Cavitation Control In Centrifugals

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WORLD PUMPS December

2009

Operating

Cavitation control in centrifugals
Cavitation is a well-known problem in centrifugal pumps, with the potential to cause serious damage and loss of function if it is not diagnosed. In this article, Chikezie Nwaoha explains the various causes of cavitation, describing the patterns of damage each inflicts and the different methods toprevent and cure them.

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n a centrifugal pump, the flow area at the eye of the pump impeller is smaller than either the flow area of the pump suction piping or the flow area through the impeller vanes. When the liquid being pumped enters the eye of the centrifugal pump, there is therefore a significant decrease in the flow area. This results in an increase in flow velocity, which is accompaniedby a decrease in pressure. The greater the pump flow rate, the greater the pressure drop between the pump’s suction and the eye of the impeller. If the decrease in pressure is

high enough, or the temperature is high enough, the liquid will ‘flash’ vapour when the local pressure falls below the saturation pressure of the liquid being pumped. Any vapour bubbles formed as a result of the pressuredrop at the eye of the impeller are swept towards the oncoming impeller vanes by the flow of the liquid. When bubbles enter a region where local pressure is greater than the saturation pressure farther out on the impeller vane, the vapour bubbles will abruptly collapse, causing a physical shock to the leading edge of the impeller

vane. This formation and subsequent collapse of vapour bubblesin a pump is the well-known process of cavitation. Cavitation degrades the performance of a centrifugal pump, resulting in a fluctuating flow rate and discharge pressure. The physical shock caused by the vapour bubbles creates small pits on the leading edge of the impeller vane. Each individual pit is microscopic in size, but the cumulative effect of millions of these pits formed over a period ofhours or days can destroy a pump impeller. In addition, cavitation may cause excessive pump vibration, which could also damage pump bearings, wearing rings and seals.

Common causes of cavitation
Cavitation occurs when bubbles in the flow stream collapse after passing into regions of higher pressure, causing noise and vibration and potentially damaging pump components, thus reducing the pump’sefficiency and limiting its ability to generate head. There are several causes of bubble formation in the flow stream, including vaporization, recirculation, vane passing syndrome, flow turbulence and air ingestion.

Vaporization
Figure 1. A pump damaged as a result of flow turbulence.
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Hydraulic fluids may contain up to 12% dissolved air by volume. Certain operating0262 1762/09 © 2009 Elsevier Ltd. All rights reserved

WORLD PUMPS December

2009

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Agitation in the supply reservoir, clogged inlet filters and corrosion can all alter the velocity of the liquid – and whenever the velocity of a fluid changes, its pressure changes. In multiple-pump arrangements, flow turbulence may occur whenever fluid streams of different velocities merge,giving rise to shear due to friction between the fluid streamlines (Figure 1). This scenario can be avoided by following piping layout best practices, such as: • Ensuring that the distance between the pump suction and the first elbow is at least ten pipe diameters • Locating the suction bells in separate bays so that one pump suction does not interfere with another.

Vane passing syndrome
Thistype of cavitation damage occurs when the outer diameter of the impeller passes too close to the pump cutwater. This causes the velocity of the liquid to increase as it flows through the small passage, thus lowering the fluid pressure and causing local vaporization and pulsation. The bubbles that form then collapse at the higher pressure just beyond the cutwater. This damage (Figure 3) is limited...