Selection of Pumps

For selecting a pumping system, preliminary data should be carefully collected future increase in consumption of at least for next few years may also be considered. The following procedure is followed for selection of a pump:

  1. Determination of number and capacity of pump units.
  2. Check is source of water has adequate capacity, if not investigate ans fix new sources.
  3. Inspect the area and fix locations for pump house, high level tank and route of pipe line.
  4. Collect essential hydraulic data, including the size of suction and delivery mains.
  5. Fix the type, size and characteristics of pump, motor and starter.
  6. Check the features of pumps of different manufacturers and select the best with due consideration to the price.
  7. Collect the particulars of the pump set like physical dimensions, weight, fittings and other accessories.
  8. Prepare the drawings showing pump house, structural supports, layout of the equipment and piping, power supply and electrical connection.

Table of Contents

Selection of Pump Capacity

First we have to ascertain the maximum daily consumption, preferably during the dry season. In the existing installation it can easily be done by conducting a full day test. This test also gives various hidden defects in the system like leaky pipes, unauthorized tapping on the mains, partially closed or clogged valves and wastage of water. In a small city the wastage of water is considerable mostly at 25 %. Even a single dripping tap can waste 100 gallons per day. If the leakages of this types are stopped, water meters are installed at suitable locations and water supply is restricted to few hours a day, considerable savings can be achieved and it may happen that we may not go for additional pump.

However for new installation, an estimate should be made by considering the per day consumption as given in the table below:

Water Requirements
Sl. No.Typical AreaWater required per head per day in litres
1.Residence135
2.Offices (Post office, Admn. Building)45
3.Hostel135
4.Restaurant/ Canteen (per seat)70
5.Auditoriums/Community Hall (per seat)15
6.Training Centers/Schools without Hostel45
7.Training Center / Schools with Hostel135
8.Hospitals (per bed)340 (up to 100 beds); 455 per bed afterwards

Other miscellaneous water requirements are as below:

1.Garden, sports ground:3.5 litres per sq.m.
2.Swimming pool:4 % addition to the capacity for makeup.
3.Fire extinction:(Demand in l/min (Q)): 3000 √P, P is the population in thousands.
4.Air Conditioning:70 litres/hour per 100 m² of the area to be conditioned.
5.Animals / Vehicle45 litres

Storage to be allowed for various fittings are:

1.Bath Tubs:200 litres
2.Water closet / Urinal / Yard Tap:180 litres
3.Shower:135 litres
4.Lavatory / basin / Sink:90 litres
5.Down take taps:70 litres

Quality of water: For Construction Purpose

(1)Acidity:≤2 ml of 0.1H caustic soda in 200 mm water to neutralize
(2)Alkalinity:≤2 ml of 0.1 H HCL in 200 mm water to neutralize
(3)Solidity:
Organic: 200ppm (0.02 %)
Inorganic:3000 ppm (0.30 %)
Sulfates: 500 ppm (0.05 %)
Alkali Chlorides: 1000 ppm (0.10 %)
PH: 6 to 8

After the calculation of this, the rate of consumption at different periods of the day shall be determined by the water consumption chart, a typical chart is shown below:

But if we design a pump to meet the peak hour demand, it may result in an unnecessarily large pump. So a comparatively small pump working continuously for say 18 hours a day and a high level tank of sufficient capacity to meet the short time peak demands shall be provided. The advantages of providing a small pump are:

  1. Its cost is less.
  2. For the same delivery pipe, a smaller rate of pumping will mean smaller friction loss and higher efficiency.
  3. In wells having a low yield, a smaller rate of pumping will reduce the drop in the water level and permit the pump to work continuously and deliver a much larger quantity of water in a day, a large pump would quickly reduce the water level in the well rendering the pump unworkable.
  4. The increase in maximum demand on the electric supply during the peak hour will be much less. The installation of additional pump need not result in any increase in MD if pump operation is cut off by a time-switch during the peak period. The delivery of water will not be adversely affected as the pump can work for the remaining 18 to 20 hours of the day.

The determination of the number of pumps depends on the importance of the installation.For small installations, however as a standard practice pumps shall be installed on (1 + 1) basis i.e. with 100 % standby.

Capacity of Storage Tank: The size of the storage tank is determined by the maximum rate of draw-off, the average demand and the size of the pump. For example, suppose in a colony some 180 000 gallons of water are required per day. If considering for likely increase in consumption, we may assume a 15 hour rate instead of 20 hour, then the capacity of each pump shall be 180 000/15 = 12 000 GPH. At a 12 hour rate 15 000 GPH pumps may be required.

Now by seeing the water consumption chart, say the demand is 60 000 gallons between 9 hours to 12 hours. So the 12 000 GPH pump would provide only 36 000 gallons, and so the high level tank should be able to provide balance 24 000 gallons. To provide for a reasonable margin the capacity of the tank should be at least 50 % higher say 36 000 gallons. If it is vital installation it may be prudent to provide a 60 000 gallon to meet peak demand of three hours. This will provide for emergencies like sudden shutdown of power or failure of a pipe joint. From the security point of view it is desirable to have supply from two independent sources with separate pipe lines.

Storage Capacities:

1.U/G Sump40 % of total O/H Storage.
2.O/H StorageHalf to one day demand for regular supply; otherwise one to two days demand.
3.Hot Water Storage25 liters per capita and if bathtubs are provided 40 liters per capita.

SELECTION OF SUBMERSIBLE PUMP

The selection of submersible pump depends on following factors:

  1. Water requirement
  2. Yield of the bore well
  3. Depth of low water level of the bore well
  4. Height and distance to which water is to be pumped

Water requirement

Water requirement for which submersible pump is required should be ascertained.

Yield of bore well

Yield of a well refers to the average amount of unrestrained water percolation into the well. The yield of a well depends on:

  1. Nature of source
  2. No. of veins of source tapped
  3. Subsoil water level

The yield is least during the dry season. So a well drilled in summer may improve yield in monsoon and a well having good yield in monsoon may go dry in summer.

The rate of pumping should not be much more than yield of the well, unless the storage capacity of the well is large. This ensures that pump does not run dry and thus increasing the life of pump. Also water does not become saline.

If the yield is insufficient:

  1. The well can be deepened or more wells can be opened in the vicinity.
  2. A deep bore hole type well may be sunk until a rich water bearing strata is stuck.

If both these options fail, a new source of supply should be located even if it is some distance away. The following precautions should be taken when in the deep well sand or mud is encountered:

  1. When mud is encountered it normally means that yield is not much, as higher yields are obtained from water bearing sand strata. An essential precaution to be taken is to pump out all muck and sand by an air or jet pump. The percolating flow of water automatically dislodges all loose particles of sand etc on the inner surface of the bore well, the process is greatly assisted by spraying the sides with water under pressure. Within a short time the sides get lined with bigger pieces of stone or gravel followed by smaller ones, forming an excellent filter.
  2. If sand is encountered, the space between the walls of the well and the perforated cover tube surrounding the pump shall be packed with a layer of clean graded gravel or stone chips. This can be achieved by fitting a thin outer cover temporarily around the pump, pack the intervening space with gravels or chips and after lowering the whole assembly to the bottom of the well, the outer cover is pulled out. The gravel will then drop out, fill the sides of the pump and protect the pump against ingress of sand.

To design a bore well pump, the highest water level (HWL) and the lowest water level (LWL) recorded during the previous 10 to 15 years should be ascertained and a datum level should be selected in relation to which all other levels should be fixed, viz. center line of pump, bottom of storage tank and delivery outlet into the storage tank by a leveling instrument. Next the route of suction and delivery pipes should be measured and their diameter determined. A note should be made of every fitting on the suction and discharge side like foot valve, strainer, non-return valve, T-bend, right angle bend etc which may offer friction in the flow.

From the data collected, the total head from all causes comprising the following shall be calculated, which comprise:

  1. Static suction head.
  2. Static delivery head.
  3. Friction head in suction and delivery.
  4. Velocity head.

The relation between head of water in feet and pressure in lb/sq.in is given by the following formulae:

Head in feet = (2.31 × pressure in lb.in) / specific gravity of liquid

So the atmospheric pressure at sea level is 14.7 lb/in². and is equivalent to 760 mm of mercury and 33.95 ft ( say 34 ft) of water. Thus theoretically 34 feet of water can be supported by the atmosphere at sea level, but actual lift possible is much less in practice due to:

  1. At reduced pressures, vaporization of liquid occurs which counter balances the air pressure.
  2. At reduced pressure dissolved gases get released.
  3. There is loss of head at the entry into suction pipe, at the foot valve, and any bends in the suction pipe.
  4. Some head is lost in the velocity acquired by liquid in the suction pipe.

If we make reasonable allowance for the above and deduct some 6 to 8 feet from the theoretical max head of 34 feet, it should be possible to get a suction lift of 26 to 28 feet. Actually however, it is not so, because of considerable fall in pressure where the water enters into the eye of the impeller, not only because of the reduction in area but also because of simultaneous increase in velocity.

Every centrifugal pump requires for stable and satisfactory operation, that its inlet pressure should be at least 10 to 15 feet above vacuum. This is called “Net Positive Suction Head” or NPSH. The NPSH required for a pump is directly related to its specific speed. The higher the specific speed the greater should be the NPSH, which varies as output multiplied by square of the speed. Representative values of specific speed and maximum suction lift is indicated below:

Relation between Specific Speed and NPSH
Type of PumpSp. Speed (rpm)NPSH (ft)Other losses (ft)Atmoshperic pressure (ft.)Maximum negative suction head (ft.)
Centrifugal1000463424
Centrifugal24001073417
Mixed Flow4600208348
Axial Flow6500258341

For higher specific speeds a positive head should be provided at the suction inlet of the pump. For pumping hot or boiling water and viscous fluids like crude oil, lubricating oil positive suction head is essential.

As a general rule, the maximum suction head for an ordinary centrifugal pumps should not exceed 20 feet and should preferably 15 feet.

Static Suction Head: It is the vertical height through which the water has to be lifted from the well i.e. the height of center line of pump from the water level. If pump is below the water level, the suction head is said to be positive and if pump is above the water level, suction head is said to be negative. Since the water level varies from season to season so the static suction head also varies.

Static Delivery Head: This is the vertical height through which water is lifted i.e. from the center line of the pump to the delivery pipe outlet where water is discharged into the high level tank. If the delivery pipe is connected to the bottom or side of the tank, two delivery heads are possible: the maximum when the tank is full and minimum when the tank is empty.

Remember that for calculation of head vertical height is considered not the length of pipe.

Friction Head: It is a measure of energy lost when water is flowing through pipes and fittings. The rougher the surface, greater will be the friction loss. It is proportional to the square of the velocity.

Also in fluid flow, consideration should be given to prevent turbulence and formation of eddies and vortices as it results in loss of energy and head. They occur whenever there is a change in the direction of flow or obstruction in the path. Also they occur at the point of entry or exit from an orifice, change in diameter of pipe and when velocity is excessive.

So for smooth flow it is essential that fluid path is streamlined. The shape of all surface and impediments in the fluid path should be modified and guide vanes provided to reduce the eddies. The surface in contact with the fluid should be smooth and the velocity should be kept as low as is possible and economical. The number of bends, T’s, valves should be kept as minimum as possible. There should not be abrupt change in size of pipe or direction, and it should be done shown in second figure below:

Friction Head Due to Bends, Fittings etc: After ascertaining the number of fittings of each type, equivalent pipe length corresponding to each is worked out and added to the route length of pipe, to arrive at a total length. This should be done for suction and delivery separately. In an existing installation, the necessary data is easily obtained, for new installation it should be estimated as closely as possible. To quickly assess the equivalent length of each fitting, multiply the diameter of pipe line by the factor given below. For more accurate assessment tables on pumps design may be referred.

Multiplying Factor for Various Fittings: 

Type of FittingMultiplying Factor
Bends (radius 3 to 5 dia)10 D
Short Bends30 D
Elbow or Tee (sharp)70 D
Sluice Valve, fully open10 D
Foot Valve & Strainer110 D
Non-return Valve, hinged flap type100 D

D is the diameter in ft.

After getting the total length on both the suction and delivery side, the pipe friction is evaluated. This can be done by using empirical formulae for different types of pipes like cast iron, galvanized iron, by reading the friction loss from table and by reading friction graphically. The empirical formulae are not convenient and calculation is done from table or graphically. Friction loss table based on William & Hazen formulae is given in the table and a graph showing the loss of head due to friction in feet per 100 feet of new wrought iron pipe is given below:

Logarithmic scales graphs are also used and they are more accurate. In these graphs we get diameter of pipe and velocity of fluid and if any two parameters are known, other can be easily obtained. Also consideration should be given for deposits that gets accumulated and make the pipe rough over years of use. So an allowance of 25 % may be made to the total head calculated from the chart. For water containing impurities or sewage, a further allowance up to 25 % may be made.

Hydraulic Gradient: As per the Bernoulli theorem the sum of potential, kinetic and pressure energy is constant at all points ignoring the frictional losses and can be given by:

Z + (V² / 2 g) + (P/W) = Constant, where Z is the potential energy, V²/2g is the kinetic energy called velocity head and P/W is pressure energy.

For a velocity of 4 fps and 20 fps, the velocity head comes to 0.25 and 6 feet respectively. If an increase of 6 feet is occurred at the eye of an impeller, it can cause cavitation trouble.

Also if there is an increase in velocity of flow such as due to constriction, there is reduction of the pressure and if velocity is increased sufficiently, the fall in pressure may bring it below atmospheric pressure. If this happens, there is possibility of air entering into the system and air accumulation can interfere with free flow.

If the pressure heads at different points along the route of a delivery pipe line are plotted on a graph and joined together, the line so obtained is called the hydraulic gradient. Any point above this line represents negative pressure i.e. below atmospheric and should be avoided and a pipe should not be placed above the hydraulic gradient as shown in fig below:

CALCULATION OF PUMP HP: After the total head is calculated, the capacity of pump can be calculated as below:

 Water HP = Work done in foot.lb/min / 33 000 = GPM × 10 lbs × total head in feet / 33 000

 HP of Motor = Water HP / Pump Efficiency = GPM x 10 lb × total head (feet) / (33 000 x η)

= LPS × total head in m / (75 × η) = (Q × H) / (75 × η)

The efficiency of a small pump is considered about 0.70. Under high water level condition, the pump selected by the above procedure is likely to get overloaded and for this we may select the next higher size of the pump. As a thumb rule, the capacity of pump can be calculated by:

Motor HP = GPH / 1000 × head in feet / 100,

In the above thump rule, suitable provision of overload is made and combined efficiency of pump and motor is considered to be 0.50.

The above procedure will be clear by the following example:

Example on Design of Pump: 

Required Discharge: 200 US gallons/minute i.e. 12.5 liter per second.

Static suction head: = 10 ft.

Static discharge head: = 50 ft.

Assuming a velocity of 2.5 ft/s for the suction pipe and 4 ft/s for delivery pipe, the nearest diameter and friction loss can be found from the W & H chart against 200 US gallons per minute and is indicated below:

We have against 200 US GPH: following speed and dia. combinations: speed in fps (dia in inches):

  9.09 (3”), 5.11 (4”), 3.27 (5”), 2.27 (6”) and the corresponding friction loss in feet per 100 feet are 17.8, 4.4, 1.48 and 0.62 respectively.

So 5” pipe for delivery and 6” pipe for suction satisfies the velocity assumption, but as pipe dia. increases, pipe cost also increases, we may select out of 4” and 5” pipes.

Now as suction dia ≥ discharge dia, following combination can be used:-

Suction100 mm125 mm125 mm
Delivery100 mm100 mm125 mm

For 125/100 mm case:

Suction pipeDelivery pipe
1. Velocity in FPS3.27 ft/sec5.11 ft/sec
2. Diameter (bore)5 in (125mm)4 in (100mm)
3. Friction head per 100 feet1.48 ft4.4 ft
4. Adding 25 % for deterioration with age, friction head per 100 feet1.85 ft5.5 ft
QtyEq. straight length
Suction side:
Length of pipe20 ft.20 ft.
Foot valve (110 D)1 no.45.8 ft.
90o bend (30 D)2 nos.25 ft.
Gate valve (10 D)1 No.4.2 ft.
TOTAL95 ft.
Delivery side:
Length of pipe150 ft.150 ft.
Gate valve (10 D)1 no.3.3 ft.
Non-return valve (100 D)1 no.33.3 ft.
90o bend (30 D)6 nos.60 ft.
45o bend (10 D)2 nos.6.7 ft.
TOTAL253 ft.


      Suction side: 125mm dia
      Static head = 10 ft.
      Friction head = 95 × 1.85 / 100 = 1.8 ft.
      Total Suction Head (A) = 11.8 ft.
      Delivery side: 100 mm dia
      Static head = 50 ft.
      Friction head = 253 × 5.5 / 100 = 13.9 ft.
      Total Discharge Head (B) = 63.9 ft.
      So TOTAL HEAD = A + B = 75.7 ft. or about 23 m.
      So a pump having 12.5 LPS discharge at a head of 38 m is required.

      So Pump Motor HP  = (Q × H) / (75 × E)  =  (12.5 × 23) / (75 × 0.70)
    = 5.47 HP.

So we have to select the next higher size viz 7.5 HP Pump to take care of pump overloading etc and considering the efficiency of motor to be 0.90, we need 7.5 × 0.746 / 0.90 = 6.20 kW of input power.

The efficiency of a pump mainly depends on the specific speed (not to be confused with the operating speed) and the discharge and is given in the table below:

Efficiency of a Pump
Specific SpeedOutput in LPS
301202401200
75060737783
100072788288
200077838690
400074808588

The selection catalogue of centrifugal, submersible, compressor and open well submersible pumps are attached and the procedure is self explanatory.

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