Noise from Cavitation: Bad for Control Valves and Equipment
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Cavitation Risks in Control Valves
Cavitation in control valves can lead to serious damage not only to the valves themselves but also to the entire piping system. This damage is primarily due to vibrational noise energy, accelerated corrosion, and process contamination. The high noise levels associated with cavitation are caused by the formation and collapse of vapor bubbles near and downstream of the vena contracta.Susceptibility of Different Valves
While cavitation typically occurs within the valve body in globe and rotary plug valves, it can also happen in the piping downstream of high-recovery valves like wafer-body segment (V) ball valves, large ball valves, and butterfly valves. Misapplication of these valves can result in weld-repaired leaks or frequent pipe replacements due to failures.Downstream Equipment Vulnerabilities
The damage extends beyond the immediate area of cavitation. Equipment downstream, such as pressure gauges, thermowells, flow meters, and heat exchangers, can suffer from large-amplitude vibrations. These vibrations can lead to oscillatory failure in thin diaphragms and springs, accelerated wear in check valves and actuators, and loosening of mounting brackets and fasteners. Hydrodynamic noise propagates effectively in liquids and metal pipes, causing potential damage over long distances.Fretting Corrosion Challenges
Fretting corrosion, a common issue near cavitating valves, accelerates wear between surfaces and contaminates the process with solid particles and corrosion by-products. This is especially problematic in high-purity processes like RODI water treatment, food or pharmaceutical production, and paper pulp bleaching.Predicting & Eliminating Cavitation Damage
Predicting cavitation in rotary valves is complex, often requiring more than just calculating the choked flow pressure drop. Some manufacturers use an incipient damage pressure drop to predict cavitation damage. If noise levels are kept below specific thresholds, significant damage can be avoided:Up to 3-inch valve size: 80 dBA
4-6 inch valve size: 85 dBA
8-14 inch valve size: 90 dBA
16 inch and larger valve size: 95 dBA
Methods to eliminate cavitation damage include selecting appropriate valve designs and modifying the process. Special valve designs can use flow division and pressure drop staging to minimize cavitation. Flow division reduces bubble size, resulting in less noise and damage. Pressure drop staging prevents the pressure at the vena contracta from dropping to the liquid’s vapor pressure.
Locating control valves where the inlet pressure is higher or the liquid temperature is lower can also help eliminate cavitation.
