engine vibration analysis guidelines_inprogress

7/26/2019 Engine Vibration Analysis Guidelines_InProgress

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SACSTM – ENGINE VIBRATION ANALYSIS

The goal of the Engine Vibration Analysis is to determine the joint

displacement due to unbalanced forces. Joint displacements are

compared versus various allowable deection specications and

epressed as displacement unity chec! ratios.

Basic Knowledge"efore starting the #ynamic analysis try to get familiar with the

following terminology$%. &hy 'nbalanced forces are present in the (ompressor

). 'nderstand *as Tor+ue ,oading (urves-. ass oment of /nertia0. 1etain #egrees of 2reedom3. ass participation factor4. ode 5hapes6. 1elation 7 2re+uency 8(95:; Time 9eriod 8sec.:; Engine 5peed 819:<. Actual #eections are compared with the Allowable #eections for

the 2ollowing 5pecications$

• 5=AE 8society of =aval Architect and arine Engineers:

• ilitary 5pecications

• #,/=E

>. ?armonic =umber

%@.(onvert Vibration ,evels$ /,5 1 /=(?E5B5E( T /=(?E5%%.=atural 2re+uency of the structure 8*lobal C local:

The fre+uency associated with the mode % is *lobal natural

fre+uency of the structure. The local fre+uency is determined by

eamining the response of structure at each mode. The fre+uency

of a structure at a mode where the interested8point of interest:

responses is the natural fre+uency of the structure. i.e if a

beamBbeams or any structural component responses 8deects: at

mode %3; then the fre+uency at the mode %3 is the natural

fre+uency of the aceess platform or beamBbeams. The local

fre+uency cannot be deretermined accurately unless the structureis modelled very accurately..



Infora!ion Needed !o "erfor Engine Vi#ra!ion Anal$sis in

SACSTM

%. Engine 5!id #rawings). perating &eights and (orresponding (.* location of (ompressor

and Engine-. 'nbalanced 2orces C (ouples and *as Tor+ue ,oading of

(ompressor 85ee 5ample data:0. =eed information on all the eisiting s!ids present on the dec!3. =eed perating weight of all the eisting E+uipment including (.*

location

/t is always important to capture all the data accurately including

the s!id beams; stiDeners The error in engine vibration anlaysis

can vary from 3 to -3 F depending on the accuracy of the model

and other modelling techni+ues.

SACSTM Engine %i#ra!ion anal$sis is "erfored in following

s!e" "rocess&%. 5et up the basic 5A(5 model with appropriate loads and retain

degrees of freedom). 5et up the dynpac le to etract mode shapes and mass les-. Etract the mode shapes and mass les0. 5et up the #ynamic 1esponse input le3. 1un the #ynamicBEngine vibration analysis4. (hec! the deection unity ratio or go through the plots

're"aring SACS Model for ($naic Anal$sis

The accuracy of SACS model is very critical in Engine

Vibration/Dynamic analysis. The results may vary approx. ! to " !

depending on the accuracy of the model

%. odel the compressor s!id beams 8complete s!id pac!age:

including the bearing pads if present.). odel the structure accurately including all the eisting s!id beams.

Apply the e+uipment weights at true Vertical (..* location.-. odel all the access platforms including the stair landings

0. (onnects all the joints together if itGs a plated dec!. 9ay attention

while meshing the plates. 2or plated dec!s connect each and every

single joint with the plate to account for the stiDeness.3. #elete the Jac!et i.e the topsides should be ade+uate for engine

vibration study



4. ,ocate the (.* of the compressor and the engine and connect the

c.g. of compressor and engine to the s!id beams with stiD members

at respective location. 85ee 2igure HH.:6. (onnect the (.* of Engine and (ompressor with horiIontal member

8(ran! 5haft:.

<. 1elease the moments at the engine end so that all shear transfersfrom compressor to engine.

>. 9repare the 5A(5 model for the #ynamic analysis.%@.&or! from top dec! level to the bottom level retaining the degrees

of freedom 8)))@@@: based on the load path.%%.(onsider the boundary joints to be pinned 81eason chec! with

*avin:%).Environmental loads are not considered

E)!rac! Mode S*a"es

+iles Needed,& SACS Model +ile-& SEATATE +ile .O"!ional/0& ($n"ac In"1! +ile .(YNIN'&NAME/

($n"ac In"1! +ile2 'sed to etract mode shape le and #ynamicmass le; which are need for the #ynamic response analysis.

%. Etract sucient number of modes to obtain fre+uency greater

than 0 times the engine running speed 8E$ %0@@19 K 0%0@@B4@

K >-.- cps; therefore etract mode shapes such that fre+uency

greater than >-.- cps:.). (alculate the mass moment of inertia of compressor and engine

assuming it to be a bo or prism.-. Apply the weight of compressor and engine as joint weights in all

directions 8H;L;M: along with the mass moment of inertia calculatedabove

0. (hec! the structural response at various mode shapes in order to

determine the any errorsBdisconnected joints in the model3. a!e sure that the mass participation is above >@ F 8min <@ F:.

1etained degrees of freedom directly aDects the mass participation

factor.



4. a!e sure that there is no negative massBloading or any other

warnings. 2i any !ind of warnings or errors..

Na!1ral fre31enc$ of #eas fro SACS and *and

calc1la!ions 1sing Roar45s can #e %eri6ed if we e)cl1de !*e

o"!ion 7incl1de s*ear defora!ions8 in SACSMoni!or !*e s!r1c!1ral res"onse a! di9eren! odes !o see

!*e a)is a#o1! w*ic* i! res"onses& :se !*a! a)is iner!ia in

;and calc1la!ions<

Vi#ra!ion Anal$sis

ass 9articipation factor$ 5hould be nearly >@F. 1etained degrees of

freedom aDect the mass participation factor.

engine vibration analysis guidelines_inprogress

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