Review and optimisation of pump suction reducer selection

Abstract

The approach flow to a pump must be undisturbed and free from unequal velocity distributions, unequal pressure distributions, entrained air or gas bubbles, vortices and excessive pre-swirl. A reducer fitting is typically used in pump station pipe work to reduce the size of the suction pipe to match the size of the pump suction end flange. Two types of reducer fittings are commonly manufactured, namely: Eccentric Reducers and Concentric Reducers. Inlet pipework design guidelines traditionally prescribe the use of eccentric reducers, with the flat side on top. This prescription is to allow the transport of air through the fitting. The flow through an eccentric reducer accelerates along the sloped side as the flow path narrows from below, thereby causing higher velocities towards this sloped side. These flow conditions are contradictory to the recommended pump inlet approach flow conditions and pump station failures have been recorded resulting from the incorrect application of eccentric reducers. Relationships exist to assess the hydraulic transportation of air through a pipe and these relationships can be applied to calculate the ability to transport air through a concentric reducer. It is therefore hypothesised that a correctly designed concentric reducer will not only provide a more uniform pressure/velocity distribution in comparison to an eccentric reducer, but will allow any free air to be hydraulically transported through the reducer to the pump. Computational Fluid Dynamics (CFD) was utilised to assess the resulting velocity distributions through various concentric and eccentric reducer geometries at various flow rates. Six concentric reducers and six eccentric reducers were simulated with four inlet velocities. The resulting velocity distributions were recorded with scalar scenes and velocity probes at four positions spaced at a distance of 1 x the downstream diameter starting at the downstream end of the reducer. These velocity distributions were then compared to the pump inlet requirements typically used in the industry. These requirements require the velocity variation along a line drawn through the centre of the pipe to be less than 10% of the average velocity along that line and the velocity variation along a circle within the pipe is less than 5% of the average velocity along the circle. It was found that the eccentric reducers with angles of 15°, 20° and 30° and the concentric reducer with an angle of 20° do not pass the requirements used in the assessment at all four velocities. From these results it was highlighted that some of the standard eccentric reducer geometries (including those specified by AWWA C208) do not pass the inlet requirements. It was then assessed if air can be hydraulically transported through the concentric reducers utilising available hydraulic air transport theory. Air can be hydraulically transported through all of the concentric reducers except for the 20° reducer (the same size that failed the velocity distribution assessment) at 1m/s for the assessed diameters. It was therefore shown that a correctly designed concentric reducer (angle less or equal to 15°) will not only provide a more uniform pressure/velocity distribution in comparison to an eccentric reducer, but will allow air to be hydraulically transported through the reducer to the pump.