Positive Displacement Pumps


A diaphragm pump is a positive displacement pump that uses a combination of the reciprocating action of a rubber or teflon diaphragm and suitable non-return check valves to pump a fluid. Sometimes this type of pump is also called membrane pump.

There are two main types of diaphragm pump:

In the first type, the diaphragm is sealed with one side in the fluid to be pumped, and the other in air or hydraulic fluid. The diaphragm is flexed, causing the volume of the pump chamber to increase and decrease. A pair of non-return check valves prevent reverse flow of the fluid.
The second type of diaphragm pump has one or more unsealed diaphragms with the fluid to be pumped on both sides. The diaphragm(s) again are flexed, causing the volume to change.
When the volume of a chamber of either type is increased (the diaphragm moving up), the pressure decreases, and fluid is drawn into the chamber. When the chamber pressure later increases from decreased volume (the diaphragm moving down), the fluid previously drawn in is forced out. Finally, the diaphragm moving up once again draws fluid into the chamber, completing the cycle. This action is similar to that of the cylinder in an internal combustion engine.

Applications
Diaphragm pumps may be low lift (flooded suction), low pressure pumps with low flow rates. They can handle sludges and slurries with a moderate amount of grit and solid content. Excessive solids cause blockages.

Diaphram pumps with teflon diaphrams, ball check valves, and hydraulic actuators are used to deliver precise volumes of chemical solutions at high pressures (as much as 5000 lbf/in) into industrial boilers or process vessels.
Diaphragm pumps can be used to make artificial hearts.


A peristaltic pump is a type of positive displacement pump used for pumping a variety of fluids. The fluid is contained within a flexible tube fitted inside a circular pump casing (though linear peristaltic pumps have been made). A rotor with a number of ‘rollers’, ‘shoes’ or ‘wipers’ attached to the external circumference compresses the flexible tube. As the rotor turns, the part of tube under compression closes (or ‘occludes’) thus forcing the fluid to be pumped to move through the tube. Additionally, as the tube opens to its natural state after the passing of the cam (‘restitution’) fluid flow is induced to the pump. This process is called peristalsis and is used in many biological systems such as the gastrointestinal tract.

Peristaltic pumps are typically used to pump clean or sterile fluids because the pump cannot contaminate the fluid, or to pump aggressive fluids because the fluid cannot contaminate the pump. Some common applications include pumping aggressive chemicals, high solids slurries and other materials where isolation of the product from the environment, and the environment from the product, are critical.

Higher pressure peristaltic pumps which can typically operate against up to 16 bar, typically use shoes and have casings filled with lubricant to prevent abrasion of the exterior of the pump tube and to aid in the dissipation of heat. Lower pressure peristaltic pumps, typically have dry casings and use rollers. High pressure peristaltic pumps typically use reinforced tubes, often called ‘hoses’, and the class of pump is often called a ‘hose pump’. Lower pressure peristaltic pumps typically use non-reinforced tubing, and the class of pump is sometimes called a ‘tube pump’ or ‘tubing pump’.

Because the only part of the pump in contact with the fluid being pumped is the interior of the tube, it is easy to sterilise and clean the inside surfaces of the pump. Furthermore, since there are no moving parts in contact with the fluid, peristaltic pumps are inexpensive to manufacture. Their lack of valves, seals and glands makes them comparatively inexpensive to maintain, and the use of a hose or tube makes for a relatively low-cost maintenance item compared to other pump types.

Typical applications

Dialysis machines
Open-heart bypass pump machines
Food manufacturing
Beverage dispensing
Pharmaceutical production
Sewage sludge