Impeller vane shape and form
Impellers can also be classified by the shape and form of their vanes as follows:
The straight-vane impeller (redial)
The mixed-flow impeller
The axial-flow impeller or propeller
Francis vane
Backward curved vane
Straight-vane impeller
In a straight-vane impeller, the vane surfaces are generated by a straight line parallel to the axis of rotation. These vanes are also called single curvature vanes.
Mixed-flow impeller
An impeller design that has both radial and axial flow components is a mixed-flow impeller. It is generally restricted to single-suctions with a specific speed above 4,200. Types with lower specific speeds are called francis vane impeller.
Axial-flow impeller
Mixed-flow impellers with a very small redial flow component are usually referred to as propellers. In a true propeller or axial-flow impeller, the flow strictly parallels the axis of rotation.
Specific speed
Calculating specific speed is one method of classifying the pump impeller with reference to their geometric similarity.
Specific speed is a correlation pump capacity, head, and rotative speed and can be described by the following formula:
Where:
Ns = specific speed
N = rotative speed,(rpm)
Q = flow at optimum efficiency, (gpm us)
H = total head (ft/stage)
Vane shape
Classification of impeller according to their vane shape is arbitrary as there is a great deal of overlapping in the types of impellers used in different types of pumps. For example: Impellers in single and double-suction pumps of low specific speed have vanes extending across the suction eye. This provides a mixed flow at the impeller entrance for low pickup losses at high rotative speeds, but allows the discharge portion of the impeller to use the straight-vane principle.
In pumps of higher specific speed operating against low heads, impellers have double-curvature vanes extending over the full vane surface.
Many impellers are designed for specific applications.
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For example: the conventional impeller design with sharp vane edges and restricted areas is not suitable for handling liquids that contain rags, stringy material, and solid such as sewage because it will become clogged. Special non-clogging impellers with blunt edges and large waterways have been designed for such service.
The impeller design used for paper pulp or sewage pumps is fully open, non-clogging and has screw and redial stream-lined vanes. The vanes leading edge projects far into the suction nozzle permitting the pump to handle pulp stocks with a high consistency of paper.
Wearing Rings
Wearing rings (for casing or impeller) provide an easily and economically renewable leakage joint.
There are various types of wearing ring designs, and the selection of the most desirable type depends on the following:
Liquid being handled
Pressure differential across the leakage joint
Rubbing speed
Pump design (i.e., sewage vs. clean liquid)
The most common ring constructions are the flat type (figure 35)-2 and the L type (figure 36)-3.
Some designers favor labyrinth-type rings, which have two or more annular leakage joints connected by relief chambers
(figure 37) - 4.
In leakage joints involving a flat-type wearing ring, the leakage flow is a function of the following:
Area
Length of the joint
Pressure differential across the joint
If the path is broken by relief chambers, the velocity energy in the leakage jet is dissipated in each relief chamber, thereby increasing the resistance. As a result, with several relief chambers and several leakage joints for the same actual flow through the joint, is less resulting in higher pump performance and operating efficiency.
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