A positive displacement pump is a pump designed to pump fluids by repeatedly enclosing a designated volume and moving it through the system mechanically. They have a cyclic pumping action and can be driven by screws, pistons, gears, diaphragms, rollers or vanes. Although there is a wide range of pump designs under this category, most of the pumps can be placed into two main categories –rotary and reciprocating pumps.
Reciprocating PD pumps
These pumps work by back-and-forth movements known as strokes of either the plunger, diaphragm or piston. The cycles are known as reciprocation. The first stroke in a piston pump creates a vacuum and opens an inlet valve. It closes the outlet valve and draws the liquid into the piston chamber. The piston reverses and the inlet valve is closed while the outlet valve is opened, allowing the liquid in the piston chamber to be discharged.
Features and Benefits
Two main families of pumps exist – centrifugal pumps and positive displacement pumps. Centrifugal is capable of higher flows. They can work with liquids of lower viscosity. In some chemical industries, 90% of the pumps in use are centrifugal pumps but there are several applications for which PD pumps are used. For example, PD pumps can handle fluids with higher viscosity and can operate at high pressures and low flows more efficiently. They are more accurate when it comes to metering.
Limitations of PD pumps
Before buying PD pumps from your positive displacement pump stockists, it is important to understand their limitations. In general, PD pumps are more difficult to maintain compared to centrifugal pumps. PD pumps are also more complex than centrifugal pumps. They are not capable of generating the high flow rate features of centrifugal pumps.
PD pumps are less capable of handling low-viscosity fluids compared to centrifugal pumps. To reduce leaks and slippage and generate suction, rotary pumps rely on the seal between the rotating elements and the housing of the pump. This is reduced with low-viscosity liquids. Similarly, it is harder to prevent leaks or slippage from the valves in reciprocating pumps with a feed of low viscosity because of the high pressures created during the pumping action.
Pulsing discharges are also a characteristic of PD pumps and especially reciprocating designs. Pulsation can cause vibration and noise in the pipe system and cavitation problems which can lead to failure or damage. You can reduce pulsing by using pulsation dampeners and multiple pump cylinders but this requires careful design. Centrifugal pumps produce a smooth constant flow.
Reciprocating pumps have a back-and-forth motion that can be a source of noise and vibration. It is therefore important to make very strong foundations for a positive displacement pump. As a consequence of high pressures created during the pumping cycle, it is crucial that either the discharge line or pump has some form of pressure relief in case of blockages. Centrifugal pumps don’t need over-pressure protection because the fluid is recirculated in this. Feeds that contain a high level of abrasive solids can cause wear and tear on the components of all pump types and especially seals and valves.