The first priority is to identify the needed capacity in terms of flow rate and discharge pressure. It is important not to oversize a metering pump. Generally, a metering pump should be sized so the maximum expected flow rate is 80% to 85% of the pump capacity. This leaves additional capacity if needed. The minimum capacity should never be less than 10% of the pump capacity to maintain accuracy. Ideally, the metering pump will operate at all times between 10% and 100% of capacity. Variable speed can be used to obtain broader turndown ranges as discussed later.
Next, consider the correct materials of construction. Metering pumps are available in a variety of metal and plastic liquid ends. Material compatibility selection must take into consideration corrosion, erosion or solvent actions. Certain solvent-based chemicals may dissolve plastic headed pumps. Acids and caustics require stainless steel or alloy liquid ends. Consider the effect of erosion when handling abrasive slurries. Soft ball- checks used with a hard seat allow pumping very abrasive chemicals.
Is the chemical viscous or is the chemical slurry? Special liquid ends are available for these specific applications. Standard metering pumps handle clear liquids with viscosity's ranging from water-like to 1500 cps. Neptune offers special liquid ends on the series "VS" pump that will handle viscosities to 5,000 cps and suspensions containing fine particles. For true slurries or higher viscosity's, the tubular diaphragm head allows pumping chemicals with viscosity's to 20,000 cps or slurries containing up to 10% solids.
Consider the environment in which the pump will operate. Is the pump indoors or outdoors? Exposed to weather? The motor should be sheltered from direct sun light. The pump will operate in freezing temperature provided it is running continuously and providing the liquid running to and from the pump will not freeze. If this is not the case, freeze protection and heat tracing must be applied. Is the environment corrosive? Is the environment hazardous with special electrical requirements? When evaluating a hazardous environment, consider dust, which can ignite as well as fumes or vapors. Defining the environment completely will be necessary to determine the correct motor, the correct pump and any special coatings required.
Stroking speeds range from 18 through 144 spm. Stroking speeds on diaphragm pumps are not as critical as they were in the days of packed plunger pumps where higher speed would cause wear and failure. The moving parts in hydraulically actuated diaphragm pumps offer long, reliable service even at high stroking speeds. The highest stroking speeds should be avoided with viscous or abrasive chemicals. The lowest stroking speeds must be avoided if variable speed drives are to be used since the pump should never be operated at less than 15 spm. Use of a 37 spm or 72-spm pump with the variable drive would severely limit the range of turndown available by use of the variable speed drive.
Metering pump flow rate is controlled manually by adjusting a micrometer dial on the front of the pump. This manual control allows the pump to be operated between 10% and 100% of nameplate capacity. In addition to the control knob, a manual variable speed drive will allow an additional adjustability or turndown over the range of the drive, depending upon the stroking speed of the pump. For example, a pump operating at 75 spm which could be turned down to 15 spm would allow a 5:1 turndown on speed using the variable speed drive and a 10:1 turndown on stroke length using the micrometer dial for a combined turndown range of 50:1.
Metering pumps can be controlled automatically by use of variable speed drives which accept a 4 - 20 mA or other process signal to change the speed of the pump or by automatic electric or pneumatic positioners to control the pump stroke length. Do realize that when a metering pump is controlled automatically by variable speed drive, the pump speed slows and the size of the dose injected on each stroke remains the same. This can produce an undesirable process result in that the same size slug of chemical is injected less frequently. In a constantly flowing line, the discreet slugs of chemical would be separated more widely in the line than if a constant speed were maintained.
Use of automatic, electric or pneumatic stroke positioners keeps the number of doses constant and reduces the size of each dose. These keeps the doses uniformly distributed and can be a very important process consideration again particularly when injecting chemicals into moving streams or lines.
The above information is intended to help you recognize the important variables, which must be considered when selecting a metering pump. This information is not intended to guide you to a specific model number - we will be glad to do that for you. Just call (215)699-8701, or click for our assistance.