Page 268 - Chemical process engineering design and economics

248 Chapter 5

amount of clearance. Clearance is necessary to prevent seizure, but it also re-
sults in leakage, called slippage. A sliding-vane pump, shown in Figure 5.25, is
similar to a sliding-vane compressor or vacuum pump. Rectangular vanes that
are free to move in a radial slot are placed at regular intervals around the rotor.
As the rotor revolves, the vanes are thrown outwards against the casing to form
a seal. In the suction side of the pump, the space between the vanes fills with
liquid, which is then compressed and discharged. Both the gear and sliding-
vane pumps are not suitable for pumping liquids containing solids.
A progressive-cavity pump is shown in Figure 5.25. A helical screw re-
volves in a fixed casing which is shaped to produce cavities. At the suction side of
the pump, the liquid flows into a partial vacuum created in a cavity, which moves
the liquid to the discharge side of the pump as the helical screw rotates. Toward
the discharge side, the shape of the casing causes the cavity to close. This action
generates an increase in pressure forcing the liquid into the outlet line. The dis-
charge pressure determines the length and pitch of the helical-screw rotor. Unlike
the other types of rotary pumps, the progressive-cavity pump, can pump liquids
containing large amounts of nonabrasive suspended solids.

DYNAMIC PUMPS
Dynamic pumps are divided into two main classes as shown in Figure 5.24, cen-
trifugal and peripheral. Dynamic pumps are characterized by their ability to de-
liver high flow rates at low pressures. To achieve high flow rates requires that the
impeller rotate at high speeds. Thus, the clearances between the impeller and the
pump housing are larger than those between moving and stationary parts of posi-
tive-displacement pumps. This, in turn, means that the pressures developed by
dynamic pumps cannot be as large as the pressures developed by positive-
displacement pumps. Dynamic pumps, with the exception of peripheral pumps,
are not self priming. The large clearances between the impeller and casing does
not facilitate the removal of air from the pump at startup. Thus, dynamic pumps
must be filled with the liquid being pumped before starting, which is called prim-
ing. The flow rate from dynamic pumps is smooth and is easily be controlled by
installing a control valve on the discharge side of the pump.

Centrifugal Pumps

About 95 % of the pumps used in the chemical industry are centrifugal pumps.
The centrifugal pump contains an impeller, usually having curved blades that are
mounted on a shaft. The blades rotate inside a volute casing, as shown in Figure
5.27. A liquid enters axially into the eye of the impeller and velocity is imparted
to the liquid by rotating blades. An appreciable amount of the kinetic energy of
the liquid is then converted into pressure in the casing. Centrifugal pumps are

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