A typical metering pump application in water treatment, chemical process or agriculture can deliver large amounts of chemicals during daily operations. While the volumes may be large, the metering also must be precise, with exact amounts delivered according to strict injection schedules. The wide range of injection pressures, from high to low, must also be governed precisely. These demanding applications require the use of a unique pumping technology, one that can ensure successful, reliable adherence to the injection schedule and fluid volumes.
Metering pumps have become a top choice for the injection of chemicals in water treatment, processing and agricultural applications. Many styles and modes of operation are available, so users must understand the different metering pump technologies in order to select the ideal equipment for their injection applications.
Know The Parameters
When determining which metering pump is the best option for a chemical injection application, users must consider several variables:
- Flow rate. Metering pumps should never be oversized, so determining the exact flow rate required for the application is paramount. A metering pump should be sized so that the maximum expected flow rate is 85 to 90 percent of the pump's capacity.
- Materials of construction. Metering pumps are available in a variety of materials, most commonly 316 stainless steel, C-20 stainless steel, PVC and Kynar polyvinylidene fluoride (PVDF). Users must consider the corrosion, erosion and solvent action of the chemical. For example, solvent-based chemicals may dissolve plastic-headed pumps, while acids and caustics may require stainless-steel models. The effects of erosion must also be considered with abrasive slurries.
- Chemical makeup. Chemicals come in many formulations, from thin to highly viscous, and they can also be classified as a slurry or off-gas when transferred. Standard metering pumps are typically able to handle clear liquids with viscosities ranging from water-like to 1,500 centipoise (cPs). Chemicals with viscosities that approach 5,000 cPs or have light suspensions will require special liquid ends. Those with viscosities up to 20,000 cPs or that contain up to 10 percent solids will require special diaphragms, while ones that automatically vent accumulated gas will need their own type of liquid ends.
- Driver. Drivers—which can be powered by electricity, water, gas, air or the sun—must be selected according to the utilities that are available. Users must also consider any environmental hazards that may be found in the operating area, with the operator realizing that pumps used in remote locations may not be able to be inspected as often as those in controlled environments.
- Environment. Determine if the pump will be operating indoors or outdoors. If used outdoors, the pump must be sheltered from direct sunlight. Any pumps that will be used in freezing temperatures can only pump fluids that will not freeze at that temperature.
- Method of control. The operator must know if the pump will be used in manual continuous operation, on/off operation or operation that is governed by a process signal.
Method of control is arguably the most important variable when choosing a metering pump. Many styles of metering pumps allow their flow rates to be adjusted manually through the use of a micrometer dial. Adjusting this dial changes the pump's stroke length and allows the pump to be operated anywhere between 10 and 100 percent of its rated flow capacity. Metering pumps with micrometer dials may also feature a variable speed drive that allows adjustment of the pump's stroke speed. Using the two in unison can allow additional flexibility or turndown capabilities, over the range of the drive, depending on the pump's stroking speed.
Flow rate set-point can be maintained automatically by using electric or pneumatic positioners to adjust the stroke length, which will deliver a full 10-to-1 turndown ratio. In this method, the number of doses remains constant with the size of each dose reduced, resulting in doses that are uniformly distributed in a constantly flowing line.
Metering pumps that use a variable speed drive will deliver a turndown ratio that is determined by dividing the pump's stroke speed by its minimum operating speed. A variable speed drive will enable the pump to inject a dose of the same size on each stroke, but because the stroke speeds will change, the doses themselves will be less frequent. Additionally, using a variable speed drive with an induction motor-driven pump, which normally operate at speeds less than 100 to 150 strokes per minute (spm), is not practical because slowing the motor causes each stroke to take longer to complete. However, electronic metering pumps, which are pulsed by a solenoid, can operate at less than one spm because the timing of each stroke, from start to finish, is uniform at every stroking speed.