Guidelines for Acoustic Induced Vibration (AIV), Flow Induced Vibration (FIV) Analysis

Introduction

The scope of this topic is to define the design criteria and guidelines to be used for Design & Supporting for lines prone to Acoustic Induced Vibration (AIV) & Flow Induced Vibration (FIV), especially with regard to small bore connections, in line with EI (Energy Institute) Guide lines. This topic does not address lines subjected to other types of vibrations, such as vibrations due to pulsation, mechanical excitation, wind, earth quake etc.

• High Frequency Acoustic Excitation more commonly referred to as Acoustic Induced Vibration (AIV).

• Flow Induced Turbulence more commonly referred to as Flow Induced Vibration (FIV).

Abbreviations

AIV       Acoustic Induced Vibration

DLF       Dynamic Load Factor

EI        Energy Institute

FIV       Flow Induced Vibration

LOF       Likelihood of Failure

NPS       Nominal Pipe Size

IFC       Issued for construction

SIF       Stress Intensification Factor

SBC      Small Bore Connections

TPI       Third party Inspection

References

Energy Institute (EI) Guide lines: Guide Lines for the Avoidance of Vibration Induced Fatigue Failure in Process Pipe Work.

ASME B31.3: Process Piping

Description of Activities

4.1  Acoustic Induced Vibration

a)  Acoustic Induced Vibration – AIV Definition

Acoustic Induced Vibration (AIV) is generally applicable to lines in gas service. In a Gas System, high levels of high frequency acoustic energy can be generated by a pressure reducing device such as a Relief valve, Control valve or Orifice plate. The amplitude of this energy is governed primarily by the Flow rate & Pressure drop. Excitation due to this can lead to fatigue failure of welded downstream connections. Piping downstream of Pressure reducing devices as below is prone to Acoustic Induced Vibration AIV.

• Relief Valves.
• Blow Down valves & Restriction Orifices.
• Pressure reducing Valves.
• Compressor recycle Valve.
• Choke Valve.

b) Impact of Acoustic Induced Vibration – AIV

Acoustic Induced Vibration can lead to Fatigue failure at Small Bore branches.

c)  Mitigation of Acoustic Induced Vibration – AIV

Detailed Acoustic Induced Vibration – AIV study shall be carried out by AIV Consultant based on the Multidiscipline responsibility Matrix as per Appendix-1. The study recommendations shall be incorporated in the design of Piping Systems.

Generally the study recommendations will be available at a later stage of the Project. In order to minimize the modifications due to Acoustic Induced Vibration AIV Consultant’s recommendations, the following guidelines shall be followed during initial design of lines prone to AIV.

• Process to identify Acoustic Induced Vibration AIV Prone lines in Line list, at early stage of the Project.

• A separate Pipe Class may be used for Flare system.

• Generally after Acoustic Induced Vibration AIV study, the Consultant comes up with typical recommendations such as increase of Pipe wall thickness particularly, for low wall thick Flare lines, Sweepolet branch fitting in place of weldolet, forged lateral/ 90 Deg Tee in place  of stub –in lateral/ 45 Degree connections, Full encirclement pad for supports at PSV outlets etc. In case of reinforced Stub-in connection, consideration shall be given to Clause 304.3.5 (b) of ASME B31.3 regarding use of Tee fittings or complete encirclement types of reinforcement for branch stub-in connection.

As this will have impact on material procurement & design, the study shall              be expedited & provision for incorporation of such recommendations shall be kept to the extent possible in preliminary piping design itself.

• For high energy level PSVs like Slug Catcher/ Export Line PSVs, in low wall thick piping, D/T (Diameter/Thickness) ratios of pipes shall be reviewed & D/T<= 72 is recommended along with use of Sweepolets in place of weldolets. However, this needs to be reviewed during initial Piping bulk procurement stage in consultation with Acoustic Induced Vibration AIV Consultant & Project Management.

• SBC tappings (2” & below) shall be braced back to header by 2-plane bracing. In header pipe sufficient spool shall be provided to accommodate this bracing support. Bracing supports shall be from the main pipe, thus ensuring that the small bore connection moves with the main pipe during start up, thermal transients and vibration. Small bore connections shall not be braced from a local structure such as steel work, Decks etc. Any mass at the free end of a cantilever shall be supported in both directions to the axis of the small bore. Any possibility of reducing mass at the free end of cantilever also to be considered. (E.g. using mono block / slim line valves instead of 2 separate block and bleed valve arrangement.)

• Pipe wall thickness for flare systems shall be agreed with Consultant at early stage of Project, particularly in low wall thick lines.

• Type of Branch fittings for Acoustic Induced Vibration AIV prone lines shall be agreed with Consultant at early stage of Project.

• For Small bore standout connections, the branch fitting and overall unsupported length shall be as short as feasible.

• In certain cases use of clamped Pipe shoes will be recommended by Acoustic Induced Vibration AIV Consultant for flare system. In such cases, specific confirmation shall be taken from Consultant for the locations where this is mandatory & at other locations confirmation for use of welded shoes shall be taken, as clamped shoes, especially in higher sizes 24” & above are to be avoided. In Flare lines from outlet of Flare Knock out Drum to Flare Stack, confirmation for use of welded shoes shall be taken from Acoustic Induced Vibration AIV Consultant.

• Provide Axial Stop support very close to relief valve at PSV outlet lines, with full encirclement pad.

• Provide Hold Down support & Guides for Flare lines.

• Support span for flare lines for offshore platforms especially running on bridge to be determined at early stage of design to finalize bridge structural drawings.

• Low noise trim, if used by Instrumentation, the same shall be informed to Acoustic Induced Vibration AIV Consultant to include in Acoustic Induced Vibration AIV calculations which will help to reduce Acoustic Energy levels in the system.

• Avoid Spool between PSV inlet/outlet Flange & reducers at upstream & downstream of relief valves, Blow down valves.

• Minimize use of Trunion supports.

• Any fastenings used shall be designed to be effective under vibration such as providing lock nuts etc.

• Particular care shall be taken when adopting small bore supports that are welded to the connection and its main pipe as these welds provide additional sites for fatigue failure. Dressing of welds by grinding shall be recommended and discussed with Construction & QC.

• Clamp type supports rely on bolted arrangements. Periodic inspection at field shall be recommended to ensure that no loosening occurs during years of operation.

• For lines where Acoustic Insulation is recommended, Support selection shall be as per support codes suitable for Acoustic Insulation, as mentioned in Pipe Support Standard.

4.2 Flow Induced Vibration

a)  Flow Induced Vibration – FIV Definition

Turbulence due to flow depends on flow regime. The main sources of turbulence   are flow discontinuities in the system. This in turn generates high levels of broad   band Kinetic Energy which can propagate through the Piping system. This leads to    excitation of low frequency vibration modes of the pipe work causing vibrations of piping and in some cases pipe supports, which is generally referred to as Flow Induced Vibration FIV.

b) Impact of Flow Induced Vibration – FIV

Flow Induced Vibration can lead to Fatigue failure at Small Bore branches.

c) Mitigation of Flow Induced Vibration – FIV

Detailed Flow Induced Vibration FIV study shall be carried out by the Consultant based on the multi discipline responsibility Matrix as per Appendix-1.The study recommendations shall be incorporated in the design of Piping Systems.

Generally the study recommendations will be available at a later stage of the Project. In order to minimize the modifications due to Flow Induced Vibration FIV requirements the following guidelines shall be followed during initial design of lines prone to Flow Induced Vibration FIV.

• Process to Identify Flow Induced Vibration FIV prone lines at the beginning of the project.

• Generally after Flow Induced Vibration FIV study, the Consultant comes up with typical recommendations such as Sweepolet branch fitting in place of weldolet, Brace supports for SBCs etc. As this will have impact on material procurement, the study shall be expedited & provision for incorporation of such recommendations shall be kept in piping deign. Bracing supports shall be from the main pipe, thus ensuring that the small bore connection moves with the main pipe during start up, thermal transients and vibration. Small bore connections shall not be braced from a local structure such as steel work, Decks etc. Any mass at the free end of a cantilever shall be supported in both directions perpendicular to the axis of the small bore.

• Type of Branch fittings for Flow Induced Vibration FIV prone lines shall be agreed with Consultant at early stage of Project.

• For Small bore standout connections, the branch fitting & overall unsupported length shall be as short as feasible.

• Provide Axial Stop support very close to Control valves. The recommended support configuration for Control valve is Rest+Guide on one side & Rest+Guide+Axial Stop+Hold Down on the other side, subject to feasibility as per line geometry & Stress analysis.

• Provide adequate Hold down supports & Guides for FIV lines.

• For High Delta P valves, provide spool on upstream & downstream piping to accommodate Anchor Support on one side & Guide support on the other side.

• For 2 Phase & Slug Flow lines, provide axial stops on main line at all changes of direction as per feasibility. It shall be ensured that Guides & Axial stops in these lines are active for all Load cases. Also provide adequate Hold down supports (types FCG, SG2 etc. as per Support Standard) for main line & braced support for Small bore stand-alone connections.

• Minimize use of Trunion supports.

• Any fastenings used shall be designed to be effective under vibration such   as providing lock nuts etc.

• Particular care shall be taken when adopting small bore supports that are welded to the connection and its main pipe, as these welds provide additional sites for fatigue failure. Dressing of welds by grinding shall be recommended and discussed with Construction.

• Clamp type supports rely on bolted arrangements. Periodic inspection shall be recommended to ensure that no loosening occurs during years of operation.

Appendix- 1(Work Process Steps and the Responsibility Matrix for AIV /FIV study)