How To Size a Pump For Continuous or Variable Flow


Selecting the right metering pump is an essential step in the development of a water treatment program. The processes described will help you select the right metering pump for your application, every time.
By Teresa Brown | Posted: Dec 10, 2012 | Views: 9946

Selecting the right metering pump is an essential step in the development of a water treatment program. A common question asked of the Technical Team is: “What size pump or flow rate, do I need for dosing a certain application?”

The following process will help you to select the right metering pump for your application, every time.

The first formula example will be sizing a pump for continuous flow applications while feeding AquaTrol C-2712 (Note: C-2712 is 12.5% Chlorine which is equal to 125,000 PPM)

  • System flow rate: 50 GPM
  • Feed: 4 PPM of AquaTrol C-2712
50 GPM x 4 PPM x 1440 Minutes / 125,000 PPM = 4.6 gallons per day (GPD)

It is also important to know the backpressure (working pressure) of the system the product is fed into in order to properly determine the pump’s pressure rating. The pump needed for the application must have a higher pressure rating than the working pressure of the system — otherwise, chemical injection will not be accomplished.

Let’s say that the system pressure is 80 psi — now we know that we need a pump that will pump at least 5 gallons per day against 80 psi. In order to allow for the dosage rate to be increased we should size the pump for 10 to 14 gallons per day against 100 psi.

Momar’s AquaTrol Division distributes Advantage, LMI, Pulsafeeder, and Walchem pumps.  Please contact your local AquaTrol Sales/Services Representative or contact Momar for the appropriate pump for your application.

The next formula example will be sizing a pump for a contacting head water meter (pulse output) feeding the same concentration of AquaTrol C-2712 (4 PPM) with the same flow rate (50 GPM). Here, the same principles come into play. You will need to know the following:

  • System pump output rate: 50 GPM flow rate
  • Required dosage: 4 PPM
  • Solution strength: 125,000 PPM
  • Water meter rating: 1 GPC (gallon per contact)

This is basically a three step calculation:

  1. Chemical injection GPM = 50 x 4/ 125,000 = .0016
    (Where: 50 = flow in GPM, 4 = dosage, 125,000 = solution strength)
     
  2. Contacts per minute from meter in strokes per minute (SPM) = 50/ 1 = 50
    (Where: 50 = flow in GPM, 1 = gallons per contact rating of water meter)
    **Note: The SPM (strokes per minute) cannot exceed 125
     
  3. Pump rated output in GPD (gallons per day) = .0016/50 x 125 x 1440 = 5.76 GPD
    (Where: .0016 = chemical injection GPM. 50 = contacts per minute from water meter. 125 = MAX Strokes per minute of the metering pump and 1440 is the total minutes in a day)

It is typically recommended to oversize the pump from the formula by multiplying the result by 2. For this application we would use a 12 GPD pump. As in the first example, we need to determine the pressure we need to pump against for a final pump selection.

With these calculations as a tool, you can use them for any solution strength, required dosage, system flow rate and contacts per gallon.

To get in touch with an AquaTrol Representative call 800-556-3967 or fill out our contact form.

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