No matter the details, all metering pumps are composed of only a few important parts, starting with a pump head and motor. Said pump head is a cylindrical cavity, in which fluids reside before they are pumped. So that fluids can flow in and out of it, the pump head has both an inlet and an outlet. Connected to the inlet and outlet are check valves. Attached to the intake check valve are tubes or hoses, which connect it to a fluid reservoir.
Tubes or hoses also connect the outlet, or exhaust, valve on the other end to the main body of the pump. Note that each valve can only flow in one direction. The pump’s main body holds the process flow to which the fluid will be introduced. Also inside the pump head is found the mechanism that will enact the motion that draws in and expels the fluid. This mechanism may be a piston, diaphragm, or bellows. Depending on a pump’s particular design, attached to this mechanism is either a manual pedal or a motor. The motor, which drives the pump head as a whole, is usually electric.
As noted, metering pumps function using one of three mechanisms: pistons, diaphragms, or bellows.
Piston pumps use pistons, which are reciprocating, plunger-like cylinders. To pump fluid, they move in and out of a correspondingly shaped chamber in the pump head. The inlet and outlet valves are both connected to this chamber, called the piston chamber.
While piston pumps use a piston to generate motion, diaphragm metering pumps use a flexible membrane made of teflon, rubber, or thermoplastic, valves on either side, and, most likely, hydraulic power, to pump liquids. These valves may be any shut-off type that suits the job, such as butterfly valves, check valves, or flap valves. Liquid will not pass through diaphragms, meaning the liquid inside is sealed off from the outside world. For this reason, diaphragm valves are often chosen to keep valves from leaking, especially those carrying sensitive, dangerous, toxic, or noxious fluids.
Bellows pumps work much like piston pumps, except that, in order to generate displacement, they use an accordion-style pleated body.
Another, lesser known, type of pump, peristaltic pumps, use motor-driven rollers to compress and move liquid inside flexible rubber tubing.
The latter three pump types each operate a little differently than one another, but they still mirror the basic operation of piston pumps. This two-part process begins with the suction, or intake stroke, which is when the piston first leaves the correspondingly shaped chamber in the pump head. When the piston is withdrawn, or, alternatively, when the bellows are extended, a vacuum is created which opens the inlet valve and pulls the fluid into the pump head. When the suction stops, the inlet closes.
After this, an operator or motor extends the membrane piston, or, alternatively, compresses the bellows in towards the fluids. Both actions put pressure on the fluid, in order to compress and pressurize it as much as possible; this pressure forces the outlet valve open, where, due to displacement, the fluid is expelled.
Metering pumps must meet a variety of unique and highly specialized needs, so they are available in many sizes, materials, and configurations. When selecting a metering pump, customers must take into consideration a number of factors, such as the type of liquid in use, maximum discharge pressure rating, flow rates, temperature ratings, motor capacity, pump apparatus, and fluid composition.
It is important to understand the type of liquid that will flow through a pump for a few different reasons. First of all, most liquids are easy to compress, but the liquid in use is a type that habitually produces gas bubbles, it will be much harder to compress. Paired with the wrong pump, such liquid may not get dispelled properly or at all.
In addition, the type of liquid may affect the material construction of the pump. Generally, metering pumps are made with durable metals or plastics. Some more than others, however, must have special qualities, such as extreme corrosion resistance. Factors like this will decide the material selection.
On top of all of this, customers may choose pumps that create and work with variable displacement constant speeds or pumps that create and work with fixed displacement variable speeds. These methods of displacement are further impacted by a pump’s individual motor capacity, stroke length and/or the degree to which the piston, membrane, or bellows can be extended.
Metering pumps are incredibly useful, but can be intimidating to shop for without professional assistance. For the best results, consult with a metering pump manufacturer.