PVM Training

Module 1 - Pipework Vibration

Objectives & Outcomes

• Gain knowledge of vibration and specifically piping vibration.

• Learn how to identify potential risks & failure mechanisms.

• Learn how to measure and quantify vibration risk.

Contents – Module 1

• General Vibration– What is Vibration / Fatigue– Amplitude and Frequency– Natural Frequency– Vibration Excitation– Vibration Measurement

• Pipework Vibration– Risks– Main Pipe

•Excitation Mechanisms•Fretting

– Small Bore•Poor Geometry Examples•SBC Supports

– Pipework Vibration Criteria

General Vibration

What is Vibration ? Video

• An oscillating motion about a reference position• Frequency is a measure of the number of cycles of this motion during a

period of 1 second – SI unit: Hertz (Hz)• Vibration occurs due to dynamic effects of tolerances, clearances,

rolling/rubbing contact and out of balance forces in mechanical systems• Vibration is often destructive, but is sometimes generated intentionally to

perform a task

Damping, ζ Stiffness, K

Mass, MX

What is Vibration ?

Stiffness (k)

Mass (kg)

mass

mass

Time

Max Positive +

Max Negative -

AM

PLI

TUD

E

Pea

k

RM

S

Pea

k to

Pea

k

PeakRMS *2/2Root Mean Square ‘useful’ unit of amplitude in a sine wave -

Frequency

• A vibrating system’s motion can consist of:

– a single component occurring at a single frequency, e.g. a tuning fork, or

– Several components occurring at different frequencies simultaneously, e.g. a car, engine, exhaust, road noise, suspension etc

• In practice the second case is correct for the majority of vibrating systems

• When looking at the different frequency components in a vibrating system, frequency analysis is required.

Time & Frequency Domains – Single Frequency

-1.5

-1

-0.5

0

0.5

1

1.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Time (sec)

Am

plitu

de

0

0.2

0.4

0.6

0.8

1

1.2

0 25 50 75 100 125 150 175 200

Frequency (Hz)

Am

plitu

de

Period

Frequency = 1 / Period

Time & Frequency Domains – Multiple Frequencies

-30000

-20000

-10000

0

10000

20000

30000

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Time (s)

Am

plitu

de

• Random Vibration with energy distributed over a wide frequency range

From Disorder (time history)…..

To Order (frequency domain)….

Important Measurement Units

• SI Unit for frequency measurement is number of cycles per second (Hertz). • Other common frequency unit is RPM – revolutions per minute.• To be understood, it is essential that units state both the parameter and

amplitude measure being used e.g.:

Velocity - mm/s RMS

Acceleration - g (peak)

Displacement - mm (peak to peak)

Natural Frequency & Resonance

• Natural Frequency

• Very little energy is required to excite the natural frequency of a system, as the system ‘wants’ to respond at this particular frequency. If damping is present then this will dissipate the dynamic energy and reduce the vibrational response.

massstiffness spring

21 :frequency Natural

nf

Natural Frequency & Resonance – Video Tacoma Narrows

Vibration Excitation - Recap

• Excitation can either be tonal i.e. energy is only input at discrete frequencies, or broadband i.e. energy is input over a wide frequency range.

• Tonal Excitation – ResonantExcitation frequency matches a natural frequency then a resonant condition is said to exist. All excitation energy is available to ‘drive’ the natural frequency of the system

• Tonal Excitation – ForcedExcitation frequency does not match a system natural frequency, then vibration will still be present at the excitation frequency, although at much lower levels than the resonant condition. High levels of vibration will occur only if the excitation energy levels are high, relative to the stiffness of the system.

• Broadband ExcitationIf the excitation is broadband then there is a probability that some energy will be input at the system natural frequencies. Generally, response levels are lower than for the purely resonant vibration case described above because the excitation energy is spread over a wide frequency range.

Measurement Techniques – Why Do We Measure Vibration?

• Vibration amplitude and frequency data provide good correlation to dynamic strain.• To be able to identify sources of undesirable vibration.• To avoid excitation of resonances in certain parts of the system.• To use for condition monitoring. • To construct or verify computer models of structures & systems.

Fatigue

• Progressive and localized structural damage that occurs when a material is subjected to cyclic loading

Fatigue Assessment

For an F2 category weld

Stress Range = 35 N/mm2

= 2.3% Probability

17.5N/mm = 0.1% Probability BS7608

Stress Ratio SR Classification Action

SR < 0.5 OK None

0.5 < SR < 1 CONCERN NDT. Modify at the earliest opportunity

SR >= 1 PROBLEM NDT. Modify immediately

Pipework Vibration

Root Cause of Reported Leaks

0

5

10

15

20

25

30

Degradationof materialproperties

Corrosion /Erosion

Fatigue /Vibration

IncorrectInstallation

Operator Error InadequateProcedures

Inadequateequipment

Inadequateisolation

Proceduralviolation

Line blockage

Perc

enta

ge o

f all

Inci

dent

s

Fatigue /Vibration

Main Pipe Vibration – Excitation

Excitation• There are a variety of excitation mechanisms which can be present in a

piping system including;

•Flow Induced Turbulence

•Mechanical Excitation

•Pulsation

Flow Induced Turbulence Excitation

• Turbulence will exist in most piping systems. The dominant sources are found at major flow discontinuities, such as: partially closed valve, short radius or mitred bends, tees or reducers. This in turn generates potentially high levels of Broadband Energy, which is typically concentrated at lower frequencies (below 100 Hz)

Fluid Velocity Profile Kinetic Energy

Mechanical Excitation

• Most of the problems encountered have been with reciprocating compressors & pumps. Dynamic forces load the pipework connected or cause vibration of the supports which in turn excites the pipework connected. Normally high levels of vibration and failures only occur when the natural frequency of the pipework system coincides with the running speed (or harmonics) of the machine.

• Transmission– Problems can also occur on pipework which shares supports with either the

machinery or associated pipework, but is not part of the system which involves the excitation.

Pulsation Excitation

• Fluids within piping systems also exhibit natural frequencies – Acoustic.

• Pressure pulsation is a tonal form of excitation whereby dynamic pressure fluctuations are generated in the process fluid at discrete frequencies.

• If these frequencies coincide with a structural natural frequency of the pipework system high vibration can occur.

•Typical sources of Pulsation are:Reciprocating Pumps & CompressorsCentrifugal Compressors (Rotating Stall)Flow over bodies within flow – (thermowells)Dead Leg Branches

Potential Failure Mode: Identifying Fretting Issues

• Typical locations to be considered include:– U-bolt pipe clamps, particularly where there is no resilient layer (e.g. tico pad)– Resting supports– Deck penetrations – Loose insulation cladding– Contact between pipes (partial clash)– Pipework in contact with other equipment items (e.g. cable racks, handrails,

other fittings, etc)– Temporary supports (e.g. scaffold poles, chain blocks etc.)

• Where fretting is identified, the items in contact should be separated and appropriate inspection performed to quantify any damage which has been sustained.

Examples of Fretting

The screw and nut used to mount a temp gauge in contact with pipe, resulting in penetration of sch. 160 pipe

Pipeline contact to cable rackresulting in fretting damage

U-bolt is attached to the connection on a reducer section and is not lined and susceptible to fretting damage

Pipework VibrationSmall Bore

Small Bore

• Excitation generally from the main pipe as previously described

• Geometry significantly effects the response– Length– Diameter– Type of fitting– Number & size of valves– Parent pipe schedule– Location

Example of High Risk Geometry

Necked down connection and large cantilevered mass

Large cantilevered mass with poor geometry

Example of High Risk Geometry

Support to first flange, however, large mass assembly overhang

Large cantilevered mass with poor geometry

Example of High Risk Geometry

Example of High Risk Geometry

Clamp – Inadequate Supports

Connection braced at small bore pipe using flat bar, no support

provided to the valve and potential punch through threat.

Brace only protects welded connection to parent pipe. Down stream elbow welded connection

unprotected

Clamp – Inadequate Supports

Connection handcuffed to adjacent pipe rather than parent

pipe

Connection braced to deck. Combination of static (axial) loading and vibration leading

to failure

Clamp – Adequate Support

• Clamp provides restraint in both the horizontal and vertical direction.

• No overhanging mass of the valve

Clamp – Maintenance & Inspection

• Example of critical clamp not been re-instated correctly after intervention work on line

• Disadvantage of Bracing Everything– High cost– Risk of corrosion– Risk of loosening resulting in ineffectiveness– On-going maintenance is required

Welded Web Supports

• Example of SBC fully supported using welded webs

• Thin-wall PipeworkPotential failure location when high

frequency vibration present

Piping Vibration Criteria

•Industry standard curves basedon 25 years experience in thepetrochemical industry

•Frequency Weighted

Pipework Vibration - Experience

Piping Vibration Criteria

ACTION

OK No action required, vibration levels are acceptable

CONCERN The level of vibration, above which, further analysis is required to determine if modifications must be undertaken to prevent possible high cycle fatigue failure

PROBLEM The level of vibration, above which, modifications must be undertaken immediately to prevent high cycle fatigue failures

Doosan Power Systems application of the pipework vibration curves is to calculate

the overall RMS value over the entire frequency range, acting at the lowest

dominant frequency, and compare it to the curves to determine the classification.

Solutions – What Role Can you Play…

• Keep your eyes and ears open• Pipework vibrating• Leaks / drips• Noise, especially tonal• Changes from yesterday / last trip

Don’t assume someone else is dealing with the concern!

Questions

Any Questions