Acoustic Induced Vibration

8/9/2019 Acoustic Induced Vibration

1/33

17 May 2009

GN 44-005

Assessment of Acoustically InducedVibration

Guidance Note

Document No. GN 44-005

Date May 2009

BP GROUPENGINEERING TECHNICAL PRACTICES


2/33

GN 44-005 Assessment of Acoustically Induced Vibration

Page 2 of 33

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 theagreement 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 contractexpressly allow, or unless disclosure is required by law.

In the event of a conflict between this document and a relevant law or regulation, therelevant 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.


3/33


Page 3 of 33

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


Annex C: Example 3 ........................................................................................................................21


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


4/33


5/33


Page 5 of 33

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.


6/33


Page 6 of 33

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, wherethese 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 designalternative 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 overThese mechanisms are beyond the scope of this document. ETP’s 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 Piping systems (ASME B31.3)

GP 44-70 Overpressure Protection Systems

GP 44-80 Relief Disposal Systems


7/33


Page 7 of 33

Industry Standards

Energy Institute Guidelines for the Avoidance of Vibration Induced Fatigue in Process

Pipework, 2nd Edition, January 2008

3. 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 Likelihood of failure

4. AIF Proceedure

It is intended that this guidance note supplements the use of the EI Guidelines and BP ETP’s 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 insection 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 theassessment using this guidance note and the EI guidelines.


8/33


9/33


5. Acoustically Induced Vibration Data Sheet

Calculations would normally be completed using a spreadsheet type format. Typical layout is shown in Table 1. Part

used in the calculation. The formula and consistent set of units shown in section 5.1 should normally be used.

Inputs Result Commen

Valve Tag Mass flow UpstreamPressure

DownstreamPressure

Gas molecularweight

Upstreamtemperature

PWL Continuoservice

kg/hr Bar(A) Bar(A) deg C dB (>5 hours

V-001 20,000 30 1 25 40 154 N

V-002 140,000 30 1 25 50 171 N

V-003 100,000 40 1 18 53 170 N

V-004 160,000 40 1 18 53 174 N

V-005 150,000 60 1 22 53 172 N

V-006 70,000 40 1 22 53 166 Y

V-007 60,000 40 1 19 53 165 N

V-008 30,000 40 1 19 53 159 N

V-009 40,000 40 1 19 53 161 N

Table 1: Typical layout for acoustic fatigue calculation


10/33


Page 10 of 33

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.

Table 2: Valve nomenc lature

SFF m

T W

P

PPPWL ++

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟ ⎠

⎞⎜⎝

⎛ ⎟⎟ ⎠

⎞⎜⎜⎝

⎛ −= 1.126log10

2.1

12

6.3

1

2110

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.

D

xPWL xPWL

06.0)( −= 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.

γ mT

DPW M 2

2

2

2 116= Equation 3

p1

T1

p2

T2

PWL

Direction of flow

x

PWL(x)


11/33


Page 11 of 33

Where:

P1 = upstream pressure Bar(A)

P2 = downstream pressure Bar(A)

T1, = upstream temperature K

T2 = downstream temperature (0oC=273 K K

PWL = sound power level dB

SFF = A correction factor to account for sonic flow . If sonicconditions exist (M2>1) then SFF=6; otherwiseSFF=0

dB

M2 = Downstream Mach number

W = flow rate of gas and liquid kg/s

D = nominal pipe diameter m

x = distance downstream of valve m

γ = ratio of specific heats Cp/Cv, see Appendix E

m = molecular weight

DL = Design limit sound power level dB

5.1.1. 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. Downst ream 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 initialevent 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.


12/33


Page 12 of 33

5.2. Design action: Continuous ly 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.

Experience shows that piping systems associated with blow-down valves that are used during

plant commissioning or start-up are particularly vulnerable.

Piping integrity improvement

Minimum schedule

LOF

rqd?CS SS M

a x .

d / s

M a c h N o . M 2

S p e c i a l i s t

a s s i s t a n c e

L o w

N o i s e

V a l v e s ( Y / N )

≤ 0.5 No ≤ 0.5 No No

0.5 < LOF ≤ 1.0 Yes STD 40S ≤ 0.5 No Yes

> 1.0 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 whichcase 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.

5.3. Design action: Non- continuously Operated System

Relief valves are normally considered to operate non -continuously.

Piping integrity Improvement

Minimum schedule

LOF

rqd?CS SS M

a x .

d / s

M a c h N o . M 2

S p e c i a l i s t

a s s i s t a n c e

≤ 0.5 No ≤ 0.75 No

0.5 < LOF ≤ 1.0 Yes STD 40S ≤ 0.75 No

> 1.0 Yes

Table 4: Design actions: Non-continuously operated systems


13/33


Page 13 of 33

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 whichcase 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 isused 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. 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 completeremove 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.

5.5. Specialist assis tance

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 controlvalve (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


14/33


Page 14 of 33

strongly recommended that a BP mechanical specialist provide oversight to the external

specialist work.


15/33


Page 15 of 33

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.

Where:

P1 = upstream pressure 30 Bar(A)

P2 = downstream pressure 1 Bar(A)

T2 = downstream temperature 313 K

SFF = A correction factor to account for sonic flow . If sonic conditions existthen SFF=6; otherwise SFF=0

0 dB


W = flow rate of gas and liquid 5.56 kg/s

D = nominal pipe diameter 0.2 m

Di = Inside pipe diameter (std wall) 0.211 m

γ = ratio of specific heats Cp/Cv 1.21

m = molecular weight 25

8” DIA

24” DIA

60m16” DIA

10m


16/33


17/33


Page 17 of 33

Annex B - Example 2

The relief valve example used in Annex A has its capacity increased to 140,000 kg/hour.

Where:

P1 = upstream pressure 30 Bar(A)

P2 = downstream pressure 1 Bar(A)T2 = downstream temperature 313 K


0 dB




Di = Inside pipe diameter (std wal) 0.211 m



8” DIA

24” DIA

60m

16” DIA

10m


18/33


Page 18 of 33

Section 1: 8” line downstream of valve

Step 1 Calculate Mach number. The developed back pressure in the system is 8 Bar(A).

41.0

21.125

313

211.0108

89.38116

116

25

2

2

2

2

=×××

=

=

M

m

T

DP

W M

i γ

Step 2 Calculate sound power level.

dB

SFF m

T W

P

PPPWL

171

01.12625

31389.38

30

130log10

1.126log10

2.1

2

6.3

10

2.1

12

6.3

1

110

=

++⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟ ⎠

⎞⎜⎝

⎛ ⎟ ⎠

⎞⎜⎝

⎛ −=

++⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟ ⎠

⎞⎜⎝

⎛ ⎟⎟ ⎠

⎞⎜⎜⎝

⎛ −=

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 andtake the pipe as 8” standard wall

Use flowchart T2-6

D=219 ; d=219

S=65.1 ; B=160.2 ; N=8.3x107

FLM1=1.75 ; FLM2=1 ; FLM3=1

N=1.4x108

Step 5 Calculate the LOF

( )

( )

7.0

1.34.2

1.31044.1ln1303.0

1.3ln1303.0

8

=

+−=

+××−=

+×−=

LOF

N LOF

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 thewall 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.


19/33


Page 19 of 33

Section 2: 16” line downst ream of valve

The 8” pipe now joins a 16” header using a fabricated tee. The 8” to 16” branch must be

assessed.

Step 1

dB

D

xPWL xPWL

168

3171

219.0

1006.0171

06.0)(

=

−=

×−=

−=


Step 3 Use flowchart T2-6

D=406 ; d=219

S=49.3 ; B=161.5 ; N=4.5x107

FLM1=1.34 ; FLM2=1 ;FLM3=1

N=5.9x107


( )

( )

8.0

1.33.2

1.3109.5ln1303.0

1.3ln1303.0

7

=

+−=

+××−=

+×−=

LOF

N LOF


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.


20/33


Page 20 of 33


The 16” pipe now joins a 24” header using a fabricated tee. The 16” to 24” branch must be

assessed.

Step 1

dB

D

xPWL xPWL

159

9168

406.0

6006.0168

06.0)(

=

−=

×−=

−=


Step 3 Use flowchart T2-6D=610 ; d=406

S=27.9 ; B=156.3 ; N=6.5x108

FLM1=1.44 ; FLM2=1 ;FLM3=1

N=9.3x108


( )

( )

4.01.33.2

1.3103.9ln1303.0

1.3ln1303.0

8

= +−=

+××−=

+×−=

LOF

N LOF

Step 6 The LOF is less than 0.5, no further corrective action is required


21/33


Page 21 of 33

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.

Where:P1 = upstream pressure 80 Bar(A)

P2 = downstream pressure 1 Bar(A)

T2 = downstream temperature 353 K


0 dB




Di = Inside pipe diameter (std wal) 0.313 m



12” DIA

24” DIA


22/33


Page 22 of 33


Step 1 Calculate Mach number. The developed back pressure is 8 Bar(A)

55.0

25.118

353

313.0108

4.94116

116

25

2

2

2

2

=×××

=

=

M

m

T

DP

W M

i γ

Step 2 Calculate sound power level.

dB

SFF m

T W

P

PPPWL

181

01.12618

3534.94

80

180log10

1.126log10

2.1

2

6.3

10

2.1

12

6.3

1

110

=

++⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟ ⎠

⎞⎜⎝

⎛ ⎟ ⎠

⎞⎜⎝

⎛ −=

++⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟ ⎠

⎞⎜⎝

⎛ ⎟⎟ ⎠

⎞⎜⎜⎝

⎛ −=

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

S=-3.2 ; B=181.4 ; N=3.6x104

FLM1=1.34 ; FLM2=1 ;FLM3=0.35

N=1.6x104


( )

( )

8.1

1.33.1

1.3106.1ln1303.0

1.3ln1303.0

4

=

+−=

+××−=

+×−=

LOF

N LOF


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.


23/33


Page 23 of 33

Annex D: Example 4

Two relief valve are designed to operate simultaneously into a common header.

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.

5m 24” DIA

10 m 8” DIA10 m 8” DIA

valve #1 valve #2

A B


24/33


Page 24 of 33

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

dB

D xPWL xPWL

169

3172

219.0

1006.0172

06.0)(

=

−=

×−=

−=

Contribution from valve #2

dB

D

xPWL xPWL

162

5.03165

610.0

506.0

219.0

1006.0165

06.0)(

=

−−=

×−

×−=

−=

Calculate the total sound power level at A using Table 5

dB170dB162dB169)(

= += APWL



25/33


Page 25 of 33

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

dB

D

xPWL xPWL

168

5.03172

610.0

506.0

219.0

1006.0172

06.0)(

=

−−=

×−

×−=

−=

Contribution from valve #2

dB

D xPWL xPWL

162

3165

219.0

1006.0165

06.0)(

=

−=

×−=

−=

Calculate the total sound power level at A using Table 5

dB169

dB162dB168)(

=

+= APWL


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.


26/33


Page 26 of 33

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;

2

2

2

2

:where

ZRT

m p

r

wv

m

RT c

c

v M

=

=

=

=

ρ

ρ π

γ

Where:

M = Mach number (dimensionless)

v = Gas velocity in pipe (m/s)

c = Speed of sound of gas in pipe (m/s)

γ = Ratio of specific heats

R = Universal gas constant

T2 = Downstream temperature (K)

m = Molecular weight of gas

w = Mass flow rate (kg/s)

r = Pipe inside radius (m)

D = Pipe inside diameter (m)

ρ = Gas density (kg/m3)

p2 = Downstream pressure (Pa)

Z = compressibility

If equation is rearranged to give the Mach number then:

γ π

γ π

m

T

D p

w R Z

RT

m

m p

ZRT

r

w M

2

2

2

2

22

2

2

2××⎟

⎟ ⎠

⎞⎜⎜⎝

⎛ =

××=

Using the normal values for R (8315) and Z (1).

γ

γ π

m

T

D p

w M

m

T

D p

w M

2

2

2

2

2

2

2

116

831512

×=

××⎟⎟ ⎠

⎞⎜⎜⎝

⎛ ××=


27/33


Page 27 of 33

Figure 2: Graph to estimate the ratio o f specific heats for hydrocarbon gases


28/33


Page 28 of 33

Annex F: Addit ion 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

dB power reference

power sound log10 10 ⎥

⎦

⎤⎢⎣

⎡=PWL Equation 4

Where reference power is 10-12

watts.

Using this equation, it can be seen that 3dB represents a doubling of energy.

PWL= 155 dB watts000,30110 power sound 1210 =×= −PWL



It can be seen that care must be taken when adding two decibel values. This can be done by using

Table 5.

Difference between thetwo levels

Add to higher level

dB dB

0 3

1 2.5

2 2

3 2

4 1.5

5 1

6 1

7 1

8 0.5

9 0,5

10 0

Table 5: Addition of sound levels


29/33


Page 29 of 33

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 reinforcementdetails

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.


30/33


Page 30 of 33

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


31/33


Page 31 of 33

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


32/33


Page 32 of 33

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.

Piping integrity improvement

Action Δ=PWL-DL (dB)

rqd?Length to apply

downstream

Min. pipethk (mm)

M a x .

d / s

M a c h N o . M 2

R e d e s i g n

L o w

N o i s e

V a l v e s ( Y / N )

No Action ≤ 0 No ≤ 0.5 No No

A ≤ 3 Yes 50 D 13 ≤ 0.5 No No

B 3 < Δ ≤ 5 Yes 50D. Δ/3 13 ≤ 0.5 No Yes

C

D5 < Δ ≤ 15 Yes 50D. Δ/3 16 ≤ 0.5 No Yes

E > 15 Yes

Table 6: Design actions: Continuously operated systems

Piping integrity Improvement

ActionΔ=PWL-DL

(dB)

rqd?Length to apply

downstream

Min. pipe

thk (mm) M a x .

d / s

M

a c h N o . M 2

R

e d e s i g n

No Action ≤ 0 No ≤ 0.75 No

A ≤ 3 Yes 50D. Δ/3 13 ≤ 0.75 No

B 3 < Δ ≤ 5 Yes 50D. Δ/3 13 ≤ 0.75 No

C 5 < Δ ≤ 10 Yes 50D. Δ/3 13 ≤ 0.75 No

D 10 < Δ ≤ 15 Yes 50D. Δ/3 16 ≤ 0.75 No

E > 15 Yes

Table 7: Design actions: Non-continuously operated systems


33/33


Bibliography

[1] Acoustically Induced Piping Vibration In High Capacity Pressure Reducing Systems. V.A. Carucciand R.J. Meuller. ASHE winter annual meeting 14-19

th 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 ASMEVol121, 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 PVP-

Vol 328

Acoustic Induced Vibration

Documents

Transcript of Acoustic Induced Vibration