Operation and Performance of Reciprocating Pump

In general a pump may be defined as a mechanical device which when interposed in a pipeline, converts the mechanical energy supplied to it from some external source into hydraulic energy and transfers the same to the liquid through the pipeline, thereby increasing the energy of the flowing liquid. Almost all the pumps increase the pressure energy of the liquid which is subsequently converted into potential energy as the liquid is lifted from a lower level to a higher level. The various pumps may broadly be classified into the following two types:

(i) Positive-displacement pumps.

(ii) Rotodynamic pumps (or dynamic-pressure pumps).

The 'positive displacement' pumps are those pumps in which the liquid is sucked and then it is actually pushed or displaced due to the thrust exerted on it by a moving member, which results in lifting the liquid to the required height. These pumps usually have one or more chambers which are alternately filled with the liquid to be. pumped and then emptied again. As such the discharge of liquid pumped by these pumps almost wholly depends on the speed of the pump. The most common example of the positive displacement type of pumps is that of reciprocating pumps.

The 'rotodynamic pumps' have a rotating element, called impeller, through which-as the liquid passes its angular momentum changes, due to which the pressure energy of the liquid is increased; As such a rotodynamic pump does not push the liquid as in the case of a positive displacement pump. The centrifugal pumps are the most common examples of rotodynamic pumps.

A reciprocating pump essentially consists of a plunger and a cylinder where the plunger is enclosed by cylinder. The cylinder is connected to suction and delivery pipes, each of which is provided with a non-return or one way valve called suction valve and delivery valve respectively. The function of non-return or one way valve is to admit liquid in one direction only. Thus the suction valve allows the liquid only to enter the cylinder and the delivery valve permits only its discharge from the cylinder. The piston Of the plunger is connected to a crank by means of a connecting rod.

As the crank is rotated at uniform speed by a driving engine or motor, the piston or plunger moves to and fro (or backward and forward) in the cylinder. When the crank rotates from e =0' to e = 180', the piston or plunger which is initially at its extreme left position (that is, it is completely inside the cylinder), move to its extreme right position, (that is, it moves outwardly from the cylinder). During the outward movement of the piston or plunger a partial vacuum (pressure below atmospheric) is created in the cylinder, which enables the atmospheric pressure acting on the liquid surface in the well or sump below, to force the liquid up the suction pipe and fill the cylinder by forcing open the suction valve.

Since during this operation of the pump the liquid is sucked from below it is known as its suction stroke. Thus at the end of the suction stroke the piston or plunger is at its extreme right position, the crank is at e = 180' (i.e., at its outer dead centre), the cylinder is full of liquid, the suction valve is closed and the delivery valve is just at the point of opening.

The reciprocating pumps can be classified according to the liquid being in contact with one side or both the sides of the piston or plunger; and according to the number of cylinders provided.

According to the first basis of classification the reciprocating pumps may be classified as:

(i) Single acting pump.

(ii) Double acting pump.

If the liquid is in contact with one side of the piston or plunger only, it is known as single acting pump. Thus as shown in Fig. a single acting pump has one suction and one delivery pipe and in one complete revolution of crank there are only two strokes-one suction and one delivery stroke. On the other hand if the liquid is in contact with both the sides of the piston or plunger, it is known as double acting pump .As shown in Fig. a double acting pump has two suction and two delivery pipes with appropriate valves, so that during each stroke when suction takes place on one side of the piston, the other side delivers the liquid. In this way in the case of a double acting pump in one complete revolution of the crank there are two suction strokes and two delivery strokes.

According to the number of cylinders provided the reciprocating pumps may be classified as :

(i) Single cylinder pump.

(ii) Double cylinder pump.

(iii) Triple cylinder pump.

(iv) Duplex double acting pump.

(v)Quintuplex pump.

The air vessel is a cast iron chamber, which has opening at the base, through which water can flow, one chamber is fitted on the suction pipe just near to suction valve and one on the delivery pipe just near the delivery valves. Each channel is converted through a small length of pipe. For efficient working, vacuum vessels should be 3 to 5 times the discharge per stroke and the air vessel on the delivery side 6 to 10 times the discharge per stroke.

During the middle of delivery stroke, when pump is facing the water into the delivery pipe at a velocity greater than the average, excess water flows into the air vessel and compresses the tapped air in upper portion of the chamber. At the end of the stroke when water flows into the delivery pipe at a rate less than the average water flows out of air vessel from the excess amount of water already stored to keep the discharge more uniform. This fluctuating water column causes the acceleration head to be reduced that in between pump cylinder and the air vessel, which allows the pump to run at higher speeds. Thus in this way it saves large amount of power lost in developing accelerating heads on suction side, water first collects in the air vessel and then flows in cylinder on delivery side. Water first goes to air vessel and then flows with a uniform velocity. An air vessel provided in a reciprocating pump acts like a flywheel of an engine.

Other functions of an air vessel :-

1. Reduces the possibility of separation and cavitation.

2. Allows pump to run at high speed.

3. Suction head can be increased by increasing the length of pipe below air vessel.

4. Large amount of power is saved due to low acceleration head.

5. Uniform discharge.

The operating characteristic curves indicate the performance of the reciprocating pump are obtained by plotting discharge, power input and overall efficiency against the head developed by the pump when it is operating at a constant speed under ideal conditions. The discharge of a reciprocating pump operating at constant speed is independent of the head developed by the pump. However, in actual practice it is observed that the discharge of a reciprocating pump slightly decreases as head developed by pump increases almost linearly beyond a certain minimum value with the increase in head developed by the pump. The overall efficiency of this pump also increases with increase in head developed by the pump.

Learning objectives :-

To calculate the efficiency of reciprocating pump

To determine the relationship between efficiency and head

Aim :- To study the performance of reciprocating pump.

Apparatus :- Reciprocating pump test rig, tachometer, stopwatch.

1. Reciprocating pump 38.11 mm bore diameter (D) 45.3 mm stroke length

Double acting with air vessel on discharge side Suction pipe 31 mm diameter

Delivery pipe 25 mm diameter

2. AC motor 3 HP – Speed variations controlled by a stepped pulley

3. Measuring tank – 400 x 400 x 450 mm wide

4. Sump tank – 600 x 900 x 600 mm height

5. Piston rod – 9.52 mm

Measurements :

1. Pressure gauge – 0 to 7 kgf/cm2 for discharge pressure.

2. Vacuum pressure – 0 to 760 mm of Hg for suction pipe.

3. 3 phase energy meter for motor input measurement.

The apparatus consists of single cylinder, double acting reciprocating pump mounted on the sump tank. The pump is driven by A.C motor with stepped cone pulley. An energy meter measures electrical input to motor. Measuring tank is provided to measure the discharge of the pump. The pressure and vacuum gauges provided to measure the delivery pressure and suction vacuum respectively

1. Fully open the discharge valve.

2. Start the pump, certain discharge is observed.

3. For that particular discharge, note down the discharge head, suction head, time required for 10 lts. Of water collection. Also note down the time taken for 5 revolutions of energy meter and speed of the pump.

4. Note down the above observations at different discharge pressure and tabulate the values.