Tuesday, 19 November 2013

Reciprocating Pump

Reciprocating Pump Points : Reciprocating Pump, Types of reciprocating pumps, Slip of the pump, Negative Slip of the Pump, Discharge of a reciprocating pump A reciprocating pump, in its simplest form, consists of the following parts as.
1. A cylinder C, in which a piston P works. The movement of the piston is obtained by a connecting rod, which connects the piston and the rotating crank.

2. A suction pipe, connecting the source of water and the cylinder.
3. A delivery pipe, into which the water is discharge from the cylinder.
4. A valve a, which admits the flow from the suction pipe into the cylinder.
5. A valve b, which admits the flow from the cylinder into the delivery pipe.

During the suction stroke, the piston P moves towards right (i.e. from 0° to 180°), thus creating vacuum in the cylinder. This vacuum causes the suction valve a to open and the water enters the cylinder. During the delivery stroke, the piston P move towards left (i.e., from 180° to 3 60°) thus increasing pressure in the cylinder. The increase in pressure causes the suction valve a to close and delivery valve b to open, and the water is forced into the delivery pipe.

A reciprocating pump is also called a positive displacement pump, as it discharge a definite quantity of liquid during the displacement of its piston or plunger. This is why a reciprocating pump is ideally suitable for grouting operations in dam foundations.

Types of reciprocating pumps The reciprocating pumps may be classified as discussed below:
1. According to action of water:
(a) Single acting pump, and
(b) Double acting pump.
2. According to number of cylinders:
(a) Single cylinder pump,
(b) Double cylinder pump, and
(c) Triple cylinder pump etc.
3. According to the existence of air vessels:
(a) With air vessel, and
(b) Without air vessel.
Slip of the pump In the last article, we have obtained the relation for the discharge of a single acting and double acting reciprocating pump. But in practice, the actual discharge is less than the theoretical discharge. The difference between theoretical discharge and actual discharge is known as slip of the pump. This theory is similar to that which discussed for coefficient of discharge. Negative Slip of the Pump Sometimes, the actual discharge of a reciprocating pump, is more than the theoretical discharge. In such cases, the coefficient of discharge will be more than unity, and the corresponding slip is known as negative slip of the pump. This happens, when the suction pipe is long and delivery pipe is short and pump is running at high speeds. This causes the delivery valve to open before completion of the suction stroke and some water is pushed into the delivery pipe, before the piston commences its delivery stroke. Discharge of a reciprocating pump Consider a single acting reciprocating pump (i.e., a pump, in which the water is acting on one side of the piston only).
Let L = Length of the stroke or piston,
A = Cross-sectional area of the pist6n, and
N = No of revolutions, per minute of the crank.
Discharge of water is one stroke
=LA
and discharge of the pump per second,
Q = LAN/60

If the pump is a double acting reciprocating pump (i.e.,. a pump, in which the water is acting on both sides of the piston) the discharge is taken to be double the discharge than that of a single acting pump. This is due to the reason that, in a double acting pump, the water is sucked on one side of the piston and delivered from the other side during the same stroke. These two processes (i.e., suction on one side and delivery from the other) are reversed during the return stoke. Therefore the discharge of a double acting reciprocating pump,
Q = 2LAN/60