How to prevent flashing and cavitation of valves? In industry, we often find wear marks, deep grooves and pits on the internal parts of regulating valves, pressure reducing valves and other throttling devices, such as valve valves and valve seats. Much of this damage stems from a phenomenon known as “cavitation”.
Cavitation is a destructive transformation of a fluid that occurs when the fluid is subjected to specific pressure and temperature conditions, and consists of two phases: flashing and cavitation. Imagine the narrow passage formed by the valve seat and valve flap as the fluid flows rapidly through the regulating valve, resulting in dramatic changes in the pressure and velocity of the fluid.
Flash phase: when the pressure of P1 fluid through the throttle orifice, the flow rate surge, the static pressure plummeted. If the pressure P2 after the hole before reaching the saturation vapor pressure Pv of the fluid, part of the fluid will vaporize into gas, forming bubbles, gas-liquid coexistence. This is like a shaken bottle of soda suddenly opening and bubbles popping up quickly.
While regulating valves cannot completely prevent flash vapor, we can take steps to reduce its destructive power. Next, when the pressure downstream of the fluid rises back above the saturation pressure, these bubbles are compressed and suddenly burst, entering the cavitation phase. The rapid bursting of the bubbles creates a tremendous impact, and this concentrated energy and destructive force is the primary cause of cavitation damage.
Cavitation will not only leave smooth abrasion marks on the surface of the part, but will also tear the surface layer like sandpaper sanding, creating a rough surface. Under high-pressure differential conditions, even the hardest valve flaps and seats will be damaged in a short time, resulting in leakage and affecting valve performance. In addition, the noise generated during the vapor corrosion process should not be ignored, the more bubbles, the more serious the noise.
So, how do we prevent the damage caused by cavitation?
Valve structure design: Although the valve structure can not prevent the occurrence of flash vapor, but a reasonable design can inhibit its destructive power. For example, the use of media flow from top to bottom of the angular valve structure, compared to spherical valves, can more effectively reduce the direct impact of flash steam on the valve body.
Material selection: high hardness materials are more resistant to the destruction of vapor corrosion. In the power industry, chrome molybdenum alloy steel valves are favored for their excellent corrosion resistance. If the angle valve is equipped with a hard pipe downstream, the valve body itself can be made of carbon steel.
Zigzag Path: By allowing fluid to pass through a throttling member that contains a zigzag path, pressure recovery can be reduced, thereby mitigating cavitation. This design is widely used in a variety of components to control cavitation.
Multi-stage pressure reduction: Multi-stage pressure reduction is designed so that each stage consumes a portion of the energy, thereby reducing the differential pressure and pressure recovery at the next stage, effectively preventing cavitation. In contrast, one-stage throttling is more prone to cavitation when subjected to the same pressure drop.
Multi-hole throttling design: This is a comprehensive program that ensures that the pressure of the high-velocity fluid through each point is higher than the saturated vapor pressure by means of a special valve seat and valve structure. At the same time, a convergent injection method is used to convert the kinetic energy of the fluid into thermal energy and reduce bubble formation. In addition, the bubbles are made to rupture in the center of the sleeve, avoiding direct damage to the valve seat and valve flap.
In conclusion, vapor corrosion is one of the main causes of internal damage of the throttle valve, but through reasonable valve structure design, material selection and the use of zigzag paths, multi-stage pressure reduction and multi-hole throttling and other preventive measures, we can effectively reduce its destructive force and prolong the service life of the valve.
Translated with DeepL.com (free version)