2023 Online Catalog - Flipbook - Page 506
17 Feed Pump Sizing
How to Size Feed Pumps
In many water treatment systems, a chemical feed pump
is necessary to successfully supply good quality water.
Bacterial iron and hydrogen sulfide may require chlorination
or potassium permanganate. Clear water iron can be treated
in the same manner. Polyphosphate can be used to treat low
hardness waters, waters containing low levels of clear water
iron, or aggressive waters.
The following formula can be used to size feed pumps for all
of these applications:
Well pump
Required
Solution
output rate x dosage x 1440 ÷ strength
(gpm)
(ppm)
(ppm)
= FEED PUMP OUTPUT IN GPD
From the formula, it can be seen we need three pieces
of information:
1. Well pump output rate
2. The required dosage
3. The solution strength of the compound to be fed
Let’s go through each one separately.
Technical & Engineering
22
WELL PUMP OUTPUT RATE
The well pump output rate can be determined quite easily.
Don’t confuse this with the flow rate delivered by one or two
taps in the home. What is required is how fast the well pump
refills the pressure tank – the reason being is that the well
pump controls the feed pump and well pump may be filling
the pressure tank more rapidly than the water is being used
in the home.
To determine the output rate open a tap until the well pump
turns on. Immediately turn off the tap and time how long the
well pump runs. Then, using a one-gallon container, close the
tap and dump the gallon of water down the drain. Continue
doing this and counting each gallon until the pump starts
again. You now have the information to calculate the well
pump output rate.
For Example: The well pump required two minutes to refill
the pressure tank and you found it took 21
gallons before the pump started again.
21 gallons = 10.5 gpm
2 minutes
DOSAGE REQUIRED
The dosage required can be estimated by the following:
• 3 ppm of chlorine is required for each ppm of
hydrogen sulfide.
• 1 ppm of chlorine is required for each ppm of iron.
• 0.7 of potassium permanganate is required for both iron
and hydrogen sulfide.
• 2 to 5 ppm of polyphosphate is required for hardness
and clear water iron treatment.
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NOTE: Don’t neglect the ppm of chlorine residual required in
chlorination. For example, treat a supply containing
two ppm of iron and four ppm of hydrogen sulfide, a
dosage of 15 ppm of chlorine is required.
(2 ppm Fe) (1 ppm Chlorine)
+
(4 ppm H2S) (3 ppm Chlorine)
+
1 ppm residual
=
15 ppm total Chlorine
SOLUTION STRENGTH
The final piece of information required is the solution strength.
Typical concentrations are:
• Straight 5.25% bleach is 52,500 ppm
• Straight 12.5% bleach is 125,000 ppm
• Potassium permanganate dissolved at 1/4 lb per gallon
is 30,000 ppm
• Polyphosphate dissolved at one lb. per 10 gallons is
12,000 ppm
We are now ready to size the feed pump. Using our example above, treat the iron and hydrogen sulfide with straight
5.25 % bleach:
(10.5 gpm) (15 ppm) [1440] ÷ ( 52,500 ppm) = 4.3 gpd
This means we need a feed pump with an output of 4.3
gallons per day. Our PM-16 can pump 20 gpd at its maximum setting, therefore we would set the pump at 2.15.
4.3 x 10 = 2.15
20
The “10” in the formula above is the maximum dial setting
on the PM-16, and “20” is the maximum feed rate of the
PM 16.
Now this is a very low setting on the pump. Try to arrive at
a feed rate that is near mid-scale on the dial of the pump.
This is easily done by adjusting the solution strength of the
compound being fed.
For example, we can dilute one gallon of bleach with one
gallon of water, double our feed pump setting and still
maintain a 15 PPM dosage. We can dilute one gallon of
bleach with two gallons of water and triple the feed pump
setting. By getting a feed rate near the mid-setting of the
pump, we give ourselves room for adjustment.
This method of sizing feed pumps will get you close to
the required dosage. However, there is no substitute for
actually measuring the residual in the treated water. Slight
adjustments in the feed rate setting are normally required.
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