transient vibration assessment for fast acting valves screening assessment.pdf
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Transient VibrationTRANSCRIPT
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
2.2 Liquid or Multiphase Valve Closure
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)
5
2.3 Liquid or Multiphase Valve Opening
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)
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.
3.1 Liquid or Multiphase Valve Closure
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
3.2 Liquid or Multiphase Valve Opening
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
Step 4: Use equations 10 and 11 to find force limit and LOF
Step 5: Proceed to small bore connection review if required
3.3 Dry Gas Rapid Valve Operation
Step 1: If valve closing LOF = 0.
Step 2: Predict maximum force using equation 9.
Step 3: Use equations 10 and 11 to find force limit and LOF
Step 4: Proceed to small bore connection review if required
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
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
Check that the basic design of SBCs is sound (see guidance given in Section 4.3 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
minimum of both inputs
minimum of both inputs
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
Check that the basic design of SBCs is sound (see guidance given in Section 4.3 of MTD Guidelines)
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