There are many types of Positive Displacement (PD) pumps, but they can be narrowed down to two categories: PD rotary pumps and PD reciprocating pumps. PD pumps ensure that the pumping action is moving forward, making it positive. It is designed so that recirculation of the fluids within the pump does not occur. These pumps maintain a consistent, fixed speed and flow rate proportional to each other, which is a huge advantage in a setting where pressure is constantly changing. Another significant advantage of PD pumps is their ability to handle a wide range of viscosity, which is the case in water flowing through wastewater treatment plants.
The PD pump shown in the image above is made by Seepex.
Rotary positive displacement pumps operate on the principle of using rotating elements to displace fluid from the inlet to the outlet. These pumps typically consist of a housing containing one or more rotating components, such as gears, lobes, screws, or vanes. As the rotor turns within the housing, the fluid is drawn into the cavities formed between the rotating elements and the pump casing. The fluid is then trapped within these cavities and carried around the pump housing until it is discharged through the outlet port. The design of the pump ensures a continuous and uniform flow of fluid.
One common type of rotary positive displacement pump is the gear pump, which utilizes intermeshing gears to generate fluid displacement. As the gears rotate, they create a seal along the outer edges of the pump housing, trapping the fluid in the spaces between the gear teeth and the pump casing - similar to a vacuum. Another example is the rotary vane pump, which employs sliding vanes attached to a central rotor. As the rotor turns, centrifugal force causes the vanes to extend outward, drawing fluid into the pump chamber. The rotation of the rotor then compresses the fluid between the vanes and the pump housing, forcing it out through the discharge port. These mechanisms enable rotary positive displacement pumps to deliver reliable and efficient fluid transfer.
Reciprocating positive displacement pumps operate by using a reciprocating motion, typically driven by pistons or plungers, to displace fluid through the pump chamber. In these pumps, the piston or plunger moves back and forth within a cylinder, creating alternating cycles of suction and discharge. During the suction stroke, the piston retracts, drawing fluid into the pump chamber through the inlet valve, which opens due to the decreased pressure. As the piston reverses direction and moves forward during the discharge stroke, the inlet valve closes, and the fluid is forced out through the outlet valve, which opens under the increased pressure. This repetitive cycle results in a continuous flow of fluid through the pump, with each stroke displacing a fixed volume of liquid.
One common example of a reciprocating positive displacement pump is the piston pump, which utilizes one or more pistons fitted with seals to create a tight seal against the cylinder walls. As the piston moves back and forth, it alternately pressurizes and depressurizes the fluid within the pump chamber, generating a pulsating flow. Another type is the diaphragm pump, where a flexible diaphragm attached to a reciprocating rod creates the displacement action. As the diaphragm moves, it compresses and expands the pump chamber, drawing in and expelling the fluid. Reciprocating positive displacement pumps are valued for their ability to provide high pressures and precise flow control, making them suitable for a wide range of applications, including hydraulic systems, chemical dosing, and high-pressure cleaning.
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