Safeguarding your process flow control system
1 Cavitation Understanding Flashing and Cavitation and flashing are major causes of metal erosion in flow control systems. It occurs when a media goes through a valve and the resulting pressure drop causes the media to increase in velocity and turn into a vapor. If it stays in a vapor state the resulting flow is called flashing and damage to the flow control system is smooth and shiny. If it returns to a liquid state, it is called cavitation. As the vapor returns to the liquid state, it forms liquid microjets and the pressure in the flow control system cause these microjets to burst, sending pressure waves of up to 100,000 PSI of force over the valve and piping surfaces. This bursting is the cause of pitting and rough surfaces in the flow control system. It also causes noise and vibration which can damage the flow control system and be harmful to workers safety. There are several factors that impact the effects of flashing and cavitation including: Intensity caused by greater pressure drops Materials the valve is made from Length of exposure Frequency of flashing and cavitation Valve size Design of the valve and trim A properly designed system using appropriate materials can add years to the productive life of capital assets and provide greater efficiency and quality to a process control system. But, where to start?
2 To Flow Control Considerations minimize the effects of turbulence and cavitation on a flow control system, several steps need to be taken. First, anything that impedes the flow of the media has to be considered. This is usually a valve, but could be changes in the flow direction like a ninety degree bend in a pipe. As the media flows past the restriction, there is lower pressure on the outlet side. With this reduction in pressure comes an increase in media velocity which can cause the liquid to move to a vapor state. So making this pressure reduction as low as possible is key. Next, the flow past the restriction should be optimized to limit the amount of turbulence that is caused by the obstruction. Figure 1 shows the impact of a butterfly valve and it s easy to see the downstream impact of the obstruction. Figure 1: Butterfly Valve during throttling Another key consideration is the materials used in the flow control system. Depending on the intensity and the duration of cavitation a system is exposed to, the materials selected are of prime importance. As patents have expired and flow control vendors compete on price rather than innovation, it is possible to get subpar materials into various parts of a flow control system.
3 With The Solution the valve as the key component of a well designed and installed flow control system, let s consider the attributes that a well-engineered valve should have. Zero pressure drop. The valve should have zero pressure drop between the inlet and the outlet of the valve. This maintains a constant velocity of flow and limits the potential for a liquid to turn into a vapor. Precise flow control. The valve should be able to manage the flow of media to reduce the introduction of turbulence. This can be achieved by having surfaces that are engineered and designed to limit flat points of contact with the media. Leak-proof closure. Even small amounts of media can cause cavitation. The valve should be leak-proof even under high pressures. Only one valve on the market today has the right set of features to achieve these benefits. The Shutter Valve from Clarke Industrial Engineering combines the correct mix of leak-proof closure up to 1,000 PSI, precision flow control using aeronautically designed control petals and has zero pressure drop so the pressure at the valve inlet equals the pressure at the valve outlet.
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