Impacting and shaking are two types of vibration normally monitored when using a continuous monitoring system. Impacting of one component against another is best detected with acceleration, while shaking is measured with velocity or displacement. An impact event is made up of high frequency components that are significantly affected by the path that they take to the sensor position, so great care should be taken to keep that path as short and rigid as possible. If you want to monitor compressor valve opening and closure impacts, then mount the accelerometer on the cylinder body near those components. If you want to monitor possible impacting of the reciprocating components, then mount the accelerometer in the path of those vibrations.

Common reciprocating components that can be involved in an impact event are piston to piston rod connection, piston rod to crosshead connection, crosshead pin and bushing, connecting rod bearing and sometimes main bearings.

Other unwanted impacts can occur with the crosshead shoes, slides and attached components. There is not a good path for these vibrations to be presented to an accelerometer mounted on the cylinder body. The crosshead housing or compressor frame near the main bearings provide the best paths for these sometime small vibration events to get to the sensor. While rigidly mounting the accelerometer vertically on the crosshead housing or horizontally on the compressor frame near a main bearing are the most common, the exact placement of the sensor will be determined by the physical characteristics of the unit.

Mounting of the velocity measuring sensor is quite different. The reciprocating impact events will not get much response from the velocity sensor until the event shakes the machine. If this happens, you may have already experienced some serious damage. The velocity sensor is mounted on the components that are subject to unwanted movement such as the compressor frame and/or compressor cylinder ends. Shaking is highly directional so mount the sensor in the direction of the most unwanted movement. Common frame locations are on opposing corners of the frame in the horizontal plane; however, if the frame is subject to unwanted vertical movement, then mount the sensors vertically. Mounting velocity sensors on the compressor cylinders ends is also common. They should be mounted in the plane of greatest anticipated movement. Cylinder “stretch” would best be monitored in the horizontal plane in relation to the crankshaft center line (axial in relation to the cylinder bore). Mounting the sensor vertically will give the greatest response to cylinder support issues. Mounting the sensor on the cylinder end, axially to the cylinder bore in the 1:30, 4:30, 7:30 or 10:30 clock positions will give a good, combined response from movement in all directions. 

Look for more thoughts on reciprocating compressor package vibration in future newsletter issues.

Happy monitoring,

Warren