Acoustic Induced Vibration

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


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


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


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


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. 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 GP 44-70 GP 44-80

Piping systems (ASME B31.3) Overpressure Protection Systems Relief Disposal Systems

Page 6 of 33


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 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 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


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


Yes


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


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

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

No action required

EDP service, used at start up

Table 1: Typical layout for acoustic fatigue calculation

Page 9 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.

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 mγ

Equation 3

Page 10 of 33


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

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

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

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


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


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


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

Page 14 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.

10m

8” DIA

16” DIA

60m

24” DIA

Where: P1 P2 T2

= = =

30 1 313

Bar(A) Bar(A) K

0

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

= = = =

γ m

=

molecular weight

25

Page 15 of 33


Section 1: 8” line downstream of valve Step 1

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

M 2 = 116

T2 mγ

W P2 Di2

5.56 1 × 10 × 0.2112 M = 0.48 = 116

Step 2

5

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


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

=

0

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

γ m

=

molecular weight

25

Page 17 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).

M 2 = 116

W P2 Di2

T2 mγ

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


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


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


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


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


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


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

=

0

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

γ m

=

molecular weight

18

Page 21 of 33


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 mγ

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


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


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

A

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


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


Page 24 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

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


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


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 :

γRT2

c=

m w v= 2 πr ρ p m ρ= 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

ρ

= = = = = = =

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

γ

If equation is rearranged to give the Mach number then:

M =

w ZRT2 m × × 2 p2 m πr γRT2

⎛ 22 Z R ⎞ w T2 ⎟× × = ⎜⎜ 2 ⎟ mγ ⎠ p2 D ⎝ π Using the normal values for R (8315) and Z (1).

⎛ 2 2 × 1 × 8315 ⎞ T2 w ⎟× M = ⎜⎜ × 2 ⎟ p D mγ π 2 ⎠ ⎝ M = 116

T2 w × 2 mγ p2 D

Page 26 of 33


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

Page 27 of 33


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

× 10−12 = 3,000 watts

PWL

PWL= 170 dB

sound power = 10

10

× 10 −12 = 100,000 watts

PWL

PWL= 173 dB

sound power = 10

10

× 10−12 = 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

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

Page 28 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 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


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


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


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

≤0

No

A

≤3

Yes

50 D

B

3<Δ≤5

Yes

C

5 < Δ ≤ 15

Yes

D E

Min. pipe thk (mm)

≤ 0.5

No

No

13

≤ 0.5

No

No

50D. Δ/3

13

≤ 0.5

No

Yes

50D. Δ/3

16

≤ 0.5

No

Yes

> 15

Yes

rqd?

Length to apply downstream

No Action

≤0

No

A

≤3

Yes

50D. Δ/3

B

3<Δ≤5

Yes

C

5 < Δ ≤ 10

D

10 < Δ ≤ 15

E

> 15

Min. pipe thk (mm)

Redesign

Piping integrity Improvement Max. d/s

Action

Δ=PWL-DL (dB)

Mach No.M2

Table 6: Design actions: Continuously operated systems

≤ 0.75

No

13

≤ 0.75

No

50D. Δ/3

13

≤ 0.75

No

Yes

50D. Δ/3

13

≤ 0.75

No

Yes

50D. Δ/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

Δ=PWL-DL (dB) Max. d/s

Action

Mach No.M2

Piping integrity improvement


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

Page 33 of 33

Acoustic Induced Vibration

Recommend Documents