In Todays article we are going to discuss about Centrifugal Pumps , its parts and how it works. Also we will be discussing about the various Terms involved in understanding the Concept.
Pump – A pump is a mechanical device helps in transferring a fluid from one place to another by increasing its pressure.
Types of Pumps
Displacement Type / Positive Displacement Pump
In positive displacement Pump the fluid is displaced from the suction to the discharge by the mechanical variation of the volume of the chamber. It is further subdivided into
1. Reciprocating Pump
2. Rotary Pump
Rotodynamic Pump / Centrifugal Pump
The centrifugal Pump operates on a rotodynamic principle in which the flow through the Pump is induced by the centrifugal force imparted to the liquid by the rotation of an impeller.
Centrifugal Pump is a type of rotodynamic pump in which the flow through the pump is induced by the centrifugal force imparted to the liquid by rotation of the impeller.
Parts Of The Centrifugal Pump
The centrifugal pump consists of two main parts:
Rotor Consists of following main components:
- Impeller – The impeller is the one whose rotary motion induce a centrifugal force on the fluid.
- Shaft – The impeller is mounted on a shaft and enclosed by casing.
Casing– It is the stationary part of the pump which acts as housing to all the internal parts and protects them from external atmosphere. It consists of:
- Suction Nozzle / Ports
- Discharge nozzle / Ports
- Seal rings
Other Parts are :
- Gland Packing
- Mechanical Seals
This is so named because of its spiral which is so constructed to convert the part of the velocity of the fluid to the pressure energy, which is the objective of a pump , to increase the fluid’s pressure.
Some of the centrifugal pump also uses diffuser in addition to the volute casing.
Diffuser : The diffuser performs the same function as volute casing i.e convert part of K.E energy of the fluid to the pressure energy. It consists of a ring of guide passages around the impeller. This design is used for high pressure as in multi-stage boiler feed pump.
Eye Of The Impeller
Eye of the Impeller : In the centre of the impeller, is the eye of the impeller which receives inlet flow of liquid into the vanes of the impeller.
Suction Pipe with Foot Valve And Strainer
A pipe whose one end is connected with the inlet of the impeller and the other end is dipped into the sump of the water is called suction pipe. The suction pipe consists of a foot valve and strainer at its lower end. The foot valve is a one way valve that opens in the upward direction so that the water does not flow back to the supply side when the pump is not in the operation. The strainer is used to filter the unwanted particles present in the water to prevent the centrifugal pump from blockage.
Delivery pipe is the pipe whose one end is connected to the outlet of the pump and the other end is connected to the required height where water is to be delivered.
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WORKING OF THE CENTRIFUGAL PUMP
The rotation of the centrifugal pump impeller causes the liquid it contains to move outward from the center to beyond the circumference of the impeller because of the centrifugal effect. And because of this movement of the fluid to the outer periphery, there is a drop of pressure at the eye of the impeller. This drop in pressure creates the suction force of the pump and hence the pump draws the fluid from the suction supply.
Now, the water due to centrifugal force continue to move towards the casing. The area of casing increases gradually in the direction of rotation. So the velocity of the water keeps on decreasing and the pressure increases and at the outlet of the pump the pressure is maximum.
From the outlet of the pump, the water goes to its desired location through delivery pipe.
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WHAT IS PRIMING AND WHY PRIMING IS NECESSARY
One important thing is to note that the centrifugal pump is not self-priming. So, in order to make it functional, it needs to be primed.
PRIMING- It is the process in which the suction pipe , casing, and delivery pipe upto the delivery valve is filled completely with liquid to be raised from outside source before starting the pump. It is done to remove the air from the pump.
If the air is not removed from the pump then only a small negative pressure is created at the suction pipe and it cannot suck the water from the water sump.
WHY DISCHARGED VALVE IS CLOSED DURING STARTING OF THE CENTRIFUGAL PUMP?
By closing the discharge valve, we can reduce the starting current.
As we know that, the current will be high during the starting of any motor. If we start the pump with the discharge valve open, The discharge head will act on the pump i.e. more resistance, so the motor has to give more starting torque to the pump which means more current is drawn by the motor.
In other words, if there is pressure in discharge side of the pump , prior to startup, it can flow back through the pump, causing a backward spin and may draw more current, thereby causing damage to the pump.
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During operation if the drop in the pressure created at the suction side of a centrifugal pump (by liquid moving radially outwards from the eye of the impeller) is greater than the vapor pressure for the temperature at which liquid being pumped, the vapor / bubbles will be drawn from the liquid in this area. Any vapour bubbles formed by the pressure drop at the eye of the impeller are swept along the impeller vanes by the flow of the liquid. When the bubbles enter from low pressure to high pressure farther out the impeller vanes, they abruptly collapse. The process of the formation and subsequent collapse of the vapor bubbles in a pump is called cavitation.
This phenomenon is likely to occur if there is a restriction in the suction pipe, or if the liquid is volatile, or has a higher temperature than anticipated , or if the impeller speed is excessive.
Cavitation degrades the performance of the pump, resulting in a fluctuating flow rate and discharge pressure. It can also be destructive to the pump’s components as collapsing of bubbles on impeller vanes can damage the blades. It can also cause excessive pump vibration which could damage pump bearings, wearing rings and seals.
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NET POSITIVE SUCTION HEAD (NPSH)
It is the difference between inlet pressure and the lowest pressure level inside the pump. It is therefore an expression of the pressure loss that takes place inside the first part of the pump housing.
Required NPSH is the lowest inlet pressure required by the specific pump at a given flow to avoid cavitation.
Available NPSH is the absolute pressure at the suction part of the pump.
Pump will operate only if:
( required NPSH > Available NPSH )
It refers to the pressure at which the vapour and liquid phases are bin equilibrium .
Suction lift exists when the source of supply is below the centre line of the pump. Thus, static suction lift is the vertical distance from the center line of the pump to the free level of the liquid to be pumped.
Suction Head exists when the source of supply is above the center line of the pump. Thus, static suction head is the vertical distance from the centerline of the pump to the free level of the liquid to be pumped .
STATIC DISCHARGE HEAD
It is the vertical distance between the center line and the point of free discharge or the surface of the liquid in the discharge tank.
TOTAL STATIC HEAD
It is the vertical distance between the free level of the source of supply and the point of free discharge or the free surface of the discharge liquid.
It is the head required to overcome the resistance to flow in pipe and fittings. Depends on the size, type of pipe, flow rate and the nature of the liquid.
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