Classification of Pumps | Pumping Stations | Water Engineering

In this article we will discuss about the classification of pumps based on their principles of operation.

1. Displacement Pumps:

In these types of pumps vacuum is created mechanically by the moveable part of the pumps. In the vacuum first the water is withdrawn inside the pumps, which on the return of mechanical part of the pump is displaced and forced out of the chamber through the valve and pipe. The back flow of the water is prevented by means of suitable valves.

The following are the two main types of displacement pumps:

i. Reciprocating pumps.

ii. Rotary pumps.

i. Reciprocating Pumps:

Reciprocating pumps may be of the following types:

(a) Simple hand-operated reciprocating pump.

(b) Power operated deep well reciprocating pump.

(c) Single-acting reciprocating pump.

(d) Double-acting reciprocating pump.

(a) Simple Hand-Operated Reciprocating Pump:

This is the simplest and cheapest type of pump which is widely used in the towns and villages of India, where the first water­ bearing stratum is within reasonable depth from the ground surface. In cities also these are provided in private houses and on roads for general public use, in case of failure of the water works supply.

Fig. 6.1 illustrates the details of a simple hand-operated single-acting reciprocating pump. It essentially consists of a strainer suction pipe, gun metal cylinder, G.I. cylinder pipe, piston, piston rod and set of valves. In ordinary pumps instead of gun metal cylinder, G.I. cylinder is used. But for longer life and more quantity of water, it is recommended to use the gun metal cylinder.

The piston or plunger moves up and down by means of handle as shown in Fig. 6.1. When the piston is raised up, vacuum gets created in cylinder below the piston, thereby opening the check valve V₁, automatically closes up, valve V₂ (piston valve) is opened and the water enters in the space above the piston. In this way with each upward stroke it moves up and starts flowing through the spout fixed for this purpose.

This pump can give continuous supply of water which is sucked from the ground through strainer, which removes sand, clay and other impurities up to reasonable limit. These pumps are usually used when the water table is 6.0 m of so from the ground level.

(b) Power-Operated Deep Well Reciprocating Pump:

This is similar to hand-operated reciprocating pump, with the difference that its piston is kept below the water table and secondly it is operated by power instead of hand.

Fig. 6.2 illustrates this type of pump. These pumps are used to lift the water when the depth of water table is more than 6.0 m or so. As the cylinder is kept below the water table, these pumps have only to lift the water. No suction pipe is provided in these pumps. The strainer is directly fixed to the cylinder.

At some places the cylinder is provided 6.0 m above the water table inside the ground. Sometimes additional check valves are provided in between the suction pipe to increase the suction capacity of the pumps.

(c) Single-Acting Reciprocating Pump:

In a single-acting pump, a piston moves inside a cylinder with the help of piston rod operated by a wheel through connecting rod. One suction and one delivery pipe are provided in the cylinder as shown in Fig. 6.3. When piston moves outwards in the cylinder vacuum is created in it, which carries the opening of suction valve and closing of delivery pipe.

The water enters in the cylinder through suction valve. Now when the piston moves inwards, it forces the inside water of cylinder outward through the delivery pipe. In this way the flow of water is not constant but it is intermittent and gives vibration, shock and loss of energy. This condition can be partly overcome by using double- acting pumps.

(d) Double-Acting Reciprocating Pump:

Fig. 6.4 illustrates the double-acting reciprocating pump. These pumps have two sets of solution and delivery valves as shown in the figure. The discharge in these pumps depends on the number of revolutions per minute of the pump and the efficiency varies from 40% to 85%.

ii. Rotary Pumps:

Fig. 6.5 shows the suction through two typical rotary pumps. The revolving blades fit closely in the casing and push the water by their displacement. The blades revolve in a downward direction at the centre as shown and the water is carried upward around the side of the casing. In this way the water is pushed through the discharge pipe and partial vacuum is created on the suction side. The intensity of the vacuum mainly depends on the tightness of the parts.

Rotary pumps are not suitable for handling liquid containing suspended matter, because of the close fitting of the rotors in the casting. The efficiency of these pumps is between 50 to 85 per cent.

Advantages of Rotary Pumps:

1. They do not require any priming as they are self-primed. Due to this advantage these are used in priming large centrifugal or reciprocating pumps.

2. The efficiency of these pumps is high at low to moderate heads up to discharge of 2000 litres/min.

3. These pumps have no valves, are easy in construction and maintenance as compared with reciprocating pumps.

4. These pumps give steady and constant flow without any pulsations.

5. These pumps can be easily employed for the individual building water supply and for fire protection.

Disadvantages of Rotary Pumps:

(i) The initial cost of these pumps is high.

(ii) Their maintenance cost is high due to abrasion of their cams and gears.

(iii) They cannot pump water containing suspended impurities as the wear and abrasion caused by the impurities will destroy the seal between the cams and the casing.

2. Centrifugal Pumps:

These pumps work on the principle of centrifugal force, therefore, named centrifugal pumps. The water which has entered inside the pumps is revolved at high speed by means of impeller and is thrown to the periphery by the centrifugal force.

The water enters in the pump at the centre of the impeller called the eye usually at right angle to the plane of the impeller.

There are two types of centrifugal pumps:

i. Volute type centrifugal pump.

ii. Diffuser or Turbine type centrifugal pump.

i. Volute Pump:

In this pump Discharge the channel into which the, water flows after leaving the impeller has a volute shape of such proportion that the velocity of flow remains the same at all points in the channel. The impeller is usually rotated by an electric motor and due to the centrifugal force the water to rise in the delivery pipe at high rate.

ii. Diffuser or Turbine Pump:

In the turbine type pump the impeller is surrounded by the stationary guide vanes, which reduce the velocity of water before the water enters the casing. Thus, these pumps convert the velocity head into pressure head in the casing itself. The casing surrounding the guide vanes is usually circular and concentric with the impeller.

In this pump, the channel into which the water flows on leaving the impeller has the same cross-sectional area throughout. The velocity of the water which leaves the impeller blades is changed in direction and the velocity head is changed to pressure head by means of diffuser vanes.

Centrifugal pumps using one impeller are known as single stage pumps. Pumps using two or more impellers are called double or multistage pumps. These pumps are employed for high lifts. In cast of deep well turbine pumps, which is commonly used in tube-well, several impellers are installed on one vertical shaft which is suspended and rotated inside the tube- well by prime mover motors installed at the ground surface.

Fig. 6.14 shows the section through pump houses installed with typical types of centrifugal pumps.

Advantages and Disadvantages of Centrifugal Pumps:

The centrifugal pumps have the following advantages:

(a) Due to compact design, they require very small space.

(b) They can be fixed to high-speed driving mechanism.

(c) They have rotary motion due to which there is no noise.

(d) They are cheap in cost.

(e) They have simple mechanisms due to which they can be easily repaired.

(f) They have very simple operation.

(g) They cannot be damaged due to high pressure.

The following are the disadvantages of centrifugal pumps:

(a) The rate of flow of water cannot be regulated.

(b) They cannot be operated without prime movers.

(c) Their speed cannot always be adjusted to the prime mover without speed regulating mechanism.

(d) For operation they have restricted suction.

(e) Any air leakage on suction side will affect the efficiency of the pump.

(f) They have high efficiency only for low head and discharge.

(g) The pump will run backward, if it is stopped with the discharge valve open.

Priming and Operation of Centrifugal Pumps:

Before starting the centrifugal pumps, usually they require priming. Priming is the process of removing the trapped air from the pump and filling it completely with water. The centrifugal pumps which are installed below the water level do not require priming. While doing the priming of these pumps, the water from an outside tank (which is kept for this purpose only) is filled in the suction pipe and the pump fully.

The trapped air is allowed to escape through the air-vent made for this purpose in these pumps. When the air has been removed completely water will start coming through the air-vent. Now the air-vent screw is tightened and the pump started.

Fig. 6.7 illustrates the installation of a typical centrifugal pump. Sometimes just after starting the pump, it may allow entrance of air through leaky joint or trapped air which could not be removed during priming comes in between and the flow of water is interrupted. Under such conditions the pump is stopped and the priming is done again.

Characteristics of Centrifugal Pumps:

All the centrifugal pumps and axial flow pumps have certain operating characteristics depending on their design, speed and discharge.

Fig. 6.8 shows typical characteristic curves of centrifugal pump. Efficiency, RHP and water head curves are plotted in Fig. 6.8. These curves are very useful while selecting the suitability of a particular pump for particular purpose.

While studying these curves following points should be noted:

(i) When the discharge of the pump is increased, there is reduction in the produced head of water i.e., in other words the lifting capacity is reduced. From the curve the maximum head up to which the water can be lifted at the maximum discharge can be determined. From the curve, it is clear that at zero discharge the head developed is maximum, which is known as shut off head.

(ii) The maximum efficiency shall be available at a particular discharge which is known as normal or discharge rated capacity of the pump at the given speed. The rating of the pumps are determined for maximum efficiency. From the curve it is clear that only the pumps can work at a particular speed for maximum efficiency.

If the speed is increased or reduced than the rated speed, the efficiency of the pump shall be reduced. Now as the demand of water by the town varies hour to hour, therefore, to meet the demand it is not possible to decrease or increase the discharge of the pump from hour to hour.

To meet up the demand one method is to install several pumps instead of one to meet the maximum demand rate. When the demand decreases the number of pumps are closed accordingly. But it is a cumbersome method. In the second method which is commonly used, the pump of maximum demand rate is installed and operated. During less demand the excess water pumped is stored in the storage reservoir.

3. Air Lift Pumps:

When the water contains suspended matter and acids or alkalies which may damage other types of pumps, the air lift pumps can be used very successfully. These types of pumps are suitable for lifting water from small wells or deep wells. These are simple in operation and give least trouble because they have no remote or submerged moving parts.

The given Fig. 6.9 shows the arrangement of an air­lift pump. It consists of a vertical pipe known as ‘educator pipe’ placed in a casing pipe. Air inlet pipe is lowered in the educator with the direction of its outlet upward as shown in the figure. Top end of the air-lift pipe is connected to the air compressor.

Working of an air-lift pump is as follows:

The compressed air is released through the air diffuser connected at the lower end of the air-inlet pipe in the educator pipe. This air is mixed with water and forms bubbles having low specific outside of the educator pipe is the same, but when compressed air is passed, the density of water inside the educator pipe is reduced due to which the mixture of water rises in the educator pipe becomes equal to the weight of only water from A to C outside the educator pipe the water starts coming out from the outlet at the level of B.

Sufficient submergence of air-pipe in the water is necessary for the lifting of water. If the well is deeper, the submergence should be more. There is minimum submergence ratio (D/D + h) required for different lifts [h] of water, which can be calculated.

4. Impulse Pump:

This is also called hydraulic ram and works on the principle of impulse. The advantage of a small fall is taken in lifting the water to great heights by means of the hydraulic ram. Fig. 6.10 shows the essentials of hydraulic ram. It essentially consists of an inlet pipe, waste valve, delivery valve, ram, air chamber and delivery pipe. When not working the waste valve remains open and delivery valve remains close due to force of gravity only. In the beginning the water enters the ram through inlet pipe and flows out through the waste valve.

But as the incoming water valve is automatically raised and suddenly closed, due to which an impact or impulse is created in the ram causing the delivery valve to get opened, and some quantity of water enters the air chamber and starts flowing through the outlet pipe.

When the impulse ceases the waste valve again opens and the same cycle is repeated. Hydraulic ram can make 200 strokes per minute and can lift the water up to a height of 30 times the working fall. The efficiency of these pumps is about 50 percent.

Following are the advantages of hydraulic ram:

(i) The ram has long life and is durable.

(ii) Its working is very simple and automatic. After stalling it, usually no attention is required.

(iii) No power, engine, fuel etc. are required for its working hence it is most economical.

Following are the disadvantages of hydraulic:

(i) While in operation, it creates noise, which may cause great inconvenience during working hours to the workers.

(ii) Considerable quantity of water is wasted during its working.