CENTRIFUGAL PUMP

The pump is a mechanical conveying liquid or liquid booster. It will be the prime mover of the mechanical energy or other external energy transmitted to the liquid, the liquid energy increase. The pump is mainly used to transport liquids including water, oil, acid and alkali liquid, emulsion, suspension, emulsion and liquid metal, etc., but also transport liquid, gas mixtures and liquids containing suspended solids. Upgrading of water for human life and production are equally important.Centrifugal pump is one of the commonly used pumps in our daily life.

CENTRIFUGAL PUMP

A centrifugal pump is a rotodynamic pump that uses a rotating impeller to increase the pressure of a fluid. Centrifugal pumps are commonly used to move liquids through a piping system. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber (casing), from where it exits into the downstream piping system. Centrifugal pumps are used for large discharge through smaller heads.

Figure Centrifugal Pump

History of Centrifugal Pump:

The invention of the pump is accredited to the Mesopotamians around 3000 B.C. Mesopotamia was an ancient empire that was located in what is now modern day Iraq, Iran, Syria and Turkey. The Mesopotamian’s were a clever lot and invented a horde of useful things like writing, the wheel, agriculture, wine and the domestication of animals – just to name a few.

Their pump was very primitive by today’s standards, but it got the job done. They placed a wooden lever next to a water bank. On one end was a bucket and the other was a counterweight. When the pole was pushed down, the bucket filled with water. The counterweight brought it back up so it could be emptied into a trough. Pretty clever idea, isn’t it?

The next big leap happened between the 3rd and 1st century BC when there were big advances in technology and science. Hellenistic engineers came up with the water wheel, which they used for irrigation and as a power source.

Figure Basic old pump

It was also around this time that Archimedes came up with what is arguably the greatest invention of all time – the screw pump – an invention that is so simple and ingenious that it is still used in part of the world that lack electrical pumps.

Another great contributor to hydraulic tech during this period was by Ctesibus of Alexandria, Egypt. He invented the force pump, which is a sort of hand pump. It comprised of a cylinder that had a plunger on top that drew water through valves.

According to Reti, the Brazilian soldier and historian of science, the first machine that could be characterized as a centrifugal pump was a mud lifting machine which appeared as early as 1475 in a treatise by the Italian Renaissance engineer Francesco di Giorgio Martini.  True centrifugal pumps were not developed until the late 17th century, when Denis Papin made one with straight vanes. The curved vane was introduced by British inventor John Appold in 1851.

 Main Parts of a Centrifugal Pump :

• Impeller
• Casing
• Suction pipe with a foot valve and a stranger 
• Delivery pipe

Figure Parts of a centrifugal pump .

Working of Centrifugal Pump

The first-step in the operation of a centrifugal pump is priming. Priming is the operation in which the suction pipe, casing of the pump and portion of the delivery pipe up to the delivery valve are completely filled with the liquid which is to be pumped, so that all the air from this portion of the sump is driven out and no air pocket is left. It has been observed that even the presence of a small air pocket in any of the portion of pump may result in no delivery of liquid from the pump.
The necessity of priming a centrifugal pump is due to the fact that the pressure generated in a centrifugal pump impeller is directly proportional to the density of the fluid that is in contact with it. Hence if an impeller is made to rotate in the presence of air, only a negligible pressure would be produced with the result that no liquid will be lifted up by the pump. As such it is essential to properly prime a centrifugal pump before it can be started. The various methods used for priming a centrifugal pump are discussed later.
After the pump is primed, the delivery valve is still kept closed and the electric motor is started to rotate the impeller. The delivery valve is kept closed in order to reduce the starting torque for the motor. The rotation of the impeller in the casing full of liquid produces a forced vortex which imparts a centrifugal head to the liquid and thus results in an increase of pressure throughout the liquid mass. The increase of pressure at any point is proportional to the square of the angular velocity and the distance of the point from the axis of rotation. Thus if the speed of the impeller of the pump is sufficiently high, the pressure in the liquid surrounding the impeller is considerably increased.
Now as long the delivery valve is closed and the impeller is rotating, it just churns the liquid in the casing. By opening the delivery valve the liquid is forced to flow out from the pump casing outlet portion.  At the eye of the impeller due to the centrifugal action a partial vacuum is created. This causes the liquid from the sump, which is at atmospheric pressure, to rush 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 utilized to flow the liquid to higher end through the delivery pipe.
As the liquid flows through the rotating impeller it 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 the leaving liquid is not wasted in eddies and efficiency of the pump thereby lowered, it is essential that this high, velocity of the leaving liquid is gradually reduced to a lower velocity of the delivery pipe, so that the larger portion of the kinetic energy is converted into useful pressure energy.
Usually this is achieved by shaping the casing such that the leaving liquid flows through a passage of gradually, expanded area, the gradually increased cross-sectional area of the casing also helps in maintaining uniform velocity of flow throughout, because as the flow proceeds from the tongue T to the delivery pipe, more and more liquid is added from the impeller.

These steps are considered as standard operating procedure for most of the centrifugal pumps in chemical industries.

1. Suction valve of the pump to be opened which cause fluid flow to the impeller and fill the volute of the centrifugal pump.

2. Open the vent valve which is on the discharge line before the discharge valve of the centrifugal pump which cause all air to move out of the casing and filled with the pumping fluid only.

3. When some quantity of the fluid comes out from the vent valve close the valve.

4. Now open the bypass valve of the discharge valve which is near or side of the discharge valve on discharge line.

5. Now start the pump and let it attain its capacity in the pressure gauge on the discharge line.

6. When the pressure gauge is stable it is time to open the discharge valve of the centrifugal pump.

Application of Centrifugal pump :

Centrifugal pumps are used in many applications and across many industries. Some of the commercial, municipal, industrial and scientific fields that make use of such pumps are listed below:

• Oil and energy companies, refineries, power plants
• Heating and ventilation, air conditioning, pressure boosting, fire protection sprinkler systems.
• Waste water processing plants, manufacturing industries, boiler feed applications, municipal, industrial applications, irrigation, drainage and flood protection
• Chemical and Process Industries, pharmaceuticals, cellulose, sugar refining, food and beverage production
• Cryogenics and refrigerants. 

Classification of the Centrifugal Pump

According to the type of casing provided, centrifugal pumps are classified into the following two classes:

(1) Volute pump.

(2) Diffuser or turbine pump

1. Volute Pump. In a volute pump the impeller is surrounded by a spiral shaped casing which is known as volute chamber. The shape of the casing is such that the sectional area of flow around the periphery of the impeller gradually increases from the tongue T towards the deliver)' pipe. This increase in the cross-sectional area results in developing a uniform velocity throughout the casing, because as the flow progresses from the tongue T towards the delivery pipe, more and more liquid is added to the stream from the periphery of the impeller. The volute casing may be designed to have the velocity of flow approximately equal to that of the liquid leaving the impeller. If the casing is designed according to this consideration then the loss of energy is considerably reduced, but the conversion of kinetic energy 'into useful pressure energy will not be possible.If at all the casing is so designed that the casing velocity may be kept down to the value of the velocity in the delivery pipe, then there will be considerable loss of energy due to the difference between the casing velocity and that of the liquid discharged from the impeller. As such a compromise design is often used in which the casing is gradually enlarged so that the velocity is gradually reduced, from the velocity of the liquid leaving the impeller to that in the delivery pipe.The vortex chamber is usually formed as a part of the casing with its side walls parallel . It acts as a diffuser where in the conversion of kinetic energy into pressure energy takes place as explained below. The liquid after leaving the impeller enters the vortex chamber with a whirling motion, that is the liquid particles move radially away from the center following a rotary path while passing through this chamber. Since no work is done on the liquid as it passes through this chamber, its energy remains constant (except for the slight loss by friction). Therefore the torque produced for the liquid does not change and hence a free vortex is formed as the liquid passes through the vortex chamber. Since for a free vortex the velocity of whirl varies inversely as its radial distance from the center, there is a reduction in velocity of flow of liquid as it passes through the vortex chamber.The reduction -in velocity is accompanied by an increase in pressure. As such a vortex chamber serves a dual purpose of reducing the velocity and increasing the efficiency of the pump by converting a large amount of kinetic energy into pressure energy. The liquid after leaving the vortex chamber passes through the volute chamber surrounding it, which further increases the efficiency of the pump.

2. Diffuser or Turbine Pump. In the diffuser pump, the impeller is surrounded by a series of guide vanes mounted on a ring called diffuser ring as shown in Fig. The diffuser ring and the guide vanes are fixed in position. The adjacent guide vanes provide gradually enlarged passages for the flow of liquid. The liquid after leaving the impeller passes through these passages of increasing area, wherein the velocity of flow , decreases and the pressure increases. The guide vanes are so designed that the liquid emerging from the impeller enters these passages without shock. This condition may however be achieved by making the tangent to the guide vane at the inlet tip to coincide with the direction of the absolute velocity of liquid leaving the impeller. After passing through the guide vanes the liquid flows into the surrounding casing which may be circular, and concentric with the impeller or it may be volute shaped like that of volute pump. However, the common practice is to adopt circular casings for these pumps.These pumps which are provided with diffuser ring and guide vanes very much resemble a reversed turbine and hence they are also known as turbine pumps. It has been found from tests that a well designed diffuser pump is capable of converting as much as 75 percent of the kinetic energy of the liquid discharged from the impeller into pressure energy. However these pumps will work with maximum efficiency only for one rate of discharge at given impeller speed. This is so because the guide vanes will be correctly set or shaped for one rate of discharge only and for other discharges a loss of energy by shock or turbulence will occur at the entrance to the guide vanes, thereby resulting in a low efficiency. Moreover turbine pumps are more costly than the simple volute pumps. As such the arrangement of diffuser ring is usually employed only in multistage pumps.

The centrifugal pumps may also be classified on the basis of certain other factors as indicated below:

(a) Number of impellers per shaft.

Based to impellers count provided, the pumps also classified as single-stage and multi-stage. A single stage centrifugal pump has only one impeller mounted on the shaft. A multi-stage centrifugal pump has two or more impellers connected in series, which are mounted on the same shaft and ate enclosed in the Same casing.

(b) Relative direction of flow through impeller.

On the basis of the direction of flow of the liquid through the impeller the pump may be classified as radial flow pump, mixed 'flow pump and axial flow pump. A radial flow pump is that in which the liquid flows through the impeller in the radial direction only. Ordinarily all the centrifugal pumps are provided with radial flow impellers. In mixed flow pumps the liquid flows through the impeller axially as well as radially, that is there is a combination of radial and axial flows.

(c) Number of entrances to the impeller.

A mixed flow impeller is just a modification of radial flow type in this respect that the former is capable of discharging a large quantity of liquid. As such mixed flow pumps are generally used where a large quantity of liquid is to be discharged to low heights. In axial flow pumps the flow through. the impeller is in the axial direction only. Axial flow pumps are usually designed to deliver very large quantities of liquid at relatively low heads. However, it is not justified to call. axial flow pumps as centrifugal pumps, because there is hardly any centrifugal action in their operation.

Depending on the number of entrances to the impeller the centrifugal pumps may be classified as single suction pump and double suction pump. In a single action (or entry) pump liquid is admitted from a suction pipe on one side of the impeller. In a double suction (or entry) pump liquid enters from both sides of the impeller. A double suction pump has an advantage that by this arrangement the axial thrust on the impeller is neutralized. Further it is suitable for pumping large quantities of liquid since it provides a large inlet area.

(d) Disposition of shaft

 The centrifugal pumps may be designed with either horizontal or vertical disposition of shafts. Generally the pumps are provided with horizontal shafts. However, for deep wells and mines the pumps with vertical shafts are more suitable because the pumps with vertically disposed shafts occupy less space.

(e) Working head.

According to the head developed, the centrifugal pumps may be classified as low head, medium head and high head pumps. A low head pump is the one which is capable of working against a total head up to 15 m. A medium head pump is that which is capable of working against a total head more than 15 m but up to 40.m. A high head pump is the one which is capable of working against a total head above 40 m. Generally high head pumps are multi-stage pumps.

Problems in Centrifugal Pump

• Cavitation—the NPSH of the system is too low for the selected pump
• Wear of the Impeller—can be worsened by suspended solids
• Corrosion inside the pump caused by the fluid properties
• Overheating due to low flow
• Leakage along rotating shaft
• Lack of prime—centrifugal pumps must be filled (with the fluid to be pumped) in order to operate
• Surge

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