Cavitation and flashing

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. 

Defining the type of fluid. This includes the physical properties (density and viscosity), corrosive properties, and the nominal pressures, temperatures and flow rate. In the case of fluids, it is necessary to know the vapor pressures to check for flashing and cavitation. Also a high temperature increase can severely damage some types of gaskets and packings. 

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 ... 

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 hardware that can withstand these severe conditions. The downstream line must be sized for two-phase flow, and a long conical adapter from the control valve to the downstream line must be used. 

SUCTION LIFT AND CAVITATION. The power calculated by Eq. (8.4) depends on the difference in pressure between discharge and suction and is independent of the pressure level. From energy considerations it is immaterial whether the suetion pressure is below atmospheric pressure or well above it as long as the fluid remains liquid. However, if the suction pressure is only slightly greater than the vapor pressure, some liquid may flash to vapor inside the pump, a process called cavitation, which greatly reduces the pump capacity and causes severe erosion. If the suction pressure is actually less than the vapor pressure, there will be vaporization in the suction line, and no liquid can be drawn into the pump. 

A saturated liquid flow through a control valve or a control valve with large pressure drop will usually cause flash or cavitation. Control valve pressure drop is the largest at minimum flow case, and the control valve should he checked for this case for possible flash or cavitation. 

Cavitation and Flashing From the discussion on pressure recoveiy it was seen that the pressure at the vena contracta can be much lower than the downstream pressure. If the pressure on a hquid falls below its vapor pressure (p,J, the liquid will vaporize. Due to the effect of surface tension, this vapor phase will first appear as bubbles. These bubbles are carried downstream with the flow, where they collapse if the pressure recovers to a value above p,. This pressure-driven process of vapor-bubble formation and collapse is known as cavitation. 

Do not confuse NPSH vdth suction head, as suction head refers to pressure above atmospheric [17]. If this consideration of NPSH is ignored the pump may well be inoperative in the system, or it may be on the border-line and become troublesome or cavitating. The significance of NPSH is to ensure sufficient head of liquid at the entrance of the pump impeller to overcome the internal flow losses of the pump. This allows the pump impeller to operate wfith a full bite of liquid essentially free of flashing bubbles of vapor due to boiling action of the fluid. 

Cavitation of a centrifugal pump, or any pump, develops when there is insufficient NPSH for the liquid to flow into the inlet of the pump, allowing flashing or bubble formation in the suction system and entrance to the pump. Each pump design or family of dimensional features related to the inlet and impeller eye area and entrance pattern requires a specific minimum value of NPSH to operate satisfactorily without flashing, cavitating, and loss of suction flowt... 

Noise generation and erosion/corrosion considerations limit the maximum water velocity in pipework systems. Noise is caused by the free air present in the water, sudden pressure drops (which, in turn, cause cavitation or the flashing of water into steam), turbulence or a combination of these. 

High water velocities can result in erosion or corrosion due to the abrasive action of particles in the water and the breakdown of the protective film which normally forms on the inside surface of the pipe. Erosion can also result from the formation of flash steam and from cavitation caused by turbulence. Publishing data on limiting water velocities are in conclusive. Table 27.9 summarizes the available information. 

Typical manufacturer s values of Cv to be used with Eq. (10-29) require the variables to be expressed in the above units, with hv in ft. [For liquids, the value of 0.658 includes the value of the density of water, pw = 62.3 lbm/ft3, the ratio g/gc (which has a magnitude of 1), and 144 (in./ft)2]. For each valve design, tables for the values of the flow coefficients as a function of valve size and percent of valve opening are provided by the manufacturer (see Table 10-3, pages 318-319). In Table 10-3, Km applies to cavitating and flashing liquids and C applies to critical (choked) compressible flow, as discussed later. 

Figure 10-20 Critical pressure ratio for cavitating and flashing liquids other than water. The abscissa is the ratio of the liquid vapor pressure at the valve inlet divided by the thermodynamic critical pressure of the liquid. The ordinate is the corresponding critical pressure ratio, re. (From Fisher Controls, 1987.)...

Recovery is a measure of the degree of pressure recovery at the valve outlet from the low pressure at the vena contracts. When flashing occurs at the vena contracts and the pressure recovery is high, the bubbles collapse with resulting cavitation and noise. The more streamlined the valve, the more complete the pressure recovery thus, from this point of view streamlining seems to be an undesirable quality. A table of recovery factors of a number of valve types is given by Chaffin (1980) such data usually are provided by manufacturers. 

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. 

The cause of cavitation is that the pumped liquid flashes to vapor at one point inside the pump, where pressure is below the vapor pressure, and as the spinning impeller throws the liquid and vapor outward, the vapor bubbles collapse as the pressure rises above the vapor pressure. When the col-... 

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