Centrifugal Pump : Notes, Application, Methods, Principle and Diagram!
Notes on Centrifugal Pump:
While a turbine is a device to convert hydraulic energy into mechanical energy, a pump is a device to convert mechanical energy into hydraulic energy.
A centrifugal pump acts as a reversed turbine except that in this device special modifications are made to increase the efficiency. Centrifugal pumps are all of the outward flow type, where the radial velocity of water is increased by centrifugal action caused by the rotating vanes. It is necessary that the pump has to be full of water when starting it and hence, it should not be left to drain. External power is used to turn the vanes which gives a centrifugal head to the water collected in the pump.
This water is forced to leave the moving vanes at the outer periphery at a high velocity and pressure. This creates a partial vacuum in the centre of the pump and accordingly, the water from the suction pipe flows into it. It is the high pressure of the water leaving the vanes, which is utilized to overcome the delivery head of the pump. In the pumps made in the earlier years, the kinetic energy of the water leaving the vanes was not utilized and there used to be considerable loss of energy in the formation of eddies in the surrounding circular chamber.
In the later models, this defect has been rectified by converting the kinetic energy into pressure energy by making the water leaving the vanes, flow through a passage of gradually increasing area. The increase in the pressure head brought about in this manner is utilized in increasing the delivery head to the pump, thereby the efficiency of the pump is increased.
Generally, centrifugal pumps are made of the radial flow type only. But there are also axial flow or propeller pumps which are particularly adopted for low heads.
The following are the methods used to convert the kinetic energy of the water leaving the vanes into pressure energy:
(i) The Volute Chamber:
ADVERTISEMENTS:
The volute chamber is a spiral casing surrounding the wheel which is also called the impeller. The water which leaves the vanes is directed to move in the volute chamber circumferentially. The area of the volute chamber increases gradually and hence, the velocity gets decreased accompanied by corresponding increase of pressure.
As the water reaches the delivery pipe, a considerable part of kinetic energy is converted into pressure energy. Observations however have shown that in this arrangement the efficiency of the pump is increased only slightly. This means eddies are not completely avoided and some loss of energy takes place in the eddies due to the continually increasing quantity of water passing through the volute chamber.
(ii) Vortex or Whirlpool Chamber:
This is an improvement over the ordinary volute chamber. In this case, the impeller is surrounded by a chamber by combining a circular chamber and a spiral chamber. See Fig. 24.2. In this arrangement the efficiency is increased considerably.
(iii) Guide Blades:
These are fixed vanes which receive the water leaving the moving vanes. The guide vanes provide a gradually increasing area of flow leading to decrease of kinetic energy and increase of pressure energy. A pump with such receiving guide vanes is called a turbine pump. The angles of the guide vanes at their receiving tips are such that the water enters the guides without shock. The guides are fixed over a circular ring. The ring with the guide blades is called a diffuser. The pump is also provided with a volute chamber which directs the water to the delivery pipe.
Single Suction and Double Suction Impellers:
ADVERTISEMENTS:
Impellers may be single suction impellers or double suction impellers. See Fig. 24.4.(a) and (b).
In a single suction impeller water enters at one side of the casing only. But in a double suction impeller, water enters the impeller on both sides of the casing. This type of impeller can accommodate a large rate of flow. While in the case of a single suction impeller, due to entry of water on one side of the casing only, a lateral thrust is transmitted to the impeller. But in a double suction impeller, the thrusts act on both sides of the impeller and are balanced.
i. The Suction Pipe:
ADVERTISEMENTS:
This is a pipe connecting the pump and the inlet of the pump. A non-return valve is provided at the lower end of this pipe. This valve does not allow the water to drain out of the pipe, when the pump is not running. By containing the water in the suction pipe, it assists in priming.
Generally, the lower end of the pipe is also provided with a strainer which allows only clear water into the pipe. The upper end of this pipe is connected to the inlet of the impeller and this section is called the eye of the pump. This pipe is called the suction pipe since the pressure in this pipe is below the atmospheric pressure.
To avoid cavitation the negative pressure at the pump inlet is maintained within limits. Obviously, from this consideration, the height of the suction pipe above the sump water level is kept as small as possible. Again, from this consideration the diameter of the suction pipe is made large. To prevent pressure drops bends should be avoided in the suction pipe. Due to larger diameter provided, the velocity in the suction pipe is lowered and accordingly, the loss of head due to friction in the pipe is minimized.
ii. The Delivery Pipe:
This is a pipe connecting the pump outlet to the delivery end or delivery reservoir. A delivery valve is provided with strainer near the outlet of the pump, in order to regulate the flow into the delivery pipe. (See Figs. 24.5 and 24.6)
iii. The Foot Value:
This is a one way valve allowing the liquid to flow only from the sump to the suction pipe. This valve is fitted at the lower end of the suction pipe and is permanently positioned well below the lowest sump water level. As this valve does not allow backward flow, it retains the water in the suction pipe when the pump is not in operation.
Strainer:
A strainer or screen is provided to the foot valve to prevent any suspended solid material from entering the suction pipe.
Priming of the Pump:
The centrifugal head generated in the pump is proportional to the specific weight of the fluid which fills the passage. If the impeller contains only air, the rotation of the-air mass can produce only a small centrifugal head. Hence, in order to operate the pump it is necessary that the pump should be filled with the liquid to be pumped. After the pump is filled with the liquid, the impeller should be rotated.
This process is called priming of the pump. Generally, priming is done by introducing the liquid into the impeller through a funnel provided. As the liquid fills the impeller, the air present in it will escape through the airvent valve. After all the air in the impeller and the casing is removed, the airvent valve is closed and the pump can be run.
The Analysis of the Pump:
The analysis of the pump is made similar to that of a radial flow turbine. The inlet and outlet diagrams are drawn as usual.
The height of the centre of the pump above the sump water level must not exceed a limit to avoid separation at the inlet of the pump.
Least Diameter of Impeller:
It is possible to work out the minimum outside diameter of an impeller to enable the pump to start pumping at its normal speed. We know the condition –
Minimum Speed to Start the Pump:
While starting a centrifugal pump, it should be realized that no flow will occur through the wheel until the difference of pressure head in the impeller is sufficiently large to overcome the total lift.
Loss of Head in a Centrifugal Pump due to Reduced or Increased Flow:
A centrifugal pump will exhibit its maximum efficiency only when it is running and discharging at the speeds for which it was originally designed. When there is an increase or decrease in the discharge from the normal discharge, a loss of head occurs at entry due to shock.
In Fig. 24.14, abd represents the inlet velocity triangle when the pump is running at its normal speed. The velocity of flow at this stage is represented by db. Suppose the velocity of flow is reduced to the value dc, while the pump continues to run at the same speed. In this condition the velocity triangle is represented by acd. Now the relative velocity is represented by ac. But the blade angle θ at inlet cannot change. This means the relative velocity will no longer be parallel to the blade and this results in shock at entry.
Since the velocity of flow has a certain definite value in the new condition, and since the flow of water has to take place along the vane, it therefore, follows that the velocity diagram will be the triangle ecd, the side ec of the triangle being parallel to ab. Thus, a sudden change ae will take place in the tangential velocity and due to this shock, a loss of head will occur.
The relation given above is also applicable when the flow becomes greater than the normal flow.
Multistage Centrifugal Pumps:
A centrifugal pump with a single impeller can develop a head up to nearly 40 metres. In order to develop greater heads or to discharge a high rate, a multistage pump is used. A multistage pump is a pump with more than one impeller.
There are two ways of arranging the impellers in a multistage pump as explained below:
(i) Impellers in Series:
In this case, a number of impellers are mounted over a common shaft. The discharge from the first impeller is guided into the inlet of the second impeller. The discharge from the second impeller is guided into the inlet of the third impeller and so on and finally, the discharge from the last impeller is directed to the delivery pipe.
As the liquid flows through each impeller, the head Hm is impressed on it. Suppose there are n impellers. The total head developed = Ht = n Hm. The same discharge Q passes through all the impellers and is finally delivered to the delivery pipe.
(ii) Impellers in Parallel:
This arrangement is meant for discharging a high rate of flow at a given small head Hm. The impellers in this case are mounted on separate shafts. The discharges from the various delivery pipes are collected in a collecting pipe which communicates to the final delivery pipe.
Water Pressure in Centrifugal Pumps:
We can determine the pressure of water at any section of the stream in the pump by using Bernoulli’s equation. Fig. 24.17 shows a centrifugal pump with its suction and delivery pipes. Let A and B be points at inlet and outlet edges of the impeller at the level of the centre of the pump.
Let C be a point on the water surface of the sump. Let D a point just at the outlet of the delivery pipe.
Specific Speed of a Centrifugal Pump:
The specific speed of a centrifugal pump is the speed at which the pump would deliver one cube metre per second under a head of 1 metre. All geometrically similar pumps have the same specific speed. The discharge of the pump is given by –
Principle of Similarity Applied to Centrifugal Pumps:
By the application of principle of similarity, it is possible to predict the performance of a prototype pump from tests on a geometrically similar model.
It is known-
