How to Operate Centrifugal Pump: Working Principle, Types and Components

Well for the operators, trainees, and freshers who just move into the process industries will have their introduction, with fluid transporting devices which are mainly "PUMPS". All chemical process industries have the basic and the mostly optimized device know as centrifugal pump. It is a superb piece of equipment where most of the fluids which are viscous and coarse form are  handled easily and  it is cheaper when compared to other pumping devices, even maintenance cost is much lesser than reciprocation pumps.  When coming to its standard operating procedure, there are sequence of steps to be followed for every centrifugal pump. which  may vary in some situation based on design but mostly the following procedure is well practiced:

Standard operating procedure to operate centrifugal pump is:


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.
These steps are considered as standard operating procedure for most of the centrifugal pumps in chemical industries.
Note: check periodical maintenance log book to confirm about cleaning of the strainer in the suction line and fitted back, to protect the impeller from damage. Know with what parts a centrifugal pump is made off


Schematic drawing of centrifugal pump set up, pipe lining to the centrifugal pumps suction and discharge connections


 THEORY :-

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, which also results in increasing the pressure of the liquid.

According to the general direction of flow of liquid within the passage of the rotating wheel or impeller the rotodynamic pumps are classified as,

(i) Centrifugal pumps,
(ii) Half axial or screw or mixed flow pumps,
(iii) Axial flow or propeller pumps. 
In the impeller of a centrifugal pump the liquid flows in the outward radial direction, while the flow of liquid in a propeller pump impeller is in the axial direction, parallel to the rotating shaft. The mixed flow pump impeller has an intermediate form so that the flow of liquid is in between the radial and axial directions. However, there are no rigid boundaries separating these three types of pumps, and often all the three types of pumps are called centrifugal pumps.
In general all the rotodynamic pumps closely resemble reaction type of hydraulic turbines and they may be regarded as reversed reaction turbines. Thus the action of a centrifugal pump is just the reverse of a radially inward flow reaction turbine. Similarly the axial flow pumps are reverse of propeller or Kaplan turbines and the mixed flow pumps are the reverse of mixed flow type turbines such as Francis turbine. In the present chapter only centrifugal pumps have been described.

The main advantage of a centrifugal pump is that its discharging capacity is very much greater than that of a reciprocating pump which can handle relatively small quantity of liquid only, A centrifugal pump can be used for lifting highly viscous liquids such as oils, muddy and sewage water, paper pulp, sugar molasses, chemicals etc. But a reciprocating pump can handle only pure water or less viscous liquids free from impurities as otherwise its valves may cause frequent trouble. A centrifugal pump can be operated at very high speeds without any danger of separation and cavitation.

As such it can be coupled directly through flanged coupling to electric motor. The maximum speed of a reciprocating pump is limited from the considerations of separation and cavitation. As such reciprocating pumps can be operated at low speeds only and for that these pumps are mostly belt driven. The maintenance cost of a centrifugal pump is low and only periodical check up is sufficient. But for a reciprocating pump the maintenance cost is high because the parts such as valves etc. may need frequent replacement. However, a reciprocating pump can build up very high pressures as high as 69 x 106 N/m2 {700 kg(f)/cm2} or even more and hence these pumps are used for lifting oil from very deep oil wells.

COMPONENT PARTS OF A CENTRIFUGAL PUMP

The main component parts of a centrifugal pump which are described below:
(i) Impeller. It is a wheel or rotor which is provided with a series of backward curved blades or vanes. It is mounted on a shaft which is coupled to an external source of energy (usually an electric motor) which imparts the required energy to the impeller thereby making it to rotate.
The impellers may be classified as.
(a) shrouded or closed impeller,
(b) semi-open impeller; and
(c) open impeller,

Closed or shrouded impeller is that whose vanes are provided with metal cover plates or shrouds on both sides. These plates or shrouds are known as crown plate and lower or base plate  The closed impeller provides better guidance for the liquid and is more efficient. However, this type of impeller is most suited when the liquid to be pumped is pure and comparatively free from debris.
If the vanes have only the base plate and no crown plate, then the impeller is known as 'semi-open type impeller'. Such an impeller is suitable even if the liquids are charged with some debris.
An 'open impeller' is that whose vanes have neither the crown plate nor the base plate. Such impellers are useful in the pumping of liquids containing suspended solid matter, such as paper pulp, sewage and water containing sand or grit. These impellers are less liable to clog when handing liquids charged with a large quantity of debris.

(ii) Casing. It is an airtight chamber which surrounds the impeller. It is similar to the casing of a reaction turbine. The different types of casings that are commonly adopted are described later.

(iii) Suction Pipe. It is a pipe which is connected at its upper end to the inlet of the pump or to the center of the impeller which is commonly known as eye. The lower end of the suction pipe dips into liquid in a suction tank or a sump from which the liquid is to be pumped or lifted up.
The lower end of the suction pipe is fitted with a foot valve and strainer. The liquid first enters the strainer which is provided in order to keep the debris (such as leaves, wooden pieces and other rubbish) away from the pump.. It then passes through the foot valve to enter the suction pipe. A 'foot valve' is a non-return or one-way type of valve which opens only in the upward direction. As such the liquid will pass through the foot valve only upwards and it will not allow the liquid to move downwards back to the sump.
(iv) Delivery Pipe. It is a pipe which is connected at its lower end to the outlet of .the pump and it delivers the liquid to the required height. Just near the outlet of the pump on the delivery pipe a delivery valve is invariably provided. A delivery valve is a regulating valve which is of sluice type and is required to be provided in order to control the flow from the pump into delivery pipe.


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. There are different types of casings that are adopted for this purpose and on the basis of the type of casing used; the centrifugal pumps are classified into different types as described in the next section.
Centrifugal Pumps Operation and Selection

TYPES OF CENTRIFUGAL PUMPS



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.
(b) Relative direction of flow through impeller.
(c) Number of entrances to the impeller.
(d) Disposition of shaft, and
(e) Working head.

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.

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.

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.

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.

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.


Performance of Pumps – Characteristic Curves :-


A pump is usually designed for one speed, flow rate and head, but in actual practice the operation may be at some other conditions of head or flow rate and for changed conditions the behavior of pump may be quite different. In order to predict the behavior and performance of pump under varying conditions tests are performed and the results of test are plotted. The curves are called characteristic curves

(a). Main and operating characteristics
(b). Constant efficiency curves
(c). Constant curves and constant discharge curves

(a). Main and operating characteristics :-
In order to obtain the main characteristic curves of pump it is operated at different speeds. For each speed rate of flow discharge is varied by means of a delivery valve and for different values of manometric head Hm, shaft power P and overall efficiency Eo, are measured or calculated. The same operation is repeated for different speeds of pump. Then Hm Vs Q, P Vs Q and Eo Vs Q curves for different speeds are plotted, so that three sets of curves are obtained, which represent main characteristics of pump.

(b). Constant efficiency curves :-
The iso-efficiency curves facilitate direct determination of range of operations of a pump with a particular efficiency.

(c). Constant head and constant discharge curves :-
These heads are useful in determining the performances of a variable speed pump for which the speed constantly varies.