Cavitation avoidance

Suction Limitations of a Pump Whenever the pressure in a liquid drops below the vapor pressure corresponding to its temperature, the liquid will vaporize. When this happens within an operating pump, the vapor bubbles will be carried along to a point of higher pressure, where they suddenly collapse. This phenomenon is known as cavitation. Cavitation in a pump should be avoided, as it is accompanied by metal removal, vibration, reduced flow, loss in efficiency, and noise. When the absolute suction pressure is low, cavitation may occur in the pump inlet and damage result in the pump suction and on the impeller vanes near the inlet edges. To avoid this phenomenon, it is necessary to maintain a required net positive suction head (NPSH)r, which is the equivalent total head of liquid at the pump centerline less the vapor pressure p. Each pump manufacturer publishes curves relating (NPSH)r to capacity and speed for each pump. 

Generation Spontaneous generation of gas bubbles within a homogeneous liquid is theoreticaUy impossible (Bikerman, Foams Theoiy and Industrial Applications, Reinhold, New York, 1953, p. 10). The appearance of a bubble requires a gas nucleus as avoid in the liquid. The nucleus may be in the form of a small bubble or of a solid carrying adsorbed gas, examples of the latter being dust particles, boiling chips, and a solid wall. A void can result from cavitation, mechan-ic ly or acoustically induced. Blander and Katz [AlChE J., 21, 833 (1975)] have thoroughly reviewed bubble nucleation in liquids. 

Cavitation Formation of transient voids or vacuum bubbles in a liquid stream passing over a surface is called cavitation. This is often encountered arouna propellers, rudders, and struts and in pumps. When these bubbles collapse on a metal surface, there is a severe impact or explosive effec t that can cause considerable mechanical damage, and corrosion can be greatly accelerated because of the destruction of protective films. Redesign or a more resistant metal is generally required to avoid this problem. 

Changing the pump metallurgy to a more corrosion- and cavitation-resistant material, such as stainless steel, is a potential solution to this type of problem. Note, however, that all other cast iron pump components that have sustained graphitic corrosion should be replaced to avoid the possibility of galvanic corrosion (see Chap. 16) between retained graphitically corroded cast iron components and new components. 

The definition of NPSHr may change in the future. A pump is in a definite state of cavitation with the 3% total head loss definition. Many pump users want a more explicit definition of NPSHr, and higher NPSHa safety margins to avoid inadequate NPSHa and cavitation altogether. 

As we ve said numerous times before in this chapter, the important thing is that the NPSHa of the system is above the NPSHr of the pump bv a sufficient amount to avoid stress and possible cavitation. If the NPSHa should be inadequate, there are ways to elevate it. Remember from the formula that five elements compose the NPSHa. Two of those elements, the Ha and the Hs, add energy to the fluid. And three elements, the Hvp, the Hf, and the Hi, subtract energ from the fluid. VVe must either increa.se the elements that add energ), or decrease the elements that subtract energy. To increase the NPSHa ... 

Plate anodes were used for corrosion protection in order to avoid damage due to erosion and cavitation. These consisted of enamelled steel bodies in which a metal oxide-coated titanium anode 1 dm in surface area was fitted. The enamel... 

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. 

A pump is designed to handle liquid, not vapor. Unfortunately, for many situations, it is easy to get vapor into the pump if the design is not earefully done. Vapor forms if the pressure in the pump falls below the liquid s vapor pressure. The lowest pressure occurs right at the impeller inlet where a sharp pressure dip oeeurs. The impeller rapidly builds up the pressure, which collapses vapor bubbles, eausing cavitation and damage. This must be avoided by maintaining sufficient net positive suetion head (NPSFl) as specified by the manufacturer. 

Under cavitating conditions a pump will perform below its head-performance curve at any particular flow rate. Although the pump may operate under cavitation conditions, it will often be noisy because of collapsing vapor bubbles and severe pitting, and erosion of the impeller often results. This damage can become so severe as to completely destroy the impeller and create excessive clearances in the casing. To avoid these problems, the fol-iotving are a few situations to watch ... 

Absolute pressure at the pump inlet must not be low enough to release non-condensables of (2). If such release can occur, then the NPSHr would need to be increased above that of the cold water requirements to avoid cavitation and poor pump performance. 

The absolute pressure at the inlet to the pump is usually the atmospheric pressure in the receiver, plus the static head from the water surface to the pump inlet and minus the friction loss through the pipes, valves and fittings joining the pump to the receiver. If his absolute pressure exceeds the vapor pressure of water at the temperature at which it enters the pump, then a net positive suction hand (NPSH) exists. If this NPSH is above the value specified by the pump manufacturer, the water does not begin to boil as it enters the pump suction and cavitation is avoided. If the water entering the pump is at a higher temperature, its vapor pressure is increased and a greater hydrostatic head over the pump suction is needed to ensure that the necessary NPSH is obtained. 

With the feedwater being raised in temperature it is necessary to avoid cavitation at the inlet to the boiler... 

Feedwater tank water temperatures on package boiler systems not fitted with deaerator heaters should be as high as practically possible, while consistent with the avoidance of FW pump cavitation. 

Avoid overheating FW tanks and the generation of steam in the tank. If the FW temperature is too high and there is inadequate net positive static head (NPSH) on the FW pump suction, the pump will not operate and cavitation problems may occur. 

Use increased pressures or rotor speed but avoid super-cavitation by operating below a certain optimum value Optimisation needs to be carried out depending on the application. Higher diameters are recommended for applications which require intense cavitation whereas lower diameters with large number of holes should be selected for applications with reduced intensity Lower free areas must be used for producing high intensities of cavitation and hence the desired beneficial effects... 

As we have mentioned before, acoustic streaming, cavitation and other effects derived from them, microjetting and shock waves take also relevance when the ultrasound field interacts with solid walls. On the other hand, an electrochemical process is a heterogeneous electron transfer which takes place in the interphase electrode-solution, it means, in a very located zone of the electrochemical system. Therefore, a carefully and comprehensive read reveals that all these phenomena can provide opposite effects in an electrochemical process. For example, shock waves can avoid the passivation of the electrode or damage the electrode surface depending on the electrode process and/or strength of the electrode materials [29]. 

The NPSH at the operating point for the pump determines where the pump can be installed in a piping system to ensure that cavitation will not occur. The criterion is that the pressure head at the suction (entrance) of the pump (e.g., the NPSHA) must exceed the vapor pressure head by at least the value of the NPSH (or NPSHR) to avoid cavitation. Thus, if the pressure at the pump suction is Ps and the fluid vapor pressure is Pv at the operating temperature, cavitation will be prevented if... 

In order that cavitation may be avoided, pump manufacturers specify a minimum value by which the total head at the suction flange must exceed the head corresponding to the liquid s vapour pressure. 

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