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January/February 2018 www.coalage.com 39 operating ideas Controlling Compressor Vibration and Surge by jim slama Controlling vibration and surge is one of the most critical aspects of keeping cen- trifugal and reciprocating compressors up and running. If improperly monitored and maintained, these types of compressors can experience frequent functional issues and ultimately shorten their life span. Vibration controls are essential con- cerning any kind of rotating equipment, and compressors are no exception. Re- garding reciprocating compressors, typ- ically the vibration sensing is not all that sophisticated. One will generally see a velocity measuring device, transmitting in inches per second, mounted to the block of a reciprocating compressor. As the vi- bration increases, the velocity of that vi- bration is going to increase along with it. This signal is then fed into a PLC or anoth- er comparable device. The signal is com- pared with a set of predetermined points. In the industry, these points are referred to as the "high" and the "high high." An alarm is set to go off on the "high" to warn the operator that the vibration is starting to exceed a velocity that is recommended. If the vibration does not level off or slow down, the "high high" point will be trig- gered, which is programmed to shut down the compressor so it does not cause any irreversible damage. What are some problems that can lead to uncharacteristically high vibration in a re- ciprocating compressor? A common cause for high vibration is a bad bearing. If a bearing is going out, it does not perform the task it needs to and will ultimately need to be replaced. Anoth- er common cause for high vibration is me- chanical fasteners that have come loose or were improperly torqued. For example, there is a mine setting where someone fails to tighten valves properly on cylin- der heads of a reciprocating compressor. Over time, these valves can vibrate loose. However, if proper vibration controls are implemented in a situation such as this, protections are put in place. Although a high-vibration shutdown can be costly, it is ultimately less expensive than com- pletely replacing the compressor, and can protect operations personnel from a dan- gerous situation in their plant. How does the velocity sensing work for a vibration control system in reciprocating compressors? The signal produced by the velocity mea- suring device is sent to a signal conditioning device that generates a 4-20 milliamp signal fed into a machine monitor or the plant monitoring system. The alarm shutdown is then programmed into the machine PLC or the plant's distributed control system (DCS). Alternatively, the signal conditioning device could have a contact closure. This needs to be configured for an alarm and a shutdown level. This will, in turn, have a contact closure on the back. The contact closure sends a signal to the plant DCS or the machine's overall control system, letting the system know it needs to shut down. What about centrifugal compressors? Centrifugal compressors are a little differ- ent in how they are controlled. Typically, one sees more high-end vibration mon- itoring systems installed on centrifugal compressors. For reciprocating compres- sors, the approach outlined above is effec- tive. However, for centrifugal compressors, there is more rotation/potential vibration at work. Equipment can be spinning at 10,000-plus rpms in a centrifugal com- pressor. Consequently, bearings and vi- bration monitoring on these compressors is crucial. If there is uncharacteristically high vibration in this situation, it needs to be caught and addressed or it will po- tentially destroy the entire machine. With centrifugal compressors, instead of moni- toring velocity in inches per second, there are probes (x,y) called proximeters. These measure the distance from the tip of the probe to the shaft. Instead of technical- ly measuring "vibration," the proximeter probes act as a distance-sensing device. Additionally, there is a keyphasor, which monitors and provides a voltage pulse on a once-per-turn-event. These three signals (the x, the y and the keyphasor) are fed into a signal conditioning box located near the machine, roughly 20 to 30 feet away. The proximeter probes sense inductance. Placed at each end of the shaft, the prox- imeter probes measure the radial location of the shaft. The rotor on the compressor may shift radially, causing major problems. When everything is running properly, the center of the shaft will stay centered con- sistently as its running. Once up to speed, the shaft will develop a high-pressure film on the bearing. At this point, there can be liftoff and the shaft will suddenly be ro- tating inside the journal bearing on a thin film of oil (1/10 of a millimeter). The prox- If it goes unnoticed, high vibration levels could severely damage a compressor.