Centrifugal pumps are the most preferred hydraulic pumps used in domestic and industrial world. In this video we will have a conceptual overview of working of centrifugal pumps.Detailed webpage version of of the video is given below.
Impeller - Heart of Centrifugal Pumps
Centrifugal pumps are used to induce flow or raise pressure of a liquid. Its working is simple. At heart of the system lies impeller. It has got series of curved vanes fitted inside shroud plates. Impeller is always immersed in water. When the impeller is made to rotate, it makes the fluid surrounding it also rotate. This imparts centrifugal force to the water particles, and water moves radially out. In Fig.1 its working is illustrated.
|Fig.1 Rotating impeller imparts centrifugal force to water particles and water moves radially out|
|Fig.2 Negative pressure created by displacement of water from the eye helps in sucking fresh stream of water|
|Fig.3 Water leaving the impeller gets collected in casing, flow direction is also marked|
Use of Casing
From illustrations of pump so far one specialty of casing is clear, it has got increasing area along the flow direction. Such increasing area will help in accommodating newly added water stream, and will also help in reducing exit flow velocity. Reduction in flow velocity will result in increase in static pressure, which is required to overcome resistance of pumping system.
As we have discussed earlier impeller is the most vital part of centrifugal pump. Successful impellers are developed with many years of analysis and development work. Fig.4 shows one of such impeller with its one shroud plate removed for better view of vanes.
These vanes are backward curved. Backward curved vanes have got blade angle less than 90 degree and they are the most preferred vane type in industry due to its self stabilizing power consumption characteristics. Forward and radial blades are less common in industry. The eye configuration of impeller shown is state of the art. This vane is extracted from a Kirloskar pump model. Such projecting eye section induces better swirl of flow and guarantees high negative pressure at the suction.
|Fig.4 More details of vanes inside the impeller|
NPSH - Overcoming Cavitation
If pressure at suction side of impeller goes below vapor pressure of water, a dangerous phenomenon could happen. Water will start to boil forming vapor bubbles. These bubbles will move along with the flow and will break in a high pressure region. Upon breaking the bubbles will send high impulsive shock waves and spoil impeller material overtime. This phenomenon is known as cavitation. More the suction head, lesser should be the pressure at suction side to lift water. This fact puts a limit to maximum suction head a pump can have.
|Fig.5 Low pressure at suction side can cause cavitation; More the suction head,lesser is the suction pressure|
V is speed of water at suction side
For a given pumping system it will have an NPSH called 'Available NPSH'. Pump manufacturer will specify the minimum NPSH required for each pump for its safe operation, known as 'Required NPSH'. If the pump needs to work without Cavitation the 'Available NPSH' should be greater than 'Required NPSH'.
Types of Impeller
The impeller type we have used for the discussion so far is called as enclosed type. Here vanes are closed from both the ends with shroud plates. Other types of impeller which are used in industry are Semi open and open impellers. If the working fluid is cloggy in nature it is preferred to use open kind of impeller. But they are slightly less efficient.
|Fig.6 Kind of impellers used in centrifugal pumps : Enclosed, Semi-Open and Open|
Mechanical Design Aspects
Mechanical design of centrifugal pump is always challenging. A shaft is used to connect between the impeller and motor. Since water pressure inside casing is huge, a proper sealing arrangement is imperative in arresting water leakage through shaft casing clearance. Mechanical seal or stuffing box based mechanism is used for this purpose.
|Fig.7 In this figure gland sealing is provided in order to arrest water leakage|
|Fig.8 Bearings of pump are fitted in the same side; A proper cooling arrangement improves life of such bearings|