During operation of choked flow gas valves, such as relief, blowdown and process control valves, significant broadband, high-frequency “jet” noise is generated by shock cell formation and turbulent mixing of the gas flow through and immediately downstream of the valve (typically in the range of 500 to 2000 Hz). This noise is clearly audible in the vicinity of the valve and since the noise travels with the gas flow into the pipework downstream of the valve, it can also be heard for some distance from the valve. Hence in extreme cases, it can be a noise hazard requiring appropriate noise control or management for personnel working in that area.

Of greater concern, however, is that the noise travelling through the downstream pipework excites the higher-frequency piping shell flexural modes (see Figure 1 for typical vibration mode shapes). In practice, this vibration is not visible as the displacements are small, but it can be felt if a hand is placed on the pipework. For typical piping designs, the piping stresses are acceptable from a fatigue perspective, even close to welded circumferential, axisymmetric (“girth”) welds. In contrast, where there are non-axisymmetric welded connections, for example weldolets, weldoflanges, tees, pipe supports etc, significant dynamic stresses can result due to the piping geometry, sufficient to be of fatigue concern close to the weldline and ultimately leading to an acoustic fatigue failure. Since the piping vibration is high-frequency, the time to fatigue failure can be short and measured in minutes, or failure resulting in hydrocarbon release can occur after one or two valve operations.

Figure 1: Piping shell flexural modes excited by internal noise

New requirements of API 521

Figure 2: Acoustic fatigue failure

The 6th edition of API 521, “Pressure-relieving and Depressuring Systems,” was released in January 2014. This latest edition of API 521 makes explicit comments regarding operator considerations and design requirements to avoid acoustic-induced vibration fatigue failures. The impetus for including these comments was due to operator experience of AIV fatigue failures, including the safety risks of hydrocarbon release, production downtime and remediation costs.

The 6th edition of API 521 requires that “the potential for acoustic fatigue” be evaluated through one of several different methods, including a first level screening of the sound power level (PWL, a measure of noise energy) generated by the flare system gas valves. According to the standard, the high-risk piping downstream of the high-level concern valves should then be assessed for the risk of acoustic fatigue failure, and appropriate mitigation measures then implemented as appropriate.

It is therefore imperative for both operators and piping design engineers to consider AIV as a significant safety and production-critical risk for gas relief and blowdown systems on gas compressor stations, LNG and gas plants, or offshore platforms.

It should be noted that relief system pipework is particularly prone to AIV as to meet the (low) pressure requirements, only thin wall piping is generally employed, typically Schedule 10/10S. This means that the piping vibration and hence dynamic stresses are usually higher for a given valve noise level, compared with more typical, thicker pipe wall process gas pipework. 

Recommended approach to meeting API 521

The requirement to meet the 6th Edition API 521 standard includes an initial calculation of the valve sound power level (which employs an empirical formula to predict the acoustic energy produced immediately downstream of the valve) to identify valves of concern. Several methods then exist to identify the downstream piping most at risk of an AIV fatigue failure:

1. Carucci and Mueller: proposed 30 years ago, these researchers were the first to collate field data to show the relationship between choked flow valve noise and AIV fatigue failures. They developed the base equation to relate flow rate, pressure, molecular weight and temperature to the valve sound power level. They then proposed acceptable PWL values for a range of pipe diameters (“D”), based on empirical data.
2. Eisinger: proposed a revised acceptance criterion for the valve sound power level, using the original Carucci and Mueller data, but related the valve PWL to the pipe diameter to wall thickness ratio (“D/t”).  
3. Energy Institute Guideline: uses a Likelihood of Failure (LOF) prediction for each welded connection, based on the previous assessment methodologies, and factoring PWL, D/t, D/d (mainline to branch line) and type of branch connection. This risk-based approach therefore identifies the welded connection most likely to fail due to AIV and hence any mitigation measures or design changes are targeted to specific piping sections.

Based on extensive field experience and joint industry project (JIP) research, the Wood Group VDN team has developed its own risk-based, AIV assessment methodology. This approach is less conservative for specific welded connections, resulting in lower assessment and mitigation costs and higher reliability than other methods. Wood Group’s method is based on three phases:

• Phase 1: Identify all gas valves subject to choked flow - typically PSVs, BDVs, PCVs, ASVs - and then predict the valve sound power level (PWL) associated with operation of each of the valves, or likely valve operating combinations
• Phase 2: For valves with a PWL of concern, carry out an assessment of the downstream pipework, based on predicting the LOF of welded side-branch and tee connections, and welded pipe supports.
• Phase 3: For high-LOF welded connections, review design changes and/or remedial actions to give an acceptable LOF: alternatively use detailed FEA to give a more accurate acoustic fatigue life prediction (hours, or valve operation events) for subsequent evaluation. 

Wood Group VDN has extensive experience of carrying out detailed FEA for acoustic fatigue life predictions, based on many years of development and supported by both field measurements and laboratory tests.

Wood Group VDN experience base

Engineers in the Wood Group vibration, dynamics & noise (VDN) service line have extensive and industry-leading experience of carrying out AIV studies, based on close involvement with an AIV joint industry project (JIP), and through their active leadership in developing the Energy Institute guidelines relating to piping vibration.

VDN engineers perform AIV assessments during the design phase for EPCs (usually upon request of the operator) and for operational assets where a study has not been carried out. In addition, Wood Group’s engineers have carried out numerous AIV failure investigations, including site measurements, detailed finite element analysis, and identification of practical short- and long-term design mitigation measures.

About the author

Jonathan Baker, PhD (Acoustics and Vibration), has 26 years of experience in the assessment, measurement and troubleshooting of piping noise and vibration concerns, predominantly in the global oil and gas industry. He was involved in the Energy Institute Guidelines, the Industry Best Practice for minimising the risk of vibration-induced fatigue failure of process pipework. Similarly, he has presented technical papers at conferences, including a joint paper with a major operator reflecting experience of AIV concerns for both new designs and operational plant. Jonathan has previously worked for Xodus and joined the Wood Group VDN service line in early 2016 along with other leading experts in static equipment dynamics.

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