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.
Pump System Inspections
Routine visual inspection of your system is a great way to be proactive instead of reactive. Starting at the chemical feed tank, follow the chemical feed system tubing or piping all the way to the injection point. Inspect for leaking tanks or piping, or corrosion on the fittings. Corrosion indicates that a leak is present and requires immediate attention. Check the ends of the tubing for splits, cracks or thin spots. If you discover any of these signs of deterioration, replace the entire length of tubing. Depending on your application, it is generally recommended that you change out your discharge tubing on a regular preventive maintenance schedule of every 12 months.
Check that you have adequate chemical solution levels in feed tanks to sustain treatment process until the next inspection. Listen for unusual noises coming from your equipment during operation. Unusual noises from the pump, excessive vibration or hotter-than-normal operation are good indications that there is a problem and the situation should be investigated immediately.
Always be aware of missing identification tags on your equipment. This information is helpful when calling for replacement parts. Finally, keep your chemical feed equipment clean-clean equipment runs cooler and is easier to adjust, inspect and maintain. It just looks better than a dirty system.
Each pump manufacturer has recommendations for how often certain components should be replaced. Depending on the type of pump and pump application, these recommendations may or not hold true. It is impossible to forecast the lives of wetted parts such as diaphragms, seats and check valves since corrosion rates and operational conditions affect functional material life. Each metering pump must be considered according to its application conditions. Regular inspections from the early stages of pump operation will indicate the type and levels of required maintenance. A preventive maintenance program based on these inspections will minimize operational problems.
Electronic Diaphragm-Style Pumps generally require the installation of a repair kit every four to 12 months, depending on your application. Repeated short-term deterioration of valve seats and check balls usually indicates a need to review the chemical compatibility of wetted materials selected for your application.
If your pump has been out of service for more than one month, clean its head/valve assemblies by pumping fresh water for approximately 30 minutes. If you're looking for a more thorough cleaning process, try pumping vinegar, citric acid or any of the calcium, lime and rust remover products. These chemicals dissolve calcium and other mineral deposits that may have accumulated over the course of the pump's life in operation. Note: Be sure to adequately flush the chemicals with water to ensure the cleaning solution has been purged before putting the pump back into service.
Motor-Driven Pumps have the same maintenance recommendations as electronic diaphragm-style pumps. Some motor-driven pumps require gear grease or drive oil for proper operation. For optimum performance under normal conditions, gear grease should be changed every 1,500 hours. Drive oil should be changed after the first 1,000 hours of operation. After that, it should be changed every 5,000 hours or on an annual basis, whichever comes first. Depending on the manufacturer, it is recommended that you install a repair kit after every 5,000 hours of operation, as well as new oil seal bellows.
Always look for leaks. If your motor flange is leaking, replace its sealing gasket. If the stroke adjustment knob is leaking, replace its adjustment seal. You may want to contact the manufacturer for assistance. Some pumps have a leak detection port or a "weep hole," which helps notify the user of leaks. If your pump has this feature, determine whether the collected product at the detection port in the liquid end mounting assembly is lubricating oil or the pumped fluid. If the product is pumped fluid, replace the diaphragm. If the product is lubricating oil, replace the oil seal bellows.
Peristaltic Pumps need little maintenance, but do require routine inspections. Always pay close attention to the pump tube, since it is the pump's main contact with the chemical and lobe or shoes on the pump. If the tube becomes swelled or discolored, replace the damaged tube immediately to eliminate the possibility of tube failure. Greasing the rollers is not necessary, but a 100 percent silicon lubricant can be used on the roller assembly if you prefer. Check with the pump manufacturer for other areas where grease should be applied.
The general cause for tube failure is a lack of routine maintenance. Cracking, crazing and discoloration during the first week of operation are signs of severe chemical attack. Check with the pump manufacturer for chemical compatibility for your application. Again, forecasting how long a tube will last is impossible, but routine replacement of the tubes will decrease the risk of future tube failures.
Monitoring Chemical Feed Setup
By choosing the right chemical feed pump for your sodium hypochlorite system, and continuously monitoring its condition, you'll ensure a healthy lifecycle for your operation. Remember to watch for corrosion and leaks and listen for unusual noises. Catching these common problems before they cause system failures will save you many headaches.