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Causes and Prevention of Flashing and Cavitation in Control Valves
Flash vaporization occurs when a non-compressible fluid, after being throttled through a control valve, experiences a drop in static pressure—from the vena contracta section all the way to the valve outlet—until the pressure equals or falls below the fluid’s saturation vapor pressure at the valve inlet temperature. This causes partial liquid evaporation, resulting in a two-phase mixture of gas and liquid downstream of the valve. The onset of flash vaporization leads to a point where the liquid flow no longer increases with further pressure drops, triggering choked flow conditions. Additionally, flash vaporization generates a gas-liquid two-phase flow, with both gas and liquid simultaneously flowing through the valve trim and downstream piping, causing erosion. A distinctive feature of this phenomenon is that the valve trim develops a smooth, polished appearance.
Release date:
2020-04-28
Flash evaporation occurs when an incompressible fluid is subjected to Control Valve After throttling, when the static pressure drops—from the constricted section all the way to the valve outlet—to a level equal to or lower than the saturation vapor pressure of the fluid at the valve inlet temperature, partial liquid vaporization occurs, leading to the formation of a two-phase gas-liquid flow downstream of the valve. This phenomenon, known as flash evaporation, causes the liquid flow rate to no longer increase with further pressure drops, resulting in choked flow. Additionally, flash evaporation generates a mixed gas-liquid flow, where both gas and liquid simultaneously pass through the valve trim and downstream piping, causing erosion. Notably, the valve trim develops a smooth, polished appearance as a result of this erosive process.
Cavitation occurs when a fluid passes through Control Valve At this point, the static pressure at the constricted flow section drops to a level equal to or lower than the saturation vapor pressure of the fluid at the valve inlet temperature, causing some of the liquid to vaporize and form bubbles. Subsequently, as the static pressure recovers to match the saturation vapor pressure again, these bubbles collapse and revert back to the liquid phase. This entire process—where bubbles form, grow, and then violently implode—is known as cavitation. Cavitation erosion refers to the material degradation caused by the intense forces generated during cavitation. When cavitation or erosion occurs, it can severely erode the valve trim, leading to significant wear and damage. Unlike flash evaporation-induced erosion, which typically results in smoother, more polished surfaces, cavitation erosion leaves both the valve trim and downstream piping with rough, coal-cinder-like textures.
To prevent flash evaporation, the main measures adopted are:
1. Increase material hardness. Use hard alloy for the valve core, or weld hard materials at locations where flash vaporization may occur, thereby enhancing material durability and reducing erosion.
2. Reduce fluid flow velocity. Design an optimized flow path to lower the downstream fluid velocity, thereby minimizing erosion rates. For example, install a reducing pipe downstream of a control valve to decrease the flow speed.
3. Select the appropriate Control Valve Type and flow direction: Different control valves and flow configurations have varying pressure recovery coefficients. Selecting a control valve with a higher pressure coefficient and optimizing the flow direction can help prevent choked flow. For instance, when handling liquids prone to vaporization, it’s advisable to avoid high-pressure-recovery ball or butterfly valves; instead, opt for low-pressure-recovery single-seat control valves.
Measures taken to address cavitation primarily include:
1. Control the pressure drop to prevent cavitation from occurring. For instance, use multi-stage pressure-reducing control valves, dividing the total pressure drop across several stages. Ensure that the pressure drop in each stage remains above the liquid’s saturation vapor pressure at the constricted flow area, thereby preventing bubble formation and eliminating the risk of cavitation altogether.
2. Reduce cavitation effects. Employ methods similar to those used to prevent flash evaporation—such as increasing material hardness and lowering flow velocity—to minimize the impact of cavitation.
3. Properly distribute pipeline pressure to increase downstream pressure. From a design and process perspective, boosting the pressure downstream of the control valve—while ensuring the same increase occurs at the throttling point—can effectively prevent cavitation. For instance, installing the control valve in a downstream location with higher static pressure, or adding a flow-restricting orifice plate, are effective strategies to achieve this.
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