Piston Metering Pumps
Piston metering pumps are utilized in such varied industries as pharmaceutical, food and beverage processing, water treatment, medical, commercial vending, irrigation, petroleum, milling, sewage, plastic fabrication, metalworking and laboratory. In each application these
metering systems are used to transfer exact amounts of fluid from an area of high pressure to an area of low pressure. The metering of the liquid can be adjusted in one of two ways. Variable displacement constant speed piston pumps alter the length of the stroke in order to extract and expel specific amounts of fluid at a given time while fixed displacement variable speed pumps use mechanical adjustments to alter the frequency of pump cycles to influence the amount of liquid being transferred through the outlet valve. Each can produce a continuously metered flow though adjusted motor pumps are more popular in industrial and commercial applications due to the ease of mid-process adjustments. Used in a number of applications these
dispensing pumps are known for reliability and accuracy but require some maintenance due to an inherent reliance on seals which encounter a great deal of friction during continuous use. Proper lubrication enhances product longevity though it can be a source for contamination.
The basic design and function of the piston pump is fairly simple. A piston, or rod, is connected to a motorized or manually operated actuating device at one end, and partially housed within a cylinder at the other. Seals made of elastomeric or metal materials are placed around the piston head to ensure a hermetic enclosure. At the opposite end of the cylinder, or pump head are two check valves designed for unidirectional fluid flow. These are the inlet and outlet connections. When the piston is withdrawn it increases the volume of the chamber, reducing pressure enough to create a vacuum and pull an exact amount of liquid in through the inlet valve. When capacity is reached the valve closes. At variable speeds, the piston is extended back towards the bottom of the barrel, compressing the liquid until the pressure is enough to force the liquid out through the exhaust valve and into the process stream. Like the seals and valves, the piston or rod as well as the pump head or cavity which houses it must be made to withstand the wear and friction of near constant use as well as hold up to the chemical composition of the fluids. Common materials are bronze, steel, brass, stainless steel or iron, though ceramic may also be used in some circumstances. Additional application specific considerations for piston metering pump selection include power source, flow rate, discharge pressure and operating temperature.
