In general, such pumps comprise a driven impeller which rotates in the centre of a spiral or volute casing. The impeller may comprise a circular plate which carries a number of blades or vanes which extend outwardly from an open central area on the plate. The side of the impeller opposite the plate is open to communicate with an inlet opening in the casing.

When the impeller is driven, the rotation of the blades forces the water in the central area of the impeller outwards into the spiral casing, creating suction at the central area which draws in water along an axial direction through the inlet into the impeller.

An annular wear plate is fixed on the inside of the casing around the inlet opening and fits close against the edges of the impeller blades.Water Control valves The clearance between the wear plate and the impeller blades is very small and, as the impeller rotates, the edges of the impeller blades pass over the surface of the wear plate. When the pump is used to raise slurry containing sand or grit, the motion of the impeller blades over the wear plate grinds the particles suspended in the slurry into the edge of the wear plate causing wear around the inlet opening.

As the edge of the wear plate becomes worn, the clearance between the impeller blades and the wear plate increases and the pump becomes less efficient because water is able to flow back towards the centre of the impeller around the edges of the impeller blades. However, the main problem arises when the pump is started up.Water Power Control Valves For convenience, such pumps are usually positioned above the level of the water being pumped and must, therefore, be capable of generating sufficient suction, when the operation begins, to raise water by a distance of several feet. Initially, the pump casing may be empty of water in which case the pump is primed by pouring water into the casing through the outlet. The clearance between the impeller blades and the wear plate is very small, for example, of the order of 0.015" (0.4 mm), so that the pump can build up a pressure difference between the inlet and the outlet sufficient to draw water into the casing. FORGED STEEL VALVESAs the edge of the wear plate becomes worn, this clearance increases until the leakage of water around the edges of the impeller blades prevents the pump creating sufficient suction to draw water up to the level of the pump. When this happens, the surface of the wear plate must be refaced or the entire wear plate must be replaced. This makes such pumps relatively expensive to maintain.

These problems can be avoided to a certain extent by using a closed impeller pump in which the impeller blades extend between two parallel discs provided with inlet openings. However, such pumps are generally more expensive than open-impeller pumps and, furthermore, in some circumstances, an open-impeller is more efficient. For example, when pumping sewage or stringy substances, such as water containing weed, which do not contain abrasive solids, the chopping action of the blades of an open-impeller helps to prevent the pump becoming clogged.

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2012-01-04

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