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Reciprocating Positive Displacement Pumps

August 10, 2021

In a previous article ‘What Exactly Is a Positive Displacement Pump?’ we gave a high-level overview of different types of Positive Displacement Pumps. In this article, we will focus on Reciprocating Positive Displacement Pumps – how they work, advantages/disadvantages, and common applications in which they are used.

As opposed to centrifugal pumps using impellers, the mechanical component used in Reciprocating Positive Displacement is either pistons, plungers, or diaphragms. The basic function is to use one of these reciprocating components to move liquid inside a cylinder by increasing or decreasing the volume and pressure inside. Depending on the design there can be more than one reciprocating component.

To gain a more in-depth understanding of the reciprocating mechanisms involved, let’s examine how the different types of reciprocating positive displacement pumps function.

Piston pump:  Within a cylinder, a piston covering the full diameter is pulled back which increases the volume of the cavity. As the volume of this cavity is increased, fluid is drawn through the inlet and fills the cylinder. When the piston has been fully pulled back the fluid is static for a moment, and then as the piston is pushed back into the cavity it creates differential pressure to overcome the outlet valve, allowing fluid to leave. The process is repeated.

Plunger Pumps: Plunger pumps operate similarly to piston pumps where a shaft moves back and forth in a cylinder, but the plunger component does not fill the diameter of the cylinder. Based on the design the seal is easier to maintain, as it is stationary at the end of the cylinder as the shaft moves, opposed to a piston pump that moves the seal with the reciprocating component.

Diaphragm Pumps: These pumps have multiple chambers or cavities where fluid is transferred through. They have a flexible membrane material that alternates the pressure and space between these chambers. The membrane expands to one side increasing the pressure, exiting the fluid through the outlet valve. At the same time, on the other side of the membrane, it is retracted increasing the volume on that side, pulling fluid through the inlet valve. The membrane design keeps these pumps completely airtight, removing the need for a traditional seal.

Advantages 

Operating in repeatable cycles, a reciprocating positive displacement pump’s output can achieve high-efficiency rates around 90% and are preferred when precise metering is required. They are designed to handle low flow, high head applications, and high viscosity fluids. The variety of materials that the input and output ends can be made from make them suitable to handle abrasive and corrosive fluids. Most of these positive displacement pumps are self-priming. The compression motion for each output means the fluid output is not affected by pressure and is more consistent.

Disadvantages

While the cyclic action creates a more predictable output because each compression can be accurately measured, it does create other complications. The repeated cyclic action creates a pulsation discharge with each back-and-forth motion of the reciprocating component. This can lead to vibration and noise that will eventually lead to cavitation and damage of components requiring regular maintenance. To counter these effects, a lot of reciprocating pumps use multiple cylinders with pistons/plungers that operate simultaneously but in opposite (alternating) cycle motions to balance out pressures. Pumps may also have some sort of damping construction to smooth the vibrations created.

Applications

Reciprocating Positive Displacement pumps offer the high pressure and accuracy needed for the chemical, refinery, pharmaceutical, and water treatment industries. Piston and plunger pumps are often used for the same applications in high-pressure washing, oil production, and paint spraying. Diaphragm pumps are seen more in water treatment, oils, and corrosive liquids such as sludge, acid, and chemical transfer. Depending on the extra cylinders to compensate with pulsation or adding greater pressures, reciprocating positive displacement pumps are classified as Simplex, Duplex, Triplex, or Quintuplex. Duplex is common in oil-line pumping, mine dewatering, chemical, and petroleum transfers. Triplex is found when low pulsation is a high priority. Quintruplex is common in cement, sand-filled fluids, crude oil, acids, and mud.

Anderson Process offers a variety of different brands and types of reciprocating PD pumps that can address the situations customers face when needing accurate fluid transfer efficiency or high-pressure outputs. If you have any questions or concerns, our experienced staff is ready to provide full support in helping you find the proper solution for your application.