Reciprocating compressors are wide-ly used in the LNG industry, andwill continue to be used because of

the many processes that require their highpressure and variable loading character-istics. These machines, however, are muchmore maintenance demanding than theircentrifugal and axial compressor counter-parts, and yet are some of the least moni-tored machines in the plant. This issuebecomes even more important as LNGprocess technology improves and con-sequently the time between plant shut-downs is increased. This makes the recip-rocating compressor increasingly the bot-tleneck when optimising plant mainte-nance and operation strategies.

Monitoring techniquesavailable

Experience shows that the cylinder com-ponents such as the valves, piston rings,rider rings (also called wear bands), pack-ing case, piston, piston rods, andcrosshead pin, have been identified as themajor cause of both planned andunplanned shutdowns. There are a num-ber of performance, vibration, acousticemission and other measurement tech-niques that can be used for both the on-line and off-line monitoring of these com-ponents. A few of the techniques are usedfor safety monitoring, others are just fordiagnosis and some for automatic condi-tion monitoring (also called predictivemonitoring, which includes early faultdetection, fault trending and performancemonitoring).

Some of the measurement techniquesused specifically for the cylinder portionof the reciprocating compressor are listedbelow. These particular measurementtechniques have proven to be accurate,repeatable and reliable for detecting anddiagnosing reciprocating compressorfaults when properly set up and used.Figure 1 shows the typical sensor configu-ration for these measurements.

Rod drop - A fixed proximity probe

automatically measures the distance from the probe tip to the rod, which gives an indication of the wear of the rider rings. Impact vibrations - An accelerometer can be used for safety monitoring of machine components that are loose (e.g. worn pins), damaged (valves, bearings, rings, rods, crossheads, crankshaft, etc.), or unbalanced, or for detecting the presence of liquids in the gas stream.Valve gas temperatures - This is one of the most effective safety and condition monitoring methods for detecting incorrect valve condition or operation.Stuffing box seal temperature - Auto-matic condition monitoring for seal leaks.

There are also a number of automaticallymonitored calculated performance moni-toring parameters for monitoring recipro-cating compressors that require minimaldiagnosis, such as flow, rod load, dis-charge temperature, volumetric efficiencyand power.

More monitoring has tobe done

Although much maintenance is needed forreciprocating compressors, and there is awide range of measurement techniquesavailable for monitoring these machines,condition monitoring is not done as wide-ly as is done for other critical machines. Infact, reciprocating compressors are someof the least monitored critical machines inmany plants. What is most disturbing isthat most of the previously mentionedmeasurement techniques, contrary to pop-ular belief, can give effective results ifproperly set up and used.

Industrial cooperation tofind a solution

An instrument supplier, Brüel & KjærVibro (BKV) and a compressor manufac-turer, Thomassen Compression Systems(TCS), formed a long-term cooperation in

order to come up with a solution for over-coming the lack of confidence in, and lim-itations of, the various measurement tech-niques so that end-users can reduce main-

tenance costs and downtime. A major partof the focus was on reciprocating com-pressors used in the stabilizing process ofan LNG plant (in addition to compressors

LNG journal March/April 2004 page 27

LNGjournal

Distinguished Speakers Include: The Hon. Eric A. WilliamsMinister of Energy and Energy Industries, Republic of Trinidadand Tobago

H.E. Luis ViermaVice Minister of Hydrocarbons, Ministry of Energy and Mines,Venezuela & Director, PDVSA

The Hon. Leslie MillerMinister of Trade and Industry, Commonwealth of the Bahamas

Frank Look KinPresident, The National Gas Company of Trinidad andTobago Limited

Guy CarusoAdministrator, Energy Information Administration, U.S.Department of Energy

Raul MonteforteCommissioner, CRE – Energy Regulatory Commission,Mexico

Luis Sánchez CastellónExecutive Secretary, Comisión Nacional de Energía,CNE, Chile

Jaime BarrenecheaPresident, YPFB, Bolivia

Jim SahagianVP Commercial Development, Sempra LNG Corp.

Raymond W. WrightCEO, PCJ Petroleum Corporation of Jamaica

Julian NebredaVice President, Integrated Utilities Dominican Republic, AESCorporation

Ernesto CordovaGeneral Manager, EcoElectrica, Puerto Rico

Eugênio Pierrobon NetoCOMGÁS, Brazil

LNG as a driver for developmentHilton Trinidad, Port of Spain, Trinidad & Tobago

La

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the

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CWC’s InternationalGas Steering Committee

Programme developedin consultation with

Conference: 29-30 June 2004

Workshops: Workshop A: Fundamentals of LNGWorkshop B: Fisher-Tropsch GTL Technology & Economics – 28 June 2004

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LAC Gas 200428-30 June

Successful monitoring strategy for LNG reciprocating compressors

The authors' companies have cooperated with the aim of helping to reduce maintenance costs and downtime of LNGreciprocating compressors. The article describes the development and refinement of automatic monitoring

techniques that require minimal diagnosis expertise from the end-user, with examples of applications.

Mike Hastings, Brüel & Kjær Vibro, Denmark and Jos Schrijver, Thomassen Compression Systems, The Netherlands

Figure 1 Typical measurement sensors for a reciprocating compressor. The minimumrecommended installation according to the findings from the TCS/BKV cooperation is

shown with the coloured symbols (boxes for vibration measurements, circles for processmeasurements).

p27-30.qxd 06/04/2004 17:56 Page 1

used in other applications, such as naturalgas compressor stations). The solution thatcame out of this cooperation was based onevaluating the customer's needs, evaluat-ing and modifying the existing monitoringtechniques, integrating these into themonitoring system, and providing low-cost yet effective services that ensureproper installation, set-up, fine-tuningand long-term operation. The monitoringsystem selected for this purpose was Brüel& Kjær Vibro's COMPASS system, a plant-wide automatic monitoring and safetysystem with integrated vibration, processand performance monitoring capability.

After evaluating the customer require-ments, the emphasis was to concentrate onthose measurement techniques that areautomatically monitored and do notrequire extensive resources for diagnosisfrom the end-user's side, yet still can giveaccurate reliable results (Figure 2 showssome of these recommended measure-ment techniques).

Other measurement diagnosis tech-niques such as PV-diagram analysis, whichcan still be offered as a solution to cus-tomers that require it, were not part of theinitial focus since these still require a lot ofdiagnostic resources that many end-users

cannot provide, and it is difficult to auto-matically monitor these to alarm limits.

The instrument supplier / machinemanufacturer reciprocating compressormonitoring solution provides the follow-ing benefits to the end-user:

Instrumentation on the machine is matched to the monitoring system and fine-tuned as a fast and efficient serviceThe machine manufacturer has com-pressor test facilities for researchingnew monitoring techniques and analysing or refining existing ones Sales and support network for the mon-itoring solution for both organizations is essentially "doubled"The machine manufacturer can providemachine dependent variables for calcu-

lated performance monitoring param-eters such as power, volumetric effi-ciency, rod load, flow and discharge temperatureThe cooperation is not a merger, so both companies are unaligned - the solutions can be used with all makes of compressorsIt is possible for both the machine man-ufacturer and the instrument supplier to have remote access to the monitoring database, so site visits can be minimized.

Improved monitoringsolution

The rod drop measurement was one of themeasurement techniques that were idealto focus on. This is because the end-user's

PLANT MAINTENANCE

LNG journal March/April 2004 page 28

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Figure 3 Typical COMPASS monitoring system configurationFigure 2 COMPASS screen view showing recommended measurements for each cylinder

of a compressor in the stabilizing unit of an LNG plant.

p27-30.qxd 06/04/2004 17:56 Page 2

confidence in the rod-drop measurementwas low, although there was consider-able downtime associated with changingthe rider rings too early or too late. Someinstrument suppliers even recommend totheir customers not to use the rod dropmonitoring technique at all, or suggestthey get advice from the OEM machine

supplier. Some of the improvements in using the

rod drop measurement include:Improved rod drop measurement tech-

nique - A proprietary dynamic measure-ment technique based on multiple meas-urements was optimised by TCS andextensively tested by both TCS and BKV.It does not have all of the shortcomings ofthe traditional measurement technique(e.g. the traditional rod drop measurementis a static measurement, which means itcould drift over time giving a false trend).

Measurement set-up and operationservice - Because TCS/BKV have remoteaccess to the end-user database, the roddrop measurement can actually be set upand fine-tuned remotely without a sitevisit.

Smart alarming techniques - TheCOMPASS System (Figure 3) has analarm strategy that allows groups of sim-ilar sensors to be compared, and alarmsare generated on deviations, not on theabsolute levels. As seen in Figure 4, thesuction valve temperatures for two cylin-ders vary over time by more than 20 ºCdue to changing process conditions (from44 to 65 ºC). The temperature deviationfrom one suction valve to another, how-ever, varies only 1.5 ºC, which means thetemperature deviation or spread meas-urement is much more sensitive in identi-fying a valve problem.

The reliability and accuracy of otherreciprocating compressor measurementtechniques were also addressed by theTCS/BKV cooperation but are not includ-

LNG journal March/April 2004 page 29

LNGjournaled in this article. This includes for examplethe calculated performance parameters(power, volumetric efficiency, rod load,flow, discharge temperature), which canalso be automatically monitored to alarmlimits. The implementation of these partic-ular measurement techniques aredescribed in a previous paper [1].

Case StudiesThe examples presented below are a directresult of the TCS/BKV cooperation. Most

of these case studies are focused on roddrop measurements for reciprocatingcompressors in an LNG and natural gasprocessing application, but other monitor-ing techniques are also described.

Improved rod drop measurements:Rod drop monitoring can give effectiveresults if properly set up and used. InTable 1, statistical deviations for the roddrop wear and actual dimensions are giv-en for six different machines for differentapplications at different locations (mostly

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Figure 4 Upper plot: Actual temperaturefor each suction valve (20 ºC deviation

over time). Lower plot: Temperaturedeviation between the four valves

(1.5 ºC deviation).

Figure 5 The 4-day average can establish a good long-term trend

p27-30.qxd 06/04/2004 17:57 Page 3

PLANT MAINTENANCE

LNG journal March/April 2004 page 30

in the LNG industry). The monitored roddrop measurement just prior to wear bandreplacement is compared to the actualphysical dimensions of the bands meas-ured during disassembly. In all situations,an optimized rod drop measurement tech-nique was implemented within theTCS/BKV cooperation. As seen in Table 1,the average difference between the roddrop wear measurement and the physical-ly measured bands is less than 0.15 mm.

Another method of using rod dropmeasurements to the maximum effective-ness is the use of both averaged and non-averaged rod drop measurements. Theaveraged measurement makes it a littleeasier to identify a trend so maintenancecan be planned ahead of time (Figure 5),as the non-averaged measurement isimportant for catching short-term faultssuch as lubrication system failure andsand in the cylinder caused by a faulty fil-ter (both occurred in the stabilizing unit

of an LNG plant).Broken piston ring detected by impact

monitoring: Impact safety monitoring pro-tects the compressor from rapid destruc-tive faults such as broken components orliquid ingestion. The measurement canalso be trended as a condition monitoringmeasurement to keep track of increasingclearances such as for worn sliders, rodbearings, etc. Figure 6 shows a compressorwith a broken piston ring.

The constant percentage bandwidth(CPB) spectrum measurement in the right-hand plot of Figure 6 indicates a 1k Hz sig-nal amplitude increase. In this case the 1kHz increase is not necessarily due to theimpact signals themselves, but is mostlikely the result of structural resonancesbeing excited by the impacting pieces ofthe broken piston ring.

The CPB measurement is a compositespectrum consisting of a series of individ-ually filtered measurement components,

each of which has a bandwidth that is aconstant proportion of the centre frequen-cy (6% in this example). It is much moresensitive to non-sinusoidal impacts than aFFT (fast fourier transform) spectrumanalysis and has much higher resolutionat lower frequencies.

Leaking suction valve: The overlayplot in Figure 7 shows the suction valvetemperature for two different cylinders ofthe same machine in an unmanned gascompression station. As seen in the plot,on Oct 20, one of the cylinders suddenlyincreases 3 ºC warmer than the other. Thistemperature spread is observed for 10days up until repair, which was done Oct30. The suction temperature spreadreturns to normal after the valve repair.

ConclusionThe TCS/BKV cooperation was aimed athelping LNG reciprocating compressorend-users to reduce maintenance costsand downtime. There are several accuratemeasurement techniques that can be usedfor monitoring most of the reciprocatingcompressor potential failure modes, butthese are not always being used. This ispartially due to the difficulty in setting upthese measurements and a lack of knowl-edge and confidence in the diagnosis.

After evaluating customer needs, it wasdetermined that using reliable automaticmonitoring techniques that require mini-mal diagnosis expertise from the end-user,will give faster, more effective results.Educating end-users in using diagnosticand analysis tools such as PV-diagramanalysis will not give immediate results,and thus is not the immediate goal of thisco-operation. Automatic monitoring tech-niques were evaluated and refined fromboth a technical and service point of viewto improve their effectiveness. As all thecompressors are remotely monitored,diagnoses can be done without site visits.This approach is proving itself successfulas demonstrated by the case studies. Thereis, however, a "learning curve" in promot-ing and refining this solution, so theTCS/BKV cooperation is intended as along-term project.

Mike Hastings is asenior applicationengineer at Brüel &Kjær Vibro in Den-mark. He graduatedfrom Purdue Univer-sity in 1980 as amechanical engineerand has worked as an

offshore field engineer and product designengineer in various oil and gas industries inthe USA, and as a project engineer in Africa.He has been with Brüel & Kjær Vibro for thepast 13 years.

Jos Schrijver got hismechanical engineer-ing degree from theHogere TechnischeSchool Arnhem in theNetherlands, and hasbeen working withThomassen Compres-

sion Systems since 1977. He worked sevenyears as a test engineer for centrifugal com-pressors, 15 years as a product engineer forgas turbines (specialisation in dynamics) andthe last four years as product engineer forreciprocating compressors.

Acknowledgement3rd EFRC Conference, March 27th /28th, 2003, Vienna. "Reliability andEconomics of Compression Systems -Recent Trends in the Market of Recipro-cating Compressors"

References[1] Lau G. M. Koop, "Performance mon-itoring on reciprocating compressors - Arational extension of condition monitor-ing", paper presented at the 2nd EFRCsymposium, Amsterdam, May 18, 2001

Wear Band DifferenceMeasurement Deviations mm %

Mean deviation 0.15 5.6

Average deviation 0.14 7.7

Minimum deviation 0 0

Maximum deviation 0.4 29.6

Standard deviation 0.11 7.6

Table 1 Statistical comparison of wear band wear for a number of different machinesas determined by monitoring rod drop just before replacement, and the physical

dimensions as measured during replacement.

Figure 7 A suction valve temperatureincrease due to a faulty valve

Figure 6 Right: Impact monitoring of a broken piston ring using a constant percentageband-width measurement (CPB) showing an increased 1k Hz signal. The green line

represents the reference for normal condition, the yellow line indicates an alert alarmcondition. Left: Spectrum made before the piston ring failed.

Distinguished Speakers Include: Sally Kornfeld, Office of Fossil Energy, US DEPARTMENTOF ENERGY

Dr Raul Monteforte, Commissioner, CRE – ENERGYREGULATORY COMMISSION, MEXICO

Commissioner Charles Matthews, TEXAS RAILROADCOMMISSION

Betsy Spomer, CEO/VP, BG LNG Services/Global LNG, BGGROUP

Darcel Hulse, President, SEMPRA ENERGY LNG CORP.

Richard Grant, CEO, TRACTEBEL LNG NORTH AMERICA

Sveinung J. Støhle, Head-North America LNG, TOTAL

Keith Meyer, President, CHENIERE LNG

Donald F. Santa, Jr., President, INGAA, INTERSTATENATURAL GAS ASSOCIATION OF AMERICA

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Kathleen M. Eisbrenner, President,EXCELERATE ENERGY LLC

Mark Morris, Director, LNG Services, PANHANDLE ENERGY

Ted Gehrig, COO, WEAVER’S COVE ENERGY

Bill Perkins, President, CRYSTAL ENERGY LLC

If you would like to receive more information on the Summit please contact: Shane JohnsonE-mail sjohnson@thecwcgroup.com Tel: +44 (0) 20 7089 4189 Or visit our website www.thecwcgroup.com

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