2023 Online Catalog - Flipbook - Page 487
Water System Selection 17
AMOUNT OF PRESSURE REQUIRED:
Discharge Pressure of the pump must be sufficient to balance Pumping Depth plus Pipe Friction plus Elevation plus
tank pressure. Water system pressure tanks usually operate
within the pressure range of 20 to 40 lbs. per square inch.
However, recent high demands for pressure, caused by automatic washers and other appliances, have resulted in many
systems being set for 30 – 50, or even 40 – 60 ranges. An
example for determining the required discharge
pressure for a typical submersible pump system is shown
below. Assume a pumping rate of 12 gallons per minute.
NOTE: While total pipe length must be used to compute
pressure loss due to pipe friction, only the distance
to pumping levels is included with elevation in the
summation for total pumping head.
PRESSURE AT PUMP DISCHARGE,
IN LBS. PER SQUARE INCH
285.5
= 123.6 PSI
2.31
TOTAL LIFT, EXCLUSIVE OF TANK PRESSURE
285.5 - 92.4 = 193.1 ft (round off to 200 ft)
To select the proper submersible pump for this installation,
first choose the appropriate table in the catalog, which would
be for a 12 gpm rated pump. Follow the 200 ft Depth-toWater column down until a pump is found that has the entire
desired pressure range covered. The 1 hp model meets the
required performance. Now check the performance at 200
ft. and 60 P.S.I. pressure to make sure that the pump will
generate sufficient pressure to actuate the pressure switch
at the cut-out point with at least 10 psi to spare.
NOTE: In selecting jet pumps, either shallow or deep well
type, the friction loss of the piping in the well is
included in the performance tables. Therefore, only
elevation and friction loss outside the well need to
be calculated. If the offset (horizontal distance between the pump and the well) is greater than 35 ft,
the offset piping should be increased one pipe size.
2. Compute pipe friction by using the tables on Pages 413 –
415.
(1) 1-1/4" Plastic Tee
= 1 x 3 = 3 ft Equivalent
Pipe Length
(2) 1-1/4" Check Valve
=
TANK SELECTION:
Selection of the proper tank completes the water system.
Pressure tanks used with water systems are of the hydropneumatic type. Compressed air in the tank acts as a giant
spring to provide a pressure range, between pump stops
and starts, during which a reasonable amount of water can
be withdrawn. This is necessary to prevent the pump motor
from cycling too often, and to provide s smooth flow of
water to the outlets, without water hammer.
2 x 7 = 14 ft Equivalent
Pipe Length
(1) 1-1/4" Elbow
= 1 x 1.7 = 1.7 ft Equivalent
Pipe Length
18.7 ft Total
for fittings
PRESSURE DROP FROM PIPE FRICTION
(200* + 130 + 18.7) x 2.33
= 8.1 ft
100
TOTAL DYNAMIC PUMPING HEAD
8.1 + 160* + 25 + 92.4 = 285.5 ft
FIGURE 2. Types of pressure storage tanks. Figure 2A shows
plain steel tanks; Figure 2B, the plain steel tank with floating wafer;
2C, the diaphragm tank; 2D the bladder tank.
Technical & Engineering
1. Convert all measurements to the same units. In this case,
we will change tank pressure from pounds per square
inch to equivalent feet of head by multiplying by 2.31, as
shown in the engineering formulas on Page 416. Using 40
P.S.I. as the average tank pressure, 40 x 2.31 = 92.4 feet
of head.
3
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