december 2009 vibration

8/12/2019 December 2009 vibration

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Dynaflow Lectures Reciprocating compressors

Acoustics and Mechanical Response

Rotterdam, December 10th 2009


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2

EXAMPLECompressor piping vibration analysis

Two parts:

1. Acoustical/pulsation study

2. Mechanical response analysis

Labor intensive modeling

Large number of load cases.

Copyright 2009 Dynaflow Research Group BV


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3

Sequence of dependence

Acoustics is about propagation of

pressure pulsations in piping systems

Source of Pressure pulsations:

Reciprocating compressors and pumps

Pressure waves are propagated thru the

piping system.

Pressure waves are reflected (partly) and

transmitted (partly) at geometrical

discontinuities

Pressure pulsations generate unbalancedforces that are the source of piping

vibration

Sustained vibration may result in fatigue

failures



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Elements of Acoustics

Aspects of Mechanical Response

Examples of Mechanical Response

Agenda



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Reciprocating compressors and pumps inherently

produce pulsations in the suction and discharge piping

5

Double acting cylinder:

Piston displacement opens and closes

suction and discharge valves



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Actual Piston movement (not purely sinusoidal) due to

finite rod length

Copyright 2009 Dynaflow Research Group BV 7


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Valve openings result in a Sawtooth type of gas flowDue the sequence of piston movement and valve opening and closing

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The shape of the sawtooth is determined by the rotational speed of thecompressor, the geometry of the cylinder and the pressure ratio.



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Flow time history for a single acting cylinderWith ideal instantaneous reacting valves



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Resulting Flow Frequency Spectrum (discrete) for single

acting cylinder



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Double acting cylinder (slightly unsymmetrical)Head end cranck end because of the piston rod volume



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Flow pulsations result in pressure pulsations

Pressure pulsations propagate thru the piping system at the speed of sound

Speed of sound depends on:

Gas composition

Gas Temperature

Gas Density

Pressure/Flow pulsations reflect at geometrical discont inuities

Wave length of propagating wave depending on speed of sound and pulsation frequency

Wave reflection and wave interaction results in system acoustical natural frequencies.e.g. for wave length/frequency that match a geometrical length scale standing waves may

occur

Presence of Acoust ical natural frequencies may result in Acoustical resonance

System wil l show an acoustical response to an acoust ical excitation

f

c=



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Example of acoustical natural frequency result



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Limited accuracy of acoustical model

Accuracy of prediction of acoustical natural frequencies relatively large

Error margin relatively small: +/- 5%

Errors controlled by limited number of parameters:

Geometry

Speed of sound

Compressor RPM



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Guidelines for acoustical pulsation levels according API618

Guidelines for acceptable pulsation levels.

Acceptable levels are related to (inversely proportional to) frequency, pipe

diameter and (proportional to) average pressure level

Measures to control pulsation levels:

Geometry changes (controlling acoustical natural frequencies)

Changing pipe diameters to reduce pulsation level

Introduction of damping (orifice plates at location of max oscillating flow)

Introduction of additional volumes with or without internals (creating filters)

Increasing size of bottles (windkessel function).



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EXAMPLEPulsation Bottles located near the compressor

Two bottles per

compressor

Multiple pistons per

compressor

Inlet scrubbers

COMPRESSORS



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Guidelines for Pulsation Bottle sizing

1. SINGLE CYLINDER BOTTLE 2. MULTICYLINDER BOTTLE



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Acoustical filters

Volumes connected by choke tubes

Filter frequency fh:

Filter frequency response



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Elements of Acoustics

Mechanical Response

Example of Mechanical Response

Agenda



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Mechanical response calculation fifth edition of API 618

Guidelines for pulsation levels.

If pulsation levels exceed guidelines system may be qualified by means of

mechanical response analysis.

Vibration control by mechanical means is a possible option

Large uncertainty margin in mechanics during design (minimum 10-20%)

Acoustic is more accurate (typically +/- 5%)

Preference for reduction of pulsations and thereby shaking forces by

means of acoustical measures e.g. filtering (e.g. Helmholtz resonator)



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Accuracy of prediction of mechanical natural frequenciesError margin: 10-20% or many time even larger

Modeling of Boundary conditions

Modeling of rack structures

Support clearance

Support lift off (thermal), support settling

Support stiffness i.e. stiffness of clamps and restraints

Influence of friction

Nonlinear supports (supports with gaps or single acting supports)

Uncertainties in masses

Differences between as built and design

Interaction between parallel pipes in pipe racks

Stiffness of concrete sleepers and pedestalsCopyright 2009 Dynaflow Research Group BV 24


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Many vibration problems related to attached components

Examples:

Valve Actuators Small bore branch connections

Instrument connections

Level indicators

Stairs & Ladders



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Mechanical properties and pulsations

Rule of thumb: minimum mechanical natural frequency 20% above

second compressor harmonic. Question: is this feasible???



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Mechanical properties and pulsations (2)

Mechanical resonance difficult to avoid due to uncertainty in mechanical nat. freq..

Variable speed compressor makes separation virtually impossible.

At resonance condition amplitude limited by damping only (typical damping factors

of 2%-3% of critical)

High sti ffness results in lower amplitudes.



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Application of filters in combination with high

mechanical natural frequencies looks ideal



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Acoustics

Mechanical Response

Example of Mechanical Response analysis in design

Agenda



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EXAMPLE

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Example: Mechanical Response of NAM Oude Pekela

Compressor plantAir cooler A-174



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Focal area

EXAMPLEAcoustical results of suction piping

31Copyright 2009 Dynaflow Research Group BV


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Nodal correspondence:

3360-3430 = C2 node 1085

3350-3360 = C2 node 10703000-3350 = C2 node 1033

EXAMPLE

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Unbalanced shaking forces in [kN peak to peak] per

pipe section and per compressor harmonic



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Nodal correspondence:3330-3340 = C2 node 5060

3340-3350 = C2 node 5076

3380-3350 = C2 node 5097

EXAMPLEUnbalanced shaking forces in [kN peak to peak] per

pipe section and per compressor harmonic



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Summary of shaking forcesConservative selection: maximum value of all harmonics

Acoustic pipe

sectionCaesar II node number Force [N.] [peak-peak] Force [N.] [0-peak]

3330-3340 5060 131 65.5

3340-3350 5076 355 177.5

3350-3380 5097 815 407.5

3360-3430 1085 535 267.5

3350-3360 1070 240 120

3000-3350 1033 81 40.5

EXAMPLE



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EXAMPLE

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Two-stage compression combined modelSuction (partly), Interstage (upto cooler), Discharge (complete)

Aircooler E-174

nozzles

Suction LineDischarge Line

Additional discharge volumes

to reduce pulsation levels in

remaining piping

Compressor discharge bottles

Interstage Line



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Additional discharge volumesEXAMPLE


EXAMPLE


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EXAMPLEHarmonic frequency assessment in CAESAR IISweep from 4 -56 Hz with 1 Hz steps


EXAMPLE


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Harmonic forces are inserted in the modelConservative Shaking force set taken from acoustic pulsation report

EXAMPLE


EXAMPLE


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EXAMPLEMaximum dynamic stress amplitude calculationMax amplitude 6 MPa


EXAMPLE


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Carbon Steel Fatigue Curve in the high cycle range

EXAMPLEAt a stress amplitude level of 6 MPa the number of

cycles is > 1011

41

6 MPa


A d


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Acoustics

Mechanical Response

Example of Mechanical Response analysis as built

Agenda


EXAMPLE


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1. Vibration Measurements: identification of main contributions in frequency

domain

2. Acoustical Resonance: verification of acoustical natural frequencies

3. Mechanical Resonance: verification of mechanical natural frequencies

4. Identification of source of vibration problem

5. Modification proposal

EXAMPLEIssue: Unacceptably high vibration level in compressor

suction pipingIn 5 steps to solution


C l t


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Compressor plant


Structure and support details around the compressor (I)


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Structure and support details around the compressor (I)

Copyright 2009 Dynaflow Research Group BV45

St t d t d t il d th (II)


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Structure and support details around the compressor (II)


D t il f th l ti


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Details of the compressor location


EXAMPLEStep 1 Vibration Measurements


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0.00

20.00

40.00

60.00

80.00

100.00

120.00

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.

Frequency (Hz)

Amplitude(dB)

66 Hz49 Hz

33 Hz

16 Hz

99 Hz

83 Hz

Step 1. Vibration Measurements


Intermediate conclusion from step 1


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Vibrations are at compressor harmonics

Vibrations must be result of

Acoustical resonance

or

Mechanical resonance

or

High pulsation forces without resonance (compressor bottle sizing

problem)


1

2

3

EXAMPLEStep 2 Acoustical natural frequencies & Compressor


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50

0

50

100

150

200

250

10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00

Frequency (Hz)

Am

plitude

16 Hz

Step 2. Acoustical natural frequencies & Compressor

Harmonics


EXAMPLEIntermediate conclusion from step 2


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Maybe near-to-resonance condition at first compressor harmonic (16.5 Hz.)

No further acoustical resonance

Vibration peak at 16.5 Hz, most probably is due high shaking forces as a

result of near resonant condition

The other vibration peaks must be the result of:

Mechanical resonance

or

High pulsation forces without resonance (compressor bottle sizing

problem)


1

2


EXAMPLE

Step 3 Vibration Measurements & Calculated Mechanical


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0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

100.00

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0

Frequency (Hz)

Amplitude(dB)

66 Hz

33 Hz83 Hz

Step 3. Vibration Measurements & Calculated Mechanical

Natural Frequencies (Search for Mechanical Resonance)

Purple vertical lines represent pipe system natural frequencies


Conclusion from step 3 & Identification of cause of


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Conclusion from step 3 & Identification of cause of

vibration problem

Apparently there is mechanical resonance at 33 Hz and 66 Hz and nearmechanical resonance at 83 Hz

No mechanical resonance condition at the first compressor harmonic(16.5 Hz.) and at 49 Hz. and 99 Hz

The high vibration levels 33 Hz, 66 Hz and 83 Hz are of mechanical

nature

The high vibration level at 16.5 Hz most probably is an acousticalresonance problem

The high vibration level at 49 Hz and 99 Hz. must be the result of Highpulsation forces without resonance (compressor bottle sizing problem)


EXAMPLEStep 4. Identification of cause of vibration problem


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The high vibration level at 16.5 Hz most probably is an acousticalresonance problem.

Apparently there is mechanical resonance at 33 Hz and 66 Hz and nearmechanical resonance at 83 Hz.

The high vibration levels 33 Hz, 66 Hz and 83 Hz are of mechanical nature

No mechanical resonance condition at the first compressor harmonic (16.5Hz.) and at 49 Hz. and 99 Hz.

The high vibration level at 49 Hz and 99 Hz. must be the result of: High pulsation forces without resonance (compressor bottle sizing

problem)

Step 4. Identification of cause of vibration problem


EXAMPLEExamination of mechanical behavior


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Large amplitude movement

in suction manifold

Example of 66 Hz. mode shape


EXAMPLEStep 5. Modifications


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1. The high vibration levels 33 Hz, 66 Hz and 83 Hz are of mechanical nature

and need a mechanical solution

Better supporting Improved support stiffness

2. The high vibration level at 16.5 Hz is due to acoustical resonance and

needs an acoustical solution, I.e. different bottles and/or orifice plates tointroduce more damping

3. The high vibration level at 49 Hz and 99 Hz. are the result of high pulsation

forces without resonance and must be resolved by compressor bottle(re)sizing.

p


EXAMPLEModified structure implemented and connected to


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p

attached piping

AS BUILT SITUATION IMPROVED AND IMPLEMENTED SITUATION


EXAMPLEConclusion from example


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Compressor vibration problems many cases are of a mixed nature

Part is mechanical

Part is acoustical

Each category requires a different approach and result in differentsolutions

Not all vibration problems can be solved by mechanical measures.



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END


december 2009 vibration

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