Be careful to limit the suction to 4 ft 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 6 or 7 ft and use an adequately sized line with minimized 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 suitable for the discharge pressure.
Other accessories to consider when planning a metering pump installation include:
*Flanges, unions or compression fittings
*Back pressure valve
*Injection quill and check valve
Remember that when replacing equipment or changing chemical programs, it is best to ask questions. Will the new program operate at the same feed rates as the previous one? Is the equipment properly sized for the new products? How well has the equipment been operating? Any problems with reliability, accuracy or unusually high maintenance requirements? There is no better start to a new chemical-feed program than to ensure that the chemical is delivered accurately with trouble-free equipment.
Determining the proper flow rate and discharge pressure cannot be done until the types of substances to be pumped are identified per the application, including the viscosity of the liquid or if it is a slurry. Standard metering pumps handle clear liquids with viscosities generally ranging from water-which has a viscosity around 1 cps-to 1,500 cps. Special liquid ends for applications outside this viscosity range are available for viscosities up to 5,000 cps. When considering true slurries or liquids with even higher viscosities, special tubular diaphragm heads are compatible with viscosities to 20,000 cps and slurries that contain 10 percent solids.
Materials of construction are another consideration. When selecting a metering pump, consider any corrosion, erosion or solvent action that may occur when handling specific substances. For example, solvents may dissolve plastic-headed pumps, acids and caustics are only compatible with stainless steel or certain steel alloys, and abrasive slurries can erode some materials. The best metering pump lines are available in a range of construction materials, allowing the end user to choose the best option for his specific applications.
When considering the type of head the pump should feature, remember that double-diaphragm heads with leak detection and alarm capabilities are available for applications where any diaphragm failure must be sensed immediately.
Selecting a driver is also an area of concern. A driver should be chosen by matching it to the available utilities, which usually include electric, air, gas or other means of driving the pump. When the pump's parameters are determined, consider the environment in which the pump will operate. Hazardous area requirements must be identified when selecting the driver. When evaluating a hazardous environment, remember to consider dust, which can ignite, just like fumes or vapors.
Will the pump be used indoors or outdoors? If it is located outdoors, it should be sheltered from direct sunlight. For temperature requirements, most pumps will operate in freezing conditions, provided that the pumped fluid will not freeze and that the correct lubricants are selected. In this case, freeze protection and heat-tracing may be required, while operation in corrosive environments may require special pump coatings.
Determining the pump's control method is next on the list of determining factors. The choices usually include manual continuous operation, on/off operation or automatic proportional control in response to a process signal.
In general, metering pump flow rates can be manually adjusted with a micrometer dial. This manual control allows the pump to be operated between 10 and 100 percent of capacity by changing the stroke length. By comparison, a manual variable speed drive changes the stroke speed. A combination of the two may allow additional adjustability or turndown over the range of the drive, depending on the pump's stroking speed. For example, a pump operating at 75 strokes per minute (which could be decreased to 15 spm) would allow a 5:1 turndown on speed when using the variable speed drive and a 10:1 turndown on stroke length when using the micrometer dial.
Metering pump flow rates can also be controlled automatically (in response to a process signal) by electric positioners 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 pump's minimum operating speed.