How to eliminate valve cavitation

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Over time, due to the degradation of its components and cavitation generated by the valve, the pipeline system is prone to severe damage. Such damage is mostly caused by the energy of vibration noise, which accelerates the corrosion process. The formation and collapse of flow constriction near and downstream of the bubbles generated by the high-amplitude vibrations corresponding to the high noise levels caused by cavitation. Although this usually occurs in ball valves and rotary valves within the valve body, it can actually happen in the piping on the downstream side of V-ball valves or, in particular, butterfly valves, which are short in length and have a high recovery rate. Cavitation tends to occur when the valve is subjected to stress at a certain position, which can lead to leaks at the valve’s piping connections and welding repair sites; as a result, the valve is not suitable for use in that section of the pipeline. Whether cavitation occurs inside the valve or downstream of it, the equipment in the cavitation zone suffers extensive damage. Ultra-thin films, springs, and small-cross-section cantilever structures: large-amplitude vibrations can induce oscillatory failures. Frequent failure points have been found in instruments such as pressure gauges, transmitters, thermocouple sleeves, flow meters, and sampling systems. Actuators, positioners, and limit switches containing springs will suffer accelerated wear, while mounting brackets, fasteners, and connectors may loosen and fail due to vibration. Fretting corrosion, which occurs when a worn surface is exposed to vibrations, is common near bubble valves. This produces hard oxides that act as abrasives, accelerating wear between the surfaces. Affected devices include isolation and check valves, as well as control valves, pumps, rotary screens, samplers, and any other rotating or sliding mechanisms. Vibrations of high amplitude can also cause cracks and corrosion in metal valve components and pipe walls. Scattered metal particles or corrosive chemical materials can contaminate the fluid inside pipes, having a significant impact on valves and pipes designed for sanitary use as well as on pipes containing high-purity fluids. This is also not allowed to occur. The prediction of cavitation damage in plug valves is more complex, as it is not simply the calculated flow resistance drop. Experience shows that it is possible for the pressure in the main flow to drop to the vapor pressure of the liquid before local vaporization and bubble collapse occur in that area. Some valve manufacturers predict erosion damage at the beginning of the year by defining an initial damage pressure drop. A valve manufacturer’s method for predicting the onset of cavitation damage is based on the fact that it is the collapse of steam bubbles that causes cavitation and noise. The manufacturer has determined that if the calculated noise level is below the following limits, significant cavitation damage can be avoided. Valve sizes of up to 3 inches – 80 decibels; valve sizes of 4–6 inches – 85 decibels; valve sizes of 8–14 inches – 90 decibels; valve sizes of 16 inches and larger – 95 decibels. Methods to eliminate cavitation damage: Special valve designs to prevent cavitation utilize flow splitting and stepped pressure drops. “Valve flow splitting” involves dividing a large flow rate into several smaller flow rates, with the valve’s flow path being designed so that the flow passes through several parallel small openings. Since part of the size of the cavitation bubbles is calculated based on the opening through which the flow passes. Smaller openings produce smaller bubbles, resulting in less noise and less damage. "\"Staged pressure drop\" means that the valve is designed with two or more series-connected control points, so rather than having the entire pressure drop in a single step, it occurs in several smaller steps. A pressure drop smaller than a certain value can prevent the pressure at the throat from dropping the vapor pressure of the liquid, thereby eliminating valve cavitation. The combination of shunting and pressure drop staging in the same valve can achieve improved cavitation resistance in the following manner. During the modification of the valve, by positioning the control valve such that the pressure at its inlet is high (for example, on the far upstream side or at a lower height), the problem of cavitation can sometimes be eliminated. Furthermore, keeping the temperature of the liquid at the location of the control valve – and thus its vapor pressure – low (such as in a heat exchanger on the low-temperature side) can help eliminate cavitation problems. In summary, it has been shown that cavitation in valves indeed leads not only to a decline in performance but also to valve damage. Downstream pipelines and equipment are also at risk. Predicting cavitation and taking measures to eliminate it is the only way to avoid the problem of costly valve replacement expenses.