The centrifugal pumps are one of the most commonly used pumps in all sorts of different applications, from domestic to industrial, and various other complex areas such as nuclear powerplants. Moreover, the centrifugal pump has become one of the most useful engineering marvels ever.
A centrifugal pump must not be operated unless it is fully primed. The pump casing should be filled before starting, the suction pipe and the branch to the discharge stop valve must also be full. If the liquid enters the pump suction by gravity, priming is usually unnecessary and the pump will remain full of liquid when shut down. To prime a pump initially, or one that has become filled with air open a small cock on the top of the pump casing to release air and close it when the liquid begins to flow.
If the pump is operating with a suction lift, it may be primed from an independent supply, for example by opening the sea suction of a ballast pump, or by means of a priming system, which will evacuate air from the pump and suction piping. The discharge valve is kept closed while priming the pump.
Working Principle of a centrifugal pump
The rotation of the impeller of the centrifugal pump which is placed on a shaft that is eventually connected to a motor produces rotational kinetic energy. This rotational kinetic energy is converted into hydrodynamic energy in a centrifugal pump.
The rotation of the impeller of a centrifugal pump causes the liquid it contains to move towards the outer surface or towards outside from the center to beyond the circumference of the impeller. Thus the liquid being revolved will eventually get impelled due to the centrifugal effect. The flow in the discharge side of the system is caused by the displaced liquid which moves from the casing to the delivery pipe. However, the liquid can only be projected towards the casing from the periphery of the impeller if the other liquid in the casing can be displaced.
The drop in pressure at the center of the centrifugal pump is caused by the movement of liquid in the impeller and the casing of the centrifugal pump. This very liquid in the casing and impeller is crucial for the proper functioning of the centrifugal pump. In addition, the drop in pressure at the center causes the suction pipe to supply the water to be pumped, in order to achieve a continuous operation.
Priming
Priming is required for a centrifugal pump because, the velocity of water at the outer tips of the impeller is very low, the suction efforts of a pump when it is empty rarely exceed 12mm when it is empty. This is obtained through a water gauge. Since the centrifugal pump cannot exhaust the air, it must be primed.
Priming is one of the critical aspects of a centrifugal pump. Without priming, most of the centrifugal pumps will never take suction. A centrifugal pump placed above the water level isn’t self priming because it can not exhaust or remove the air contained in its casing. Hence a means of priming is required to create a vacuum in the suction line.
How is priming done ?
One of the common methods of priming is the air handling method. The removal of air from the pump suction pipes is achieved using a liquid ring primer. This method can create vacuum conditions inside the pump so that atmospheric pressure acting on the surface of the liquid will help the inflow of water into the pump and thus priming it.
A separate pipe from the suction pipe of the main pump is taken to a rotary sir pump which is directly driven by a friction clutch operated spindle. The rotor revolves in a special variable shaped chamber which is supplied with fresh water from a separate water tank attached to the air pump casing.
How does a priming pump work?
It works on the water ring principle. As the impeller vanes pass through the suction port, sir is drawn in and trapped firmly between the water ring and the pump shaft. This sir is carried and delivered to the discharge port. The air pump is easily disengaged by the discharge pressure of the pump, which operates a hydraulic mechanism that clutches in and out the operating spindle of the air pump.
Different types of casing
Centrifugal pumps use different shapes of the casing to attain better efficiency and flow rates. Another advantage of the appropriate casing is better flow rate can be obtained.
1. Volute casing
The volute pump is named after its casing, it is one of the most common casings used in centrifugal pumps. The main goal of this particular casing is to reduce the velocity of water after it leaves the impeller and so convert part of its kinetic energy into pressure energy.
2. Diffuser casing
The diffuser casing consists of a ring of stationary guide vanes surrounding the impeller. The passage through the diffuser vanes is designed to change some of the velocity energy in the fluid to pressure energy. A large amount of pressure energy is obtained by converting the kinetic energy through the diffuser passages. The diffuser pump is also called the turbine pump. When compared to an ordinary volute pump, the diffuser pump can deliver a much greater head. Multi-stage pumps used for bulk liquid cargoes are called turbine pumps.
3. Regenerative casing
A regenerative pump is used where relatively high pressure and small capacity are required. However regenerative pumps are subject to rapid wear and tear. They are ideally suitable for hot fluids since they do not get vapour locked.
Construction and parts of a centrifugal pump
The centrifugal pump consists of a casing made of two halves and fitted with an impeller mounted on a spindle or a shaft. The casing is always provided with inlet and outlet pipes. The area of the casing where the impeller is situated is arranged such that area beyond the impeller increases from minimum to maximum at the inlet to the discharge side. The discharge side is made tapered to the outside to suit the diameter of the corresponding pipe size.
In single stage pumps, a single impeller rotates in a casing of spiral or volute form and in multistage pumps two or more impellers are fitted on the same shaft. Fluid enters the centrifugal pump through the eye and then by centrifugal action continues radially and discharges around the circumference. In double inlet pumps fluid enters from two sides to the impeller eye as if there were two impellers back to back giving twice the discharge at the given head. In multi stage pumps the fluid from one impeller is discharged via suitable passages to the eye of the next impeller so that total head developed or discharge pressure is the product of the head per stage and the number of stages. These types of pumps are often used for high pressure discharge at moderate speed, for example, turbo feed.
Double eye inlet
The casing usually has the suction and discharge branches arranged at the back so that the impeller and spindle can be removed from the front without breaking pipe joints. The discharge branch is usually on the pump centerline so that the pump is not handed. The number of impeller vanes is not fixed but usually, there are six to ten. The volute casing is like a divergent nozzle that is wrapped around the impeller and serves two main functions
1. Enables velocity energy to be converted into pressure energy, the degree of conversion is governed by the degree of divergence.
2. It accommodates a gradual increase in the quantity of fluid that builds at the discharge from the circumference of the impeller
For the velocity to be constant the volute is made so that cross sectional pipe area increases uniformly from cut water to throat. With the impeller having six vanes then the cross sectional area of the volute at no 1 vane will be 1/6 th of the throat area as one vane is pumping 1/6 th of the water quantity, similarly 1/3rd at no 2 and so on, taking vanes in turn from cut water to throat. A common fault for repair with these pumps is the increase of clearance due to wear at the wearing ring or sealing ring faces. This allows connection between suction and discharges, so drastically reducing efficiency.
Parts of a Centrifugal Pump
A modern centrifugal pump consists of a lot of different parts built to their specifications. We have listed some of the basic parts found in a centrifugal pump
Motor
Motor is where the pumps shaft is connected. The power for pumping action is delivered by the motor. There are different types of motors used. The most common motors use 3 phase alternating current(AC) to rotate the centrifugal pump.
Impeller
The impeller is used to provide a rotating action to the liquid. It is mainly made of bronze material and can withstand forces.
Casing
The casing is the covering of the pumps. It houses the wear ring in it. The wear ring provides a minute clearance between it and the impeller. The wear of the wear ring must be checked periodically.
Pressure gauges
These are installed at the suction and discharge sides of the pumps. They will indicate the pressure of the fluid leaving or entering the pump. They are critical in letting the operator know whether the pump is working satisfactorily.
Shaft sealin
It is preferable on vertical pumps to have shaft sealing at the pump upper end only. This permits observation and adjustment of the shaft seal. It also ensures that the pump does not drain through a leaking gland during idle periods. The pump will remain free because deposits will not harden. In smaller designs, the shaft gland seal is by ordinary stuffing box. Stuffing box glands may be packed with soft or metal foil type packing. These packing must be carefully installed as they may score or nip the shaft, thus damaging it.
Advantages of centrifugal pumps
- At constant speed, it does not require overpressure protection
- Liquids with low lubricating properties can be pumped
- Can handle liquids containing particles, dirt, solids
- Produces very little noise
- The rate of wear is extremely low compared to other pumps.
- Frictional losses are low
Disadvantages
- It requires priming initially
- Problems due to leakage along shaft seals may occur
- Can get overheated due to low flow rate
- The pump is prone to corrosion
- The impeller can get significantly worn out or fail completely dye to cavitation
- Viscous liquids may lead to reduced efficiency
Materials of a centrifugal pump
Non salt water pumps
The material used for non salt water pumps is different when compared to their counterparts. For non salt water centrifugal pumps, the casing is made up of high grade cast iron with bronze Internal. Its shaft comprises bronze or Stainless steel. Whereas the impeller is made from bronze material.
Saltwater pumps
For salt water pumps the casing is produced from gunmetal which is bronze metal free of zinc. The shaft is made of an alloy of Stainless steel. However, the impeller is made from aluminum bronze, or stainless steel.
Cavitation in centrifugal pumps
During the operation of a centrifugal pump, if the drop in pressure created at the suction side of a centrifugal pump (mostly by the liquid moving radially outwards from the eye of the impeller) is greater than the vapour pressure for the temperature of the liquid being pumped the vapour will be drawn from the liquid in this area. If there is a restriction in the suction pipe then this process may occur. It may Laos be caused if the speed of the impeller is higher or has a higher temperature than anticipated.
A vapour cavity of this type is likely to cause loss of suction or erratic operation. A lesser cavitation problem occurs when the net positive suction head required by the pump is only just matched by the net positive suction head available from the system. This is because the centrifugal pumps have features that promote local cavitation.
The impeller entry from the side away from the shaft has a profile that resembles that of a hydrofoil. And local liquid flow creates a drop in pressure which starts at the effective leading edge and extends along the surface. A vapour pocket created by a drop in pressure at this surface would collapse in an area of higher pressure and cause cavitation damage. A change of flow direction from axial to radial causes the fluid to experience different velocities and at the extremity, a drop in pressure could produce cavitation again with subsequent bubble collapse and damage. The types of cavitation described could cause surface roughening and generate some tell-tale noises. The corrosion could cause wastage of the impeller and pump casing.
Procedures for operating a centrifugal pump
Starting the pump
- Open the suction valve completely and keep the discharge valve closed
- Open the air vent and if the water comes out of it close it. If the water comes out, it means the pump is primed.
- If the pump is of the self-priming type with an attached vacuum pump keep the air vent closed and open the check valve on the vacuum pump line
- Start the motor
- When the discharge pressure has risen, open the discharge valve gradually. For a self-priming arrangement close the check valve connecting to the vacuum pump
- Changeover the suction and discharge as required gradually
- Monitor the suction and discharge pressures, load on the motor, vibrations, and leakages through valves and glands. If necessary ‘nip up ‘ the glands.
Stopping the pump
- Close the discharge valve
- Stop the motor
- Close suction and all the other valves that were opened for use