introduction to vibration problems at compressor stations

8/13/2019 Introduction to Vibration Problems at Compressor Stations

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Introduction to Vibration Problemsat Compressor Stations

Presented by:

Gary Maxwell, Chris Harper, Shelley Greenfield(Beta Machinery Analysis)


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Welcome

Purpose: Introduction to compressor vibration(for more detail, recommend the 2.5 day GMRC Course in May)

Focus on practical issues.

Audience participationdemos, case studies,questions, etc.

(We cant take you to the field, so we are bringing the field to the classroom)

Presenters introductions

Questions for the parking lot?


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Vibration Induced Pipe Fatigue Failure

/

, , ,


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Todays Topics

1. Vibration Overview

2. Sources of Vibration

3. Pulsation Control4. Mechanical Resonance

5. Torsional Analysis

6. Pipe Strain7. Small Bore Piping

8. Start-up Vibration Survey

9. Summary


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1. Vibration Overview

Presented by: Chris Harper


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How Equipment Fails

Vibration is the leading cause of mechanicalproblems

Equipment and piping fail due to excessiveSTRESS (fatigue failure)

Pulsation Forces Vibration Stress Failure


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What is vibration?

Vibration = periodic motion about anequilibrium position

Vibration can be described with: Amplitude and

Frequency (number of

cycles per time) or Period (time to

complete one cycle)


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Vibration amplitude

Three related units

Displacement

(m, mils) Velocity

(mm/s, in/s or

ips) Acceleration

(mm/s2, in/s2,gs)

Only related whenvibration is simple,like in a spring-

mass system


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Two Ways to Look at Vibration

Frequency-domain

Individual vibration

Time-domain

Overall vibrationUnits = seconds

Units = Hz

Time domain amplitudetypically higher

than frequencydomain amplitude


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Time domain frequency domain


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Another way to visualize it

Time domain and frequency domain show thesame information, just in different ways

Frequency domain breaks out thecomponents of the time domain

Time domain is

measured Frequency domain

is calculated

O ll ti d i ib ti


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Overall time-domain vibration -terminology

pe

ak

Peak-to-peak

RMS p

eak

Peak-to-peak

RMS

Peak (measure of deflection) is used more

often than RMS (measure of energy) Frequency domain is either peak or RMS (not

peak-to-peak)


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Vibration Directions (common terminology)

Axial: along crankshaft

Horizontal: direction

of piston motion

Vertical


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Video #1 Vibration Equation


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Demo #1 Scrubber Vibration


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BETA guidelines - velocity

Dashed linesadapted from

SwRI Piping guideline

also applicable forvessels, and forsmall borepiping ( 2 NPS)

At individual

frequencies, notoverall (time-domain) vibration


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Comparison

Many differentvibration guidelines

Remember thanvibrations overguideline mean

moreinvestigationneeded

Use 1 ips (FD) or1.5 ips (TD) as ascreening guideline

for piping18


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2. Sources of Vibration

Presented by: Shelley Greenfield


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Vibration Risk Areas

TorsionalPulsation(Acoustics)

Mechanical

Skid & Foundation (Dynamics)

Small Bore


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Risk Areas and Design Considerations

Off-skid Pulsations

Thermal Expansion:Piping Layout and

Supports

InteractionBetween Other Units


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Risk Areas and Design Considerations

Off-skidPulsations

Thermal Expansion;Piping Layout and Supports

System PressureDrop (performanceissue, losses)


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Responsibility

Pulsations and thermal growth crossboundaries of responsibility

Vibration consultant hired by packager

may be acceptable for small gatheringsystems

good specificationsand communication

Owner

Engineering firm

Packager

Vibration consultant

Large critical pipeline,storage, oroffshore units -recommend vibrationconsultant hired byowner

Dynamic force on


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Dynamic force onreciprocating compressor

Unbalanced Forces and Moments

due to Reciprocating MotionPulsation Shaking

Forces in Piping

Gas Forces

(Cylinder

Stretch)

Crosshead Guide

Forces


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Forces occur at multiples of runspeed

1x Compressor primary forces & moments

Cylinder gas forces (rod load)

Pulsation shaking forces (single-acting)

2x Compressor secondary forces & moment

Crosshead guide forcesCylinder gas forces

Pulsation shaking forces (double-acting)

3x Cylinder gas forces

Pulsation shaking forces


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/2

(0.605 ) 7042 (7001200

)

880 1000 , 1058

1270

, , 1

How High Can Pulsation Forces Get?

Cooler Nozzle Failure

Pulsation Shaking Forces Can Be Very


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Pulsation Shaking Forces Can Be VeryHigh

Original

Bottles

Guesses as to how high forcecould be in this run of piping?

No acoustical study had been performed

To solve problem, Beta conducted acoustical studyand recommended new bottles

6 pipe - area = 26 in2


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As Found Unbalanced Forces

11000 lbf pk-pk at 38 Hz

What speed?

Vertical

Riser toCooler

38 Hz x 60 s/min 2

= 1140 RPM


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Gas Forces Cause Cylinder Motion

Act on cylinder, bottles, scrubber and piping

Create high vibrations around compressor

:


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Vibration Risk: Compressor APPLICATION

Lower Risk Vib. Risk Factors Higher Risk

1 # of Units Online Many

Convenient LocationOffshore or

Remote

Not Unit CriticalityCritical to the

Process

NotImportant

Efficiency Important


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Vibration Risk: Compressor CONFIGURATION

Lower Risk Vib. Risk Factors Higher Risk

Sweet Gas Composition Sour, Heavy

1 Step, DA Load Steps DA + SA(>50% turndown)

Fixed Suct./Disc. Pressure Wide range;

Fixed Speed Wide Range

2 stg (4 or6 cylinder)

Compressor Stages 1 stg (manycylinders)

CR > 1.7 Compression Ratio < 1.3

< 150 HP/ Cylinder > 750

Vibration Study Scope


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CompressorPackage

Vibration Study Scope

/

,

Small Bore

Piping

Foundation& Structure

Off-Skid PipingVibration

& ()

/


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3. Pulsation Control


l i i i


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Pulsation animation

Pulsations in non-flowing gas

Notice change in pressure and velocity

Vid #2 P l ti d Oth F


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Video #2 - Pulsations and Other Forces

P l ti F I Pi i S t


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Pulsation Forces In Piping System

Example: Interstage System

Discharge

Piping

Suction

Piping

Cooler

P l ti F DA SA


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Pulsation Forces DA vs. SA

Cylinder vertical forces


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Cylinder vertical forces

Can be significant

Pulsations controlled

with orificeplates

Vibration controlled

with outboardsupports


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Compressor Vibration

Case Study:

C I t ll d Vib ti P bl


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Compressors Installed Vibration Problem

6 1700

Vibration Problems


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( )

Customer tried to fix problem no success

Units not fully operational very expensive

called BETA for help

Vibration Problems

Example: Piping to Cooler (Riser)


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Example: Piping to Cooler (Riser)

:

> 3

( )

( )

Other Problem Locations(U b l d F lbf k k)


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(Unbalanced Forces, lbf pk-pk)

&

Recommendations


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New Bottles(Suction; Discharge)

Modify Piping andSupports (includingoff-skid)

Recommendations Implemented


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:

Case Summary


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Case Summary

Vibration problems are expensive

Small errors during design stage are avoidable

Illustrates how vibration analysis techniquesused to solve or prevent problems (comparedto trial and error fixes)

What Happens to Pulsations ifOperating Envelope Changes?


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Operating Envelope Changes?

ACCEPTABLE BottleShaking Forces

Design Change: Increased #of Load Steps and Ps Range

Bottle Shaking Forces >200% of

Guideline. High Risk of VibrationProblem

Initial Operating Points


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Pulsation mitigation


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g

Surge volumes and resistive elements (orificeplates) are simple but can be costly (capital

and pressure drop)

Acoustic filtering offers much more pulsationcontrol with some capital cost but very littlepressure drop

Factors affecting pulsation mitigation


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g p g

Speedrange

Valveunloaders

Difficultycontrollingpulsations

Difficultycontrolledvibration

Fixed Very low Low

Narrow Low Medium

Wide Medium High

Fixed Medium Low

Narrow High Medium

Wide Very high High

For example, fixed speed =1200rpm, narrow speed range = 900 - 1200rpm,wide speed range = 600 - 1200rpm

Optimizing Pulsation Control


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p g

Case study - Impact of off-skid piping


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y p p p g

Case study:

One stage, two-throw Dresser-Rand 5BVIP2

1200 RPM, gas speed of sound 1200 ft/s

Both single-acting (SA) and double-acting (DA)

Off-skid piping comes several weeks after pulsationstudy was completed - two units with two coolers

Stages of analysis:

Bottle sizing with a damper check

On-skid design with infinite pipe termination

Off-skid piping added

On-skid design is volume-choke filter

Piping layout


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Damper Check Piping LayoutOn-Skid Piping Layout

Off-Skid Piping Layout

Pulsations:

- Cylinder nozzle

- Bottle outlet nozzle

- Skid edge

Shaking Forces:

- Cylinder

- Bottle

- Crossover piping

Pulsations - discharge nozzle


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0

5

10

15

20

25

Pulsations

,psipk-pk

1x, SA 1x, DA 2x, SA 2x, DA

Damper Check

On-SkidOff-Skid

Shaking force - crossover piping


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0

50100

150

200

250

300

350

400

450

Sha

king

Forc

es,

lbfpk-pk

2x, SA 2x, DA

On-Skid

Off-Skid

What was the difference?


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Hint: the length betweenthe discharge bottle and thecooler header is 15 feet

Half-wave between bottleand cooler header box

volume amplified pulsations

Multiple compressors beat frequency


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Animation courtesy of Dr. Dan Russell, Kettering University

Unit A

Unit B

Unit A

Unit B

Combined Pulsations

Unit A and B run at slightly different speeds

Because of this, the pulsations go in andout of phase

Total pulsation amplitude is sumof pulsations from each unit

Beat frequency is related to thespeed differential between Unit A

and B

Summary


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Shaking forces are more important to controlthan pulsations

Acoustic filters are more effective than orificeplates for controlling pulsations

More pressure drop is required to filter

pulsations when wider speed ranges are usedor unit single-acting

The more information included in a pulsation

study improves accuracy and reduces risk


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4. Mechanical Resonance


Summary


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Example of Mechanical Analysis Model


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Mechanical Analysis - MNFs


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Frequencies wheresmall forces result

in large vibrationresponse ofstructure

Modal Analysis


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Finite Element Analysis(FEA) used to calculateMechanical Natural

Frequencies (MNFs) Elastic Modulus

Geometry

Density

Measure MNFs with Bump

Test

Demo #2 Mechanical Natural Frequency


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Mechanical Resonance


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|

6x

|

1x

|

2x

|

3x

|

4x

|

5x

Frequency

Forces

MNFs

We define resonance when force frequency is +/- 10% of MNFAt resonance, displacement can be magnified by 40 times can cause fatigue

failure

What happens at 3X? What about 4X? 6X? Potential resonance,but insufficient forceto cause problems

Change design to shiftMNF away from resonance

Mechanical Analysis Design Goal


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MNF

|

1x

|

2x

|

3x

|

4x

Forces

API 618 Design GoalMNF > 2.4 x

Wide speed range


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MNF

Frequency avoidance becomes challenging asspeed range is increased

Blocking out speeds may help avoidresonance

|

1x

|

2xFrequency (orders of run speed)

Speed of

driver

Magnitude

of Force

1200 rpm

700 rpm

No room forMNF to hide

MNFs of Main Components in Relation toCompressor Harmonics


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60 Hz40 Hz 50 Hz

ScrubberMNFs:

15-30 Hz Typ.

Cylinder MNFs:

30-50 Hz Typ.

Bottle MNFs:40-70 Hz Typ.

70 Hz20 Hz

Example: Scrubber Design

Move MNF to Higher Frequency

= Extra costs; design modification

2.4 X 900 RPM 2.4 X 1200 RPM


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Case Study 3rd Stage MNF (API 618 Step3a)


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Case Study 3rd Stage MNF


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Case Study Cylinder Gas Loads at 3x?


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TABLE L.2 - Cylinder Gas Forces (kips,0-Pk) in Horizontal direction

Unit: Ariel KBZ/6

STAGE#3 CYLINDER# 2

COND# 01X 02X 03X 04X 05X 06X 07X 08X 09X 10X

1 75.0 5.7 4.3 0.7 3.4 1.2 0.7 0.8 0.9 0.8

2 69.6 5.5 6.5 1.1 2.9 1.2 0.7 0.9 0.9 0.8

3 67.8 5.4 7.0 1.2 2.7 1.2 0.9 0.9 0.8 0.8

4 65.8 5.3 7.5 1.3 2.5 1.1 1.0 1.0 0.7 0.8

5 49.1 4.5 5.2 4.1 1.6 1.1 0.4 0.4 0.2 0.6

6 48.4 4.3 5.5 4.1 1.6 1.2 0.4 0.3 0.3 0.5

Therefore 7500 lbs (0-pk) at 3x compressor run speed.

(Weight of large SUV fully reversing 43.5 times per second!)

Causes cylinder stretch

Case Study Forced Response Analysis(API 618 Step 3b1)


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Case Study 3rd Stage MNF, with LWN


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Case Study 3rd Stage MNF, with LWN


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Case Study Cylinder Gas Loads at 4x?


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TABLE L.2 - Cylinder Gas Forces (kips,0-Pk) in Horizontal direction

Unit: Ariel KBZ/6

STAGE#3 CYLINDER# 2

COND# 01X 02X 03X 04X 05X 06X 07X 08X 09X 10X

1 75.0 5.7 4.3 0.7 3.4 1.2 0.7 0.8 0.9 0.8

2 69.6 5.5 6.5 1.1 2.9 1.2 0.7 0.9 0.9 0.8

3 67.8 5.4 7.0 1.2 2.7 1.2 0.9 0.9 0.8 0.8

4 65.8 5.3 7.5 1.3 2.5 1.1 1.0 1.0 0.7 0.8

5 49.1 4.5 5.2 4.1 1.6 1.1 0.4 0.4 0.2 0.66 48.4 4.3 5.5 4.1 1.6 1.2 0.4 0.3 0.3 0.5

Gas Loads are less at 4x compressor run speed than at 3x

Case Study Forced Response Analysis


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Conflict Between Thermal and Dynamic Study


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Thermal solution has large distance betweenclamps, thermal loops, and resting supports

Dynamic solution has short distance betweenclamps and avoids elbows

Balanced solution has clamps

and thermal loops API 618 recommends same

company conduct

both studies

Solutions - Scrubber Bracing


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Increase MNF of scrubbersto guideline levels, orinter-tune if possible

May be required on somehigh RPM compressors

Scrubber attachmentsmore likely to fail


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5. Torsional Vibration


Torsional Vibration Crank Failures


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Torsional Vibration Coupling Failures


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Purpose of Torsional Analysis


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To predict excessive vibratory stress or amplitudeproblems in driveline of driver / coupling /compressor

Potential Torsional Problems Compressor/Engine Crankshaft failure Motor Shaft Failures or Spider Failure (welded joints) Coupling Failure (Disk Pack, Rubber, Other) Damper/Coupling Heat Loads Compressor auxiliary drive amplitudes Engine Free End Amplitudes (Gear Problems) Motor Free End Amplitudes (Fan) Current Pulsation

Torsional Vibration - Applications


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A TVA should be done for: Any new driver or compressor combination Any change in compressor configuration (different cylinders) Different motor (same frame rarely means same rotor inside) Different operating conditions (than what was originally studied) Drive trains experiencing failures VFD applications Critical applications

Risk Chart May help to determine if a Study is requiredhttp://www.betamachinery.com/uploadedFiles/001_-_Design_Services/001_-_Reciprocating_Compressors/Recip_RISK_Chart_Vibration_Control_3.

1.xls

Thorough Checks Required

A l f ll i PLUS di i


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Analyze full operating map PLUS upset conditions Include tolerance band to consider fabrication and

installation uncertainty

Motor stub shaft to be thesame diameter as thecompressor stub

Risk of Failureat somepressures andspeeds


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6. Pipe Strain


Pipe Strain

S l t j b h


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Several recent jobs wherewe encountered unexplainedhigh frequency vibrations

and failures

Isolated the cause as pipestrain

Effects

Pi t i


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Pipe strain can:

Increase natural frequencies (like aguitar string)

Reduce damping (high frequencyvibrations increase)

Increase mean stress in system (makingit more likely to fail due to vibrations)

Contributing Factors

Mi li d fl


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Misaligned flanges

Gaps between pipe and support

Flange Misalignment

ASME B31 3 offers guidance for flange


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ASME B31.3 offers guidance for flangealignment

Solutions

Custom or modified spool pieces orifice


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Custom or modified spool pieces, orificeplates, etc.

Shim between piping and supports, ratherthan just tightening clamp bolts

Post-weld heat treating (e.g., vessel nozzles)

Designing more flexibility into system

Small details are

important!


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7. Small Bore Piping Vibration


Small Bore Piping - Introduction


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2 (50)

(, , ),

, , , .

, , .

Demo #3 Small Bore Piping


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Video #4 Small Bore Piping Vibration


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Why is SBP a High Risk Problem?

Small bore piping is often overlooked:


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Small bore piping is often overlooked:

May not be explicitly designed - fieldinstalled

Not shown on compressor package GAs

Not included in typical pulsation/vibration

study Difficult to measure properly in the field

Failure can lead to significant downtime

Field Measurements

Measure Relative Vibration


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Measure Relative Vibration,if required

Steady State (Running)

Transient (Start-up)

Further check/investigation ifexceeds screening guideline

Assessment Methods

Energy Institute


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Energy Institute

Need dynamic force &poor design & poorlocation = high likelihood of failure

GMRC

Tables of lengths

and weights FEA

Calculate allowable

vibration before failure


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8. Start-up Vibration Survey



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Typical vibration measurement points

Scrubber: Top seam


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Bottle:Both ends of bottle (seam); sometimesmiddle

Cylinder: Cylinder head end

Compressor frame& engine:

Crank height drive andnon-drive ends

Pipe: Elbows, between supports

PSV: Top of valve body

Main skid: Front and rear corners

Small Bore Piping: End of cantilever; between supports

Plus other points if vibrations at above points are suspect!

Not all vibrations are alike

Be clear what is being measured and what


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gguideline is being applied

Overall vs. individual frequencies

Units: mm/s vs. inches/second

Peak or RMS (or pseudo RMS)

Frequency range Apply appropriate guidelines (time-domain

vs. frequency domain guidelines)

When do I call an expert?

Basic repairs/modifications do not work


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p /

Try temporary bracing first

Very high vibration levels

Vibrations are high in multiple areas

Vibrations are high for multiple operating

conditions Suspect pulsations are high

High vibrations away from compressor

Need help measuring or interpreting data

Solutions

Vibration = Dynamic Force x Dynamic Flexibility


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y y y

Control forces

Pulsation control devices like orifice plates

Moving acoustic natural frequencies

Control flexibility Gussets

Bracing

Modified or additional clamping

Moving mechanical natural frequencies

Braces Test temporary brace


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Add wooden braceas field test


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9. Summary


Video #5 - Summary


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Summary - Vibration

Vibration = Dynamic Force x Dynamic Flexibility


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Vibration cannot be eliminated, but can becontrolled through a balance between cost,

performance and reliability

The earlier vibration risk is identified, theeasier (and cheaper) it is to deal with

Draft Vibration Specification (GMRC)

Study Analysis Step Description

A. Preliminary Design Review &

Preliminary Pulsation Bottle Sizing

Project Planning Stage:

Assess operating range, unloading plan, piping

layout options.

Provide preliminary pulsation control scheme and estimated vessel sizing.

Scope of Work for Compressor System (Pipeline, Gas Injection/Withdrawal, Critical Application)


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B. Torsional Vibration Analysis (TVA) Assess stress and vibration on crankshaft(s) (driver and compressor system), and coupling dynamic torque

effects.

C Pulsation Analysis Pulsation study of compressor and piping system (including package and station piping). Provide final

recommendations on pulsation control solution.

D Pressure Drop and Performance

Report

Evaluate pressure drop of pulsation control devices and piping system concurrently. Evaluate impact on

compressor performance.

E Mechanical

Analysis

Mechanical dynamic analysis of on-skid piping, supports, and vessels. FEA modelling can be applied where

necessary.

Provide recommendations for small bore piping support and vibration control.

Optional: Forced Response Analysis of the Compressor Manifold and Vessels when necessary.

(Proper design practices using resonance avoidance can eliminate the need for this task.)

Optional: Forced Response Analysis of Off-Skid Piping System when necessary.

(Proper design practices using resonance avoidance can eliminate the need for this task.)

F Piping Flexibility (Thermal Stress)

Analysis

Static Analysis of piping and vessels to evaluate stress and equipment loads due to weight, pressure and

temperature changes.

G Skid Dynamic and Static Analysis Evaluate vibration of the skid and equipment mounted on the skid due to dynamic loads from the compressor

and driver. The foundation and the geotechnical properties should be considered. Evaluate skid design relative

to lifting.

H Commission Testing Evaluate vibration of compressor, piping, skid, foundation and small bore piping. Evaluate pulsation, pressure

drop, performance, and torsional vibration.

Key Take-Aways

Properly specify vibration studies (scope, etc.)

b k d d ff k d (


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Assess vibrations on-skid and off-skid (acrossoperating envelope)

Thermal/Mechanical: performed by same group

Consider small bore vibration survey

Attention to details (alignment, installation, etc.)

Start vibration study early

Attend GMRCs 2.5 day course, Compressor StationVibration, for more training.

Questions?


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Chris Harper ([email protected])

Shelley Greenfield

Gary Maxwell

introduction to vibration problems at compressor stations

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