Metering Pump Technology
Motor-driven, solenoid and peristaltic metering pumps provide proper control for specific applications and multiple needs.
In use today are three basic pump technologies for water disinfection. Each has advantages and disadvantages that need to be understood to pick the best product for the specific application. Pump technologies include mechanical (motor) driven, electromagnetic (solenoid) driven and peristaltic—a self-priming pump that achieves pumping action by moving a system of rollers against a flexible tube, providing a constant seal on the pumping tube.
The mechanical (motor) driven diaphragm-metering pump has been around the longest and is used for its simplicity. This pump provides consistent output with simple controls, typically a single output adjustment knob. For preventative maintenance, the pump requires cleaning of the valves about every three to six months and changing of the diaphragm once a year. The disadvantage of this pump is it does contain a motor that, when exposed to extreme conditions such as moisture and strong chemical fumes, can cause premature failure. Overall, this pump typically is very price competitive when compared to the other technologies and has a loyal following in those markets.
The electromagnetic (solenoid) driven diaphragm-metering pump provides accurate output with more advanced controls. The solenoid driven pump can have a turn-down ratio of 10:1, 100:1 or in some microprocessor-based units provide 1,000:1 turn-down ratio, depending on the application, for extreme accuracy in output. The turn-down ratio is expressed as the rated capacity divided by the minimum capacity that can be obtained while maintaining specified flow repeatability, accuracy and linearity.1 The solenoid driven pump also has the ability to accept a contacting-head water meter signal to treat based on flow. The contacting-head water meter sends a signal to the pump when a defined volume of fluid passes through it. This causes the pump to actuate and feed a predetermined volume of chemical. This pump also can be connected to a level wand to shut down when the chemical tank is low or feed proportionally via a 4-20 mA electronic signal. As with the motor driven pump, the unit requires preventative maintenance on the valves and diaphragm but is more tolerable to environmental conditions. The solenoid pump with controls costs slightly more than the motor driven pump, but its ability to handle environmental conditions, accuracy and dependability make it a favorite for the disinfection marketplace.
The peristaltic pump differs from the mechanical and electromagnetic pumps by using a tube instead of a diaphragm to introduce chemicals into the system. It provides a consistent feed without the problem of losing prime for gassing chemicals. Peristaltic pumps are able to feed more viscous chemicals without problems. While they operate without loss of prime, the feed rates do change as the tube wears. Additionally, tube life varies and leaks are common when a tube wears out. To ensure consistent operation, replacement of the tube is required every 1,000–1,500 operating hours. Also peristaltic pumps currently do not have the external pacing and control features of the solenoid driven pump. Overall, the peristaltic is a good choice for applications where loss of prime is a problem. The cost is comparable to the solenoid driven pumps.
Proper pump sizing is key to the success of any application. To begin, the following information on the application is needed.
Well pump output rate in gallons per minute (gpm). This is the amount that fills the pressure tank, not the flow rate delivered to the facility. The reason for this is the well pump may be filling the pressure tank more rapidly than what’s being used in the home.
Required dosage in parts per million (ppm). You must know how much chemical is needed to treat the hydrogen sulfide (H2S), iron (Fe), manganese (Mn) or any other oxidant demands in the raw water. This can be established by testing or estimated if certain demands are unknown.
Solution strength as a percentage and ppm. For example, 5.25 percent bleach is 52,500 ppm, and 12.5 percent bleach is 125,000 ppm. Potassium permanganate dissolved at 1/4 lb. per gallon is 30,000 ppm and polyphosphate dissolved at 1 lb. per 10 gallons is 12,000 ppm.
Next, plug the information into the following equation.
Well pump output rate (gpm) x Required dosage x 1,440 ÷ Solution strength (ppm) = Feed Pump Output in gallons per day (gpd)
For example, a well has 10.5 gpm of output, the required dosage is 15 ppm and we’re feeding 5.25 percent bleach. The chemical pump required would be [(10.5 gpm) x (15 ppm) x (1,440)] ÷ (52,500 ppm) = 4.3 gpd pump.
To properly size the pump, the minimum required would be 4.3 gpd. But to cover potential changes or upsets in the system, doubling the result will provide optimum operation. Thus, the pump required would be an 8.6 gpd pump. Finally, note the pressure that’s being injected, ensuring the proper pump is used.
Many problems with metering pumps can be avoided with proper installation. To ensure a trouble-free application, the following guidelines should be followed.
The metering pump should be mounted via a wall or shelf-mount bracket, flooded suction or on a tank lid. In the case of the wall or shelf mount and tank mount, the strainer should be kept above the tank bottom by 1?2 inch and a sinker weight should be utilized to keep the suction tubing down (Figure 1). The pump should be mounted in an area that provides proper ventilation and isn’t in direct sunlight. All the connections should be “hand tight” only and the use of thread tape is discouraged. The injection valve or “probe” always should be installed into the center of the flow stream to provide proper chemical mixing. Place the injection point above the solution supply tank to prevent any siphoning. The pump voltage must match the voltage of the circuit in use and must be properly grounded. In cases of “dirty” power, a surge protector should be used. These simple rules will provide a clean, painless installation.
As needs have changed, new products have been developed to meet these requirements. The integration of multiple pieces of equipment into one is a trend that manufacturers rapidly are progressing toward. Some new enhancements include the integration of a seven-day timer into the pump and one with a built-in, adjustable timer triggered by a contacting-head water meter. The seven-day timer allows the addition of chemicals at specific times without the need for multiple pieces of equipment. Traditional methods just allow the pump to stroke for every water meter count. The built-in, adjustable timer allows more flexibility when feeding based on a contacting-head water meter giving the ability to feed at user-set times. This allows one pump to handle a changing application without having to purchase a new product and makes accuracy of feed much more reliable. Future products under development include “smart pumps” that let the user know when and how much they’ve pumped and allow the user to remotely control settings via a communication port (dial-up or LAN based).
Selective use of motor-driven, solenoid and peristaltic metering pumps for respective applications for which they are best suited allows proper chemical feed to ensure continued safe disinfection of drinking water. Advanced instrumentation options allow further control for multiple needs, special applications or remote monitoring.