SPECIAL TECHNIQUES – ROD DROP

Rod Drop is an ingenious measurement technique forincreasing the productivity of industrial plant by providing a reliable and accurate warning of rider band wear on reciprocating compressors,thus eliminating the need to shutdown the machine for inspection.The Sensonics system consists of a special type of eddycurrent proximity probe (non-contacting displacement sensor) and a real time monitor module,which measuresthe vertical position of the rod and calculates the wear on the rider bands.This measuring method is a well-proven technique;equipment to do this has been available for many years. The Sensonics system is different in that the probe used has a dramatically increased measurement range,which allows a different monitoring philosophy to be utilised.Traditionally,rod drop monitoring systems have beenconfined to using standard eddy current probes with a measuring range of just 2mm,which is often not sufficient to cover the full range of lateral (or radial) movement of the connecting rod on most types of machine.This has meant that industrial monitoring system designers have hadto utilise a ‘snap shot’ measuring technique where the vertical position of the rod is measured instantaneously at the same point on each cycle of the machine.This measurement is usually triggered by a pulse from a second eddy current proximity probe looking at a single ‘phase reference’ slot on the crankshaft.This measuring method relies on the assumption that the rods position is identical from one stroke to the next except for the gradual change in position caused by rider band wear. Research has shown that this is often not the case and in fact each rod stroke cannot always be guaranteed to be repeatable,which is often a cause of inaccuracy in the calculated rod drop reading.The Sensonics system uses a 4mm range probe (in thestandard 8mm body) and is therefore able to measure theposition of the rod throughout its 360

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stroke (evenwhere the rod is coated with ceramic).This in turn enables the monitor to calculate the true mean position of the rod more accurately and give a truly reliable measurement of the rider band wear.Due to the increased measurement range,the unit can also provide a peak to peak vibration measurement which can provide additional information on the machinery health.The Sensonics system was first installed at a chemicalplant in the UK,alongside numerous ‘traditional’ rod drop monitoring systems.Twelve months later the operating staff reported that the new system was 100% reliable and a vast improvement.They have since replaced their old equipment with the new Sensonics system.The system is not just for monitoring rod drop;casingvibration and valve temperature can also be incorporated for early warning of other types of mechanical fault.Fitted through the distance box of the compressor,thesmall size of the probe means that the Sentry system can be economically retrofitted to installed machines of any make in the field with minimal modification.The whole system can be installed and commissioned from scratch in less time than it takes to strip the machine to take a traditional rider band wear measurement.As illustrated opposite,compressor systems are complexmachines and several techniques are normally utilised for machine health monitoring.An effective additional method to rod drop is to fit an impact sensor above the piston housing to detect transient vibration events generated from cracked parts,leaks or general machine wear.Conventional vibration monitoring is excellent fordetecting sinusoidal vibration events due to unbalance or bearing wear.However for reciprocating industrial compressors,the transient shock and impact events are key,therefore simple RMS evaluation techniques cannot determine suitable trip levels for plant protection.The impact sensor in combination with a suitable monitor can count the number and magnitude of shock events providing a more reasonable metric to enable effective plant shutdown.
A typical compressor with rod drop

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