## Vortex

FIGURE 2 Typical vertical dry-pit pump intake. Improper spacing of suction bell relative to floor and back wall caused subsurface vortexing. Model test revealed problem and correction (Reference 4).

reduced pressure in the vortex core causes fluctuating load on the pump impeller along with associated vibration and noise,4,5 increased possibility of cavitation, higher inlet losses, and decreased pump efficiency, especially when the core pressure is sufficiently low to release dissolved air or other gases from the fluid.

Prerotation and Swirl Swirl is a general term for any flow condition (due to vortexing or a pipe bend) where there is a tangential velocity component in addition to a usually predominating axial flow component. Prerotation is a specific term to denote a cross-sectional average swirl in the suction line of a pump or, in case of a vertical wet-pit pump, upstream of the impeller.

The prerotation angle 6 is a measure of the strength of the tangential velocity component u, relative to that of the axial velocity component u in the flow approaching the pump impeller; that is, 6 = tan _1(u/u). Adverse effects on the pump are decreased capacity and head when the rotation is in the direction of pump rotation and increased capacity and head when the rotation is opposite the pump rotation (antirotation). The increased capacity is associated with an increase in power requirement and may cause motor overheating.

Prerotation will influence pump performance because the flow approaching the impeller already has a rotational flow field that may oppose or add to the impeller rotation, depending on direction. The design of the pump blades (that is, shape and angle) usually assumes no prerotation, and the existence of prerotation implies flow separation along one side of the impeller blades. The degree of prerotation that should be of concern depends on the type of pump and may not always be known. Prerotation could be quantified in a model by an average cross-sectional swirl angle, determined by detailed velocity measurements, or by readings on a swirl meter. Because swirl decays along a pipe as a result of wall friction, internal fluid shear, and turbulence, the swirl meter in a model suction pipe should be located near the impeller.

Losses Leading to Insufficient NPSH A poorly designed pump intake could result in large inlet losses. Losses caused by screens, poor entrance conditions, vortexing and swirl, and vortex suppression devices may add up to a value so great that the required NPSH of the pump is not satisfied. Increased inlet losses due to swirl have been reported in laboratory studies.6 In a nuclear reactor residual-heat-removal sump model, inlet losses in a preliminary design wherein air core vortices and a high degree of swirl were present were 20% higher than in a revised design with no strong vortices and swirl, and with similar pipe entrance geometry and flows. Because the degree of vorticity and swirl cannot be predicted and it is therefore not possible to calculate inlet losses reliably; they are usually obtained from model studies. With the experimentally derived values of the inlet losses, the NPSH available should be checked by recalculation.

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