Transient Vibration Assessment for Fast Acting Valves Screening Assessment.pdf

HSEHealth & Safety

Executive

Transient vibration guidlines for fast acting valves

screening assessment

Prepared by Acoustic Technology Limited

for the Health and Safety Executive

OFFSHORE TECHNOLOGY REPORT

2002/028

HSEHealth & Safety

Executive

Transient vibration guidlines for fast acting valves

screening assessment

Acoustic Technology Limited36-38 The Avenue

SouthamptonSO17 1XN

United Kingdom

HSE BOOKS

© Crown copyright 2002Applications for reproduction should be made in writing to:Copyright Unit, Her Majesty’s Stationery Office,St Clements House, 2-16 Colegate, Norwich NR3 1BQ

First published 2002

ISBN 0 7176 2511 7

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmittedin any form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.

This report is made available by the Health and SafetyExecutive as part of a series of reports of work which hasbeen supported by funds provided by the Executive.Neither the Executive, nor the contractors concernedassume any liability for the reports nor do theynecessarily reflect the views or policy of the Executive.

Summary

To avoid the hidden threat posed by transient excitation due to the rapid operation of valves on

process pipework, a screening methodology has been developed. This screening methodology has

been designed to fit the existing screening methods supplied in the MTD document “Guidelines for the

avoidance of vibration induced fatigue in process pipework”.

This report sets out the theory and screening methods to assess piping local to fast acting gas, liquid

and multiphase valves. The output from the screening is a Likelihood of Failure (LOF) term that when

used in conjunction with the small bore screening assessment in the MTD Guidelines provides a risk

rating for a connection. The risk can then be mitigated against by applying the recommended

modifications outline in the MTD document.

CONTENTS

1. INTRODUCTION

2. TRANSIENT EXCITATION DUE TO RAPID VALVE OPERATION

2.1 Nomenclature

2.2 Liquid or Multiphase Valve Closure

2.3 Liquid or Multiphase Valve Opening

2.4 Dry Gas Rapid Valve Operation

2.5 Transient Limits

3. TRANSIENT SCREENING

4. SMALL BORE CONNECTION REVIEW

5. CONCLUSIONS

REFERENCES

FIGURES

APPENDICES

1. Support Arrangement Selection

2. MTD Small Bore Connection Assessment

1

1. INTRODUCTION

Steady state vibration of process piping is becoming a better understood phenomenon with

publications such as the MTD “Guidelines for the Avoidance of Vibration Induced Fatigue in

Process Pipework” (Reference 1); steady state vibration is additionally more easily assessed

as part of a vibration survey. Transient events can result in excess levels of vibration for a

short duration, and this can pose a threat to process piping which is not always evident.

A recent investigation into an offshore piping failure in the North Sea concluded that the

failure was due to fatigue damage of a small bore drain connection as a result of several

years of operation of a valve local to the drain connection. Although steady state vibration

levels were minimal at the failure location, the line was exposed to a transient ‘kick’ each time

the adjacent automatic valve was operated.

As a result of the hidden threat of transient excitation to process piping the following

guidelines for the avoidance of piping failures have been developed. The objective of the

guidelines is to provide a ‘first level’ screening assessment method, that could be carried out

using the minimum of readily available process or valve information.

Inherent in this technique are a number of “worst case” assumptions that could potentially

identify high risk systems, a more detailed surge analysis may be required to assess the

system in more detail. The assessment methodology has been developed to be consistent

with the MTD Guidelines by generating a risk ranking in the form of likelihood of failure (LOF)

value for each valve.

For transient events impacting piping there are two limiting acceptance criteria:

�� Exceedance of the line pressure rating

�� Force sufficient to induce fatigue damage to the piping

If exceedances of the line pressure rating are predicted a full surge analysis should be

considered. For the case where the piping could be potentially damaged by fatigue, the most

likely failure location is at a small bore connection. If the LOF value predicted for the valve is

assigned to the attached pipework, the system risk assessment can be carried out as per the

MTD guidelines. The MTD guidelines provide the LOF information for small bore connections

and provide the appropriate recommendations as to remedial modifications.

2

2. TRANSIENT EXCITATION DUE TO RAPID VALVE OPERATION

The methodology for assessing the potential transient excitation due to rapid valve operation

(valve closure/opening in less than 30 seconds) can be split into the following cases:

(i) Liquid or multiphase valve closure;

(ii) liquid or multiphase valve opening;

(iii) dry gas valve operation.

Sections 2.2 to 2.5 describe the theory developed to screen process piping for potential

damage resulting from transient excitation.

2.1 Nomenclature

do - pipe outside diameter (m)

di - pipe inside diameter (m)

c - sonic velocity (m/s)

E - Young’s modulus (N/m2)

Fmax - peak force (kN)

Flim - limit force (kN)

K - fluid bulk modulus (N/m2)

L - pipe length (m)

M� - fluid mass flowrate (kg/s)

Mw - molecular weight

P0 - static pressure (Pa)

P� - pressure difference (bar)

R - universal gas constant (8314 J/kmol.K)

T - temperature (K)

t - time (s)

tc - valve closure time (s)

wt - pipe wall thickness (m)

� - ratio of specific heat capacities

� - fluid velocity (m/s)

0� - steady state fluid velocity (m/s)

� - fluid density (kg/m3)

� - pipe wall thickness / schedule 40 wall thickness

LOF - likelihood of failure

3


The transient force that effects liquid and multiphase piping systems when a fast acting valve

is closed is due to the pressure surge generated by the change of momentum of the fluid.

The maximum surge pressure for a rapid valve closure is given by the Joukowski equation

(equation 1).

0max cvP �� Pascal………(1)

The sonic velocity, c, takes into account the combined wave speed of the fluid and the

containing pipe, which for a thin walled pipe gives:

��

��

��

�

wtEd

K

co1

1

�

m/s…………(2)

This value is correct for a valve closure time that is less than (2L/c), where L is the effective

upstream pipe length. A closure time of less than (2L/c) is termed ‘sudden’, and defines the

condition where the fluid entering the upstream pipework is unaffected by the initial movement

of the valve. In Figure 1 below, the red line indicates the acoustic wave travelling at a speed

‘c’ over the pipe length L. For a valve closure to be ‘sudden’ this wave does not have

sufficient time to travel the distance 2 x L. For typical liquid systems and a valve closure time

of 2 seconds, the upstream pipe length would have to be greater than 800 m for the closure to

be deemed ‘sudden’.

FIGURE 1: Acoustic path for valve closure

For slower valve closures the peak pressure is dependant on the rate of change of valve flow

area, initial flow conditions and upstream pipe length. The rate of change of valve flow area is

valve type dependant, however, the maximum rate of change of area is predominantly over

the last few percent of closure.

Vessel or alternative flow path

Auto valve

L(m)

4

The pressure at the valve can be estimated by solving a differential equation (see equations 3

& 4).

��

��

�

��

� 2

22

01

41

2PPsurge Pascal………(3)

where 0

0 )(

P

tFdtdLv�

� …………….(4)

If the valve is downstream of a pump the shut in head of the pump also needs to be included

for the assessment as to whether the piping pressure rating is exceeded.

0_ PPPP inshutsurgetotal �� Pascal……………(5)

The function )(tF is the function defining the flow area of the valve at a time t. If it assumed

that the peak surge pressure occurs at the point where the valve is closed, ie. t equals total

time to closure (tc). This simplifies the differential of )(tF such that a simple term based on

total valve closure time can be expressed, see Table 1. The following terms are valid for

valve closure times up to 30 seconds.

Valve Type )(tF

dtd

Full bore ball 27.0281.1

�

�

ct

Reduced bore ball 362.0168.1

�

�

ct

Butterfly 275.0877.2

�

�

ct

Globe 32.0266.2

�

�

ct

Gate 315.041.3

�

�

ct

TABLE 1: Valve closure functions

The maximum transient force due to the rapid closure of a valve in a liquid system is:

4000

2

maxi

surged

PF �� kN………………………………(6)

J0380336

Rectangle

5


If a closed valve in a liquid or multiphase system can be depressurised on the downstream

side, there is the potential for cavitation or a phase instability problem when the valve is

opened. The pressure profile with distance for a valve typically follows the form shown in

Figure 2. The horizontal scale goes from upstream (negative) to downstream (positive) of the

valve, which is located at zero. The minimum pressure occurs at the valves vena contracta,

there is then a pressure recovery zone resulting in a final pressure drop for the valve.

The values for maximum pressure drop and total pressure drop are dependent on fluid

density, fluid velocity and valve loss coefficient. The loss coefficient for a valve is the constant

that relates the pressure drop across the valve to the flow velocity.

FIGURE 2: Typical Static Pressure profile across valve.

For a liquid, if the pressure drop across the valve is such that pressure downstream of the

pressure recovery zone is below the vapour pressure for the liquid, a large increase in the

vapour fraction (flashing) can occur. The downstream pipework causes the system to be

semi-bounded, this results in a situation where the rapid increase in specific volume due to

the phase change from liquid to vapour can cause a localised pressure increase which results

in a reversal of the phase change. The resulting forces from large bubbles forming and then

collapsing can be sufficient to cause excessive vibration.

In the case where:

a) the static pressure at the valves vena contracta is less than the liquids vapour pressure

and

-4 0 4 8 12 16Length/diameter

Stat

ic p

ress

ure

max dp

Total dp

flow

Pressure recovery zone

6

b) the static pressure after pressure recovery (ie. 5 to 6 diameters downstream of the valve),

is greater than the vapour pressure then cavitation will occur.

Cavitation is a more localised effect than the ‘flashing’ described above. Small bubbles are

formed at the vena contracta, these bubbles then collapse in the pressure recovery region.

This effect commonly occurs on fire water and water injection overboard dump lines. As the

opening of a valve is a transient event and the maximum pressure drop is changing with valve

position, there can be cavitation for a short duration. Typically, the dynamic forces due to

cavitation are not as extreme as those for flashing, but are still capable of causing pipework

failures.

The calculation for the pressure at the vena contracta for various valve types is ongoing.

Initial indications are that ball valves are unlikely to have a cavitation problem, whereas with

globe, butterfly and gate valves cavitation is possible. A conservative estimate for the

pressure at the vena contracta would be to take the downstream pressure minus 20% of the

pressure across the valve.

If neither flashing nor cavitation are likely to occur, there is still the possibility of high dynamic

forces due to the rapid change in momentum. The force in kN due to the change in fluid

momentum can be calculated using equation 7.

�

PMF ��

58.11

max kN ………(7)

2.4 Dry Gas Rapid Valve Operation For a dry gas any potential surge pressure due to a rapid closure is taken up via compression

of the gas, hence the likelihood of failure due to a gas valve closing is negligible.

For a rapid opening of a gas valve the transient forces are due to the sudden change in

momentum.

vMF ��max ……………(8)

where � can be expressed as the choking velocity, to give the peak force in kN by:

MwTRMF

��

��

)1(2

1000max�

��

kN……..(9)

J0380336

Rectangle

J0380336

Rectangle

7

2.5 Transient Limits

If the total pressure for a valve closure in a liquid system exceeds the pressure rating for the

line a detailed surge analysis should be undertaken.

The second limiting factor is governed by the potential of the transient excitation to cause

fatigue damage to the local piping system. To assess the potential of a transient force to

result in piping damage a Likelihood of Failure (LOF) term has been developed. This LOF

value is dependant on pipe diameter, schedule and support structure.

lim

max

FF

LOF � ………………………………(10)

� �4

25.257 + 525.67 + 1.8139 - 16.813 2

23lim

io

ddF �� ……….(11)

where � is the ratio of the actual pipe wall thickness to the Schedule 40 pipe wall thickness

for the respective nominal pipe size.

� is the correction for the support type as detailed in table 2.

Support type

(see Appendix 1) �

Stiff 4

Medium stiff 2

Medium 1

Flexible 0.5

Table 2: Support type correction

The actions depending on the LOF value are as follows:

LOF < 0.3 - OK

0.3 < LOF < 0.5 - undertake small bore review for line

LOF> 0.5 - check support structure and undertake SBC review

8

3. TRANSIENT SCREENING

The assessment methodology is shown in flowchart format in Figures 1 to 3.


Step 1: Is 4000

2

0id

cv �� > 1 kN

If Yes proceed to step 2

If No then LOF = 0

Step 2: Use equations 3 to 5 to attain maximum pressure

Step 3: If maximum pressure exceeds line pressure rating – detailed surge analysis required

Step 4: Use equations 6 to attain maximum force

Step 5: Use equations 10 and 11 to find force limit and LOF

Step 6: Proceed to small bore connection review if required


Step 1: Is the fluid vapour pressure at upstream conditions > static downstream pressure

If Yes then flashing will occur – LOF = 1.0 go to Step 5

If No go to Step 2

Step 2: Is vapour pressure at upstream conditions > static pressure at vena contracta for any

valve position

If Yes then cavitiation will occur – LOF = 0.7 go to Step 5

If No go to Step 3

Step 3: Predict maximum force by equation 7



3.3 Dry Gas Rapid Valve Operation

Step 1: If valve closing LOF = 0.

Step 2: Predict maximum force using equation 9.



9

4. SMALL BORE REVIEW

The LOF value predicted from the transient screening is equivalent to an LOF predicted due

to flow induced turbulence or the other excitation sources considered in the MTD Guidelines.

For lines with an LOF, predicted from the transient screening, greater than 0.3 a review of the

small bore connections shall be assessed according to Appendix 2. For a small bore

connection to be at risk there needs to be both an excitation and a poor small bore connection

design.

The likelihood of failure of the small bore connection is the minimum of:

�� the Process LOF (i.e. from Transient Screening), see Figure A2.1

�� the small bore connection LOF (SBC LOF) from Appendix 2, see Figure A2.2

The minimum of the two inputs is required because both a badly placed/designed small bore

connection and an excitation source need to be present for the small bore connection to have

a higher likelihood of failure. This gives a ‘Total LOF’ value, as shown in Figure A2.3.

The following are recommended actions as a result of the detailed screening of small bore

connection analysis.

1.0 > Likelihood of Failure > 0.7

· Modify the connection at the design stage or brace the small bore connection by means of

suitable support. Remove unnecessary or redundant small bore connections. Further

possible design solutions are contained in the MTD Guidelines (Reference 1) Section 4.3

(Design solutions for small bore connections).

0.7 > Likelihood of Failure > 0.4

Monitoring is required during commissioning to determine if bracing is required. In the event

of bracing being required, design solutions are itemised in Section 4.3 of the MTD Guidelines

(Design solutions for small bore connections). Alternatively modify the connection at the

design stage, as above.

Likelihood of Failure < 0.4

To ensure that design features of small bore connections are acceptable, a visual survey

should be conducted.

10

5. CONCLUSIONS

A methodology has been proposed for assessing the vibration induced in pipework due to

operation of fast acting gas, liquid or multiphase valves. The likelihood of failure (LOF)

predicted for each valve should be applied to the attached pipework and then combined with

the small bore connection modifier LOFs to determine what vibration control measures, if any,

are required.

REFERENCES

1. “Guidelines for the avoidance of vibration induced fatigue in process pipework”; MTD

Publication 99/100.

2. “Handbook of Industrial Pipework Engineering”; Holmes E; McGraw Hill (1973).

FIGURE 1: FAST ACTING VALVE ANALYSIS

Start

Process

Operation

Peak ForceAssessment

OK

CalculateMain Line LOF(Eqs 10 & 11)

Operation

List of LinesWith Fast

Acting Valves

DetailedAssessment

(Figure 3)

�-c-vAssessment(Eqs 1 & 2)

Force

OKDetailedAssessment

(Figure 2)

OPENING OPENING

FORCE > 1 kN

FORCE < 1kN

CLOSING CLOSING

LIQUIDGAS

FIGURE 2: DETAILED ASSESSMENT OF LIQUID SYSTEMS FOR VALVE CLOSING

Start

Liquid SystemValve WithHigh Peak

Force

Peak ForcePrediction

PredictForce

(Eqs 3 & 6)

CalculateMain Line LOF(Eqs 10 & 11)

Valve Type

UpstreamPipe Length

ProcessData

ValveClosingTime

FIGURE 3: DETAILED ASSESSMENT OF LIQUID SYSTEMS FOR VALVE OPENING

Start

Liquid SystemOpening Valves

Upstream VapourPressure > Downstream

Pressure?

Upstream VapourPressure > Vena Contracta

Pressure?

Main LineLOF = 1

Main LineLOF = 0.7

Predict Maximum Forceand Main Pipe LOF

(Eqs 7, 10 & 11)

NO

NO

YES

YES

Appendix 1: Support Arrangement Selection

The screening method is designed for four support arrangements; stiff, medium stiff, medium and

flexible, as detailed below. The principal response of the pipe to low frequency flow induced

turbulence is associated with the low frequency bending modes of piping spans, either between

supports or, if the supports are poorly designed, the supports should also be included.

‘Stiff Support Arrangement’: applicable to piping systems which are well supported (as per

recommendations given in Reference 2). The fundamental natural frequency of the piping span is

approximately 14 to 16 Hz.

‘Medium Stiff Support Arrangement’: applicable to piping systems which are well supported. The

fundamental natural frequency of the piping span is approximately 7 Hz.

‘Medium Support Arrangement’: applicable to piping systems which are well supported. The

fundamental natural frequency of the piping span is approximately 4 Hz.

‘Flexible Support Arrangement’: applicable to piping systems where long unsupported spans are

encountered and the fundamental natural frequency of the piping span is approximately 1 Hz. An

example of such a system is a wellhead flowline where increased flexibility is required to

accommodate riser movement.

The selection of support arrangement can be simplified as follows (Figure A1.1):

Support Arrangement Span Length Criteria Typical Natural

Frequency

Stiff 0563.202.010*2346.1 25��

� DDL 14 to 16 Hz

Medium Stiff

3601.3025262.010*1886.10563.202.010*2346.1

25

25

��

��

�

�

DDLDDL

7 Hz

Medium

429.4033583.010*5968.13601.3025262.010*1886.1

25

25

��

��

�

�

DDLDDL

4 Hz

Flexible 429.4033583.010*5968.1 25��

� DDL 1 Hz

(mm)diameter outside actual (m),length span where �� DL

Table A1.1 Support Arrangement

Figure A1.1 Different support arrangements as a function of span length and outside diameter

0

5

10

15

20

25

0 100 200 300 400 500 600 700 800 900

Actual Outside Diameter - D - (mm)

Flexible Support Arrangement

Medium StiffSupport Arrangement

Stiff Support Arrangement

L = -1.2346*10-5D2 + 0.02D + 2.0563

L = -1.1886*10-5D2 + 0.025262D + 3.3601 Medium Support Arrangement

L = -1.5968*10-5D2 + 0.033583D +4.429

Appendix 2: Small Bore Connections Screening

1.0 Small Bore Connection Modifier

The calculation of the small bore connection modifier is categorised into two parts:

�� Likelihood of failure in branch due to branch geometry

�� Likelihood of failure due to main pipe geometry.

These are combined to give the small bore connection modifier. The small bore connection modifier

is the minimum of the likelihood of failure in branch due to branch geometry and the likelihood of

failure due to main pipe geometry.

2.0 Likelihood of Failure due to the Branch Geometry

The factors governing the likelihood of failure of the branch are:

�� type of fitting;

�� overall length of branch;

�� number and size of valves;

�� main pipe schedule;

�� small bore pipe diameter.

The various factors are combined as shown in Figure A2.1 to give an overall probability of failure in

the small bore branch connection.

2.1 Type of Fitting

A weldolet involves two welds and hence (in comparison to a contoured body fitting or short

contoured body fitting) has double the number of sites at welds for potential fatigue failures.

Additionally contoured body fittings and short contoured body fitting have higher natural frequencies

than weldolets.

Fitting Likelihood of Failure (LOF)

Weldolet 0.9

Contoured body fitting 0.6

Short contoured body fitting 0.4

2.2 Overall Length of Branch

The length also determines the natural frequency. Again a longer unsupported branch results in

lower natural frequencies and hence greater likelihood of failure. Length is measured from the main

pipe wall to the end of the branch assembly (including valve(s) if fitted).

Length Likelihood of Failure (LOF)

over 600mm 0.9

up to 600mm 0.7

up to 400mm 0.3

up to 200mm 0.1

2.3 Number and Size of Valves

This is the element of likelihood of failure associated with the unsupported mass. Higher mass results

in lower natural frequencies and hence greater likelihood of failure.

Number of Valves Likelihood of Failure (LOF)

2 or more 0.9

1 or integral double block and bleed valve 0.5

0 0.2

2.4 Main Pipe Schedule

Thin walled main pipe is at higher likelihood of failure than the heavier schedules as its lower stiffness

results in low natural frequencies and high levels of stress at the joint between the small bore branch

and the main pipe.

Schedule Likelihood of Failure (LOF)

10S 0.9

20 0.8

40 0.7

80 0.5

160 0.3

>160 0.3

2.5 Small Bore Pipe Diameter As the diameter of the small bore fitting increases the natural frequency will also increase and hence

likelihood of failure will be reduced.

Fitting Diameter (Nominal Bore)

Inches DN (mm)

Likelihood of Failure (LOF)

0.5 15 0.9

0.75 20 0.8

1 25 0.7

1.5 40 0.6

2 50 0.5

3.0 Likelihood of Failure due to Location on the Parent Pipe The likelihood of failure of a connection due to the geometry of the main pipe is dependent on:

�� pipe schedule;

�� location of the connection on the main pipe.

3.1 Main Pipe Schedule Thin walled main pipe has a higher likelihood of failure than the heavier schedules as its lower

stiffness results in low natural frequencies and high levels of stress at the joint between the small bore

branch and the main pipe.

Schedule Likelihood of Failure (LOF)

10S 0.9

20 0.8

40 0.7

80 0.5

160 0.3

>160 0.3

3.2 Location on Main Pipe

A small bore connection located at rigid supports for the main pipe is unlikely to vibrate as the support

will force a node of vibration on the main pipe and as a result no forcing for the small bore branch.

Conversely small bore branches located near bends, reducers or valves are more likely to experience

high levels of excitation and therefore a higher likelihood of failure.

Location Likelihood of Failure (LOF)

Valve 0.9

Reducer 0.9

Bend 0.9

Mid span 0.7

Partially Fixed Support * 0.4

Fixed support** 0.1

* Braced in one direction : (1 translational degree of freedom perpendicular to the axis of the small

bore is fixed and the remaining degrees of freedom are free)

** Braced in two directions : (two translational degrees of freedom perpendicular to the axis of the

small bore are fixed (braced in two directions), please note this means no allowance for movement).

4.0 Small Bore Connection Modifier

The LOF values are combined as shown in Figure A2.2 to give the small bore connection modifier.

The LOF for the connection is defined as the minimum of the likelihood of failure in the branch due to

branch geometry and the likelihood of failure due to main pipe geometry; this is termed the SBC LOF.

As both an excitation and a poor small bore geometry are required for the connection to be at a ‘high’

risk; an overall LOF for the fitting is attained by taking the minimum of the SBC LOF and 1.42 times

the predicted process LOF, as shown in Figure A2.3.

Figure A2.1 Process LOF Screening Flowchart

Stage 1 (yes/no)

Flow

Indu

ced

Turb

ulen

ce

Hig

h Fr

eque

ncy

Acou

stic

Mec

hani

cal E

xcita

tion

Puls

atio

n (R

ecip

roca

ting

Com

pres

sor/P

ump)

Pu

lsat

ion

(Rot

atin

g St

all)

Puls

atio

n (F

low

Indu

ced

Exci

tatio

n)

Stage 2 (LOF)

Maximum of all inputs

Main Pipe LOF

Is main pipe LOF>1.0?

Yes No

SBC=small bore connection LOF=likelihood of failure

Assess all SBC's on problem system. Place Main Pipe LOF value in Figure A2.3.

Is main pipe LOF >=0.5 ?

Is main pipe LOF >=0.3?

Redesign as per Section 4.2 of MTD Guidelines

Can system be redesigned or supported as per Section 4.2 of the MTD Guidelines?

Alternatively survey the main pipe as per Section 5.0 of MTD Guidelines. If above acceptance limit redesign as per Section 4.2 of MTD Guidelines

Check that the basic design of SBCs is sound (see guidance given in Section 4.3 of MTD Guidelines)

Yes

Yes

No

Yes

No

No

Project/Plant: System: Subsystem: Assessed by: Ref. No.

Line number: Main Pipe LOF: Actions:

Is detailed analysis possible?

No

LOF greater than 1.0 see Section 4.2 of MTD Guidelines

Yes

Tran

sien

t Exc

itatio

n (F

ast a

ctin

g va

lves

)

Date:

Like

lihoo

d of

sm

all

bore

fa

ilure

due

to lo

catio

n

on p

aren

t pip

e

Like

lihoo

d of

sm

all

bore

failu

re d

ue to

ge

omet

ry o

f bra

nch

Wha

t is

the

pare

nt p

ipe

sche

dule

?

Whe

re is

the

SBC

on

the

pare

nt p

ipe?

Wha

t is

the

SBC

di

amet

er?

Wha

t is

the

type

of f

ittin

g?

Wha

t is

the

num

ber a

nd

size

of v

alve

s?

Wha

t is

the

over

all l

engt

h of

the

bran

ch?

Min

imum

of b

oth

inpu

ts

Wha

t is

the

pare

nt p

ipe

sche

dule

?

LOF

Valu

es

10S

0.9

20

0.8

40

0.7

80

0.5

160

0.3

>160

0.

3

LOF

Valu

es

DN

15 –

0.5

” D

N20

– 0

.75”

D

N25

– 1

” D

N40

– 1

.5”

DN

50 –

2”

LOF

Valu

es

>2

0.9

1 0.

5 0

0.2

LOF

Valu

es

>600

mm

0.

9 <6

00 m

m

0.7

<400

mm

0.

3 <2

00 m

m

0.1

LOF

Valu

es

Wel

dole

t

0.9

Con

tour

ed B

ody

0.6

Shor

t Con

tour

ed B

ody

0.4

Mea

n M

ean

Smal

l Bor

e C

onne

ctio

n M

odifi

er

SBC

=Sm

all B

ore

LOF=

Like

lihoo

d of

Fai

lure

Star

t

0.9

0.9

0.9

0.7

0.4

0.1

LOF

Valu

es

Valv

e R

educ

er

Bend

M

id S

pan

Parti

al S

uppo

rt Fi

xed

Supp

ort

0.9

0.8

0.7

0.6

0.5

LOF

Valu

es

10S

0.9

20

0.8

40

0.7

80

0.5

160

0.3

>160

0.

3

Figure A2.2 Small Bore Connection Screening Flowchart

Project/Plant: Line Number: System: SBC number: P&ID No.: SBC LOF: Assessed by: Date: Actions: Ref. No.

Figure A2.3 Overall LOF Screening Flowchart

LOF of SBC due to geometry of branch

Type

of f

ittin

g

Ove

rall

Leng

th

Num

ber a

nd

size

of v

alve

s

Pare

nt P

ipe

Sche

dule

Smal

l bor

e di

amet

er

Loca

tion

on

Pare

nt P

i pe

Pare

nt P

ipe

Sche

dule

5 2

LOF due to location on parent pipe

Main pipe LOF From Figure A2.1

multiply by 1.42

SBC Modifier From Figure 2.2

minimum of both inputs


Overall SBC LOF

Is SBC LOF ≥ 0.7 ?

Is SBC LOF ≥ 0.4 ?

Monitor during commissioning or operation as per Section 5 and Appendix D of MTD Guidelines. In the event bracing is required, brace as per Appendix C of MTD Guidelines.

Redesign as per Section 4.3 or support as per Appendix C of MTD Guidelines


SBC = small bore connection LOF = likelihood of failure

no

no

yes

yes

Appendix 3: Worked Examples

Example 1: Valve closure on liquid / multiphase system ESD valve closure on the liquids line out of a separator:

Main line 12” Schedule 10S (OD 0.3239 m, wt 0.0046 m, di 0.3147 m) Pipe support type medium Static Pressure 4 bara Initial flow rate 800000 kg/hr Bulk modulus of fluid 1.5 GN/m2 Fluid density 400 kg/m3 Pipe length from sep to valve 18 m Valve type full bore ball Valve closure time 2 secs

1” drain located 250mm upstream of valve (weldolet fitting with 1 valve fitting length 480 mm)

Step 1: Is 4000

2

0id

cv �� > 1 kN

Speed of sound in fluid given by eq(2)

sm

eewtEd

K

co

/1561

91960046.03239.0

95.11400

1

1

1�

��

��

�

�

�

��

��

�

�

�

Initial velocity = smdi

/14.73600400

48000002�

��

�

�

kNdd

cv ii 9.3464000

14.715614004000

22

0 �� (>1kN therefore proceed to Step 2)

Step 2: Calculate surge pressure, eqs(3 to 5)

For full bore ball valve closure profile is )(tFdtd = 9105.027.0

2281.127.0281.1

��

�

��

�

ct

117.0104

9105.01814.74005

��

�

��

PaPPsurge 49617141

2 22

2

0 ��

�

�

��

�

�

��

��

Step 3: Check max pressure against line rating

Line rating 150 lb, for this line a pressure rating of 20 bar was supplied listed.

Max transient pressure 4.99 bar - therefore acceptable.

Step 4: Find maximum force due to surge eq(6)

Ndd

PF iisurge 3859

449617

4

22

max ��

Step 5: Calculate LOF value using eqs(10 and 11)

483.0525.9

6.4_40

��wtSch

wt

pipe support medium, hence � = 1

55.007.7

859.3

lim

max��

FF

LOF

LOF is greater than 0.5 therefore check parent pipe support structure, and undertake a small bore

review of connections upstream and downstream of the valve (as per Appendix 2).

For 1” drain 250 mm upstream:

The small bore screening is undertaken for this fitting as detailed in Appendix 2. The worksheet on

next page shows the results of this screening. The LOF due to the fitting geometry is 0.74 and the

LOF for the location is 0.9, therefore the SBC LOF is 0.74.

The LOF predicted due to transient excitation is 0.55. The overall LOF is defined as the minimum of

the process LOF x 1.42 and the SBC LOF, which equates to 0.74.

With this overall LOF of 0.74, the fitting should be either re-designed (reducing the length, avoid using

a weldolet etc), or braced.

Project/Plant: Example 1 Line Number: line 1 System: S.B.C. number: sbc 1 P&ID No.: S.B.C. L.O.F.: 0.74 Assessed by: WJS Date: Actions: Redesign or Brace Ref. No.

LOF of SBC due to geometry of branch

Type

of f

ittin

g

Ove

rall

Leng

th

Num

ber a

nd

size

of v

alve

s

Pare

nt P

ipe

Sche

dule

Smal

l bor

e di

amet

er

Loca

tion

on

Pare

nt P

i pe

Pare

nt P

ipe

Sche

dule

0.9 0.743.7

0.7 0.5 0.9 0.7 0.9 0.9 0.9 1.8

5 2

LOF due to location on parent pipe

0.74

0.74

Main pipe LOF

multiply by 1.42

SBC Modifier



0.78 0.55

Overall SBC LOF

Is S.B.C. L.O.F. ≥ 0.7 ?

Is S.B.C. L.O.F. ≥ 0.4 ?

Redesign as per Section 4.3 or support as per Appendix C of MTD Guidelines

SBC = small bore connection LOF = likelihood of failure

no

no

yes

yes


Monitor during commissioning or operation as per Section 5 and Appendix D of MTD Guidelines. In the event bracing is required, brace as per Appendix C of MTD Guidelines.

Example 2: Valve Opening on Liquid System

The liquid from an oil drum requires an automatic condensate drain valve controlled by a level switch

in the drum.

Upstream pressure 10 bara Downstream pressure 6 bara Fluid vapour pressure 5.5 bara Flow rate for fully open valve 8 kg/s Fluid density 600 kg/m3

Step 1: Is vapour pressure (5.5 bara) > downstream pressure (6 bara) – NO

Step 2: Is vapour pressure > pressure at valve vena contracta (6 – 20%x(10 - 6) = 5.2 bara) – YES

Cavitation across the valve is likely therefore LOF is 0.7.

LOF is greater than 0.5 therefore check parent pipe support structure, and undertake a small bore

review of connections upstream and downstream of the valve (as per Appendix 2).

Example 3: Gas Valve Opening

This example predicts the dynamic forces due to a relief valve opening.

Flow rate for fully open valve 3 kg/s Ratio of specific heat capacities 1.4 Molecular weight 21 Upstream temperature 45 °C Main line 8” Sch 40S (OD 0.2191 m, wt 0.0082 m, di 0.2027 m) Support type medium stiff

Step 1: Is this a valve closure scenario? - NO

Step 2: Predict force due to valve opening, eq(9).

� ��

kNMwTRMF 15.1

2114.12734583144.12

10003

)1(2

1000max �

��

��

�

��

��

�

�

��

Step 3: Calculate LOF value, eqs(10 and 11)

Pipe is sch 40, hence 1�� and pipe support medium stiff, hence � = 2

� � kNd

dF io 00.8

425.257 + 525.67 + 1.8139 - 16.813

223

lim ��

14.0815.1

lim

max��

FF

LOF

LOF is less than 0.3, therefore line is OK.

Printed and published by the Health and Safety ExecutiveC0.50 06/02

OTO 2002/028

£10.00 9 780717 625116

ISBN 0-7176-2511-7

Transient Vibration Assessment for Fast Acting Valves Screening Assessment.pdf

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