Rod Drop Monitor

Rod drop monitoring is a monitoring system used on horizontal reciprocating compressors. Most horizontal reciprocating compressors use wear bands also known as rider rings to support the piston in its cylinder The wear bands are generally made of a plastic material which in time abrades and requires replacement. Should the wear go unchecked, contact with the cylinder wall by the piston will occur with the potential for serious damage to the cylinder and the piston. Also the packing may be damaged which could lead to a gas release.

Compressors should be stopped periodically to check for wear. The inspection may occur every few months. To accomplish this the compressor must be stopped, blocked in, and purged if the gas is hazardous. A valve is removed from the cylinder and a feeler gauge inserted through the valve opening. A measurement is taken between the lower side of the piston and the cylinder wall.

An alternate method of measurement is the use of a rod drop monitor. The monitor consists of a proximity probe located in a vertical position in the packing case. Another probe is used to develop a once-per-revolution timing pulse. The probe gap voltage is read with a remote monitor. The timing pulse is used to gate the probe reading to allow for an instantaneous rod position reading. The advantage of taking the reading at a discreet point in the stroke is that the effect of scratches or coatings on the rod can be minimized. Also, the reading can be timed to occur at or near bottom dead center (BDC). Here the dynamic forces have a minimal effect. Also the location of the center of the piston is known at that instant. With this information, the monitor can make a geometric correction to rod position reading providing a display that indicates the amount of wear band wear. Figure 8-32 shows a diagram of the rod drop sensors.

Figure 8-32. Line drawing showing instrumentation locations for a rod drop monitor,

Timing

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Timing

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Figure 8-32. Line drawing showing instrumentation locations for a rod drop monitor,

Packing Box

Packing Box

The rod drop monitor is recommended for all non-lubricated compressor applications, because there is no lubricant to act as a buffer to prevent piston-to-cylinder contact on the loss of the wear band. Lubricated com pressors handling gases with traces of water or gas components that can degrade the local lubricant are candidates. Hydrogen compressors should be considered for monitors because hydrogen is a difficult gas in itself and may contain trace quantities of water. While sweet gas compressors, as are found in pipeline service, would normally not be considered a problem, the rod drop monitor may be used to signal a loss of lubricant and the compressor can be shutdown before damage can occur.

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