Choosing a Metering Pump for Boiler Water Treatment Metering Pumps Are Used to Pump Many Chemical T Their Simplest, Metering Pumps Solutions and Additives

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Fire Fire www.process-heating.com re re | August2011 | Volume 18,Number8 Pumps Choosing a Metering Pump for Boiler Water Treatment Metering pumps are used to pump many chemical t their simplest, metering pumps solutions and additives. For boiler water applications, are used to inject liquids at pre- these devices must dispense accurate chemical Acisely controlled, adjustable flow rates, which is a process that often is called dosages every time to ensure the proper chemical metering. As defined by the Hydraulic treatment for the conditioning of boiler feed water. Institute’s Metering Pump Section, controlled volume metering pumps are recip- rocating, positive-displacement pumps By Tom O’Donnell, Neptune Pump Co. that typically are used for the injection of chemical additives, proportional blending of multiple components, or metered trans- fer of a single liquid. Metering pumps normally consist of a Hydraulic diaphragm pumps are a popular selection because of their solenoid drive or a gearbox with a motor, longevity in process applications. a control mechanism, and a pump head with valves. It is through the latter that the liquid being pumped enters the inlet connection and exits the discharge connection. Metering pumps are designed to pump into low or high discharge pressures at controlled flow rates. For boiler water treatment applications, the metering pumps used must be able to dispense accurate chemical dosages every time in order to ensure the proper chemical treatment of the boiler feed water. Nearly every process within boiler feed water treatment requires metering pumps to dose treatment chemicals to help eliminate erosion, corrosion and scale buildup. 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 suited for boiler water treatment applications that include the injection of phosphates to control phosphate levels in boiler feed water, and the injection of hydrazine, mor- phaline and ammonia for corrosion control. Taking all of these parameters into account, metering pumps rightfully remain one of the most versatile and relied upon technologies for the safe,

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accurate and efficient injection of an usually include electricity, air, gas or other array of chemicals in boiler water treat- means of driving the pump. When the ment applications. To employ them effec- pump’s parameters are determined, you tively, the user must effectively define the must then consider the environment in process-specific variables that need to be which the pump will operate. evaluated when choosing and installing Hazardous area requirements also must the proper metering pump or complete be identified when selecting the driver. chemical feed system. Having the proper When evaluating a hazardous environ- metering pump and chemical feed system ment, remember to consider dust, which can help users inject liquids or slurries can ignite just like fumes or vapors. Is regardless of viscosity and help ensure the pump to be utilized indoors or that the metering is done in an effi- outdoors? If it is located outdoors, cient, environmentally friendly and it should be sheltered from direct energy-wise manner. sunlight. As far as temperature requirements, most pumps will operate in freez- Metering ing conditions provided that the fluid to Pump Selection be pumped will not freeze and that the As seems to be the case with most correct lubricants are selected. In this everything, size really does matter when case, freeze protection and heat tracing determining the proper metering pump to may be required. Finally, operation in be used in a boiler water treatment applica- corrosive environments may require tion. More specifically, selection begins by special pump coatings. understanding the proper size in terms of capacity of both the pump’s flow rate and Total Control discharge pressure. Metering pumps normally consist of a Determining the pump’s method of con- Simply put, metering pumps should solenoid drive or a gearbox with a motor, trol is next on the list of determining fac- not be oversized. In fact, a metering pump a control mechanism, plus a pump head tors. The choices usually include: with valves — through which the liquid should be sized so that its maximum being pumped enters the inlet connec- expected flow rate is 85 to 90 percent of tion and exits the discharge connection. • Manual continuous operation. the pump’s capacity, which leaves addi- • On/off operation. tional capacity, if needed. At the other end • Automatic proportional control in of the spectrum, a metering pump’s mini- Another key determination is materials response to a process signal. mum capacity should never be less than 10 of construction. Selection of a metering percent of the capacity. Anything less will, pump must take into consideration any In general, metering pump flow rates in many cases, affect the pump’s accuracy. corrosion, erosion or solvent action that can be adjusted manually through the use Selecting a metering pump cannot be may occur when handling specific sub- of a micrometer dial. This manual control done until the proper flow rates, discharge stances. For example, solvents may dissolve allows the pump to be operated between pressures and type of fluids are identified pump heads constructed of plastic. Acids 10 and 100 percent of capacity by chang- for the specific application. Once that is and caustics only may be compatible with ing the stroke length. determined, the type of metering pump stainless steel or certain steel alloys, and By comparison, a manual variable-speed can be selected, i.e., hydraulic diaphragm, abrasive slurries can erode some materials. drive changes the stroke speed. A combi- mechanically actuated diaphragm, sole- Fortunately, metering pumps are available nation of the two may allow additional noid driven or piston style. in a range of materials of construction. adjustability or turndown over the range Hydraulic diaphragm pumps are a When considering the type of head the of the drive, depending upon the stroking popular selection because of their longev- pump should have, consider whether spe- speed of the pump. For example, a pump ity and robust design. Most hydraulic cial features are required for your applica- operating at 75 strokes per minute (which diaphragm pumps handle clear liquids tion. For instance, some models are offered could be turned down to 15 strokes per with viscosities generally ranging from with double-diaphragm heads with leak minute) would allow a 5:1 turndown on water (which has a viscosity around 1 cps detection and alarm capabilities for appli- speed when using the variable-speed drive, at 68°F [20°C]) to 1,500 cps. Special liq- cations where any diaphragm failure must and a 10:1 turndown on stroke length uid ends or special heads are available for be sensed immediately. when using the micrometer dial. applications outside this viscosity range Selecting a driver is another area of Metering pump flow rates also can be (fluid viscosities to 20,000 cps and slur- concern. A driver should be chosen by controlled automatically — in response to ries up to 10 percent solids). matching it to the available utilities, which a process signal — by electric positioners

26 | August 2011 • Process Heating Pumps that change the pump’s stroke length, or by variable-speed drives that alter the stroking speed. Using a positioner gives the operator a full 10:1 turndown, which is the full adjustable range. Using a variable-speed drive will supply only as much turndown as the ratio of the pump stroking speed divided by the minimum operating speed of the pump. Remember that it is not practical to use a variable-speed drive on motor-driven pumps that normally operate at less than 100 strokes per minute. Slowing the motor causes each stroke to take longer from start to finish and, as a practical matter, hydraulic motor-driven pumps should not be operated at less than 15 strokes per minute. Electronic diaphragm pumps, which are pulsed by a solenoid, can operate Metering-pump flow rates can be con- at less than a single stroke per minute trolled automatically (in response to a because the characteristic and timing of process signal) by electric positioners, each stroke, from start to finish, is the which change the pump’s stroke length, same at all stroking speeds. The moving or by variable-speed drives, which alter the stroking speed. parts in modern diaphragm pumps offer long service at all stroking speeds. The highest stroking speeds should be avoided with viscous or abrasive chemicals. In boiler water treatment applications, When a metering pump is controlled metering pumping technology that compre- by automatic or electric stroke position- hensively offers the best in operation, reli- ers, the number of doses remains constant ability and energy efficiency when handling and the size of each dose is reduced, thus an array of liquids, chemicals and slurries is keeping the doses uniformly distributed the way to go. When boiler feed water must in a constantly flowing line. Use of a vari- be treated to eliminate erosion, corrosion able-speed drive changes the stroke speed. and scale buildup, having the best chemical Also, the size of the dose injected on each metering pumps on hand makes meeting stroke remains the same, but the variable- those goals much easier and safer. PH speed drive makes the doses less frequent as the motor speed is slowed. This, how- Tom O’Donnell is senior product specialist for ever, can produce an undesirable process Neptune Chemical Pump Co., Lansdale, Pa., an result in a constantly flowing line as the operating company with the Pump Solutions discreet slugs of chemical are more widely Group of Dover Corp. For more information separated than if a constant dose interval from Neptune, call (215) 699-8700 or visit were maintained. www.neptune1.com. Finally, consider the application and level of quality. WEB EXCLUSIVE • Is the unit to be used for intermittent operation in light-duty applications where economy is an important consideration? Planning a Metering • Is the unit for an industrial plant, refin- Pump Installation ery, power plant or waste-treatment A metering pump installation must facility where ruggedness and additional be planned from the day tank or liq- features are required? uid source up to the injection point. • Is initial cost or lifecycle cost more READ MORE @ WWW.PROCESS-HEATING.COM important?