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Tuesday, 19 November 2013

Centrifugal pump

Centrifugal pump Points : Centrifugal pump, Types of casings, for the impeller of a centrifugal pump, Volute Casing (Spiral Casing), Vortex Casing, Volute Casing with Guide Blades, Piping System of a Centrifugal Pump, Delivery Pipe A pump in general may be defined as a machine, when driven from some external source lifts water or some other liquid from a lower level to a higher level. Or in other words, a pump may also be defined as a machine, which converts mechanical energy into pressure energy. The pump which rises water or a liquid from a lower level to a higher level by the action of centrifugal force, is known as a centrifugal pump. It will be interesting to know that the action of a centrifugal pump is that of a reversed reaction turbine. In a reaction turbine, the water at high pressure is allowed to enter the casing which gives out mechanical energy at its shaft; whereas in pump, the mechanical energy is fed into the shaft and water enters the impeller (attached to the rotating shaft) which increases the pressure energy of the outgoing fluid. The water enters the impeller radically and leaves the vanes axially.
Types of casings, for the impeller of a centrifugal pump We have seen in that a centrifugal pump consists of an impeller, similar to that of a turbine, to which curved vanes are fitted. The impeller is. enclosed in a water-tight casing, having a delivery pipe in one of its sides. The casing for a centrifugal pump is so designed that the kinetic energy of the water is converted into pressure energy before the water leaves the casing. This considerably increases the efficiency of the pump. Following are the three types of casing or chambers:
1. Volute casing,
2. Vortex casing, and
3. Volute casing, with guide blades.
Volute Casing (Spiral Casing) In a volute chamber, the impeller is surrounded by a spiral casing. Such a casing provides a gradual increase in the area of flow; thus decreasing the velocity of water, and correspondingly increasing the pressure. A considerable loss takes place due to the formation of eddies in this type of casing. Vortex Casing It is improved types of a volute casing, in which the spiral casing is combined with a circular chamber. In a vortex casing, the eddies are reduced to a considerable extent and an increased efficiency is obtained. Volute Casing with Guide Blades In this type of casing, there are guide blades surrounding the impeller. These guide blades are arranged at such an angle, that the water enters without shock, and forms a passage of increasing area, through which the water passes and reaches the delivery pipe. The ring of the guide blades is called diffuser and is very efficient. Piping System of a Centrifugal Pump The successful working of a centrifugal pump depends upon the correct selection and layout of its piping system. An extreme care should always be taken in selecting the sizes of the pipes and their arrangement. In general, a centrifugal pump has (a) suction pipe, and (b) delivery pipe. Suction Pipe The, suction pipe, of a centrifugal pump, plays an important role in the successful and smooth working of the pump. A poorly designed suction pipe causes insufficient net positive suction head (brief written as NPSH), vibration, noise, water hummer, excessive wear etc. While laying the pipe, a great care should be taken to make it air tight. A strainer foot-valve is connected at the bottom of the suction pipe to avoid the entry of foreign matter. Since the pressure at the inlet of the pump is suction (i.e., negative) and its value is limited to avoid cavitations, it is therefore essential that the losses in the suction pipe should be as small as possible. For this purpose, bends in the suction pipe are avoided and its diameter is often kept larger. Sometimes to reduce the axial thrust, the, suction pipe is branched into two parts and the liquid is allowed to enter the impeller from both sides. Such a pump is called double suction pump. Delivery Pipe A check valve is provided in the delivery pipe near the pump, in order to protect the pump from hammer and also to regulate the discharge from the pump. The size and length of the delivery pipe depends upon the requirement.

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