Pumps & Systems, September 2008
Ensuring your sodium hypochlorite chemical feed system is properly set up and maintained is crucial to effective operation. Failure to address certain issues will decrease reliability, flexibility and performance. For optimal performance, follow these simple steps:
Chemical Storage and Day Tanks
Main storage tanks should be sized to hold a minimum 30-day supply of sodium hypochlorite. However, remember that the longer it is stored, the more chlorine degradation becomes an issue. To retain maximum potency, store your chemical in a climate-controlled building to avoid contact with heat and ultraviolet (UV) radiation. If an indoor location is unavailable, use UV-protected containment units.
Day tanks hold a diluted form of concentrated sodium hypochlorite. Tanks vary in size and chemical concentration to achieve seven to 10 days of chemical storage. Typical ratios of dilution water to chlorine range from 3:1 to 10:1. The dilution water should be finish water (not source water) to maintain potency and reduce sediment in the tank. The tank's discharge (located near the bottom) should be filtered to protect pumps from any debris or sediment in the tank.
All tanks should feature over-fill protection, vents for expansion and contraction, and containment barriers for protection against leaks or ruptures. Each tank needs the ability to be isolated from other tanks, while not interrupting the chlorine flow to the system. Sodium hypochlorite tanks should be separated from other chemical tanks.
Choosing a Chemical Feed Pump
Choose your chemical feed pump based on application. Factors to consider include flow requirements, type of chemical being pumped, and suction and discharge head material. Selecting a pump by application ensures a low maintenance, cost effective system. Use the following pump selection criteria to help you achieve your desired results:
1. Identifying Your Feed Output (GPD)-Size and variations in flow determine pump size and setup. To properly size your pump, you must first identify your feed output (GPD). Use the following formula to find your GPD:
Small systems usually have one pump on-line and may have one pump as a backup. Larger systems or systems with wide variations in flow typically use a multiple-pump system, consisting of two or more pumps in parallel, with an additional pump as a backup. Multiple-pump systems have a great deal of flexibility for adjustments in flow and demand.
2. Amount of Suction Head-Diaphragm-style pumps are typically the best choice for sodium hypochlorite applications. Since they have difficulty overcoming a suction head greater than 5-ft, flooded suction is your best option, especially if there is a differential pressure across the check valves with the discharge side having 5- to 10-psi greater than the suction side. This is usually sufficient to ensure proper seating of the check balls into the valve seats.
Peristaltic style pumps are another option, and should be used to overcome much higher suction head (up to 25-ft). Peristaltic style pumps do not require the differential pressure across the pump.
3. Discharge Pressure-Diaphragm style pumps are able to overcome high system pressures-many can pump up to 250-psi or more. Peristaltic pumps typically pump to a much lower discharge pressure, usually less than 140-psi.
4. Chemical compatibility-When pumping a sodium hypochlorite solution, your pump should be constructed of chemical-resistant materials such as PVC, PVDF (Kynar®) and/or an approved acrylic. It is strongly recommended that you do not use polypropylene-based items, as they tend to dissolve in the presence of chlorine.
Typical pump operation calls for one of two methods. The first involves a pump controller that automatically turns on the chemical feed pump when the source pump turns on. In this case, the source pump's output usually remains the same, so the chemical feed pump is set accordingly. The second type of operation involves a control method using a 4-20 mA signal that adjusts the pump's output as the source pump's flow changes. A 4-20 mA signal can be received from a SCADA system, a pump featuring a variable feed drive or a flowmeter with a 4-20 output.
The pump's injection point needs to be considered since the relationship of chlorine and demand will affect its location. For instance, if a system is using a sequestering agent for iron control, the injection point for sodium hypochlorite must be injected sufficiently downstream as not to oxidize the iron prior to the completion of sequestering the iron. If using chlorimination as a primary disinfectant, the ammonia should be injected near the injection point to allow sufficient mixing for the formation of chloramines.