ME100 Basics of Mechanical Engineering Module 2 : 'Pumps'

Pump is a mechanical device generally used for raising liquids from lower level to a higher level.The hydraulic machines which convert the mechanical energy into hydraulic energy are called as pumps. The pumps are broadly classified into two as

· Positive displacement Pumps (Hydrostatic pumps)

Eg Gear pumps, vane pumps etc.

· Roto-dynamic or Non- positive displacement pumps

Eg. : Centrifugal pumps, axial pumps,

Positive displacement Pumps (Hydrostatic pumps)

The positive displacement pump operates by alternating of filling a cavity and then displacing a given volume of liquid. Positive displacement pumps physically displace the fluid; hence closing the outlet valve of a positive displacement pump will result in a continual build up in pressure resulting in mechanical failure of either pipeline or pump.

— Reciprocating Pumps:-

The reciprocating pump is a positive displacement pump in which the liquid is sucked and then it is displaced or pushed due to the thrust exerted on it by a moving member, which results in pumping liquid to the required height. Reciprocating Pump has an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pumps as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The volume is constant for each cycle of operation.

Roto-dynamic (Non- positive displacement pumps.)

The roto-dynamic pump produces a head and a flow by increasing the velocity of the liquid through the machine with the help of a rotating vane impeller. Roto-dynamic pumps can be safely operated under closed valve conditions (for short periods of time as heating of the fluid takes place) and hence there is no need for safety devices to prevent pump damage as air pressure developed automatically balances out on each side of the pump casing and stops the pumping.

Centrifugal pumps

The basic principle on which a centrifugal pump work is that when a certain mass of liquid is made to rotate by any external force it is thrown away from the central axis of rotation because a centrifugal head is impressed which enables the liquid to rise to higher level. The first step in the working of centrifugal pump is priming. It is an operation by which suction pipe, casing of the pump and portion of delivery pipe up to the delivery valve is filled with the liquid to be pumped, so as to remove air gaps. The necessity of priming in centrifugal pump is due to the fact that the centrifugal head generated by the impeller is directly proportional to the density of liquid that is in contact with it.

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