Choosing Correct Metering Pump – A Must in Water-Treatment [July 2012]

July 18, 2012 11:54 am

Ravi Prasad, Director of Sales with Pump Solutions Group outlines the different aspects of selecting the appropriate metering pump for water treatment industry
The ChallengeGenerally metering pumps are used to inject liquids at precisely controlled, adjustable flow rates, which is a process that is often called “metering.” As defined by the Hydraulic Institute’s Metering Pump Section, controlled-volume metering pumps are reciprocating positive-displacement pumps that are typically used for the injection of chemical additives, proportional blending of multiple components or metered transfer of a single liquid.
Metering pumps are used to pump chemical solutions and additives that are used in products manufactured in a wide variety of industries. They are designed to pump into low or high discharge pressures at controlled flow rates. Metering pumps normally consist of a solenoid drive or a gearbox with a motor, a control mechanism, plus a pump head with valves – through which the liquid being pumped enters the inlet connection and exits the discharge connection.
Specifically in the municipal water-treatment industry, the metering pumps that are used must be able to dispense accurate chemical dosages the first time and every time thereafter in order to ensure proper chemical treatment of drinking water or waste water while being processed within the plant. Nearly every process within the water and waste water plant requires metering pumps to dose treatment chemical for disinfection, filtration, clarification, coagulation, flocculation, etc. Metering pumps meet these requirements because they have the flexibility to easily vary the amount of chemical dosage rates, according to the process conditions. They are also ideal in handling a wide variety of corrosive or viscous liquids, as well as acids, bases and solvents.
Taking all of these parameters into account, metering pumps rightfully remain one of the most versatile and relied upon technologies for the safe, accurate and efficient injection of a unique array of chemicals in municipal-water and wastewater applications. The goal of this article is to help the user define the variables that need to be evaluated when choosing and installing the proper metering pump or complete chemical-feed system. Selecting the proper metering pump and planning a complete chemical feed system will not only help inject liquids or slurries regardless of viscosity, but also ensure that it is done in the most efficient, environmentally/community friendly and energy-wise manner.
The SolutionIn most of the cases size really does matter when determining the proper metering pump to be used in a water-treatment application. More specifically, the size in terms of capacity of both the pump’s flow rate and discharge pressure. Simply put, metering pumps should not be oversized. In fact, a metering pump should be sized so that its maximum expected flow rate is 85 per cent to 90 per cent of the pump’s capacity, which leaves additional capacity, if needed. At the other end of the spectrum, a metering pump’s minimum capacity should never be less than 10 per cent of the capacity; anything less will in many cases affect the pump’s accuracy.
Planning a Metering Pump Installation A metering-pump installation must be planned from the day tank or liquid source up to the injection point. Remember that hydraulic metering pumps will “push” against great pressures but they will not “pull” for great distances. Since it is easier to prime and more forgiving, a flooded suction is always preferred, and must be used for fluids where vapour pressure might be less than the suction lift.
Be careful to limit the suction to four feet in a suction-lift application, and a foot valve must be used in a top-mount application. Limit the length of a flooded suction to six or seven feet and use an adequately sized line with minimised bends, elbows or other restrictions. When considering the piping, the safest rule-of-thumb for selecting suction pipe size is to use one size larger than the pump suction connection. For discharge piping, specify piping that is suitable for the discharge pressure.
Other accessories to be considered when planning a metering-pump installation include: • Suction strainer • Flanges, unions or compression fittings • Isolation valves • Calibration column • Relief valve • Back pressure valve • Pressure gauge • Pulsation dampener • Injection quill and check valve.

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