To conduct a test at various heads of given centrifugal pump and to find its efficiency.
The hydraulic machines, which convert the mechanical energy into hydraulic energy, are called pumps. The hydraulic energy is in the form of pressure energy. If the mechanical energy is converted into pressure energy by means of centrifugal force acting on the fluid, the hydraulic machine is called centrifugal pump. The centrifugal pump acts as a reversed of an inward radial flow reaction turbine. This means that the flow in centrifugal pumps is in the radial outward directions.
The centrifugal pump works on the principle of forced vortex flow, which means that an external torque rotates a certain mass of liquid, the rise in pressure head of the rotating liquid takes place. The rise in pressure head at any point of the rotating liquid is proportional to the square of tangential velocity of (i.e. rise in pressure head = V2/ 2g or 2r2/2g) the liquid at that point. Thus at the outlet of the impeller where radius is more,w the rise in pressure head will be more and the liquid will be discharged at the outlet with a high- pressure head. Due to this high-pressure head, the liquid can be lifted to a high level.
Centrifugal Pump is a mechanical device, which consists of a body, impeller and a rotating mean i.e. motor, engine etc. Impeller rotates in a stationary body and sucks the fluid through its axes and delivers through its periphery. Impeller has an inlet angle, outlet angle and peripheral speed, which affect the head and discharge. Impeller is rotated by motor or i.c. engine or any other device. This means that the flow in centrifugal pumps is in the radial outward directions.
Classification of centrifugal pump:
- Single stage centrifugal pump:
The single-stage centrifugal pump, consisting of one impeller, is the most widely used in production operations. They are used in pumping services of low-to-moderate TDHs. The TDH (total dynamic head) is a function of the impeller’s top speed, normally not higher than 700 ft/min. Single-stage pumps can be either single or double suction. The single-stage pump design is widely accepted and has proved to be highly reliable. However, they have higher unbalanced thrust and radial forces at off-design flow rates than multistage designs and have limited TDH capabilities.
- Multi stage centrifugal pump:
The multistage centrifugal pump consists of two or more impellers. They are used in pumping services of moderate-to-high TDHs. Each stage is essentially a separate pump. All the stages are within the same housing and installed on the same shaft. Eight or more stages can be installed on a single horizontal shaft. There is no limit to the number of stages that can be installed on a vertical shaft. Each stage increases the head by approximately the same amount. Multistage pumps can be either single or double suction on the first impeller.
If a liquid is rotated with a sufficiently high velocity so as to enable it to raise beyond the walls of container and if more liquid is constantly supplied at the center of some suitable means, the tendency of the liquid will be to flow out as illustrated in the figure. This is the principle of centrifugal pump.
The first step in the operation of pump is priming, i. e. suction pipe and casing is filled with water so that no air pockets are left. Rotation of impeller in casing provides a forced vortex which is responsible for imparting centrifugal head to the water. Rotation of impeller effects a reduction of pressure at the center. This causes the water in the suction pipe to rush into the eye. The speed of the pump should be sufficiently high to produce centrifugal head to initiate discharge against delivery head.This is achieved in the adjustable guide pump shown in figure. Centrifugal pumps are classified as rotary dynamic type of pumps in which a dynamic pressure is developed which enables the lifting of liquids from a low datum height source to a higher position.The basic principle on which a centrifugal pump works is that when a certain mass of liquid is made to rotate by an external force, it is thrown away from the central axis of rotation and a centrifugal head is impressed which enables it to rise to a higher level. Now if more liquid is constantly made available at the center of rotation, a continuous supply of liquid at higher level may be ensured. Since in these pumps the lifting of the liquid is due to centrifugal action, these pumps are called centrifugal pumps. In addition to the centrifugal action, as the liquid passes through the revolving wheel or impeller, its angular momentum changes, this also results in increasing the pressure of the liquid.
Figure: Centrifugal Pump
PRINCIPLE CENTRIFUGAL PUMP
An increase in the fluid pressure from the pump inlet to its outlet is created when the pump is in operation. This pressure difference drives the fluid through the system or plant.
The centrifugal pump creates an increase in pressure by transferring mechanical energy from the motor to the fluid through the rotating impeller. The fluid flows from the inlet to the impeller centre and out along its blades. The centrifugal force hereby increases the fluid velocity and consequently also the kinetic energy is transformed to pressure.
COMPONENTS OF THE CENTRIFUGAL PUMP
Impeller: An impeller is a rotating component of a centrifugal pump, usually made of iron, steel, bronze, brass, aluminum, rubber or plastic, which transfers energy from the motor that drives the pump to the fluid being pumped by accelerating the fluid outwards from the center of rotation. The velocity achieved by the impeller transfers into pressure when the outward movement of the fluid is confined by the pump casing. Impellers are usually short cylinders with an open inlet (called an eye) to accept incoming fluid, vanes to push the fluid radially, and a splined, keyed, or threaded bore to accept a drive-shaft.
Impeller Seal: Vice impeller dynamic seal is a rotary sealing structure, also called hydrodynamic seal. It can overcome some shortcomings of mechanical seals and packing, especially suitable for other occasions it is difficult to seal. Such as high speed, high temperature and sealing with strong corrosive or suspension of a solid medium industry etc.. Vice impeller dynamic seal has the advantages of simple structure, reliable sealing, the use of the advantages of long service life, the liquid does not leak, has been widely used in metallurgy, petroleum, chemical industry, food, pharmaceutical, light industrial pump. Especially in thermal power plant used to pumping pulverized coal, ash and slag will pump has been widely used, and achieved obvious effect.
Casing: The volute of a centrifugal pump is the casing that receives the fluid being pumped by the impeller, slowing down the fluid’s rate of flow. A volute is a curved funnel that increases in area as it approaches the discharge port. The volute converts kinetic energy into pressure by reducing speed while increasing pressure, helping to balance the hydraulic pressure on the shaft of the pump.
Shaft: A pump shaft is a major part whose function is to transmit input power from the driver to the impeller/s mounted on the shaft. It is subjected to several stresses – flexural, shear, torsional, tensile, etc. Of these, the torsional stress is most significant and is usually used as basis for sizing the shaft diameter.
Bearing: A bearing is a machine element that constrains relative motion to only the desired motion, and reduces friction between moving parts. The design of the bearing may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Most bearings facilitate the desired motion by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or to the directions of the loads (forces) applied to the parts.
Coupling:A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. Couplings do not normally allow disconnection of shafts during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded.
- Size : 25×25 mm
- Motor : Variables Speed DC current Motor 230 V
- Shaft Speed : 0-3000 RPM
- Pump : 1 HP Centrifugal Pump
- Supply Tank : 80 Ltrs. Made of MS
- Measuring Tank : 40 Ltrs. Made of Ms fitted with
- piezometer tube & scale
- Piping : GI/PVC
- Pressure Guage and Vacuum Guage
POWER AND EFFICIENCY EXPRESSIONS:
- Loosen the V-belt by rotating the hand wheel of the motor bed and position the V-belt in the required groove of the pulley.
- Prime the pump with water if required.
- close the delivery gate valve completely.
- Start the motor and adjust the gate valve to required pressure and delivery.
- Note the following readings
(a) The pressure gauge reading P kg/sq.cm
(b) The vacuum gauge reading V mm of Hg
(c) Time for 10 revolutions of energy meter disc – T secs
(d) Time for 10 cm rise in the collecting tank – t secs
(e) Pump speed in RPM
Take 3 to 4 sets of readings by varying the head from a maximum at shut off to a minimum where gate valve is fully open. The experiment is repeated for other pump speeds.
|Pump Speed N rpm|
|Pressure Guage P kg/cm2|
|Vacuum guage V mm of Hg|
|Total Head H m of Water|
|Total Head H m of Water|
|Time for 10 cm rise in coll tank t sec|
|Time for 10 rev of energy meter disc T sec|
|Output Power KW|
|Output Power KW|
Calculations :Using the appropriate equations, calculate the efficiency of Centrifugal Pump.
- Explain the function of main components of centrifugal pump.
- Difference between the volute casing and vortex casing for the centrifugal pump.
- Derive an expression for the work done by impeller of a centrifugal pump on water per second per unit weight of water.
- Different types of impellers of centrifugal pump.
- Explain the working of a centrifugal pump with next sketch.
- A centrifugal pump has the following dimensions:
Inlet radius = 80mm
Outlet radius = 160 mm
Width of impeller at inlet = 50 mm
B1 = 0.45 radians
B2 = 0.25 radians
Width of impeller at outlet = 50 mm
Assume shock less entry. Determine the discharge and the head developed by the pump when the impeller rotates at 90 radians/sec.
- A centrifugal pump having 36 cm outlet diameter and 18 cm inlet diameter is to deliver water against a net head of 25 m at 1200 rpm. The width of impeller at outlet is 6 cm and flow velocity is constant from inlet to outlet. The entry is radial and the impeller vanes are bent back at 30° to the tangent at outlet. Taking manometric efficiency 90% , determine the width of impeller at inlet, the angle of vane tip at inlet and the discharge from the pump.