Centrifugal Pump: Selection and Installation | Water Lifting Devices

In this article we will discuss about:- 1. Total Head in a Centrifugal Pump Installation 2. Net Positive Suction Head 3. Power Requirements of Pumping 4. Selection of Centrifugal Pumps 5. Specific Speed of Centrifugal Pumps 6. Affinity Laws 7. Installation of Centrifugal Pumps 8. Operation of Centrifugal Pumps 9. Common Troubles of Centrifugal Pumps Installation.

Contents:

Total Head in a Centrifugal Pump Installation
Net Positive Suction Head
Power Requirements of Pumping
Selection of Centrifugal Pumps
Specific Speed of Centrifugal Pumps
Affinity Laws
Installation of Centrifugal Pumps
Operation of Centrifugal Pumps
Common Troubles of Centrifugal Pumps Installation

1. Total Head in a Centrifugal Pump Installation:

The energy relations in a pumping installation can be understood using the Bernoulli equation. In Fig. 10.12, considering points (1) and (2), the energy relationship in terms of head can be written as –

Where, V₁ and V₂ are velocities, P₁ and P₂ are pressures, and Z₁ and Z₂ are elevations from a datum at points 1 and 2 respectively. H_m is the energy imparted to the liquid in moving it from point 1 to 2 and H_f is the friction loss in the piping system.

The terms in the above equation are to be considered depending on the physical situation. Considering Fig. 10.12 the velocities and pressures at points 1 and 2 can be taken as zero and as such H_m is given by –

Where, h_fs and h_fd are the total friction losses in the suction side and the delivery side respectively. When the delivery side is discharging to the atmosphere, the delivery head V²_d/2g may be added to H_m and the pumping head becomes as –

Velocity head V_d²/2g is calculated using the relationship V = Q/A where Q is the discharge and A is the cross-sectional area of the pipe. The head due to frictional losses is calculated by knowing the pipe lengths on the suction and delivery sides and the pipe fittings and estimating the frictional losses through them. Standard tables giving the frictional losses through different pipe fitting are used for the purpose.

Considering points S and d in Fig. 10.12, the energy imparted by the impeller can be written as –

The above equation is used in testing of the centrifugal pumps. Knowing the rate of flow and the diameter of the pipe lines in the suction and delivery sides, V_s and V_d values can be calculated. The pressure or the suction side is negative and is measured using a vacuum gauge and the pressure on the delivery side is positive and is measured using a pressure gauge. However, allowance has to be given for the location of the gauges.

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2. Net Positive Suction Head:

In case of centrifugal pumps, installed above the water level, certain amount of energy is required to move the water into the eye of the impeller. The source of energy available for this purpose is the atmospheric pressure.

The maximum suction lift of centrifugal pumps is dependent upon the atmospheric pressure. Atmospheric pressure varies with elevation and at sea level, its value is 1.03 kg/cm² (14.7 psi) or 10.34 m (34 ft) of water. Theoretically, pump should be able to operate with this suction lift at sea level.

But because of air leaks past the impeller and through other openings the effective suction lift is of the order of 6.34 m (18 ft) at sea level and about 4.5 m (15 ft) for most inland conditions. For obtaining higher pump efficiencies, the suction lift should be as small as possible.

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In a given installation, to assure that the required energy is available, an analysis – can be made to determine the net positive suction head available NPSHA.

It can be computed as follows (Fig. 10.13):

Values of the atmospheric pressure at various altitudes, the vapour pressure and specific gravity of water as a function of temperature can be obtained from standard tables.

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The atmospheric pressure and vapour pressure are un-adjustable, but suction lift and friction head of suction side are able to adjust to be minimum for maximize the NSPHA which means energy to drive water to pump will be increased.

The amount of energy required to move the water into the eye of the impeller is referred so as the net positive suction head required (NPSHR). The NPSHR is a function of the pump speed, impeller shape, liquid properties and discharge rate. Its value is determined for a particular condition using laboratory tests.

If sufficient energy is not present in the liquid on the suction side of the pump to move the liquid into the eye of the impeller, then the liquid will vaporize and what is known as cavitation will occur. Cavitation could remove metal particles, cause severe vibrations and damage the functioning of the pump. The occurrence of cavitation should therefore be avoided.

At a given location, for the satisfactory operation of the pump, the NPSHA should be greater than NPSHR. In order to satisfy this condition the pump may have to be lowered towards the water surface, or the suction pipe could be changed to reduce the friction loss. If NPSHA is less than NPSHR, driving energy is not sufficient to requirement air and water will be pumped together which will damage the pump.

3. Power Requirements of Pumping:

Power is rate of doing work. If a force or a load moves over a distance, energy is consumed and work is done, therefore-

The scientific unit of power is the watt (W) and because this is small it is often expressed as kilowatt (1,000 W). The unit of work or energy corresponding to the Watt is the joule (J), which is defined as the work done when a force of one Newton (1 N) moves through a distance of 1 m. (Watts = joules/sec)

Another unit of power used in case of pumping installations is the horsepower (HP). This is defined as the work done at the rate of 75 m kilograms per sec. in the metric units or 550 ft.lb. per second in the British units.

The Horsepower is not the same in both units but has the following relationship –

The pump efficiency is calculated from the above formula knowing the other two terms. Brake horsepower (BHP) refers to the power supplied by the engine or electric motor as its output. When there is direct drive from the engine or electric motor to the pump and the drive efficiency being 100 per cent, brake horsepower is equal to shaft horsepower.

4. Selection of Centrifugal Pumps:

As every pumping installation has different operating head and discharge it is necessary to select the pump such that it operates under maximum efficiency with the given head and giving the required discharge. This is done by plotting what are known as characteristic curves, both for the well and the pump.

The characteristic curves represent the behaviour of the pump or the well under various operating conditions with the help of these curves different pumps can be conveniently studied and compared.

1. Head Capacity Curve:

This curve for a pump shows how much water, a given pump will deliver with a given head (Fig. 10.14a). The curve represents the behaviour of the pump at one particular speed, head being the variable. The length AO in Fig. 10.14a gives the shutoff head or the head developed when the discharge valve is closed.

At pump installations where there is considerable delivery head, at the instant of starting, the pump is not operating against the total head. Since the head is low, the discharge tends to be high and the engine or motor gets overloaded. Hence at the time of starting, the discharge valve should be kept closed and gradually opened afterwards.

2. Overall Efficiency Curve:

The relationship between the efficiency of the pump and the discharge at a particular speed is represented by the efficiency curve. Also efficiency curves are obtained for different speeds of the pump. The general patterns of the curves are shown in Fig. 10.14b.

3. Break-Horse Power Curve:

The necessary engine or motor horse power needed for a particular pump installation is obtained from the brake horse power curve. Knowing the head and the discharge and the efficiency at that discharge, the BHP is calculated and is plotted against discharge (Fig. 10.14c).

In case a centrifugal pump has to be selected for pumping from an open water source, the total head has to be calculated for selecting the suitable pump. In case of wells the head-capacity curve of the well is made use of in the selection of the pump. The head capacity curve for a given well is constructed from the data obtained when the well is tested.

This curve indicates the amount of water which the well will yield with different drawdowns. Matching the pump and well characteristics is

illustrated in Fig. 10.15. Let the head-capacity curves of the pump and the well intersect at point P. The efficiency can be read from the efficiency curve. The efficiency obtained should be maximum or near about it.

5. Specific Speed of Centrifugal Pumps:

The performance of a pump is determined by the ability of its impeller to impart energy to the water. The design of the impeller therefore, is a basic factor for deciding the type and structure of the pump. An index to operating characteristics of pumps is the specific speed n_s, expressing the relationship between speed, discharge (Q) and head (H_m).

The specific speed of a centrifugal pump (n_s) may be defined as the speed in revolutions per minutes of a geometrically similar pump of such a size that under corresponding conditions it would deliver 1 litre of liquid per second against a head of 1 m. The value of specific speed is useful in comparing the performance of different pumps.

Let H_n, Q_n, D_n and n represent the head, discharge, diameter and speed of a centrifugal pump and H_s, Q_s, D_s and n_s similar values of a model pump.

n_s, however, is not a pure (non-dimensional) number. It depends on the units chosen to express Q_s and H_s. Common units are 1 m for H_s and 1 m³/sec or 1 m³/hr or 1 1/s for Q_s. In English units H_s = 1 ft, but Q_s may be 1 ft³/sec, 1 gal/min, etc. As the value of n_s increases, the pump type changes from radial flow to mixed flow type and then to the axial flow type.

Fig. 10.16 shows the general characteristics of the different types of pumps. It can be seen from these graphs that in general for low heads and high discharges the axial flow type are more efficient compared to other types at its normal working speed. In case of single impeller centrifugal pumps the value of n_s varies from 300 to 5,000.

6. Affinity Laws:

Total dynamic head (H), discharge (Q) and brake horse power (P) of a pump are related to size (W, width) and speed (N, rpm) of impeller. Changing the size and speed of the impeller modifies the operational characteristics of a pump.

This allows pump manufacturers or users to alter the performance of a single pump to match the system needs or understand the pump performance under different operating conditions. These relationships are known as affinity laws are given by the following wherein the subscript zero refers to the original condition.

Eq. 10.15 indicates that a 50 per cent increase in impeller speed, diameter or width will increase discharge by 50 per cent. Eq. 10.16 shows that a 50 per cent increase in impeller speed, diameter, or width will increase the head developed by (1.5)² or 2.25 times. Eq. 10.17 shows that when speed, diameter, or width increases by 50 per cent, the power required increases by (1.5)³ or 3.37 times.

7. Installation of Centrifugal Pumps:

Installation of the pumps consists of:

(1) Location of the pump,

(2) Proper foundation, and

(3) Alignment of the coupling.

In order to minimize the suction lift, the pump should be located as near the water surface as possible. In open wells if there is possibility for water to rise during monsoon season, there should be a provision for installing the pump above the water level. Thus, the pump will be having two locations—one for the low water conditions and one for high water conditions.

The pump and the prime mover can be fixed on a trolley or on permanent foundation. Permanent foundation consists of the base plate fixed to foundation bolts embedded in concrete. The pump shaft and the drive shaft should be aligned straight.

Wedges placed below the base plate are useful in alignment of the pump with the prime mover as the wedges can be used to raise or lower the pump unit.

For pumping from rivers, the centrifugal pumps are installed on pantoons.

8. Operation of Centrifugal Pumps:

In order to start the pumping process in a horizontal type centrifugal pump, it is necessary to fill the suction pipe and pump case with water to expel the air. This operation of filling the suction and pump case is called ‘priming the pump’. Priming is essential because of the non-positive action of the impeller.

Priming can be done by adding water to the suction line from an outside source or by sucking out the air with various devices thereby lowering the pressure in the suction line causing the water to rise from the source to the pump by atmospheric pressure. Hand primers and dry vacuum pumps are some of the devices commonly used for priming.

Pumps Operating in Series and Parallel:

Centrifugal pumps are joined in series or in parallel to meet certain operational requirements. They are joined in series (Fig. 10.18a) when the delivery head requirements are high as in case of pumping in high rise buildings or operating sprinkler systems etc. Two or more pumps may be joined in such a way that the delivery of one becomes the inflow into the other.

These are sometimes referred to as multistage pumps wherein the impellers are mounted on a common shaft driven by a single motor and connections between impellers are shaped in casing ducts and channels. The discharge passing each of them is same (Q), but each pump in turn adds to the head according to its characteristics, H₁(Q), H₂(Q) and so on. The total head H(Q) = H₁(Q) + H₂(Q) + … the sum obtained graphically by vertical superposition.

Pumps are joined in parallel (Fig. 10.18b) when they have a common source and pumping head is same. In situations like pumping from rivers or a pond or dewatering of foundations pumps could be joined in parallel.

The pumps will now operate against the same head H = H₁ = H₂ =…. If the characteristics of the pumps differ too much, this relationship may not be applicable.

9. Common Troubles of Centrifugal Pumps Installation:

Some of the common troubles that occur with centrifugal pumps installations and the remedies are listed below:

1. Pump Fails to Deliver Water:

(i) There may be an air leak in the suction line. Threaded connections are a common source. These should be coated with white lead or pipe cement and tightened.

(ii) Gaskets at the inlet and outlet of the pump may be admitting air. These should be tightened.

(iii) The foot valve or the reflux value may be defective, so that water is not retained in the suction side. Often the flap in the foot valve does not operate properly. This should be checked and replaced if necessary.

2. Pump Fails to Develop Sufficient Pressure or Capacity:

(i) The pump speed to be checked to see whether it is as per prescribed speed or not.

(ii) Suction line and foot valve to be checked for any clogging with debris or any other foreign material.

(iii) The suction lift should be within the prescribed limits.

(iv) A wornout impeller reduces the capacity of the pump.

3. Pump Takes too Much Power:

(i) The speed of the pump may be higher than the rated speed.

(ii) The head may be lower than the rated head for the pump, thereby pumping too much water.

(iii) Mechanical defects such as bent shaft, tight stuffing box, misalignment of the pump and the driving unit should be checked.

4. Pump Leaks Excessively at the Stuffing Box:

(i) The packing material used may be wornout, incorrectly inserted or may not be of the right kind.

(ii) The shaft may be worn out.

5. Pump is Noisy:

(i) The suction lift may be too high.

(ii) Mechanical defects such as bent shaft, improper alignment between the pumps and the driving unit, broken or worn-out bearing may cause noise during the operation of the pump.