monitoring centrifugal compressors

7/28/2019 Monitoring Centrifugal Compressors

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Application Note

Monitoring Centrifugal Compressors

Turbo machinery employing centrifugal effects for increas-ing fluid or gas pressure have been in use for more than acentury. The earliest machines using this principle were,

undoubtedly, hydraulic pumps followed later by ventilatingfans, blowers and compressors. The centrifugal compres-sor has become the reasoned choice for refrigeratingplants, petrochemical and oil & gas industry.

The centrifugal compressoris used widely in more orless all-modern processing applications. Where extremehigh pressure is required the reciprocating compressor isthe preferred type. When looking at gas turbines andespecially jet engines, axial compressors are typicallyused.

Horizontal split compressor

Recognising the need for different types of compression technology, the three above-mentioned compressor

types are covering more or less all applications. B&K Vibro offers both standard and customised monitoringsolutions that consider the application rather than the general monitoring approaches, based on in-depthknowledge about specific machinery elements influencing the machinery behaviour.

The centrifugal compressorhas been thereasoned choice for refining plants and the oil &gas industry for more than 50 years. Thesecompressors have capabilities ranging from below1 MW up to around 50 MW and are preferredbecause of their high efficiency, good economy,low maintenance and high degree of reliability.

Centrifugal compressors are as standard equippedwith proximity probes measuring either an overallshaft vibration level or the Smax level, at the bearinglocations, depending on vender monitoringrecommendations for judging the vibration state.

The centrifugal compressor is required to meetwider and wider performance conditions (wideoperational speed range combined with large flowvariations resulting in large variation in head ordelivered pressure). In time with superior demands

for high performing flexible compressors it is oftenseen that designs are stretched to their limit.

In the resent years one of the main problems whenconsidering high performing centrifugal compressor

applications, has been control of the design, espe-cially when considering aerodynamic stability(e.g. surge prediction, flow instabilities, like rotatingstall both in the stationary and rotational flow path).

surge

rotorstability maxspeed

minspeed

flow

hea

d

ratedspeed

surge

rotorstability maxspeed

minspeed

flow

hea

d

ratedspeed

Centrifugal Compressor performance map showingsome typical design limitations

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stalll

ston

ewal

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Application Note

Rotordynamic stability, which was the main issuefrom the mid-fifties up to early-nineties, is todayone of many areas of concern, when designinghigh performance applications. Consequently,when choosing monitoring solutions forcentrifugal compressors it is important to considernot only shaft vibrations originating form therotational speed, but also any specialcharacteristics of various machine elements andaerodynamic impact.

Potential Failures have different distributiondepending upon the application (mechanical

design, operation range, environment etc.).Investigations show that around 50% of all failurescan be assigned to product faults, i.e.

Material faults (e.g. due to fatigue or off-design)

Faults in planning

Manufacturing faults

Assembly and repair

Around 35% of all failures can be assigned tooperational faults, i.e.

Maintenance

Mishandling

Most of these faults may be detected before theybecome critical by a system that combines vibrationand performance monitoring. In order to minimizethe machine installation costs, such monitoringsystems are rarely recommended by the machinemanufacturer. It is up to the end user of thecompressor to balance the investment cost, andthe cost of running a predictive maintenancesystem up against:

Higher production output

Less unexpected repairs

Optimized maintenance scheduling

Optimized machine performance.

Predictive Monitoring should include monitoringof both process and vibration parameters. Theprocess parameters are used in makingperformance and efficiency calculations and thevibration parameters are necessary when judgingthe condition of the rotating parts.

It is important that the monitoring system is able ofgenerating alarms on both measures andcalculated input and each of the parameters are

able to generate a relay output (alarms) forsafetyreasons.

The alarms, which are functions of one or morecalculated or analog parameters (process and/orvibration), are with regard to process parameterstypically based on:

Compressor isentropic efficiency

Compressor isentropic head

Compressor volume flow

Data corrections against atmosphericpressure.

With regard to vibration parameters:

Overall vibration level (according to APIstandards)

Rotor synchronous amplitude and phasemeasurements (Minimum 1X & 2X)

Casing and blade passing frequencies

Journal Bearings Gap, Orbit

Rolling element Bearings Bearingfrequencies and high frequency noise

- Vibration parameter measurement tools: DC

Band Pass / Smax measurements

Dual Time function with limiting filters

Vector measurements SelectiveMagnitude and Phase

FFT - Auto spectra with zoom capability

Selective Envelope Spectra

CPB Spectra

The Monitoring Set-up depends on the installedmonitoring equipment and the monitoring strategyrelevant for the specific application. As themanufacturer typically installs sensors inaccordance to monitoring requirement (installation& design manual), complying with the prescriptiveguarantee obligations, Brel & Kjr Vibromonitoring strategies starts with basicconsiderations in respect to vendor specifications,respecting any guarantee requirements.

In dialog with the customer (end user or contractor)the overall monitoring strategy is scheduled andimplemented accordingly.

The Brel & Kjr Vibro conditioning monitoringsystem imports values from the DCS system, asscalar values (normally no additional processsensors needs to be installed).

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Application Note

Typical compressor process measuring positions (these signals are easily imported to COMPASSTM

and used

for judging the performance condition of the centrifugal compressor).

The Brel & Kjr Vibro conditioning monitoringsystem requirements are considered as equalparts of warning of imminent failure and adequateprotection (safety) of personnel and facilities fromundesirable accidents. Furthermore, the goal is toprovide actionable information to the operationsand maintenance personnel, where actionableinformation is accurate and timely information thatcan be used to make intelligent machinery

management decisions, i.e.:

Continue or not?

If it fails, what are the consequences?

Can the machine be re-started?

What should be inspected?

Is the machine operating efficiently?

Will it last until next scheduledoverhaul/service?

The conditioning monitoring system COMPASSTM

is a fully automatic integrated monitoring system.Through its modular concept, COMPASS can beadapted to a large range of different machines sothat all of the requirements of a modern conditionbased maintenance strategy are fulfilled by one,plant-wide system

All in one COMPASSTM

safety & predictivecondition monitoring

COMPASSTM

is a rack based (19 inches) modularsafety and conditioning monitoring system, whichcan be tailored to meet any compressor

application.

NPTN

TLP2PLP2

System boundaryfor heat balance

Gearbox

LP

Filter

PHP1 THP1

HP

Anti-surge

Scrubber

DPLP

Anti-surge

System boundaryfor heat balance

PHP2

THP2

MLP

MHP

TLP1PLP1

DPHPIntercooler

Filter

ScrubberIntercooler

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Application Note

The compressor signals are conditioned, filtered and(if necessary) rectified, by the processor modules.Narrow-band filtering (order tracking) with regard toshaft speed, band pass measurements and anyspecial analysis requirements, like Smax, orbitmeasurements, CPB, time function measurementsetc., is done by the processor modules.

The processed signals is stored in the COMPASSTM

database and displayed on the central computer, X-terminal or remotely accessed through the modemconnection or Internet (using standard PC).

The sensor selection is based on measuringvibration, process and other relevant parameters. Ifany additional sensors are required you shouldconsult your Brel & Kjr Vibro representative.

Compressors require vibration sensors measuringvibration severity (or overall vibration levels), thedirection vibration (reference sensor) and the motionof the shafts (position relative to bearings and seals)giving:

Amplitude

Phase

Frequency

When monitoring of shaft vibration the best choice isthe displacement sensor. A reference sensor,typically a displacement sensor, measures thevibration phase.

Bearing cap or case mounted seismic sensors (e.g.accelerometers) are useful for evaluating casingvibrations and to provide absolute vibrationsmeasurements. The sensor should be selectedbased on its response curve, frequency range,

sensitivity and noise susceptibility.

The seismic sensoris often the preferred choicewhen obliging harsh environment, i.e. where thetemperature is far too high for traditionaldisplacement and velocity sensors. Therefore, whenusing seismic sensors, to measure shaft vibrationconsiderations of where to mount the sensors aredecisive for the result. The sensor should bemounted at a location where the vibration istransmitted directly to the sensor, without much loss.

The thrust position is extremely critical since thrustbearing failures and axial rubs can be catastrophicfor centrifugal compressors. The thrust position isbest measured with the displacement sensor(s)monitoring the thrust disc movement.

The bearing type of centrifugal compressors isusually fluid film bearings and rarely rolling elementsbearing. The bearing type depends on manufacturertraditions and years of experience with particulardesigns; however, the tilting pad bearing is thepreferred bearing design today.

The tilting pad bearinghas the advantage ofproviding no destabilizing forces to the rotor system,but on the expense of low bearing damping. This iswhy centrifugal compressor manufacturer decide toinstall machinery elements like squeeze film damperssupporting the bearing(s), shunt holes in the divisionwall, use honeycomb or hole pattern seals instead oftraditional labyrinth seals, inserts gas swirl- or thrust-brakes in the flow path etc. to improve the stability ofthe rotor-bearing system, by introducing dampingand/or removing destabilizing gas forces.

All these machinery elements or components have

their own characteristic and impact the rotor dynamicor aerodynamic stability differently. When definingthe overall monitoring strategy for a centrifugalcompressor the specific design of the applicationshould be taken into account. Furthermore, differentmeasurement and diagnosis techniques must beapplied depending on the type of centrifugalcompressor application.

SummaryBrel & Kjr Vibro Conditioning monitoring systemCOMPASS

TMused together with a well-considered

monitoring strategy for the individual applicationyields the optimum proactive protection of centrifugalcompressors.

The benefits of optimizing monitoring strategies toconsider the criticality of the specific design andpotential failure modes clearly distinguish inimproved performance and reliability turning directlyinto revenue improvements or cost reductions.

Brel & Kjr Vibro A/S2850 Nrum DenmarkTel.: +45 4580 0500Fax: +45 4580 2937E-mail: [email protected]

Internet: www.bkvibro.com

Brel & Kjr Vibro GmbH65293 Darmstadt GermanyTel.: +49 (0) 6151 428 1100Fax: +49 (0) 6151 428 1200E-mail: [email protected]

Internet: www.bkvibro.com BAN0012-EN-12

monitoring centrifugal compressors

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