analisis vibracion cmpresor centrifugo

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[This is the irst installment in a mini-series o Recip Tip articles that is

planned by or experienced Italian Field Application Engineer (FAE),

Gaia Rossi. Editor]

VibrationAnalysis orReciprocatingCompressors

(Part 1)

G R

Bently Nevada Field

Application Engineer

[email protected]

Vibration analysis o reciprocating machines

creates some nie challenges. This article

explains the reasons and gives clarity on

recommended monitoring and analysis

practices and tools. Years o ield experience

have demonstrated that technies which

may be well nderstood or measring and

analying the vibration o prely rotating

machinery can prodce consing reslts

when applied to reciprocating machinery.

Vibration associated with rotational speed is the

dominant motion or most indstrial rotating

machines. This synchronos (1X) behavior

allows the direct application o traditionalvibration analysis concepts towards addressing

common machinery malnctions sch

as rotor nbalance. The typical reencies

observed with those common rotor-related

malnctions generally occr between a arter

o rnning speed

and twice rnning

speed and correlate

excellently with

machine mechan-ical conditions.

Conseently,

principles and

diagnostic method-

ologies or these

machines are broadly accepted

and harmonied within the

machinery diagnostic commnity.

This is not ite tre or reciprocating

compressors. Vibration analysis o these

machines creates some nie challenges;

many orcing nctions prodce a complex

vibration signatre that makes any attempt

o sing standard analysis technies

sed or rotating eipment ineective.

ORBIT Vol .2 No.2 Apr.2012

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FiGuRe 1: This drawing shows typical vibrationmonitoring locations or a reciprocating compressor.Sensors are installed at the crosshead gides (4red hexagons) and on the rame (4 ble diamonds).[Reerence 1]

Apr.2012 No.2 Vol .2 ORBIT

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Compressor Frame Vibration

Vibration measred at the rame

reslts principally rom the response

o the mechanical system to the

orces and moments that are

occrring in the machine at the

normal rnning conditions. These

inclde the ollowing actors:

G L Fr: These orces act

on the piston and stationary compo-

nents at 1X and at integer mltipleso rnning speed. They are generally

signiicant p to abot 10X and in the

direction o the piston rod travel. For

large slow speed compressors (p to

roghly 500 rpm), gas orces are typi-

cally the largest contribtor to piston

rod and compressor rame load.

irtl L Fr: These orces

are cased by the acceleration

o the reciprocating components

(piston, piston rod, and crosshead).

These components represent

a large amont o mass to be

accelerated back and orth with

each stroke. Inertial loads o

400,000 Newton (~90,000 ponds)

o orce or more are not ncommon

with very large compressors.

Rrtg & Rttg M

ul Fr: These orces

are predominant at 1X and 2X

compressor speed, and are cased

by asymmetrical crankshat design

and imperect manactring toler-

ances. They are sally mch smaller

than inertial and gas load orces.

FiGuRe 2: Time waveorm plot o the velocity signal rom a rame-monted vibrationsensor. Observe that many dierent reency components are present in the signal.

FiGuRe 3: Freency domain (spectrm) plot o velocity signal shown in Figre 2. FastForier Transorm (FFT) processing allows s to see the varios reency componentsthat are inclded in the complex waveorm.

G ul Fr: These arecased by pressre in the plsation

bottles and plsation at the cylinder

nole area and on piping. Allowable

plsation levels are deined in API-618.

Althogh these plsating orces are

sally mch smaller than the orces

listed above, they can be destrctive

to piping and piping spport systems

i they happen to correspond to reso-

nant reencies or the strctres.

As a conseence o these actors,the extent o vibration is inherent

with the reciprocating compressor

design and its response to all the

applied orces and moments. This

cases these machines, even when

in good condition, to vibrate mch

more than a comparable rotating

machine. The examples in Figres

2 and 3 show that many harmonics

are prodced by the complex shape

o the rame velocity waveorm.


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Frame vibration reencies typically

inclde components below 10 H.For this reason, a velocity transdcer

(with extended low reency

response) is sally better sited than

an accelerometer or detecting an

increase o rotation-related orces

(de to gas load or inertial loads,

imbalance, ondation looseness,

excessive rod load, etc.). The preerred

location or the rame vibration

transdcer is on the side o the rame

oriented in the direction o piston

rod travel, on the centerline o the

crankshat and at a main bearing

where dynamic load is transmitted

(Figre 1). Magnitde or a iltered

rame velocity signal is sally low

(less than 7 mm/s); however, at low

reencies, even small amplitdes o

measred velocity may correspond

to large amonts o displacement.

On the other hand, measring only

rame vibration can be insicient

or eective condition monitoring,

as the increase in rame velocity

rom incipient ailres developing

at the rnning gear or cylinder

assembly will be small and typically

covered by the larger signal that

is prodced by normal machine

movement. Experience has shown

that by the time the malnction has

been detected by the rame velocity

transdcer and the compressor sht

down, major secondary damage may

have already occrred becase o

the malnctions. These malnctions

inclde liid or debris carryover,

loose piston or piston nt, loose

crosshead nt, or loose cylinder liner,

and typically maniest themselves as

impacts transmitted at the crosshead.

Monitoring Vibration& Impact

Vibration transdcers monitoring

rotating machinery generate station-

ary signals; this means they have

constant reency content over each

revoltion o the rotor (Figre 4).

In contrast, vibration measrements

on reciprocating compressors present

both stationary and non-stationarycontent. In particlar, the signal gen-

erated by an accelerometer placed

vertically on a crosshead gide is

characteried by dierent reencies

with dierent amplitdes that occr

at speciic points in the revoltion.

Figre 5 shows a typical waveorm

rom a crosshead accelerometer.

The signal shows high amplitde,

FiGuRe 4: Example o stationary vibration sample taken at an electric motor bearing. Thehigher reency components are typical o the characteristic vibration prodced by theinteraction o the rolling elements with the bearing races.

FiGuRe 5: Timebase waveorm o a crosshead acceleration signal.


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short dration implse peaks ol-

lowed by a ring down that occrat certain parts o each crankshat

revoltion. This signal is not iltered

so the transdcer is picking p

the widest range o reencies

(typically rom 10 H to 30 kH).

These acceleration peaks can be

reerred as responses to implse

events occrring dring compressor

operation (valve opening and closing,

gas low trblence, crosshead

pin shiting at load reversal, etc.).

Sch implses excite the strctral

resonances o the machine compo-

nents - reslting in high reency

ree vibration and the characteristic

impact/ring-down proile.

As mentioned, the main sorce o

vibration on the compressor rame

is related to periodic orces. Whilethe overall rame vibration increase

is certainly a concern, the primary

interest o crosshead vibration

monitoring is detecting peaks

associated with strctre response

to implsive events. Conditions

that increase the excitation o sch

resonances are generated by develop-

ing alts sch as ractred or loose

components or excess clearance.

Loose rod nts, loose bolts, excessive

crosshead slipper clearance, worn

pins as well as liid in the process

can be detected at early stages o

development sing crosshead impact

monitoring, ths allowing appropriate

contermeasres and avoiding

potential catastrophic conseences.

O all vibration measrements that

can be applied to reciprocatingcompressors, crosshead accelera-

tion is probably the most eective

protection measrement available,

i appropriately employed.

While crosshead acceleration has

proven itsel to be a sond measre-

ment or detecting mechanical

ailres, indstry has little experience

in applying and analying it, reslting

in increased risks o alse or missed

alarms, and poor diagnostic vale

rom diagnostic systems. The ollow-

ing paragraphs describe some basic

reirements or a reliable monitoring

system and diagnostic sotware.

Reirements or

Monitoring Systems

General considerations on the

eective employment o crosshead

acceleration or monitoring and

protection are described here:

Transdcer SelectionAmplitde measrement nits shold

be generally selected based pon the

reencies o interest. For crosshead

vibration monitoring an accelerometer

shold be selected as it emphasies

the higher reency components.The nit o measrement sed shold

be the natral nits o the transdcer

sed (signal integration is not a

recommended tool or this prpose).

Transdcer MontingFreency response is sensitive

to monting technies and may

be aected by any redction o

the mechanical copling between

accelerometer and monting srace

sch as the se o an adhesive,magnetic isolation base, or non-lat

monting srace. The transdcer

shold be installed directly on the

machine strctral component to

be measred, avoiding brackets or

plates as a spport, or monting on

langes or covers. Accracy o an

accelerometer can also be aected

by grond loops, base strains, and

cable noise. These can be minimied

by ollowing the recommendations

rom transdcers and monitoring

systems manactrers as well as

applying appropriate cable tie-downs.

Signal Processing & Alarming

One o the concerns in applying

crosshead vibration measrement

or compressor shtdown is the risk

o alse alarms de to sprios peaks

in the signal. The peak detection

circit in the protection system shold

be designed to manage implsive

vibration in order to avoid nisance

alarms; this can be accomplished

by conting the nmber o readings

that exceed an alarm threshold in a

set time beore triggering an alarm.

Additionally, an appropriate time delay

needs to be conigred or the alert

and shtdown thresholds. Carel set-ting o these thresholds, conts and

alarm delays will allow s to minimie

the possibility o alse alarms. The

recip Impact/Implse channels

in the Bently Nevada* 3500/70M

monitor inclde these eatres.

Signal FilteringAnother essential aspect to care-

lly consider is signal iltering. As


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described previosly, an accelerom-

eter can detect vibration componentsp to very high reencies. While

acceleration analysis in a broad

reency range may have diagnostic

vale, the main object o crosshead

impact monitoring is protecting the

machine rom the conseences

o mechanical ailres. A signal

with too high corner reency or

the low-pass ilter may introdce

the risk o alse alarms de to the

presence o high reency content

not related to mechanical malnc-

tions (and conseent impacts

transmitted to the crosshead gide

Amplitde Measrement

Or last important note is abot

vibration measrements taken ineither root mean sare (rms), ero-

to-peak (peak or pk), or peak-to-peak

(pp) amplitde measrement systems.

A ew international standards

recommend rms measrement or

assessing machinery health based

on overall casing vibration and this

is traditionally adopted by many

practitioners. Rms vales provide an

indication o the energy content o a

signal, and or malnctions sch as

loose ondation or load nbalance,this energy content relates well with

machine condition, as well as opera-

tor perception o machine condition.

However, rms calclation applied to

an implsive reency-rich signal

sch as crosshead vibration (Figre

5) does a poor job in correlating with

other critical conditions sch as

mechanical knocks, which have rela-

tively little energy content, bt prove

vital in assessing machine condition.

For these types o malnctions,

peak amplitde measrement is

recommended as it correlates

well with both high-energy and

low-energy malnctions typical o

reciprocating compressors. Applying

rms processing to crosshead

vibration signals wold provide

nder-predicting vales.

Crank Angle Domain Analysis

When viewed in the time domain, the

non-stationary crosshead vibration

signal looks like mltiple disconnected

events (Figre 5), so diagnostic

methodologies sch as spectral

analysis provide little vale de to the

discontinos reencies involved.

The most appropriate analytic

methodology is thereore based on

signal timing; Bently Nevada 3500

monitors synchronie the vibration

signal with crankshat rotation to

associate peaks to a piston posi-

tion along the stroke. Individal

monitoring and alarming on crank

angle bands allows association

o peaks to the problem area.

For example, a peak occrring when

the piston is travelling toward the end

o its stroke near Top Dead Center

(TDC) can be correlated to liid or

debris ingression in the compression

chamber. When the piston moves

towards its TDC position, the impact

with the non-compressible material

will generate an implse event. The

monitoring system will then raise

an alarm or the corresponding

crank angle band (or example,

starting 10 degrees beore top

dead center and ending 10 degrees

ater). Figre 6 shows case o

liid ingestion as detected by the

crosshead gide accelerometer.

YEARS OF FIELD EXPERIENCE HAVE DEMONSTRATED THAT

TECHNIquES WHICH MAY BE WELL uNDERSTOOD FOR

MEASuRING AND ANALYzING THE VIBRATION OF PuRELY

ROTATING MACHINERY CAN PRODuCE CONFuSING RESuLTS

WHEN APPLIED TO RECIPROCATING MACHINERY.


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FiGuRe 6: Crosshead acceleration in crank angle domain, presenting a high peak at TopDead Center (TDC). The horiontal axis represents 360 degrees o crankshat rotation (onell revoltion), where 0 indicates TDC. The System 1 plot also displays a Throw Animation(in the pper right corner o this plot) showing the piston movement synchronied with theplot crsor. In this example the crsor is set at 2.5 degrees, and the animation shows thatthe piston is very close to the TDC position

FiGuRe 7: The 3500/70M modle retrns two waveorm samples to System 1 sotware

rom a single crosshead acceleration signal with two dierent fltering characteristics.

understanding Freency

ContentAdditional advanced analysis tools

are available in System 1* diagnostic

sotware. As noted beore, not all

implse response events within the

crosshead accelerometer signal

contain the same reencies.

Mechanical knocks excite resonances

o the reciprocating compressor

components sch as crosshead

gides, distance pieces, etc. thatgenerally lie below 2 kH. In contrast,

events originating in gas low noise,

valve opening or valve closing events

express a mch higher reency.

Searching or a mechanical event in

an acceleration signal that contains

the whole transdcer reency

response range is practically impos-

sible de to the high amplitde and

reency peaks that cover smaller,

yet more critical, peaks related to

mechanical events. Sch overlap

prevents early indication o an incipi-

ent malnction. It is or this reason

the signal mst be iltered. Figre

7 shows crosshead acceleration in

the crank angle domain sing 3 to

30 kH (let plot) and 3 to 2 kH (right

plot) band pass iltering. The peaks

present in the narrower pass-bandcorrespond to mechanical impacts,

which are diiclt to distingish in

the signal with broader ilter corners.

System 1 sotware is integrated with

the 3500/70M monitor to allow dal

signal processing and both storing

and displaying the accelerometer

signal with two dierent ilter settings.

0 ORBIT Vol .2 No.2 Apr.2012

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Diagnostic Approach

To wrap p this irst installment, let s

consider how we can eectively asso-

ciate a malnction to a speciic vibra-

tion pattern and to obtain an early

ailre diagnostic. Experience has

shown that associating vibration with

additional measred dynamic param-

eters sch as rod load have proven

to be o great vale in pinpointing a

speciic component ailre. Details o

these other dynamic parameters willbe presented in ollowing Orbit isses.

De to the complexity o the signal

content and the vibration signatres

that dier rom case to case based

on operating conditions and ailre

modes, several dierent atomated

diagnostic approaches have been

developed. This incldes rle-based

and model-based approaches

that are driven by data or by irst

principles o Physics relationships.

Each approach presents pros and

cons and will be rther discssed

in ollowing isses as well.

Reerences1. GE Energy Brochre, Condition

Monitoring Soltions or Reciprocating

Compressors, GEA-14927

*denotes a trademark o Bently Nevada, Inc.,

a wholly-owned sbsidiary o General

Electric Company.

Copyright 2012 General Electric

Company. Al l rights reserved.

expeRience has shown

ThaT associaTinG vibRaTion

wiTh addiTionaL MeasuReddynaMic paRaMeTeRs such

as Rod Load have pRoven

To be oF GReaT vaLue in

pinpoinTinG a speciFic

coMponenT FaiLuRe.

Apr.2012 No.2 Vol .2 ORBIT1

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