Excessive vibration can cause damage to a control valve, measurement equipment and the piping system. It’s often difficult to find the source of the vibration, which can originate not only with the control valve, but also with upstream and downstream piping and equipment. This article discusses modern techniques to help monitor, predict and solve problems caused by vibration.
Solving valve vibration problems is difficult for many end users. Shawn Anderson, engineering specialist at Emerson, and Adin Mann, principal engineer at Emerson, explain the situation in their article Analyzing Vibration in and Around Control Valves in the March 2018 issue of Flow Control magazine:
Plants often try to fix problems by measuring vibration, moving equipment, stiffening the valve structure, rewelding broken pipes, changing the piping system and other remedies — only to find problems still exist.
Vibration experienced in control valve installations can be caused by pump cavitation, flow pulsation, flow-induced excitations, vortex shedding, rapid valve closure, vapor pocket collapse, pump startup and shutdown, slug flow, water hammer and many other hard-to-diagnose conditions. The exact problem can be hard to diagnose, especially if conditions around the valve are hazardous, they say:
In some cases, valves and piping can shake so badly that workers are afraid to go near the system for fear the piping will burst. In other cases, the valve noise generated is excessive.
Shawn and Adin explain that two common types of vibration are associated with control valve installations. Acoustically induced vibration (AIV) generates excessive levels of high-frequency acoustic energy that can cause fatigue failure of welded downstream connectors. The noise levels generated by the valve and other piping elements are high enough to cause damaging vibration to the piping system. Flow-induced vibration (FIV) generates high levels of kinetic energy that can cause piping vibration, loosen piping supports, and cause fatigue failure at piping branches.
Excessive vibration can damage a control valve’s accessories, such as mounting brackets, actuator tubing, transmitters and actuators. Actuator tubing can suffer fatigue failures at or near the nut/ferrule location, causing pressure to be lost in the tubing. Vibration can also cause fatigue failures in nearby measurement components, such as pressure or temperature instruments installed in the downstream or upstream piping.
One of the most difficult problems is determining what causes the vibration, they note. Vibration can be measured with accelerometers placed at various points on the valve and actuator assembly, and upstream and downstream of the valve. While vibration can be measured with handheld devices, the preferred solution is to use wireless, battery-powered accelerometers.
Vibration sensors are installed for two basic reasons: First, to measure the vibration in and around the valve and assess its severity in various locations; second, to assess changes and reductions in vibration after corrective measures are taken.
Data from the vibration sensors can be sent to a plant’s control and monitoring system, but analysis can be difficult because it often requires a high level of expertise, along with specialized training and tools, to extract usable information from the data. Shawn and Adin say that specialty analysis software, such as Emerson’s ValveLink, cuts through the complexity of vibration analysis to provide a simple, reliable indication of equipment health via a single trend:
Using the software, vibration data is analyzed for frequencies, amplitudes of displacement, velocity and acceleration, and characteristics of the vibration and changes in the vibration over time.
The article includes several examples of companies that had severe vibration problems that were solved by Emerson experts. As Shawn and Adin point out, sometimes it takes more than just vibration measurements to solve problems:
Although sensors can measure vibration, locating the root cause of problems can be difficult. What is needed in many cases is specialty software to analyze the vibration, and extensive domain expertise to correlate vibration data with process conditions. If this expertise is not available in-house, external experts—such as from Emerson—can often provide the required assistance.