Control valve cavitation

When a pulsation frequency coincides with a mechanical or acoustic resonance, severe vibration can result. A common cause for pulsation is the presence of flow control valves or pressure regulators. These often operate with high pressure drops (i.e., high flow velocities), which can result in the generation of severe pulsation. Flashing and cavitation can also contribute. 

When there is a choice, design for no flashing. When there is no choice, locate the valve to flash into a vessel if possible. If flashing or cavitation cannot be avoided, select hardw are that can withstand these severe conditions. The dowmstream line will have to be sized for tw o phase flow. It is suggested to use a long conical adaptor from the control valve to the downstream line. 

In summary, the lowest pressure that can be reached at point D in Fig. 11.6 is the pressure at point A. When these two pressures are equal, we say that the draw-off nozzle is limited by cavitation. If we were to lower the pressure downstream of point D, say, by opening a control valve, the increase in flow would be zero. 

I once tried to increase the flow of jet fuel from a crude distillation column by opening the draw-off, flow-control valve. Opening the valve from 30 to 100 percent did not increase the flow of jet fuel at all. This is a sure sign of nozzle exit loss—or cavitation limits. To prove my point, I increased the level of liquid in the draw-off sump from 2 to 4 ft. Since flow is proportional to velocity and head is proportional to (veloc-... 

Suddenly, the flow of softened water would increase exponentially. The level-control valve would rapidly open. Yet, the level in the deaerator would slowly continue to fall, until the pump lost suction and cavitated. 

Let s now assume that we wish to pump 300 GPM, not 250 GPM. If we open the flow-control valve shown in Fig. 25.1, the flow will momentarily increase. But, within a few seconds, the flow will become erratically low as the pump cavitates. The problem is that, according to Fig. 

It is positively my experience that the most common reason for pumps cavitation is partial plugging of draw nozzles. This problem is illustrated in Fig. 25.5. This is the side draw-off from a fractionator. Slowly opening the pump s discharge control valve increases flow up to a point. Beyond this point, the pump s discharge pressure and discharge flow become erratically low. It is obvious, then, that the pump is cavitating. 

In thermal solar power plants, high-temperature oil flows need to be controlled in hydrogen processes, very-low-temperature LH2 flows have to be controlled. For these reasons, the emphasis in this treatment will be on control valve designs that are suited for these applications and on phenomena (such as noise and cavitation) that are common in these applications. 

The noise sources in control valves include mechanical vibration (usually below 100 dBA) hydrodynamic noise caused by liquid turbulence, cavitation, or flashing (usually below 110 dBA) and aerodynamic noise (can reach 150 dBA). In control valve design, aerodynamic noise can be a major problem. Aerodynamic noise generation, in general, is a function of mass flow rate and the pressure ratio (p /pf) across the valve. The point at which sonic speed is reached in the valve vena contracta is a function of the valve design. 

Using the sizing formulas of the Fluid Controls Institute, Inc., size control valves for the cavitation, subcritical, and critical flow situations described below. Show how accurate the FCI formulas are. 

Cavitation Select a control valve for a situation where cavitation may occur. The fluid is steam condensate inlet pressure P is 167 psia(1151.5kPa) A P is 105 lb/in2 (724.0 kPa) inlet temperature T is 180°F (82.2°C) vapor pressure Pv is 7.5 psia (51.7 kPa). 

Leak in diaphragm of control valve Debris is stuck in opening to control valve A plugged or obstructed instrument air line Plug/seat erosion in the control valve A bypass line open or leaking Flashing and cavitation Improperly tuned valve positioner ... 

---