Feed Pump Sizing - by Nelsen Corporation

Polyphosphate Treatment for Water Systems

In order to size a MEC-O-MATIC feed pump for polyphosphate feeding it is necessary to know:

a)  the polyphosphate residual (ppm), and
b)  the well pump rate (gpm).

Rule 1: One pound of polyphosphate per 10 gallons water = 12,000 ppm (solution strength)
Rule 2:  One pound of polyphosphate typically treats 40,000 gallons of water at a 2 ppm concentration (residual).

Formula:  [Well pump rate (gpm) x polyphosphate residual

(ppm) x 1440 (conversion factor)] ÷ solution strength = required feed pump output (gpd)

Example:

Well pump rate                  =      10 gpm
Polyphoshate residual      =      4ppm
(10 x 4 x 1400) ÷ 12,000  =
56,000 ÷ 12,000                =      4.66

Feed pump output (gpd), with 10 = well pump rate;
4 = the desired concentration of polyphosphate;
1440 = the conversion factor to change gpm to gpd;
12,000 = the solution strength based on the above Rule
1; and 4.66 = the feed pump output (gpd) based on well pump rate and polyphosphate residual.

It is advisable to slug the system initially at 10 ppm for approximately 30 days to clean out the lines at a faster rate.

Useful Data

  Polyphosphate @ 1 lb/10 gal = 12,000 ppm
5.25%  bleach = 52,500 ppm
12.5% bleach = 125,000 ppm
Potassium Permanganate @ 0.25 lbs/gal = 30,000 ppm

• It requires 3 ppm chlorine for each ppm of hydrogen sulfide.
• It requires 1 ppm chlorine for each ppm of iron.
• It requires 0.7 ppm potassium permanganate for each ppm of hydrogen sulfide or iron.
• Do not neglect the residual required for chlorine applications.
• Do not mix potassium permanganate stronger than 0.25 lbs/gal or the maximum solubility will be exceeded.

Sizing Feed Pumps:

In sizing feed pumps for iron and hydrogen sulfide removal using chlorine bleach or potassium permanganate, the following formula can be used:

Well pump flow rate in gpm

x

Required dosate in ppm

x

Solution (1440) in ppm

/

strength

=

Feed rate required in ppm

Size the chemical feed pump so that its maximum capacity is double the required feeding.

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
20 x 10
=2.15

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.