Noise from Cavitation: Bad for Control Valves(2)

Regardless of whether the cavitation occurs inside or downstream of the valve, equipment downstream of the cavitation zone can see extensive damage. The large-amplitude vibrations can excite oscillatory failure in thin diaphragms, springs, and small cross-section or cantilevered structures. Frequent points of failure are found in instrumentation such as pressure gauges and transmitters, thermowells, flow meters, and sampling systems. Heat exchangers can fail prematurely from fatigue as the heat transfer fins oscillate due to vibration. Check valves, actuators, positioners and switches that contain springs will suffer accelerated wear, and mounting brackets, fasteners and couplings will loosen and fail because of the vibration. Unfortunately, hydrodynamic noise propagates extremely well in liquids and metal pipe walls, so it can take hundreds of pipe diameters for the noise to dampen to non-dangerous levels once it occurs.

A globe control valve plug that has been damaged by cavitationFretting corrosion, which occurs between wearing surfaces exposed to vibration, is common near cavitating valves. This generates hard oxides which act as abrasives to accelerate wear between the wearing surfaces. Equipment affected includes isolation and check valves in addition to the control valve, pumps, rotating screens, samplers and any other rotating or sliding mechanism.

The high-amplitude vibrations also microscopically flex the metal valve parts and pipe walls, encouraging grain-boundary cracking and corrosion. The solid particles and corrosion chemical by-products released represent potential contaminants to the process. For instance, if the process is a RODI water treatment loop, a food or pharmaceutical process, a paper pulp bleaching line, or any number of high-purity processes, these contaminants degrade the quality of the product.

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