Journal of the Department of Agriculture, Journal of the Department of Agriculture, Western Australia, Series 4 Western Australia, Series 4
Volume 8
Number 3 1967 Article 7
1-1-1967
Pumping and horsepower Pumping and horsepower
R P. Harington
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Recommended Citation Recommended Citation
Harington, R P. (1967) "Pumping and horsepower," Journal of the Department of Agriculture, Western Australia, Series 4: Vol. 8: No. 3, Article 7.
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F A R M W A T E R SUPPLIES
PUMPING AND HORSEPOWER
By R. P. HARINGTON, Field Technician, Irrigation Branch
WATER is vital to the survival of plants and animals and its availability will influence the site of the homestead and the farm layout generally.
This article describes some of the factors involved in the installation of a pumping outfit for domestic and stock requirements.
In the consideration of any pumping problem it is necessary to know.
• The amount of water required in a specified interval of time.
• The total head against which the pump is expected to operate.
Amount of water required
It is important that the supply of water meets the demand during the period of greatest need. The following table shows
Flow in Gallons per Minute
.5 .6 .7 .8 .9 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
« . 0 50.0 55.0 60.0 65.0 70.0 75.0 80.0 90.0 100.0
Table 1.—Friction head per 100 feet of polythene pipe
i in. J in.
1.5 2.0 2.8 3.5 4.4 5.0 17.0 35.0 58.0 85.0
.22 .31 .4 .5 .65 .75 2.5 5.0 8.5 12.5 17.0 22.0 28.5 35.0 42.0 85.0
Diameter of Pipe in Inches
1 in.
.13 .16 .19 .65 1.3 2.2 3.2 4.4 5.7 7.3 9.0 10.8 22.0 36.0 55.0 73.0 100.0
l i in.
.22 .45 .75 1.1 1.5 2.0 2.5 3.1 3.7 7.5 12.5 19.0 25.0 33.5 42.0 52.0 62.0 73.0 85.0 100.0
....
....
l i in. 1
.19 .31 .46 .63 .85 1.05 1.3 1.55 3.1 5.25 7.75 10.5 14.0 17.5 22.0
26.0 1 31.0 1 36.0 « 40.0 II 47.0 r 57.0 1.
60.0 1.
74.0 1 X
in.
.15 .17 .21 .27 .32 .39 .8 1.3 2.0 2.7 1.5 1.4 5.5 1.75 9.0 ) 0 3.75 2.0 1.5 S.O 1.75
to 119
the approximate maximum amounts re- quired for various purposes.
Sheep—li gallons per head per day.
Horses—10 gallons per head per day.
Beef cattle—10 gallons per head per day.
Dairy cattle—15 gallons per head per day.
Pigs—1J gallons per head per day.
Per 100 fowls—10 gallons per day.
Per member of a family—50 gallons per day.
Home garden—900 gallons per day.
Dairy cleaning per milking—15 gallons per cow.
Total head
The total head involves three factors—
Suction head Delivery head Friction head
and is expressed in feet.
THE SUCTION HEAD is the vertical distance between the water level and the pump centre. To this must be added the
losses due to friction in pumping a given quantity of water.
THE DELIVERY HEAD is the vertical distance from the pump centre to the point of discharge.
THE FRICTION HEAD is the loss due to friction cuased by the flow of water in the system. These losses vary with the size and nature of the pipe and the rate of flow through the pipe. Typical friction losses are shown in Tables 1 and 2.
In any system there must of necessity be various fittings such as valves, elbows and tee pieces. In calculating the friction loss it is necessary to equate these in terms of equivalent lengths of straight pipe. In computing the total friction loss the sum of these equivalents must be added to actual length of pipe.
Table 3 shows the equivalents in linear feet of the various pipe fittings.
An example
To illustrate the above, consider the problem in which it is required to pump
7T~ 1 T 7 T
R E S E R V O I R
s # ^ ^
y /A B X
_J/_ Y
C D E
D E F I N I T I O N S O F H E A D IN P U M P I N G S T A T I C S U C T I O N H E A D OR L I F T S T A T I C D E L I V E R Y H E A D
F R I C T I O N H E A D L O S S IN S U C T I O N P I P E F R I C T I O N H E A D L O S S IN D E L I V E R Y PIPE T O T A L S U C T I O N h E A O [A + X ]
T O T A L D E L I V E R Y H E A D [B + Y ] S T A T I C H E A D [A + B ]
F W O R K I N G OR T O T A L H E A D [A + B + X + Y ]
50 gallons per minute through a 2 inch DELIVERY SIDE galvanised iron (GJ) pipeline 300 ft. long
with the water level 10 ft. below the surface to a height of 75 ft. above the surface. In dealing with the problem it is easier to deal with the suction side, then the delivery side and add the two totals.
The example is illustrated in the sketch.
ft.
Length of 2 in. delivery pipe .... 300 Equivalent length of 2 in. bend 4 Equivalent length of 2 in. check
valve 10 Total length of delivery pipe 314 ft.
SUCTION SIDE
Length of 2 in. suction pipe ....
Equivalent length of strainer and foot valve
Equivalent length of 2 in. bend ft.
10 15 4
Total length of suction pipe 29 ft. *= 113 ft The friction loss in 2 in. G.I. piping with TOTAL HEAD.
Friction head on 314 ft. of 2 in. G.I. pip- ing with a flow of 50 G.P.M. is 12ft. per 100 = 38 feet. (Table 2.)
Total delivery head = Vertical height to which the water must be pumped plus friction head.
=75 + 38
flow of 50 gallons per minute (G.P.M.) is approximately 12 ft. per 100 ft. (Table 2). Hence in 29 ft. the friction loss is 3i ft.
The total suction head is 34 -f 10 = 134 ft.
Then the total head against which the pump must operate is
Total suction head + total delivery head
= 134+113
t= 1264 say 127 ft.
Gallons
Per Hour
100 120 ISO 180 240 300 400 500 600 900 1,000 1,200 1,500 1,800 2.100 2,400 2,700 3,000 3,300 3,600 3,900 4,200 4,500 4,800 5,400 6,000
Per Min.
1.66 2.0 2.5 3.0 4.0 5.0 6.6 8.3 10.0 15.0 16.6 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0 65.0 70.0 75.0 80.0 90.0 100.0
rable 2 . — Friction he ad per 10( feet of new galvanised iron pipe Diameter of Pipe in Inches
J i n .
3.0 3.7 5.4 7.8 13.4 21.0 35.0 55.0
1 in.
.7 .9 1.3 1.8 3.2 4.7 8.2 13.0 18.0 38.0 47.0
....
IJ in. \i in.
.2 .3 .5 .6 1.0 1.6 2.7 4.2 5.9 13.0 16.0 22.0 33.0 47.0 60.0
.2 .2 .3 .5 .7 1.1 1.7 2.3 5.0 6.0 8.7 13.0 19.0 24.0 32.0 40.0 48.0 56.0
2 in. 2J in. 3
™."l .1
....
.2
.3 .1 .4 .2 .6 .2 1.2 .4 1.4 .5 2.1 .7 3.2
4.4 5.8
1.0 1.4 1.9 7.6 2.5 9.3 3.1 12.0 3.7 14.0 4.4 17.0
19.0 22.0 25.0 28.0 35.0
5.3
6.2 : 7.1
8.1 9.3 12.0 42.0 14.0
in. 4 in.
.1 .1 .2 .2 .3 .4 .6 8 1.0 1.3 1.5 .7 2.2 IS 2.8 J.3 J.7
(.6 1
5.7 1
.1 .2 .2 .3 .3 .4 .5 .5 .6 .7 .8 .9 .1 .3 121
HORSEPOWER
Once the total head against which the pump must operate has been determined the calculation of water horsepower is the next step in the process of arriving at the motor H.P. required.
To operate the plant a source of power is required. The familiar unit of power is the horsepower and this is equal to the working rate required to move 33,000 lb.
one foot in one minute. As one gallon of water weighs 10 lb. this is equivalent to lifting 3,300 gallons of water one foot in a minute. When calculating power requirements for pumping water the term
"water horsepower" is used. This is defined as the power theoretically required to lift a given quantity of water each minute to a specified height.
The water horsepower is given by the equation—
W.H.P. = G.P.M. x 10 x H
33,000
where-
G.P.M. = gallons per minute H = total head in feet
10 = weight of 1 gallon of water in pounds
33,000 = the number of foot-pounds of work per minute to equal
1 horsepower.
In the example given earlier—
W.H.P. = 50 x 10 x 127
33,000
= 1.9 PUMP EFFICIENCY
The horsepower delivered by an electric motor or engine to the shaft it turns is
known as the brake horsepower (B.H.P.).
Pump efficiency is the ratio of the useful water horsepower delivered by a pump (the output) to the brake horsepower (the input to the pump).
Centrifugal pumps specific types of work for example
volume but
efficiently in a position where it is required to pump a low volume to a great height.
A good practical suggestion is to assume the efficiency of a centrifugal pump at 65 per cent. This allows a fair margin of safety for pump wear.
In the problem then—
B.H.P. == W.H.P.
are designed for One cannot expect, a pump designed for large low lift work to perform
% efficiency of pump.
= 1.9 0.65
= 3 H.P. approx.
POWER SUPPLY
It is of practical value always to keep some reserve "up your sleeve" to allow some margin for wear and tear in the unit supplying the power. Centrifugal pumps must run at recommended speeds and any fall in revolutions will seriously impair the pump's efficiency—even to the level where it could fail to pump at all-
A 5 H.P. engine in this scheme would provide an adequate margin.
The chief advantage of electric motors over stationary engines is the ease of operation. When considering the use of electric motors keep in mind that a squirrel cage motor has an output efficiency of 86 per cent.
Table 3.—Friction loss in pipe fittings
The figures shown are the equivalent in length in linear feet of straight pipe from which friction head must then be calculated.
Bend Elbow 90°
Tee
Foot Valve and Strainer Check Valve
Gate Valve
i in.
2 2 4 5 1
| in.
2 2 5 6 1
1 in.
3 3 6 8 1
1! in.
3 4 8 10 8
l i in.
4 5 9 12 9 1
2 in.
4 6 12 15 10 2
2] in.
6 7 14 18 IS 2
3 in.
7 8 17 21 18 2
4 in
9 12 23 30 25 3
In the plant under consideration there- 3
fore a — = 3.5 H.P. or nearest above, .85
electric motor would need to be installed.
PUMPING COSTS
To understand clearly the consumption of different fuels in pumping, it is helpful to note t h a t by definition:
Power = Work Time
and hence that Work == Power x Time.
The expression horsepower hour is used to designate the continuous consumption of power or fuel of 1 H.P. for 1 hour.
The average diesel engine consumes about 0.45 lb. fuel per horsepower hour on lull load. One gallon of fuel weighs about 9 lb. Therefore 5 H.P. will consume 0.45 x 5 lb. = 2.25 lb. per hour or i gallon. The unit of cost in electricity is the Kilowatt hour, and is equal to 1.34 H.P.
Therefore a
5 H.P. motor would consume 5 Kw.
1.34 3.6 Kilowatts per hour.
Selection of pump size
For a given centrifugal pump, the delivery, head and power used will all increase as speed is increased.
All centrifugal pumps operate within the following laws:—
(1) The quantity of fluid discharged varies directly as the speed.
(2) The head or pressure developed by the pump varies as the square of the speed.
(3) The power required varies as the cube of the speed.
It is most undesirable to drive the pump
:>t speeds higher than recommended be- cause the wear on impeller and bearings becomes very great.
Manufacturers' tables should be referred to when selecting a suitable size pump.
They are designated by the size of the
discharge pipe. For example, a 2 in. pump has a 2in. discharge pipe. A 2:3 pump has a 2in. delivery and a 3in. suction.
There is no power loss when pump and engine are directly coupled. In the case of belt drives however, an additional margin must be added to both engine H.P.
and pump speed to allow for slip. This is 5 per cent, for "V" belts and 10 per cent, for flat belts.
USEFUL RULES A N D I N F O R M A T I O N
A column of water 1 f t . high exerts a pressure of .433 lb. per sq. in.
1 Imp. gal. = 10 lb.
1 c u . foot = 6 ^ Imp. gallons.
= 6 2 . 3 5 lb. fresh water.
1 Cusec. = 1 c u . f t . per second.
= 3 7 5 . 8 G.P.M.
= 2 2 , 5 4 8 G.P.H.
1 Acre-inch = 2 2 , 6 1 5 gallons.
For quick estimating a 1-cusec flow is equal to an acre-inch per hour.
Sharp angles in a pipe line cause a large increase in f r i c t i o n , therefore use bends and easy curves instead of elbows.
By doubling the diameter of a pipe its capacity is increased four times.
By trebling the diameter of a pipe its capacity is increased nine times.
Reducing pipe sizes can increase p u m p i n g costs because of the steep rise in f r i c t i o n head.
123
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