17 May 2009
Document No.
GN 44-005
Date
May 2009
GN 44-005
Assessment of Acoustically Induced
Vibration
Guidance Note
BP GROUP
ENGINEERING TECHNICAL PRACTICES
GN 44-005
Assessment of Acoustically Induced Vibration
Foreword
This is the first issue of Guidance Note GN 44-005.
Copyright 2008 BP International Ltd. All rights reserved.
This document and any data or information generated from its use are classified, as a
minimum, BP Internal. Distribution is intended for BP authorized recipients only. The
information contained in this document is subject to the terms and conditions of the
agreement or contract under which this document was supplied to the recipient's
organization. None of the information contained in this document shall be disclosed
outside the recipient's own organization, unless the terms of such agreement or contract
expressly allow, or unless disclosure is required by law.
In the event of a conflict between this document and a relevant law or regulation, the
relevant law or regulation shall be followed. If the document creates a higher obligation, it
shall be followed as long as this also achieves full compliance with the law or regulation.
Page 2 of 33
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Assessment of Acoustically Induced Vibration
Table of Contents
Page
Foreword............................................................................................................................................2
Introduction ........................................................................................................................................5
1.
Scope........................................................................................................................................6
.......................................................................................................................................6
1.2. BP experience ...............................................................................................................6
2.
References ...............................................................................................................................6
3.
Symbols and abbreviations.......................................................................................................7
4.
AIF Proceedure.........................................................................................................................7
5.
Acoustically Induced Vibration Data Sheet...............................................................................9
5.1. Calculation formula......................................................................................................10
5.2. Design action: Continuously Operated System ...........................................................12
5.3. Design action: Non- continuously Operated System ...................................................12
5.4. Piping integrity improvement .......................................................................................13
5.5. Specialist assistance ...................................................................................................13
Annex A: Example 1 ........................................................................................................................15
Section 1: 8 line downstream of valve ...................................................................................16
Annex B - Example 2 .......................................................................................................................17
Section 1: 8 line downstream of valve ...................................................................................18
Section 2: 16 line downstream of valve .................................................................................19
Section 3: 24 line downstream of valve .................................................................................20
Annex C: Example 3 ........................................................................................................................21
Section 1: 12 line downstream of valve .................................................................................22
Annex D: Example 4 ........................................................................................................................23
Location A: valve #1 tail pipe to header..................................................................................24
Location B: valve #2 tail pipe to header..................................................................................25
Annex E: Mach Number Calculations ..............................................................................................26
Annex F: Addition of sound power levels.........................................................................................28
Annex G: Input Required For Detailed Analysis ..............................................................................29
Annex H: Design Limits for Acoustically Induced Vibration .............................................................30
Bibliography .....................................................................................................................................33
Page 3 of 33
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Assessment of Acoustically Induced Vibration
List of Tables
Table 1: Typical layout for acoustic fatigue calculation
Table 2: Valve nomenclature
Table 3: Design actions: Continuously operated systems
Table 4: Design actions: Non-continuously operated systems
Table 5: Addition of sound levels
Table 6: Design actions: Continuously operated systems
Table 7: Design actions: Non-continuously operated systems
9
10
12
12
28
32
32
List of Figures
Figure 1: High frequency acoustic induced fatigue process
Figure 2: Graph to estimate the ratio of specific heats for hydrocarbon gases
Figure 3: Design limits for acoustically induced vibration
Figure 4: Design limits for acoustically induced vibration
8
27
30
31
Page 4 of 33
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Assessment of Acoustically Induced Vibration
Introduction
Experience in the gas production, petrochemical and other industries has demonstrated that acoustic
energy in high capacity gas pressure reducing systems can cause severe piping vibrations. In extreme
cases, these have led to piping fatigue failures after a few hours of operation. Typical systems where
such problems may occur include large compressor recycle systems, Emergency Depressurisation
systems (EDP) or blowdown valves and high capacity safety valve pressure let-down systems. The
trend in recent years towards higher capacity systems has increased the likelihood of experiencing
such failures.
The most vulnerable systems have the following characteristics:
a.
High mass flow rate
b.
High pressure drop
c.
Weldolet connection into large diameter, thin walled pipe.
Page 5 of 33
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Assessment of Acoustically Induced Vibration
1.
Scope
This document is intended to provide the design engineer with methods for assessing potential failures
from acoustic induced fatigue (AIF) at an early stage of design. This note should also be used to assess
existing systems. The style of this document, with its use of decision diagrams, is intended to limit
ambiguities of interpretation which are often inevitable in guidelines of this sort. However, where
these exist and require clarification full consultation with the BP Representative should be made. Only
valves in vapour or mixed flow service need be considered. Valves in liquid service do not need to be
considered.
Engineering design criteria and methodology has been developed and is most appropriate for single
phase vapour fluids. The effects of AIF in multi phase fluids is not as well developed. It is
conservative to use the total mass flow rate on the assumption that the liquid will flash off. It is
important to note that industry experience shows that it is connections to the main pipe, rather than
circumferential or longitudinal butt welds, which are most vulnerable. Therefore any design
modifications need to consider either thickening up the main pipe; applying attention to the detail of
the connection or a combination of both of these design solutions. The connection may be a branch
pipe, pipe support or small bore vent, drain or instrument connection. It is an acceptable design
alternative to locally thicken up a pipe header in vulnerable areas and then use a reduced wall
thickness where there are no branch connections or other structural discontinuities.
Further, it is important to note that other vibration mechanisms in piping systems need to be
systematically considered. Other mechanisms include
a.
Flow induced turbulence
b.
Pulsation past a dead leg
c.
Momentum change due to fast opening valve
d.
Surge due to liquid carry over
These mechanisms are beyond the scope of this document. ETPs GP 44-80 and GP 44-70 outline
issues associated with design of pressure relief systems and should be used in conjunction with this
GN
1.2.
.BP experience
BP have had multiple cases where failures have been caused by AIF, these include:
a.
Alaska[4]
b.
Krechba, Algeria
c.
Schiehallion. North Sea
d.
ALNG, Trinidad
This GN has been developed considering latest industry practices and application of the
principals of AIF should reduce the likelihood of these types of failures.
2.
References
BP
GP 42-10
GP 44-70
GP 44-80
Piping systems (ASME B31.3)
Overpressure Protection Systems
Relief Disposal Systems
Page 6 of 33
GN 44-005
Assessment of Acoustically Induced Vibration
Industry Standards
Energy Institute
3.
Guidelines for the Avoidance of Vibration Induced Fatigue in Process
Pipework, 2nd Edition, January 2008
Symbols and abbreviations
For the purpose of this GN the following symbols and abbreviations apply:
AIF
Acoustically induced fatigue. Sometimes referred to as acoustically induced vibration
(AIV). Fatigue and vibration due to high frequency acoustic excitation. Typically, the
dominant frequency is between 500 Hz and 2 kHz.
dB
decibel
Lw, PWL Sound Power level
LOF
4.
Likelihood of failure
AIF Proceedure
It is intended that this guidance note supplements the use of the EI Guidelines and BP ETPs GP 44-70
and 44-80. This guidance note should be used on new projects in the design phase and for the
assessment of existing facilities.
It would be anticipated that new projects would normally implement the piping integrity
improvements described in section 5 as required. However, this may not be practical for existing
facilities That were either built to different standards or are being re-rated for higher capacities. In this
case it may be possible to ensure integrity by completing a thorough acoustic structural finite element
analysis and carry out local modifications to ensure acceptable integrity. This approach is described in
section T10.7 of the EI guidelines. This detailed analysis is beyond the scope of this guidance
document, however the input needed to complete this type of analysis is included in Annex G. This
design approach should be supplemented by an inspection program on downstream piping. Inspection
should be focused on looking for surface breaking defects that would act as fatigue initiation points.
Any defects should be removed.
It would normally be expected that the first stage of this analysis work (which is predominantly data
gathering and a single simple calculation) would be completed by a BP engineer or qualified
engineering contractor. Whilst the second stage which involves consideration of multiple relief valves
operating simultaneously and a full assessment to the EI guidelines would normally be completed by a
specialist consultant or engineering contractor who is familiar with the application method. However,
there is sufficient detail in this document for a competent engineer to complete this stage of the
assessment using this guidance note and the EI guidelines.
Page 7 of 33
GN 44-005
Assessment of Acoustically Induced Vibration
Calculate sound power level
using Equation 1
Is the sound power
level
Lw>155dB?
Analysis
normally
completed by BP
or contractor
engineer
No
No further action
for this valve
Yes
Identify which valves can
operate simultaneously.
Complete EI assessment in
accordance with module T2.7
Analysis
normally
completed by
specialist
consultant
Is the LOF>0.5?
No
No further action
for this valve
Yes
No further action
for this valve
Yes
Can system be designed
in accordance with
section 5.1.5?
No
Redesign piping system or consult
specialist consultant
Figure 1: High frequency acoustic induced fatigue process
Page 8 of 33
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Assessment of Acoustically Induced Vibration
5.
Acoustically Induced Vibration Data Sheet
Calculations would normally be completed using a spreadsheet type format. Typical layout is shown in Table 1. Particular attention has to be paid to the units
used in the calculation. The formula and consistent set of units shown in section 5.1 should normally be used.
Inputs
Valve Tag
Gas molecular
weight
Result
Comment
Upstream
temperature
PWL
Continuous
service
deg C
dB
(>5 hours)
Mass flow
Upstream
Pressure
Downstream
Pressure
kg/hr
Bar(A)
Bar(A)
V-001
20,000
30
25
40
154
V-002
140,000
30
25
50
171
V-003
100,000
40
18
53
170
V-004
160,000
40
18
53
174
V-005
150,000
60
22
53
172
V-006
70,000
40
22
53
166
V-007
60,000
40
19
53
165
V-008
30,000
40
19
53
159
V-009
40,000
40
19
53
161
No action required
EDP service, used at start up
Table 1: Typical layout for acoustic fatigue calculation
Page 9 of 33
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Assessment of Acoustically Induced Vibration
5.1.
Calculation formula
There are a number of formulas used to assess AIF. The sound power level immediately
downstream of the valve may be calculated using Equation 1. This equation is only valid for the
pressure, temperature and mass flow measurement units shown in this section.
Direction of flow
p1
T1
p2
T2
PWL
PWL(x)
x
Table 2: Valve nomenclature
P P 3.6 T 1.2
PWL = 10 log10 1 2 W 2 1 + 126.1 + SFF
m
P1
Equation 1
Credit may be taken for attenuation of acoustically induced vibration due to pipe length
downstream of the valve. For the purpose of these design calculations, assume attenuation of
3dB/50 diameters downstream of the pressure let-down device. The sound power level at a
distance x from the valve may be calculated using Equation 2.
Normally there is no need to extend the analysis beyond the feature that acts as an acoustic
block such as a flare knock out drum. In many cases, the piping isometrics will not be
available at the time this study is required. It is conservative to ignore attenuation due to
piping.
PWL( x) = PWL
0.06 x
D
Equation 2
The Mach number of gas downstream of the valve may be calculated using Equation 3. The
derivation of this formula is given in Annex C. This equation is only valid for the units shown
in this section.
M 2 = 116
W
P2 D 2
T2
m
Equation 3
Page 10 of 33
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Assessment of Acoustically Induced Vibration
Where:
5.1.1.
P1
upstream pressure
Bar(A)
P2
downstream pressure
Bar(A)
T1,
upstream temperature
K
o
T2
downstream temperature (0 C=273 K
PWL
sound power level
dB
SFF
dB
M2
A correction factor to account for sonic flow . If sonic
conditions exist (M2>1) then SFF=6; otherwise
SFF=0
Downstream Mach number
flow rate of gas and liquid
kg/s
nominal pipe diameter
distance downstream of valve
ratio of specific heats Cp/Cv, see Appendix E
molecular weight
DL
Design limit sound power level
dB
Downstream Pipe Diameter
The diameter used in setting the LOF limit may not be that at the pipe outlet. Careful
consideration must be given to all pipework downstream of the valve. Pipework with a larger
diameter is more vulnerable to AIF See example 2 calculation in Annex B.
5.1.2.
Downstream pressure
It is normal to use 1 Bar(A) as the downstream pressure for an atmospheric relief line. The back
pressure will build up during relief, but the maximum damage is likely to be during the initial
event when the flow rates are greatest and the back pressure is lowest.
5.1.3.
Low Noise Valves
Where low noise type valves with small passages are required, strainers should be installed
upstream of the valve to avoid debris accumulation in the valve itself. The strainers may be
omitted if there are sufficient, similar devices to ensure a debris-free system fitted elsewhere
upstream of the valve.
5.1.4.
Sonic flow correction factor
Sonic flow conditions in the downstream pipework should normally be avoided. To avoid sonic
flow a larger pipe is normally used downstream of the pressure device.
The selection of Mach number as the sole criteria that should be used in determining the flare or
relief header piping sizing where the process conditions are transient, can be problematic. It is
too conservative to use the Mach number when the downstream pressure is atmospheric. If this
is a critical factor then expert advice should be taken.
5.1.5.
Design limits for Acoustically induced vibration
Using the EI guidelines, an LOF score may be calculated. Depending on the LOF score, the
design actions shown in the following sections should be followed. Examples of calculations are
given in Annex A and Annex B. It is important that the design of the piping is balanced between
the pipe wall thickness and attention to the connection type.
For example, a weldolet connection into schedule 10S piping is particularly vulnerable.
Page 11 of 33
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Assessment of Acoustically Induced Vibration
5.2.
Design action: Continuously Operated System
For the purposes of AIF, any valves that are used for greater than 5 hours during the life of the
plant are normally considered to be operating continuously. The high frequency nature of AIF
(typically 500 Hz to 2KHz) means that a very high number of cycles can be rapidly
accumulated.
0.5
No
0.5 < LOF 1.0
Yes
SS
0.5
No
No
STD
40S
0.5
No
Yes
Minimum schedule
> 1.0
Valves (Y/N)
Low Noise
CS
Max. d/s
rqd?
Specialist
assistance
Piping integrity improvement
LOF
Mach No.M2
Experience shows that piping systems associated with blow-down valves that are used during
plant commissioning or start-up are particularly vulnerable.
Yes
Table 3: Design actions: Continuously operated systems
Table 3 should be interpreted such that either:
a.
the pipe thickness may be increased such that the LOF is less than or equal to 0.5. In which
case a low noise valve or piping integrity improvements detailed in paragraph 5.4 are not
required. Experience has shown that it would not be normal to increase the wall thickness
of the pipe above 19mm (3/4) to meet the requirements of AIF. Documented failures to
date have been in pipe work with wall thickness less than 19mm.
or
b.
an LOF of between 0.5 and 1.0 is acceptable provided that a low noise valve is used and
the pipe minimum schedule is used and the piping integrity improvements detailed in
paragraph 5.4 are included in the design.
If the LOF is greater than 1, then refer to paragraph 5.5.
Design action: Non- continuously Operated System
rqd?
0.5
No
0.5 < LOF 1.0
Yes
Minimum schedule
CS
STD
SS
40S
Specialist
assistance
Piping integrity Improvement
LOF
Mach No.M2
Relief valves are normally considered to operate non -continuously.
Max. d/s
5.3.
0.75
No
0.75
No
> 1.0
Yes
Table 4: Design actions: Non-continuously operated systems
Page 12 of 33
GN 44-005
Assessment of Acoustically Induced Vibration
Table 4 should be interpreted such that either:
a.
the pipe thickness may be increased such that the LOF is less than or equal to 0.5. In which
case piping integrity improvements detailed in paragraph 5.4 are not required. It would not
be normal to increase the wall thickness of the pipe above 19mm (3/4) to meet the
requirements of AIF.
or
b.
an LOF of between 0.5 and 1.0 is acceptable provided that the pipe minimum schedule is
used and the piping integrity improvements detailed in paragraph 5.4 are included in the
design.
If the LOF is greater than 1, then refer to paragraph 5.5.
5.4.
5.5.
Piping integrity improvement
a.
Use welding tees or sweepolets at all branch connections 80mm diameter and larger.
Weldolets, partial reinforcing pads and reinforced branch connections shall not be used.
b.
Small diameter branch connections 50mm diameter should be made using 6,000 lb
Nipolets.
c.
Use full wraparound reinforcement at welded-on support shoes or restraint points.
Consider bolted-on shoes or clamps to eliminate all welding to pipe at supports or anchors.
d.
In the piping length requiring Integrity Improvement branches shall be avoided wherever
possible. Pressure gauge, pressure tapping, vent and drain branches and similar free-end
branches shall be braced back to the header (run pipe).
e.
Eliminate small vents and drains where possible. Where not, when testing is complete
remove flanged valve(s) from hydrotest vents and drains and blank off flanged connection
with a blind flange.
f.
All small diameter valves and instrument components (50mm diameter and smaller)
attached to the main line and packing nuts for in-line control valves and block valves,
should employ locking nuts, e.g. elastic stop nuts, or double locking nuts to prevent
loosening due to vibration.
Specialist assistance
If the LOF is greater than 1, then specialist assistance should be sought. An additional check
may be made using Figure 3 in Annex H. If the PWL exceed the design limit by about 15dB or
more, then more significant system changes will probably be required. Consideration should be
given to splitting the flow into parallel paths not terminating at the same point or taking the
pressure letdown across stages in series such as by orifice plates downstream of the control
valve (where possible). The design approach to be used in these extreme cases will depend on
the particular system involved and the amount of attenuation needed. These should be discussed
as separate issues and resolved on an item by item basis.
It is possible to perform an acoustic structural finite element calculation to determine the actual
design fatigue life of the piping configuration. This is an analysis that should only be completed
by specialists who are competent and experienced in this technique. The data that is typically
required to carry out this analysis is given in Annex G.
A possible design modification is to use circumferential stiffening rings, however, this should
only be considered for existing facilities and should not be used on new designs.
Should it be determined that specialist assistance is required, then an external specialist
consultant should be brought in to assist in the analysis and develop recommendations. It is
Page 13 of 33
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Assessment of Acoustically Induced Vibration
strongly recommended that a BP mechanical specialist provide oversight to the external
specialist work.
Page 14 of 33
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Assessment of Acoustically Induced Vibration
Annex A: Example 1
The relief valve has a 30 Bar(A) upstream pressure and discharges into a header that is initially at
atmospheric conditions. The mass flow rate is 20,000 kg/hour. The relief temperature is 40oC, the gas
molecular weight is 25 and the ratio of specific heats is 1.21.
10m
8 DIA
16 DIA
60m
24 DIA
Where:
P1
P2
T2
=
=
=
30
1
313
Bar(A)
Bar(A)
K
dB
5.56
0.2
0.211
1.21
kg/s
m
m
upstream pressure
downstream pressure
downstream temperature
A correction factor to account for sonic flow . If sonic conditions exist
then SFF=6; otherwise SFF=0
Downstream Mach number
flow rate of gas and liquid
nominal pipe diameter
Inside pipe diameter (std wall)
ratio of specific heats Cp/Cv
SFF
M2
W
D
Di
=
=
=
=
molecular weight
25
Page 15 of 33
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Assessment of Acoustically Induced Vibration
Section 1: 8 line downstream of valve
Step 1
Calculate Mach number. The developed back pressure is 1 Bar(A)
M 2 = 116
T2
m
W
P2 Di2
5.56
1 10 0.2112
M = 0.48
= 116
Step 2
313
25 1.21
Calculate sound power level.
1.2
P P 3.6
T
2
W 2 1 + 126.1 + SFF
PWL = 10 log10 1
m
P1
1.2
30 1 3.6
2 313
= 10 log10
5
.
56
+ 126.1 + 0
25
30
= 154dB
Step 3
154<155 no further action required for this valve
Page 16 of 33
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Assessment of Acoustically Induced Vibration
Annex B - Example 2
The relief valve example used in Annex A has its capacity increased to 140,000 kg/hour.
10m
8 DIA
16 DIA
60m
24 DIA
Where:
P1
P2
T2
=
=
=
upstream pressure
downstream pressure
downstream temperature
A correction factor to account for sonic flow . If sonic conditions exist
then SFF=6; otherwise SFF=0
Downstream Mach number
30
1
313
Bar(A)
Bar(A)
K
SFF
dB
M2
W
D
Di
=
=
=
38.89
0.2
0.211
1.21
kg/s
m
m
flow rate of gas and liquid
nominal pipe diameter
Inside pipe diameter (std wal)
ratio of specific heats Cp/Cv
molecular weight
25
Page 17 of 33
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Assessment of Acoustically Induced Vibration
Section 1: 8 line downstream of valve
Step 1
Calculate Mach number. The developed back pressure in the system is 8 Bar(A).
M 2 = 116
W
P2 Di2
T2
m
38.89
8 10 5 0.2112
M = 0.41
= 116
Step 2
313
25 1.21
Calculate sound power level.
P P 3.6 T 1.2
W 2 1 + 126.1 + SFF
PWL = 10 log10 1
m
P1
1.2
30 1 3.6
2 313
= 10 log10
38
.
89
+ 126.1 + 0
25
30
= 171dB
Step 3
171>155 additional analysis in accordance with EI guidelines is required
Step 4
There is a welded support immediately downstream of the relief valve. Neglect attenuation and
take the pipe as 8 standard wall
Use flowchart T2-6
D=219 ; d=219
7
S=65.1 ; B=160.2 ; N=8.3x10
FLM1=1.75 ; FLM2=1 ; FLM3=1
8
N=1.4x10
Step 5
Calculate the LOF
LOF = 0.1303 ln ( N ) + 3.1
= 0.1303 ln 1.44 108 + 3.1
= 2.4 + 3.1
LOF = 0.7
Step 6
As the LOF is greater than 0.5, corrective action is required
Increasing the wall thickness to 11 mm reduces the LOF to 0.5. An alternative to increasing the
wall thickness would be to increase the wall thickness to 40S (for a stainless line) or STD wall
(for a carbon steel line) and use a wrap around reinforcement or bolted pipe shoe.
Page 18 of 33
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Assessment of Acoustically Induced Vibration
Section 2: 16 line downstream of valve
The 8 pipe now joins a 16 header using a fabricated tee. The 8 to 16 branch must be
assessed.
Step 1
0.06 x
D
0.06 10
= 171
0.219
= 171 3
= 168dB
PWL( x) = PWL
Step 2
168>155 additional analysis in accordance with EI guidelines is required
Step 3
Use flowchart T2-6
D=406 ; d=219
7
S=49.3 ; B=161.5 ; N=4.5x10
FLM1=1.34 ; FLM2=1 ;FLM3=1
7
N=5.9x10
Step 5
Calculate the LOF
LOF = 0.1303 ln (N ) + 3.1
= 0.1303 ln 5.9 10 7 + 3.1
= 2.3 + 3.1
LOF = 0.8
Step 6
As the LOF is greater than 0.5, corrective action is required
Increasing the wall thickness to 16 mm reduces the LOF to 0.5. An alternative would be to
increase the wall thickness locally to 40S (for a stainless line) or STD wall (for a carbon steel
line) and use a sweepolet or forged tee.
Page 19 of 33
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Assessment of Acoustically Induced Vibration
Section 3: 24 line downstream of valve
The 16 pipe now joins a 24 header using a fabricated tee. The 16 to 24 branch must be
assessed.
Step 1
0.06 x
D
0.06 60
= 168
0.406
= 168 9
PWL( x) = PWL
= 159dB
Step 2
159>155 additional analysis in accordance with EI guidelines is required
Step 3
Use flowchart T2-6
D=610 ; d=406
8
S=27.9 ; B=156.3 ; N=6.5x10
FLM1=1.44 ; FLM2=1 ;FLM3=1
8
N=9.3x10
Step 5
Calculate the LOF
LOF = 0.1303 ln ( N ) + 3.1
= 0.1303 ln 9.3 10 8 + 3.1
= 2.3 + 3.1
LOF = 0.4
Step 6
The LOF is less than 0.5, no further corrective action is required
Page 20 of 33
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Assessment of Acoustically Induced Vibration
Annex C: Example 3
The relief valve has a 80 Bar(A) upstream pressure and discharges into a header that is initially at
atmospheric conditions. The mass flow rate is 340,000 kg/hour. The relief temperature is 80oC, the gas
molecular weight is 18 and the ratio of specific heats is 1.25.
12 DIA
24 DIA
Where:
P1
P2
T2
=
=
=
upstream pressure
downstream pressure
downstream temperature
A correction factor to account for sonic flow . If sonic conditions exist
then SFF=6; otherwise SFF=0
Downstream Mach number
80
1
353
Bar(A)
Bar(A)
K
SFF
dB
M2
W
D
Di
=
=
=
94.4
0.323
0.313
1.25
kg/s
m
m
flow rate of gas and liquid
nominal pipe diameter
Inside pipe diameter (std wal)
ratio of specific heats Cp/Cv
molecular weight
18
Page 21 of 33
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Assessment of Acoustically Induced Vibration
Section 1: 12 line downstream of valve
Step 1
Calculate Mach number. The developed back pressure is 8 Bar(A)
M 2 = 116
W
P2 Di2
T2
m
94.4
8 10 5 0.313 2
M = 0.55
= 116
Step 2
353
18 1.25
Calculate sound power level.
1.2
P P 3.6
2 T1
1
W + 126.1 + SFF
PWL = 10 log10
m
P1
1.2
80 1 3.6
2 353
= 10 log10
94.4
+ 126.1 + 0
18
80
= 181dB
Step 3
181>155 additional analysis in accordance with EI guidelines is required. Consider the 12
to 24 connection. This is a fabricated tee. Both lines are schedule 10S stainless steel.
Step 4
Use flowchart T2-6
D=604 ; d=323
4
S=-3.2 ; B=181.4 ; N=3.6x10
FLM1=1.34 ; FLM2=1 ;FLM3=0.35
4
N=1.6x10
Step 5
Calculate the LOF
LOF = 0.1303 ln (N ) + 3.1
= 0.1303 ln 1.6 104 + 3.1
= 1.3 + 3.1
LOF = 1.8
Step 6
As the LOF is greater than 0.5, corrective action is required
Increasing the wall thickness to 19 mm reduces the LOF to 1.4. This is not acceptable. Checking
the sound power level (181 dB) against pipe size (24) shows that these values fall into the
range where a substantial redesign is required. It would be recommended that specialist
assistance is sought or the flare design is substantially revised.
Page 22 of 33
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Assessment of Acoustically Induced Vibration
Annex D: Example 4
Two relief valve are designed to operate simultaneously into a common header.
valve #1
valve #2
10 m 8 DIA
10 m 8 DIA
5m 24 DIA
B
Valve #1 has a sound power level of 172 dB and valve #2 has a sound power level of 165 dB. The
flow is not sonic.
Page 23 of 33
GN 44-005
Assessment of Acoustically Induced Vibration
Location A: valve #1 tail pipe to header
The 8 tail pipe from valve #1 joins the 24 header using a fabricated tee. The 8 to 24 branch
must be assessed by first finding the total sound power level at the junction.
Step 1
Contribution from valve #1
0.06 x
D
0.06 10
= 172
0.219
= 172 3
PWL( x) = PWL
= 169dB
Contribution from valve #2
0.06 x
D
0.06 10 0.06 5
= 165
0.219
0.610
= 165 3 0.5
PWL( x) = PWL
= 162dB
Calculate the total sound power level at A using Table 5
PWL( A) = 169 dB + 162 dB
= 170 dB
Step 2
170>155 additional analysis in accordance with EI guidelines is required
Page 24 of 33
GN 44-005
Assessment of Acoustically Induced Vibration
Location B: valve #2 tail pipe to header
The 8 tail pipe from valve #2 joins the 24 header using a fabricated tee. The 8 to 24 branch
must be assessed by first finding the total sound power level at the junction.
Step 1
Contribution from valve #1
0.06 x
D
0.06 10 0.06 5
= 172
0.219
0.610
= 172 3 0.5
PWL( x) = PWL
= 168dB
Contribution from valve #2
0.06 x
D
0.06 10
= 165
0.219
= 165 3
PWL( x) = PWL
= 162dB
Calculate the total sound power level at A using Table 5
PWL( A) = 168 dB + 162 dB
= 169 dB
Step 2
169>155 additional analysis in accordance with EI guidelines is required
Locations A and B may now be assessed using the same method as shown in previous
examples. This calculation is conservative as no account of attenuation at the header connection
or energy loss split between upstream and downstream. However, it is normally adequate for
initial screening purposes.
It is normally good practice to start with the valve with the highest sound power level.
Page 25 of 33
GN 44-005
Assessment of Acoustically Induced Vibration
Annex E: Mach Number Calculations
The mach number would normally be calculated at as part of the system process design. If this
calculation is not readily available, then the Mach number downstream of the valve may be calculated
as follows;
M =
v
c
where :
RT2
c=
m
w
v= 2
r
p m
= 2
ZRT2
Where:
M
v
c
=
=
=
=
Mach number (dimensionless)
Gas velocity in pipe (m/s)
Speed of sound of gas in pipe (m/s)
Ratio of specific heats
R
T2
m
w
r
D
=
=
=
=
=
=
=
Universal gas constant
Downstream temperature (K)
Molecular weight of gas
Mass flow rate (kg/s)
Pipe inside radius (m)
Pipe inside diameter (m)
Gas density (kg/m3)
p2
Z
=
=
Downstream pressure (Pa)
compressibility
If equation is rearranged to give the Mach number then:
M =
w ZRT2
m
2
p2 m
r
RT2
22 Z R
w
T2
=
2
m
p2 D
Using the normal values for R (8315) and Z (1).
2 2 1 8315
T2
w
M =
2
p D
m
M = 116
T2
w
2
m
p2 D
Page 26 of 33
GN 44-005
Assessment of Acoustically Induced Vibration
Figure 2: Graph to estimate the ratio of specific heats for hydrocarbon gases
Page 27 of 33
GN 44-005
Assessment of Acoustically Induced Vibration
Annex F: Addition of sound power levels
Equation 1 shows that the sound power level is measured in dB (decibel). The Bel is a unit which
gives the number of tenfold changes between two quantities, whilst the deci indicates that that the Bel
is divided into units of ten. Sound power level is defined as shown in Equation 4
sound power
PWL = 10 log10
dB
reference power
Equation 4
Where reference power is 10-12 watts.
Using this equation, it can be seen that 3dB represents a doubling of energy.
PWL
PWL= 155 dB
sound power = 10
10
1012 = 3,000 watts
PWL
PWL= 170 dB
sound power = 10
10
10 12 = 100,000 watts
PWL
PWL= 173 dB
sound power = 10
10
1012 = 200,000 watts
It can be seen that care must be taken when adding two decibel values. This can be done by using
Table 5.
Difference between the
two levels
dB
Add to higher level
dB
2.5
1.5
0.5
0,5
10
Table 5: Addition of sound levels
Page 28 of 33
GN 44-005
Assessment of Acoustically Induced Vibration
Annex G: Input Required For Detailed Analysis
The input required to complete an acoustic structural finite element analysis includes the following:
Relief system design philosophy and details including
a.
P&ID showing scope of system
b.
Isometrics showing all connections and details of supports and any pad reinforcement
details
c.
Valve data sheets
d.
Simultaneous relief scenarios
e.
Relief system design simulation output (flarenet or equivalent)
For non-relief systems, similar information will be required to complete the acoustic structural finite
element review.
The analysis results in a design time to failure. For non-continuously operated valves this design life
will normally be assessed against a required life of five hours.
Page 29 of 33
GN 44-005
Assessment of Acoustically Induced Vibration
Annex H: Design Limits for Acoustically Induced Vibration
Prior to the introduction of the EI guidelines, assessment of AIF was often based on figures such as
shown in Figure 3 and Figure 4. Such charts has been included in this section to act as a second design
check against the EI LOF method. The figure clearly demonstrates that AIF is a function of pipe
diameter. Whilst such figures are useful they do not highlight the particular vulnerability of weldolet
branch connections and other structural discontinuities.
Figure 3: Design limits for acoustically induced vibration
Page 30 of 33
GN 44-005
Assessment of Acoustically Induced Vibration
The data from Carrucci and Mueller[1] were plotted using the diameter to thickness ratio by
Eisenger[5]. This is shown in Figure 4.
Figure 4: Design limits for acoustically induced vibration
Page 31 of 33
GN 44-005
Assessment of Acoustically Induced Vibration
The assessment of the various action levels is given in Table 6 and Table 7 for continuously and non
continuously operated valves respectively. Relief valves are normally considered to operate non continuously. If the PWL exceed the design limit by about 15dB or more, then more significant system
changes are normally required.
No Action
No
Yes
50 D
3<5
Yes
5 < 15
Yes
D
E
Min. pipe
thk (mm)
0.5
No
No
13
0.5
No
No
50D. /3
13
0.5
No
Yes
50D. /3
16
0.5
No
Yes
> 15
Yes
rqd?
Length to apply
downstream
No Action
No
Yes
50D. /3
3<5
Yes
5 < 10
10 < 15
> 15
Min. pipe
thk (mm)
Redesign
Piping integrity Improvement
Max. d/s
Action
=PWL-DL
(dB)
Mach No.M2
Table 6: Design actions: Continuously operated systems
0.75
No
13
0.75
No
50D. /3
13
0.75
No
Yes
50D. /3
13
0.75
No
Yes
50D. /3
16
0.75
No
Yes
Table 7: Design actions: Non-continuously operated systems
Page 32 of 33
Valves (Y/N)
Length to apply
downstream
Low Noise
rqd?
Redesign
=PWL-DL (dB)
Max. d/s
Action
Mach No.M2
Piping integrity improvement
GN 44-005
Assessment of Acoustically Induced Vibration
Bibliography
[1]
Acoustically Induced Piping Vibration In High Capacity Pressure Reducing Systems. V.A. Carucci
and R.J. Meuller. ASHE winter annual meeting 14-19th November 1982.
[2]
Acoustic Fatigue in Pipes. Concawe Report No. 85/52, 1985
[3]
Acoustically induced structural fatigue of piping. F.L.Eisenger and J.T.Francis. Transactions of ASME
Vol121, November 1999.
[4]
Prudhoe Bay Central Gas Facility Start-up planning, commissioning and early operation. C.B.Nan.
MD.D.Kyrias, Gas Processors Annual Convention Proceedings, 1988
[5]
Designing piping systems against acoustically-induced structural fatigue. F.L.Eisenger, ASME PVPVol 328
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