A pump which lifts a liquid from a lower level to a higher level by the action of a centrifugal force is called as a centrifugal pump. Centrifugal pumps are very widely used in the chemical and petroleum industries because of its many advantages such as simplicity of design, low initial cost, low maintenance and flexibility of operation. The centrifugal pump can handle liquids of a wide range of properties. It is equally suitable for handling suspensions with a high solid content. It may be constructed from a wide range of corrosion resistant materials and it may be directly coupled to an electric motor as it operates at high speeds.
In a centrifugal pump, power from an outside source is applied to the shaft, the impeller then rotates within the stationary casing. The impeller blades in revolving produce a reduction in pressure at the eye of the impeller. Due to this, liquid flows into the impeller from the suction pipe. The liquid is thrown outward by the centrifugal action along the blades. As a result of high speed of rotation, the liquid acquires a high kinetic energy. The kinetic energy acquired is then converted into pressure energy when it leaves the blade tips and the liquid passes into the volute chamber and finally it is discharged through the outlet (discharge) on the pump. This action of the centrifugal pump is shown in Fig. 1.
Centrifugal Pump Parts :
Centrifugal Pump Parts : Impeller
It is a wheel or rotor that is provided with a series of curved blades or vanes. It is mounted on a shaft which is coupled to an electric motor (an external source of energy). The blades/vanes are shaped in such a way that the flow within the pump is as smooth as possible. The impeller is the heart of the centrifugal pump.
Centrifugal Pump Impeller types :
(i) Open impeller, (ii) Semi-open impeller, and (iii) Closed or shrouded impeller.
The open impeller has the blades fixed to a central hub. Such impellers are suited for pumping liquids containing suspended solids, e.g., paper pulp, sewage, etc.
The closed impeller has the blades held between two supporting plates / shrouds (crown plate and base plate). This impeller provides better guidance for the liquid and is more efficient. This type of impeller is suited for pumping clear liquids (liquids containing no suspended particles, dirts, etc.).
The semi-open impeller has only one plate (base plate) and no crown plate (i.e., it has a plate on one side of the blades/vanes). Such impellers are suitable for liquids containing some solid particles or dirts.
For viscous liquids or liquids containing solids, open or semi-open type impellers are used. The most efficient impeller is the closed or shrouded type. The impeller may be a single suction type or double suction type. In the former type, the liquid enters the impeller from one side; while in the latter type, the liquid enters from both the sides.
Casing in Centrifugal Pump :
It is an airtight chamber in which the impeller rotates. It is provided with an inlet (suction) and outlet (discharge) for the liquid to be pumped. The function of the casing is to convert the kinetic energy imparted to the liquid by the impeller into useful pressure energy.
Types of casing in Centrifugal Pump :
(i) volute type casing and (ii) diffuser type casing.
In a volute type casing, the liquid is discharged by the impeller into the volute - a chamber of gradually increasing cross-sectional area towards the outlet [Fig. 3 (a)]. In the volute, the fluid velocity decreases gradually thereby increasing fluid pressure, i.e., the volute converts the kinetic energy of the liquid imparted by the impeller into pressure energy. In this design, a considerable loss of energy takes place due to formation of eddies.
A vortex chamber is an improved version of the volute design. In this case, a circular chamber is provided between the impeller and the volute chamber [Fig. 3 (b)]. This design reduces eddies to a considerable extent with increase in efficiency.
In a diffuser type casing (turbine pump), guide vanes or diffusers are interposed between the chamber and the impeller. The impeller is surrounded by a series of guide vanes mounted on a ring called diffuser ring as shown in Fig. 8.6. The conversion of kinetic energy into pressure energy is more efficient with this type compared to the volute type. There is a gradual change in the direction of fluid so that the losses are kept minimum.
Other component parts of the centrifugal pump are : shaft, bearing, stuffing box, or mechanical seal, etc. Shaft must be of a corrosion resistant material and must have good mechanical properties. The shaft transmits power from the drive unit to the impeller. Sometimes, a shaft sleeve of corrosion resistant material is provided over the corrosive shaft material from strength criteria.
Shaft is mounted on bearings which handle shaft load without excessive wear. Stuffing box is a means of reducing/avoiding leakage which would otherwise occur at the point of entry of the shaft into the casing. With this the maintenance costs are high. A considerable reduction in expenditure on maintenance can be effected at the price of a small increase in the initial cost by fitting the pump with a mechanical seal. For high pressures and corrosive fluids, mechanical seals are used.
Centrifugal Pump Working Principle Animation
Centrifugal Pump Working
In the operation of a centrifugal pump before the pump is started, priming of the pump is done. In the priming operation, the suction pipe, pump casing, and portion of the delivery pipe upto a delivery valve are completely filled with the liquid to be pumped so that all the air, gas or vapour from this portion of the pump is expelled out and no air pocket is left. In presence of even very small air pocket in any of these portions, pump will not discharge the liquid. The need to do priming of pump is due to the fact that the pressure generated by a centrifugal pump impeller is directly proportional to the density of fluid that is in contact with it. Therefore, if the impeller is rotated in the presence of air, only negligible pressure would be produced and thus no liquid will be lifted by the pump.
After the pump is primed properly, the delivery valve is kept close and power from an outside source (electric motor) is applied to the shaft. The delivery valve is kept close in order to reduce the starting torque for the motor. The impeller then rotates within the stationary casing. The rotation of the impeller produces a forced vortex which imparts a centrifugal head to the liquid and thus results in an increase of pressure throughout the liquid mass. As long as the delivery valve is closed and impeller is rotated, there will be just churning of the liquid within the casing. When the delivery valve is opened, the liquid is made to flow in an outward radial direction thereby leaving the vanes of the impeller at the outer circumference with high velocity and pressure. Due to centrifugal action, a partial vacuum is created at the eye of the impeller. This causes the liquid from the sump/reservoir (at atmospheric pressure) to flow through the suction pipe to the eye of the impeller thereby replacing the liquid which is being discharged from the entire circumference of the impeller. The high pressure of the liquid leaving the impeller is utilised in lifting the liquid to the required height through the delivery pipe.
During the operation, liquid receives energy from the vanes which results in an increase in both pressure and velocity energy. As such the liquid leaves the impeller with a high absolute velocity. In order that the kinetic energy corresponding to the high velocity of liquid leaving is not wasted in eddies and efficiency of the pump thereby lowered, it is essential that the high velocity of the leaving liquid is gradually reduced to the lower velocity in the delivery pipe, so that a large portion of the kinetic energy is converted into useful pressure energy. This is usually achieved by shaping the casing such that the leaving liquid flows through a passage of gradually expanded area. The gradual increase in the flow area of the casing also helps in maintaining uniform flow velocity throughout.
Centrifugal Pump Advantages :
1. It is simple in construction.
2. Due to its simplicity of construction, it can be made in a wide range of materials.
3. Low initial cost and simplicity of design.
4. It operates at high speed and hence, can be coupled directly to an electric motor. In general, higher the speed, smaller the pump and motor required for a given duty.
5. It gives a steady delivery / discharge.
6. Lower maintenance (compared to other pumps) costs.
7. It does not get damaged even if the delivery line becomes blocked, or the delivery valve is closed, provided the pump does not run in this condition for a prolonged period.
8. It can handle readily liquids containing high proportions of suspended solids.
9. For equal capacity, the centrifugal pump is much smaller than any other type of pump. Therefore, it can be made into a sealed unit with the driving motor and immersed in the suction tank.
Centrifugal Pump Disadvantages :
1. It is not usually self-priming.
2. It operates at low efficiencies (50 – 65%).
3. It cannot handle very viscous liquids efficiently.
4. It does not develop a high pressure. Multistage pumps will develop greater pressure heads but they are much more expensive and cannot be made into corrosion resistant materials because of their greater complex construction.
5. If a non-return valve is not provided in the delivery or suction line, the liquid will run back into the suction tank (reservoir) as soon as the pump stops.