Many industrial applications require pressurised gas. Compressors, which are

typically driven by electric motors or engines, are used to pressurise the gas, and there are a number of types of compressor available, including centrifugal, axial, lobe, rotary andreciprocating.

Of these, reciprocating compressors are particularly popular, as they canachieve high pressure ratios betweenthe inlet and the exhaust.They rangefrom small units driven by motors ratedat just a few kW, as might be used in asmall workshop, to multi-cylinder industrial compressors that work 24/7.

Reciprocating compressors have one or more cylinders, each with a pistonand at least one inlet and one exhaustvalve. Some might also have a pressurereservoir in instances where high-pressure air or other gas might be required infrequently, and it is the compressor’s job to maintain this reservoir’s pressure.

Working too hard?Compressor efficiency depends on thehealth of a number of critical sealingcomponents. For example, as valveseals wear less high-pressure gas is delivered for the mechanical energy(piston movement) expended.Gas doesnot physically escape the system but isrecycled. Inlet valve wear results inhigher pressure gas leaking into the low or atmospheric side of the systemonly to be drawn in again, while exhaustvalve wear results in high-pressure gasleaking back into the cylinder.

While the manufacturers of compressorstake a scientific approach to the design,production and use of critical sealingcomponents, once a compressor is installed and operational the onus isvery much on the user to monitor itshealth. Monitoring the high-pressuregas flow is of course an option, as ismonitoring the power (or fuel)

consumption, provided the power orfuel can be monitored in isolation.However, on a multi-cylinder compressor, neither strategy wouldindicate which parts are wearing.

Physical inspection during maintenanceroutines is of course possible, but thereis a fine line between risking unexpected failure if maintenance intervals are too long and costly ‘over-servicing’, which can never guaranteeagainst unexpected failure in any event.

Acoustics have the answerAs a result of recent industrydevelopments,suitable monitoringequipment can nowbe used to assessthe health of the valves while thecompressor is running.This assessmentcan be used to underpin a site’s formalcondition-based predictive maintenance(CBPM) routines.

Specifically, much work has been doneon the measurement of the ‘acousticprofiles’ of compressor valves. Sensors,similar to vibration sensors but adaptedto frequencies higher than typical mechanical vibrations, can be used to acquire acoustic data.This is the principle on which Compressor Products International’s Valve alert isbased. It is available as a handheld unit for manual inspections or as a permanently installed system accessible via the web.

In the handheld configuration, abespoke sensor (manufactured by UKbased company Monitran) is used to

pick up acoustic emissions andtemperature data.The sensor isattached to a quick-release connectorbonded to the valve cover.

Once collected and stored in thehandheld unit,data can be transferredto a computer and analysed.Thealgorithms in CPI’s Valvealert softwareautomatically detect valve anomaliessuch as leaking valves, stiff or weaksprings,slamming, valve flutter, late orearly openings and closings, and canalso give early indications of valvecrack propagation and sticking valves.

The condition of the valves can bedisplayed through a series ofgraphical displays, ideal for spottingwear trends and enabling operatorsto schedule suitable maintenancebefore the compressor starts usingmore power or fuel than necessary.

Article reproduced with thanks to Water, Energy & Environment Magazine

THE PRESSURE IS ONTO SAVE ENERGYLEE ALEXANDER, PRODUCT MANAGER WITH COMPRESSOR PRODUCTS INTERNATIONAL, AND ANDY ANTHONY, OPERATIONS DIRECTOR OF MONITRAN,EXPLAIN HOW WORN COMPRESSOR VALVES WASTE POWER AND MONEY – AND WHAT TECHNOLOGY EXISTS TO HELP IDENTIFY PROBLEMS

“IDEAL FOR SPOTTING WEARTRENDS AND ENABLING OPERATORS TO SCHEDULEMAINTENANCE BEFORE THE COMPRESSOR STARTSUSING MORE POWER ORFUEL THAN NECESSARY”

Above, a six cylinderreciprocatingcompressor, used tocompress natural gasfor piping over longdistances. All sixcylinders must remainhealthy to maintainthe required pressureand flow rate