Selecting the right chemical feed pump may be done by a professional engineer and it simply shows up in a spec that is installed by a mechanical contractor. Or it may be that your aging pump in the field is beyond repair and you consult someone like me. After a few key questions, I help you select the right pump for the job.

The key ingredients for selection are: dosage required, pressure in the line to be overcome and how does the particular chemistry effect the liquid end of the pump. Beyond that we have some instrumentation interface issues such as is the chemical feed to be proportional to flow? If so the signal could be a digital pulse or an analog signal which is 4-20milliamp. The water meter in place can usually be retrofitted to provide a digital pulse output from the contacting head (multi-jet or turbine meters). Insertion meters or mag meters will usually furnish a 4-40ma signal.

Let us look at the key points above individually:

Dosage required - let us take the example of chlorine or sodium hypochlorite. This may be fed undiluted or diluted at a rate of two, three or four parts per million so as to leave a residual of .2-.5ppm free chlorine after exposure to iron or any bacteria in the distribution system. There is an easy formula I use to size the chemical pump:

200gal/minute x 3785 (cc per, US gal) x 3ppm (desired dosage) divided by 1,000,000 x concentration of sodium hypochlorite (.125 for 12.5%) equals cc/minute of chemical pump output. In this example it comes to 18 cc/minute.

The manual setting ona chemical pump with stroke length and speed would be as follows: 18cc is needed divided by the max pump output, say 1 gal/hour is 63 cc/min, that gives us 28.8% of the maximum or 30% rounded. We can set the speed at 100% and stroke at 30%, or we can do this in reverse or we can multiply two numbers together that will give us 30%, ie 60% and 50% (speed and stroke). We must look at the spec sheet covering the pump to see the minimun exceptable stroke length.

In an automated pulse input proportional to flow, the controller tied in with an insertion meter and controller that has a set P program which acts as a divider. If the flow is 200 gal/minute, and we need 60 pulses per minute as described above, 200 divided by 60 is 3.3 as a set P on the controller.

If the pump is being paced by the 4-20ma signal, refer to the chart on page 20 of the A9 manual. 60% speed (strokes per minute) happens at in incoming signal of about 14ma.

The second ingredient of pump selection is the line pressure. This is straight forward as the psi rating on the chemical pump must exceed the line pressure. That is, the line pressure, say it is 80psi and then add the lOpsi of spring pressure in the injection check valve for 90 psi. If we selected the A151 chemical pump at llOpsi, that would give us adequate ability to handle the 90 psi required.

The third ingredient is the materials of construction on the pump liquid end that would stand up to the chemistry being used. Let us take an example of what doesn;t work. Stainless steel is pretty rugged material, but it doesn't stand up to sodium hypochlorite. You would have to use most items in the plastics family such as acrylic, pve, or PVDF (Kynar).

The mainstream liquid ends are meant for liquids with viscosity under 500 centipoise. For polyelectrolyte that is used in flocculation and coagulation, ie separating solids from water, the high viscosity liquid ends are available. They end in a suffix ofHV ie 85HV or 86HV.

For slurries such as alum there are heads made from Ultra High Molecular Weight Polyethylene. The head is ported with larger openings to permit material with undissolved items that must pass through. This would be liquid ends ending in 9 such as 29 or 79.

Lastly, there are liquid ends withFI in the suffix that tells you these are designed for sodium fluoride or hydrofluosilicic acid. A fluoride saturator with sodium fluoride saturates the solution to 4%. One gallon of that treats 18,000 gallons of drinking water to Ippm. With the concentrated hydrofluosilicic acid at say 23% x 10,000 says that one gallon will treat 230,000 gallons to Ippm.

One other helpful formula is that soda ash should not exceed 10% by weight when diluted in a gallon of water (ie .8 lbs soda ash to be dissolved in a gallon of water). To exceed this in cold weather conditions will make an impassable slurry in the pump's discharge line.

Installation and Maintenance Tips on Chemical Metering Pumps

Flooded suction versus 5' vertical lift

Priming the pumps with four function valves, three function valves, bleed four function valves and AutoPrime Valves for use with offgassing chemicals like sodium hypochlorite.

Spare parts replacement - using the SP or RPM kits designated by the pump liquid end.

Liquifram replacement.

Troubleshooting guide from instruction manual.

Helpful accessories:

Agitators
Calibration Cylinders
Corp Stop and Nozzle
Digi-Pulse Flow Monitors
Pulse Water Meters
Liquid Level Switches
Micropace Analog to Digital Converters, Dividers, Multipliers
Pressure Relief/Back Pressure Valves
Fluoride Saturator
Primer Flush Kit - for manually/automatically flushing a pump's liquid end
Pulse Transmitter - for pacing multiple pumps from one flow meter
Pump Mounting Bracket
Splash Guards
Syphon Breakers Tank
Assemblies