High-precision and cost-effective metering pumps ensure consistent potable water quality, eliminate weekly maintenance and protect water authority operators from noxious fumes.
by Shawn Chong

The treatment of municipal drinking water demands precision and consistency. The Environmental Protection Agency requires that exacting chlorine disinfectant concentrations be maintained to ensure water quality, so utility managers are quick to replace unreliable equipment.

That’s what happened at a water utility district in rural Illinois, where repeated pump failures had raised the cost of maintaining chlorine residual levels—until a smart digital dosing pump solved the problem.

The troubled chlorine dosing pump at one of the water district’s booster stations could not handle the continuously high water pressure and was developing repeated leaks.

These leaks caused insufficient levels of residual disinfection at customer taps, adding maintenance/labor costs while also posing a safety hazard to the plant’s maintenance staff.

Big Challenge for a Small Town

Following a sand filtration system that removes mud and other particulates, the water supply operation adds chlorine disinfectant to inactivate any disease-causing organisms. The treated water is then pumped four miles to the booster station, where a second dosing pump adds additional sodium hypochlorite disinfectant to maintain EPA-mandated levels throughout the utility’s water distribution system.

The controller is equipped with an intelligent drive and microprocessorImage 1. The controller is equipped with an intelligent drive and microprocessor to ensure that each dose is performed precisely and with low pulsation, even when pumping high-viscosity chemicals or those with off-gassing properties. (Images courtesy of Grundfos)

The booster station pumps an average of 20,000 gallons of water per day through a six-inch main for delivery to four rural communities, the furthest of which is approximately 16 miles away. To reach these distant residential service connections, the pump must achieve a pressure of between 110-120 psi.

“High pressure and heavy use was overstressing the dosing pump, which sits downstream of the booster pump and has to overcome the incoming line pressure in order to be effective,” explains the water utility plant manager.

The failing pump was a peristaltic hose type that had been installed as a replacement for a traditional solenoid-actuated diaphragm pump. The initial diaphragm pump had failed due to vapor lock—a condition caused by off-gassing chemicals, in which air becomes entrapped in the pump, preventing it from maintaining prime.

This phenomenon commonly occurs when the pump is sitting idle, often at night or when flow demands are low. It’s a common issue with solenoid-actuated diaphragm pumps.

By contrast, a peristaltic hose pump can easily handle fluids containing particulates, bubbles or trapped gases without the danger of vapor lock. However, its hoses are prone to leaking under system pressures greater than
100 psi.

That’s what happened in this application. Even though the system’s 110-120 psi water pressure was only slightly above the dosing pump’s capacity, its hoses still developed repeated leaks.

Those leaking hoses were responsible for upwards of $3,000 in annual maintenance and repair costs. Moreover, the caustic chemicals leaking from the pump were corroding the pump room’s steel floor and emitting dangerous fumes, creating a safety hazard for maintenance workers.

Most importantly, the malfunctioning dosing pumps were not maintaining the necessary residual chlorine levels in the treated water.

Water Down the Drain

When water quality was compromised—about once every month—the utility was required to dump the nearly 100,000 gallons in the pipe between the booster station and the remote customer taps. This was done by flushing it through a fire hydrant at the end of the line. The flushed water also had to be dechlorinated, at an additional cost.

The plant manager sought guidance from a pump supplier, who recommended a smart dosing pump. The pump is designed to accurately inject chemicals into system pressures up to 232 psi without losing prime. In other words, the pump could accurately meter gaseous fluids without vapor locking and its pressure capacity was nearly twice that required by the utility station.

Still, the plant manager had his doubts. “I was skeptical about trying another diaphragm pump because of our prior experience,” he said, noting that the vapor lock issue is what led him to install the peristaltic model in the
first place.

digital dosing pumpImage 2. The digital dosing pump builds high pressure without vapor locking.

Smart Pump Technology

But the smart dosing pump has some important differences. Neither a conventional solenoid actuated diaphragm nor a peristaltic style pump, this particular pump offers the ability to build high pressure without vapor locking.

While most diaphragm pumps vary output by adjusting the length and frequency of the pump stroke, the electronically controlled stepper motor in the smart dosing pump allows the duration of each discharge stroke to vary according to the set flow capacity, while keeping the speed and duration of each suction stroke constant. Said differently, the pump always operates at full stroke length, regardless of injection volume, ensuring optimum accuracy, no loss of prime and no suction disturbances.

An intelligent drive and microprocessor controller ensure that each dose is performed precisely and with low pulsation, even when pumping high-viscosity chemicals or those with off-gassing properties. Metering accuracy is further increased by a 3000:1 turndown ratio to provide increased flexibility at small volumes.