Bombas

FEATURE

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Selection criteria for suction impellers of centrifugal pumps
In the third and final part of this article, J.F. GUlich of Sulzer Pumps, discusses the prevention of cavitation damage and concludes by presenting hydraulic criteria that can be used for the selection of suction impellers.

6. Prevention of cavitation damage
The risk of cavitation damage must be minimised by thefollowing means: (1)An appropriate hydraulic design of the structures and components upstream of the impeller must provide a flow distribution as uniform as practical. This applies for the suction piping layout, the sump design for a wet pit installation, or the radial inlet bends of multi-stage and double-entry pumps. (2)The hydraulic design of the impeller must optimise the pressure distributionon the blades in order to create as low cavity volumes as possible for any given NPSH and to minimise the driving pressure differential in the zone of bubble implosion (as discussed in section 3.1 "concept L" and illustrated by case histories 5.1 to 5.3). (3)Selection of the right pump for the specified service: most pumps will operate at partload with a higher risk of cavitation damage than nearBEP - be it due to higher incidence or to vortices created by suction recirculation. Over-sizing of the pump usually entails economic losses due to additional power consumption and frequently in terms of increased maintenance
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(4) and (5) attempt to exclude the risk of damage when operating with a limited amount of cavitation. This is precisely what most pumps do, since it is neithereconomical nor necessary in the majority of applications to suppress cavity formation at the pump inlet completely by providing plenty of NPSHA or choosing a sufficiently low pump speed. The risk of cavitation damage can be assessed quantitatively - albeit with considerable scatter [12, 7]. The damage prediction correlations given in ref. 12 have been validated since by additional testing [25] andextended to procedures for cavitation diagnosis [26]. The damage prediction method has been used to calculate the cavity length allowable to obtain an impeller life of 40000 hours with stainless steel impellers, Fig. 10. The allowable cavity length has been plotted against the available NPSH; the reason is that the hydraulic cavitation intensity increases with the cavity volume (length) and thedriving pressure differential in the implosion zone as quantified by the suction pressure (i.e. NPSHa). The calculation is based on a metal loss due to cavitation of 6 mm in 40000 hours with stainless steel tensile strength of R m = 800 N/mm 2. To estimate the cavity length allowable for other conditions read Lcav from the graph and transform to actual conditions by use of Eq. (8):
f

variousparameters influencing cavitation erosion; Fig. 10 can therefore just show trends and give an order of magnitude. Cavity thickness and volume increase with cavity length. For LcaffTia > 0.3 to 0.5 this effect represents a considerable uncertainty (refer also to case 5.1). 'When selecting the speed and size of a pump for a given NPSHA or when determining the NPSH A necessary for the safe operation of agiven pump typical applications may be distinguished:

A: Service when pumping water below 200 *C
Pumps with an impeller eye tip speed u 1 > 75 m/s must operate virtually above cavitation inception, since very small cavities can lead to erosion. For these pumps the impeller eye diameters are minimised in order to keep the tip speed u 1 and NPSH i as low as possible. This group of pumps includehigh-speed injection pumps and boiler feedpumps for very large nuclear or fossil power plants. When selecting pumps with impeller eye tip speeds between 50 < u 1 < 75 m/s it must be ensured that the cavity length is known and that it is limited indeed in order to avoid excessive cavitation damage (usually a booster pump is required to create a sufficient NPSH A in such applications). Typical...