Digital Dosing Pump Prevents Vapour Lock
BJWSA is capable of reclaiming up to 5 MGD of water for irrigation, including South Carolina's first residential irrigation service. It operates a lagoon-style wastewater treatment plant (WWTP) that was upgraded in 2009 to collect and treat wastewater from the small community of Point South, as well as a new planned community designed for 2,500 homes. Due to the recession of 2009, however, the planned community was put on hold and to date, no homes have been built. As a result, monthly flow to the new plant averages only 50,000 gallons per day, with night time flows falling to as little as 12gpm.
The Point South WWTP plant discharges effluent by pumping to a spray field. BJWSA's goal is to maintain a 1.5ppm chlorine residual in the final effluent. During the upgrade of the plant, the contractor installed two 1/2 HP motor-driven mechanical diaphragm pumps, equipped with an auto-degassing liquid end to dose 12.5 per cent sodium hypochlorite. The mechanical diaphragm pumps, equipped with an internal variable frequency drive (VFD) and stroke length adjustment, were sized for up to 26.7gph at 150psi. The amount of 12.5 per cent hypochlorite required to treat the effluent ranged from a high of 2.5gph on summer weekends and major rain events to a low of 0.1gph during the winter months.
The original pumps worked satisfactorily at higher chemical flow rates but repeatedly vapour locked at lower flows. To manage the problem of off-gassing and the resultant vapour lock, plant personnel were maintaining higher rates of chemical injection than required, and constantly bleeding off the vapours from the pump head during their morning inspection. The degassing device on the pumps failed to evacuate the gas from the pump head. Eventually plant personnel were forced to cut the sodium hypochlorite concentration from 12.5 per cent to six per cent in order to increase the reliability of the chemical metering pumps.
This manual dilution operation had to be done by the plant operator about every seven days, resulting in increased operational and chemical costs for BJWSA. When asked about a Beta test opportunity with Grundfos' new Smart Digital dosing pump, BJWSA mentioned the problems they were having with vapour lock at the Point South WWT plant. Their interest was to go back to using full-strength 12.5 per cent hypochlorite and avoid the process of diluting the chemical to accommodate the existing pumps. In this application, the Beta pump (Smart Digital DDA) would be required to follow a 4-20mA signal from an open channel flow meter installed in the chlorine contact chamber.
It was agreed that this was the right opportunity to test the new pump's ability to handle off gassing liquids while operating at low flows. With support from BJWSA, the Beta version of the Smart Digital DDA was installed. It was powered up and primed still using the dilute 6 percent hypochlorite, and performed as desired. The Smart Digital controls were set to auto deaeration, flow control and auto flow adapt. These features allow the pump to automatically adjust the speed to remove air bubbles while matching the capacity set point, even if there are pressure fluctuations in the line.
The 4-20mA input was scaled to match the minimum and maximum flow desired. BJWSA decided to try the Smart Digital DDA on the full-strength 12.5 per cent solution. After the switch to the full-strength chemical, the pump continued to perform without a problem. In the weeks following, coastal South Carolina experienced a very hot summer with temperatures often exceeding 100F. Throughout the hot summer, the fall and winter, the Smart Digital DDA never experienced a vapour lock problem. Beyond the substantial increase in reliability and the ability to meter full-strength sodium hypochlorite, BJWSA noted two additional benefits. First, its chemical usage dropped from an average of 11gpd with the six per cent concentration and down to 2gpd with the 12.5 per cent concentration.
This has been acknowledged as a cost savings to BJWSA. Secondly, the Smart Digital DDA installation is connected to a Universal Power Supply which displays the electrical load. The pump consumes only 1 per cent of the UPC load. The original pumps were not reconnected to determine the load, but the plant operator noted that during a power failure the UPC would last only 15 minutes with the motor-driven pumps and now lasts 45 minutes with the Smart Digital DDA. By reducing chemical use and power consumption through the use of the Smart digital, BJWSA is also reducing its foot print and environmental impact. BJWSA reports that it is pleased with the performance of the Beta version of the Smart Digital.
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