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

8/28/2014 Guidelines for Acoustic Induced Vibration (AIV), Flow Induced Vibration (FIV) Analysis

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Guidelines for Acoustic Induced Vibration (AIV), FlowInduced 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 InducedVibration (AIV) & Flow Induced Vibration (FIV), especially with regard to small bore connections, in line with EI (Energy Institute) Guidelines. 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

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Noise Vibration Vibration Damping Vibration Welding Vibration Testing



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 acousticenergy can be generated by a pressure reducing device such as a Relief valve, Control valve or Orifice plate. The amplitude of this energy isgoverned 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 asper 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 AcousticInduced Vibration AIV Consultants 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 Pipewall thickness particularly, for low wall thick Flare lines, Sweepolet branch fitting in place of weldolet, forged lateral/ 90 Deg Tee inplace of stub in lateral/ 45 Degree connections, Full encirclement pad for supports at PSV outlets etc. In case of reinforced Stub-inconnection, consideration shall be given to Clause 304.3.5 (b) of ASME B31.3 regarding use of Tee fittings or complete encirclementtypes 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 suchrecommendations 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 shallbe reviewed & D/T



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 bridgestructural drawings.

Low noise trim, if used by Instrumentation, the same shall be informed to Acoustic Induced Vibration AIV Consultant to include inAcoustic 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 weldsprovide 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 occursduring years of operation.

For lines where Acoustic Insulation is recommended, Support selection shall be as per support codes suitable for Acoustic Insulation, asmentioned 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 generateshigh levels of broad band Kinetic Energy which can propagate through the Piping system. This leads to excitation of low frequency vibrationmodes of the pipe work causing vibrations of piping and in some cases pipe supports, which is generally referred to as Flow InducedVibration 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 perAppendix-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 FlowInduced Vibration FIV requirements the following guidelines shall be followed during initial design of lines prone to Flow Induced VibrationFIV.

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 branchfitting in place of weldolet, Brace supports for SBCs etc. As this will have impact on material procurement, the study shall be expedited

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& 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 connectionsshall 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 inboth 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 onone 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 & Guidesupport 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 thatGuides & Axial stops in these lines are active for all Load cases. Also provide adequate Hold down supports (types FCG, SG2 etc. asper 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 weldsprovide 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 duringyears of operation.

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

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

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