Just as sound can have negative effects on the human body, certain frequencies can wreak havoc on industrial equipment when exerted. When control valves are properly selected, there is an increased risk of cavitation, which will result in high noise and vibration levels, leading to very rapid damage to the valve's interior and downstream piping. Additionally, high noise levels often cause vibrations that can damage pipes, instruments, etc.
Valves are susceptible to severe damage to the piping system as a result of time-lapse, degradation of parts, and cavitation from valves. This damage is mostly caused by vibrational noise energy, which accelerates the corrosion process. The formation and collapse of the bubble near and downstream of the vena contracta is generated by large-amplitude vibrations with high noise levels reflected in cavitation. While this typically occurs with ball valves and rotary valves in the valve body, it can actually occur with short, high recovery piping on the downstream side of the valve like a wafer body section V-type ball valve, especially a butterfly valve. When the valve is stressed at one position, it is easy to produce cavitation phenomenon, so it is easy to leak at the piping and welding repair of the valve, and the valve is not suitable for this section of pipeline.
Whether cavitation occurs inside the valve or downstream of the valve, equipment in the cavitation zone can suffer extensive damage. Ultra-thin films, springs and cantilever structures with small cross-sections, large-amplitude vibrations can excite oscillatory faults. Frequent failure points are found in instrumentation such as pressure gauges, transmitters, thermowells, flow meters, and sampling systems. Actuators, positioners and limit switches containing springs will experience accelerated wear, mounting brackets, fasteners and connectors will loosen and fail due to vibration.
Fretting corrosion, which occurs between worn surfaces exposed to vibration, is common near cavitation valves. This creates hard oxides that act as abrasives to accelerate wear between the wear surfaces. Affected equipment includes isolation and check valves, in addition to control valves, pumps, rotating screens, samplers and any other rotating or sliding mechanisms.
High-amplitude vibrations can also crack and corrode metal valve parts and pipe walls. Scattered metal particles or corrosive chemical materials may contaminate the medium in the pipeline, which will have a significant impact on sanitary valve pipelines and high-purity pipeline media. This is also not allowed.
The prediction of cavitation damage in plug valves is more complicated than simply calculating the choke pressure drop. Experience has shown that it is possible for the pressure in the main liquid stream to drop to the vapor pressure of the liquid before a region of localized vaporization and vapor bubble collapse occurs. Some valve manufacturers predict incipient erosion damage by defining an incipient damage pressure drop. A valve manufacturer's starting method for predicting cavitation damage is based on the fact that it is vapor bubbles that collapse, causing cavitation and noise. The manufacturer has determined that significant cavitation damage can be avoided if the calculated noise levels are below the limits listed below.
Up to 3" Valve Size - 80 dB
4-6 inch valve size - 85 dB
8-14 inch valve size - 90 dB
Valve sizes 16" and larger - 95 decibels
Methods to Eliminate Cavitation Damage
Special valve design to eliminate cavitation employs split flow and staged pressure drop:
"Valve diversion" is to divide a large flow into several small flows, and design the flow path of the valve so that the flow passes through several parallel small openings. Part of the size of the air bubbles due to cavitation is calculated for the openings through which the flow passes. Smaller openings allow for smaller air bubbles, resulting in less noise and less damage at the time.
"Staged pressure drop" means that the valve is designed with two or more regulation points in series, so that instead of the entire pressure drop in a single step, it takes several smaller steps. A pressure drop of less than one individual prevents the pressure in the vena contracta from dropping from the vapor pressure of the liquid, thus eliminating valve cavitation.
The combination of flow splitting and staging of pressure drop at the same valve can achieve improved cavitation resistance in the following ways. During valve modification, positioning the control valve so that the pressure at the inlet of the valve is higher (such as on the farther upstream side, or at a lower altitude) can sometimes eliminate cavitation problems.
Alternatively, positioning the control valve at a location where the temperature of the liquid, and therefore the vapor pressure, is low (such as the low temperature side of the heat exchanger) can help eliminate cavitation problems.
Summarize
It has been shown that valve cavitation does more than just degrade performance and damage the valve. Downstream pipelines and equipment are also at risk. Predicting cavitation and taking steps to eliminate it is the only way to avoid costly valve consumption issues.