Valve Vibrations: Examining the Complexities Associated with Control Valves – Part One

In this two-part series, TK Arasu and Chris Vergos of Emerson explore how to identify and reduce equipment-damaging vibrations linked to control valve installations. Part One focuses on two of the three main vibration sources and provides effective mitigation methods.

By TK Arasu and Chris Vergos, Emerson

Figures Courtesy of Emerson.

Extreme vibrations are often attributed to control valves. These conditions can create hazardous levels of sound, affect control valve performance, damage valve internals, and even crack the piping systems to which the valves are affixed. Interestingly, the valve may not even be the cause of the condition.

This article explains the sources of extreme vibrations and sound that can occur around control valves, as well as the damage that can result. It then turns to different solutions for mitigation once the cause has been identified.

Vibration Sources

Most plants have a screaming control valve, maybe more than one. It may be a high pressure drop application that howls like a banshee, or perhaps it is a valve that sounds like gravel is passing through it.

Other valves shriek only during certain process conditions and are quiet otherwise. Sometimes, the sound seems to emanate from the area around the valve in such a way that it is difficult to identify the actual source.

The first step to address a vibration and/or noise issue is understanding what is causing the situation. There are many potential sources of noise and vibration that could be involved with a particular application.

Reciprocating compressors, water hammer, pump pulses, and slug flow are all possible causes. However, this article — in two parts — will focus on the three main sources associated with control valves: flow induced vibration (FIV), acoustic induced vibration (AIV), and buffeting.

All these conditions usually result from flow turbulence (see figure 1). The energy of that turbulence may ultimately be propagated through FIV that resonates in the piping, or it may create extreme buffeting within the valve itself. In other situations, the flow turbulence creates acoustic waves that vibrate the downstream piping via AIV.

Flow Induced Vibration

One of the more common causes of vibration and noise around a control valve is FIV. In liquid applications, FIV can be created by cavitation, flashing, or outgassing. With these conditions, the liquid undergoes a phase change due to reduced pressure, either in the throat of the valve or downstream.

In the case of cavitation, the liquid turns to vapor as it moves through the restricted valve port, and it then collapses back to a liquid as pressure is recovered.

Flashing occurs when the downstream pressure is below the fluid’s vapor pressure, permanently vaporizing some or all of the liquid. Outgassing is a phenomenon where dissolved gas within a liquid mixture is released when there is a pressure drop.

All these conditions can damage the valve internals and downstream piping, but all are well understood, so they will not be discussed in further detail in this article.

Mitigation for these conditions usually involves specialized anti-cavitation trims that utilize pressure staging, hardened valve internals, and/or carefully designed valve bodies and downstream piping to minimize the damage.

With compressible process media like gases, FIV typically arises from rapid gas expansion, caused by a significant pressure drop or drastic velocity changes.

This situation can occur when one flow stream merges with another, or as the fluid navigates the complex flow path of a control valve (see figure 2).

FIV excitation of pipe resonances is typically a low frequency (< 500 hertz) phenomenon and can be exacerbated by inadequately sized piping—which increases velocity—or by convoluted piping configurations, such as close-coupled expanders, tees, elbows, and dead legs. It can also occur when there are long, unsupported piping runs, which tend to set up consistent, high amplitude vibrations, concentrating stress at the endpoints. The resulting vibration may quickly result in catastrophic pipe fatigue failures (see figure 3).

Fortunately, some FIV can be anticipated using piping modeling software and analytical techniques, which can predict problems with piping arrangements prior to construction. FIV damage can be mitigated by using larger diameter piping, which reduces velocity.

It can also be mitigated by spacing out piping transitions — especially downstream of a control valve taking a large pressure drop — to minimize flow turbulence, and by adding additional piping support to reduce movement and metal fatigue.

Acoustic Induced Vibration

AIV is typically higher frequency (> 500 hertz), and it occurs in the downstream of a control valve or in nearby piping. AIV is often the result of extremely high pressure drops across a valve, which creates supersonic flow, resulting in extreme turbulent shear regions that generate high frequency sound.

Very high sound levels can damage hearing, and they can create metal fatigue in the control valve and surrounding piping.

AIV and FIV can occur simultaneously in close-coupled piping configurations (see figure 4). When an installation lacks adequate upstream and downstream straight pipe runs before and after a control valve, the FIV induced by a valve can be amplified by the increased turbulence from the close-coupled piping. It can also generate noise and potentially AIV.

Sound not only emanates from the valve, but it also radiates from downstream piping. Sound radiating from piping systems loses energy slower with distance than noise from a point source, so it can be bothersome to even far-flung site personnel, and to neighboring communities.

Mitigation and Conclusion

AIV risks can be mitigated in several ways, for example with special, low-noise control valve trim, which dramatically reduces sound levels. AIV can also be reduced by using thicker walled pipe or by downstream sound diffusers, which extend the pressure drop across multiple devices.

Lower intensity AIV can be addressed with acoustic insulation or sound absorbing blankets. Often a combination of these solutions may be employed to address very high AIV applications.

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