Request the whitepaper here »
Impact measurements are used as a reliable method to monitor excessive mechanical movements of reciprocating compressors. Previously, vibration measurements were used, but this did not provide accurate measurements to monitor impact. This is because of the significant difference between a vibration signal and an impact signal.
As the piston moves through the cylinder, impacts are caused, which produce high amplitude spikes of a very short duration (figure 1). Traditional vibration measurement sensors are not suitable to monitor these impact signals as impacts barely affect the vibration signal. Impact sensors (or transmitters) are suitable to detect mechanical clearance and looseness on reciprocating compressors at an early stage.
Excessive mechanical clearance can develop quickly, which is why impact measurements are used for monitoring and, if necessary, shutting down the reciprocating compressor. Automatic shutdowns or trip functions are activated based on pre-set alarm values, which are related to the intensity of the impact and the amount of impacts above a defined threshold, within a certain timeframe.
An impact sensor can be placed on the distance piece perpendicular to the direction of movement of the piston rod in the compressor cylinder. An impact sensor can also be placed on the crosshead (figure 2).
The IT6810 / IT6811 / IT6812 impact transmitters from Metrix use a new technology to measure the impact intensity on reciprocating machinery. Merely measuring the peak g’s (acceleration) to determine mechanical clearance / looseness is not sufficient. In order to increase the reliability a technique is developed that qualifies the impact events, preventing false alarms. This is based on the amplitude and whether it concerns individual or repeated events. After the detection of impact events that meet the requirements of the threshold levels, the events are counted within a configurable time frame.
The impact transmitter (IT6810 / IT6811 / IT6812) has a 4-20 mA output signal. The 4-20 mA signal represents the amount of impact events that are detected within a certain time frame and exceed the threshold level. This time frame is called the reset time. An output signal of 4 mA indicate that no impact events occurred that exceed the threshold level, 6 mA indicate two impact events, 8 mA indicate four impact events, et cetera. A PLC / DCS / SCADA then determines whether alarm or trip action is required based on the impact intensity.