Volute Pumps
Most of these pumps have a casing of the spiral type which forms a gradually increasing water space about its impeller. In the space the liquid handled slowly loses velocity, as it flows to the discharge outlet, without the aid of diffusion vanes. The pump 
shown in the picture has a single stage, a single liquid inlet, a horizontal shaft, a volute casing, and either an open or an enclosed impeller. The open type of impeller somewhat resembles a paddle wheel with its vanes supported at one side. Rotation of the open impeller produces excessive eddying of and leakage of the liquid handled between the blades and the casing wall, and with its single inlet produces an axial thrust on the shaft which must be counterbalanced.
The open impeller gives low efficiency of operation and is used mostly with low cost units to handle thick liquids such as mash, pulp, and sewage. The shrouded type of impeller, as in the picture has a plate on each end of the vanes to form enclosed water passages. The liquid handled enters the impeller at each side at its center; the enclosing shrouds or walls together with casing rings, prevent it from escaping from the wheel blades.
Impellers are usually made of some type of bronze, as it resist corrosion and maintains the original shape and smoothness of the surfaces in contact with the liquid handled. Other non corrosive materials may be used in pump parts, depending upon the nature of the liquids handled.
Sampling theory
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