Avoiding Flow-Induced Sympathetic Vibration in Control Valves

By Asher Glaun, PE, GE Energy

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Compressible fluid flow through control valves will inevitably cause some form of flow-induced vibration in the fluid system. Identifying the type and cause of the vibration requires detective work. Determining the design changes required in the valve and fluid system to prevent the vibration from occurring requires advanced analytical techniques.

There are very good reasons for power plant designers to specify reciprocating control valves oriented in the flow-to-open direction (flow tending to lift the plug) where compressible fluids are involved. One good reason is that this configuration allows the use of noise-abatement trims, such as drilled-hole cages and tortuous path stacks. Another application advantage is to protect a compressor from surges by allowing the flow to lift the plug and open the valve in the event of stem or actuator failure (Figure 1).

1. Flow-to-open cutaway. Control valves equipped with noise-abatement trim, such as a drilled-hole cage or a tortuous path stack, are typically oriented in the flow-to-open direction. This orientation maximizes noise attenuation by allowing the expanding gases to exit the valve trim and continue downstream without re-converging through the valve’s seat ring, as would happen if the flow direction were reversed. Source: GE Energy

There can, however, be unintended, and unwelcome, consequences. Under certain conditions, low-pressure cells will detach from the valve body wall, creating eddies that are carried downstream with the flow until they dissipate. These eddies produce periodic pressure pulsations that can damage the valve trim and body, leading to unplanned and costly downtime for repairs.

Although there are many forms of vibration present in a typical power plant, this article focuses on the cause of flow-induced sympathetic vibration (see sidebar). In addition, it discusses the interaction of the fluid with the valve trim, using computational fluid dynamics (CFD) to analyze the fluid flow in the valve under real-world conditions. Strategies to help prevent vibration and resulting performance problems are also offered.

Sources of Vibration in Valves

Control valve vibration may be caused by many factors, some generated internally by the process stream and others generated externally. Some of the different forms of flow-induced vibration are explained below.

Process Stream Energy Release. A flowing process contains considerable energy, a portion of which is released during its journey through a valve. The released energy takes the form of fluid turbulence, viscous drag, generated noise flow instabilities, and other forms of released energy. The released energy also interacts with the valve trim and may cause noticeable vibration.

Upstream Equipment Pulsations. Compressors, pumps, and other upstream equipment generate mechanical vibration and pressure pulsations that interact with other process equipment and piping in the system. This vibration energy is efficiently carried downstream by the piping system. In addition, pipe configurations such as elbows and tees upstream of the valve can create swirling flows and introduce turbulence, which may affect the valve.

Pipeline Resonance. Pipelines with long runs between supports can resonate at the first or second bending mode of the system, vibrating up and down in a “U” or “S” shape. Higher-order pipe natural modes (radial and circumferential) may also be excited. These natural modes can be excited by other vibration sources, causing vibration levels to build up and affect process equipment and the supporting structure.

Support Structure Vibration. Some form of vibration is always present in a running plant, but normally, it is not severe enough to be problematic. However, if the support floor resonates in tune with this vibration, the vibration amplitude can increase to the point where it may become destructive.

Acoustic Noise Creation. Very high sound power levels can release enough energy to create serious valve vibration and, in extreme cases, cause metal fatigue.

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