Specifications

152
14. CENTRIFUGAL PUMP
14. Introduction
A centrifugal pump is a machine that imparts energy to a fluid. This energy infusion can
cause a liquid to flow, rise to a higher level, or both. The centrifugal pump is an extremely simple
machine. It is a member of a family known as rotary machines and consists of two basic parts:
1) the rotary element or impeller and 2) the stationary element or casing (volute). The increased
popularity of centrifugal pumps is due largely to the development of high speed electric motors,
steam turbines, and internal combustion engines. In operation, a centrifugal pump slings liquid
out of the impeller via centrifugal force.
14.2 The Constructional Features of Centrifugal Pump
(i) Bed plate : This is usually made of cast iron or welded steel of rigid construction. It is
bolted to the foundation.
(ii) Pump casing (volute) : This is usually made of close-grained cast iron. It is split in the
vertical plane at the centre of the casing and the two halves held together by a number
of bolts and one or two locating pins and a gasket to make it leak proof. For easy
removal of the impeller an end cover is usually provided.
(iii) Impeller : This is usually of close-grained cast iron or cast steel, hydraulically and
dynamically balanced to avoid end-thrust and vibration. To eliminate possibility of rusting,
impellers are frequently made of gun-metal. Two types of impellers are widely used.
They are closed type and open type as shown in Fig.1.
(iv) Spindle : This is made of steel. Where corrosive liquids are to be handled, stainless
steel spindles may be fitted. The portion of the spindle which works inside the casing is
usually fitted with a renewable gun-metal sleeve so that it may be replaced when it gets
worn out and the spindle may have a long life.
(v) Stuffing box and gland packing : This serves two purposes: on the suction side it prevents
leakage of air and on the delivery side leakage of water under pressure. The packing
material consists of rings of soft, cotton, woven yarn, impregnated with graphite and
tallow. The gland bolts should only be tightened lightly, just enough to prevent leakage.
Water is admitted into the stuffing box either through an internal hole or by an external
connection. It is very important that a centrifugal pump should not be allowed to run dry,
as it may result in seizure of the spindle.
Modern pumps are often fitted with mechanical leak-proof seals. Basically it consists
of a mirror-finished, hard, flat surface against which a spring loaded ring of a softer
material like carbon, rubber, leather, or plastic is used. This combination permits free
rotation without allowing any leakage. The seal is kept cool by the liquid pumped.
(vi) Bearings : These may be of the ball, roller or sleeve type. Grease lubricators are provided
on the bearing housing. Fig. 2 and 3 clearly illustrates the parts of a pump.