POOL HYDRAULICS
"Understanding Basic Pool Hydraulics"
By Jim Ashton
Filtration, circulation and chemistry are the three basic factors
affecting overall water quality in swimming pools. For residential
swimming pools, maintaining water quality means that we need to
circulate and filter all of the water in the pool two to three times
per day. In order to accomplish this, we have to design our pool
installation with properly sized equipment. Our design must consider
the volume of water in the pool, how frequently we want to achieve
water turnover, how much resistance the water will encounter moving
through the system and how we will size our pump and filter to overcome
this resistance.
Let's consider:
|
| Pipe Size |
GPM (Gallons
Per Minute) |
| 1¼ " |
33 |
1½" |
44 |
| 2" |
75 |
|
As you can see from this table, only so much water will flow through
a particular diameter of pipe. We base the flow on the maximum standards
for velocity, which is 7 feet per second.
A typical 16 x 32 swimming pool, which holds 17,000 gallons and
is plumbed with 1½" pipe, has a maximum turnover rate
of approximately 6.5 hours. The same pool plumbed with 2”
pipe would turnover in just under 4 hours! If all we were considering
was the pipe diameter, the typical pool would have to run almost
20 hours per day, while our pool with 2” plumbing would have
to run only 12.
However, we also have to consider that a number of things create
resistance within a closed plumbing system. The vacuum from the
suction side of the pump, the length of the pipe, the number of
fittings, the type of fittings, the pool filter and the elevation
of the pump compared to the pool will all contribute to the amount
of resistance within the system.
Pump Output (GPM) vs.
Total Resistance to Flow (Feet of Head) |
| Make / Model |
20ft |
30ft |
40ft |
50ft |
60ft |
70ft |
| Hayward 1HP Super Pump |
85 |
76 |
65 |
50 |
27 |
-- |
| Sta-Rite ¾HP Max-E-Glas II |
83 |
77 |
70 |
65 |
55 |
44 |
This resistance, measured in Feet of Head, will determine the volume
of flow that any particular pump can produce. Notice in the chart
above that a more efficient pump, with a smaller motor can actually
move more water . By understanding hydraulics, we can actually design
systems that not only improve water quality, but also save money!
Consider that a 3/4 HP pump motor consumes approximately 20%
less electricity than a 1HP pump motor. But further consider that
a pump motor converts electricity into two products; mechanical
energy to move the water, and heat. The amount of heat generated
depends on the efficiency of the motor, but, it is not unreasonable
to expect that we can save an additional 20% with a more efficient
pump motor. Therefore, in our typical pool, a 1HP pump consuming
1500 watts will cost approximately $100 per month to turn over our
water three times per day. The same pool using a ¾HP high
efficiency pump will cost only $60 per month.
Now, adding 2” plumbing lines, our high efficiency pump
with 50 feet of head could turn the water over three times in 13
hours, reducing the run time by 46% and the monthly electric bill
to only $32.00! By properly designing the system, a five and a half
month season will cost only $176 instead of $550. Would you consider
taking me out for dinner with the difference?
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