Understanding Positive Displacement Pumps: Mechanism, Applications, and Advantages

Unlike the commonly misunderstood term 'negative displacement pump', positive displacement pumps, or more accurately termed as 'positive displacement pumps', are a type of pump that traps a fixed amount of fluid and forces that volume into the discharge pipe. This article delves into the working mechanism, applications, and advantages of positive displacement pumps, along with their limitations.

Mechanism of Positive Displacement Pumps

Positive displacement pumps work by trapping a fixed amount of fluid and then forcing that volume into the discharge pipe, as opposed to centrifugal pumps, which move fluid via the pressure differential created by the impeller. They can be categorized into two main types: rotary pumps and reciprocating pumps.

Rotary Pumps

Rotary pumps include gear pumps, screw pumps, and vane pumps. These pumps use rotating elements to move fluid. For example, gear pumps use two meshing gears to trap and push fluid from the suction side to the discharge side. Screw pumps use two screws that rotate within a pipe to push fluid along the length of the pump. Vane pumps use a slotted rotor that captures and transfers fluid as it rotates.

Reciprocating Pumps

Reciprocating pumps include piston pumps and diaphragm pumps, which use a back-and-forth motion to displace fluid. These pumps operate by moving a piston or diaphragm back and forth, thereby creating suction and discharge cycles. The piston pump uses a moving piston within a cylinder, while the diaphragm pump employs a flexible diaphragm to transfer fluid.

Flow Rate in Positive Displacement Pumps

The flow rate of positive displacement pumps is determined by the pump’s design and speed, not by the pressure in the discharge line. This characteristic means that these pumps can deliver a consistent flow rate even under varying system pressures. If the discharge is blocked, the pump will continue to push fluid until something fails or the pump is turned off. This is a significant advantage in systems where consistent output is crucial.

Applications of Positive Displacement Pumps

Positive displacement pumps are widely used in various industries due to their ability to maintain a consistent flow rate under varying pressure conditions. Some of the primary applications include:

Oil and Gas Industry: Used in drilling, hydraulic fracturing, and refining processes. Chemical Processing: Employed in mixing, transferring, and recirculating fluids in chemical plants. Food and Beverage Manufacturing: Utilized for transferring and dosing liquids in production processes. Water Treatment: Used for pumping slurries, solids, and wastewater.

Advantages of Positive Displacement Pumps

Positive displacement pumps offer several advantages:

High Efficiency with Viscous Fluids

These pumps are highly efficient when transferring viscous fluids, such as those with high shear sensitivity. They can handle fluids with higher viscosity compared to centrifugal pumps, making them ideal for applications with thick, dense, or shear-sensitive fluids.

Constant Flow Rate

One of the key advantages of positive displacement pumps is their ability to provide a consistent flow rate, regardless of the pressure in the discharge line. This makes them suitable for applications where the flow rate must be maintained under varying system conditions.

Wide Range of Fluid Properties

Positive displacement pumps are versatile and can handle a wide range of fluid properties, including highly viscous liquids, gases, and slurries. This versatility makes them suitable for a myriad of applications across different industries.

Disadvantages of Positive Displacement Pumps

While positive displacement pumps offer many advantages, they also have some limitations:

Less Efficient at Higher Pressures

At higher pressures, positive displacement pumps can become less efficient compared to centrifugal pumps. Centrifugal pumps can operate more efficiently at higher pressure due to their constant pressure conversion capability.

Necessity for Relief Valves

Positive displacement pumps are prone to overpressure conditions, and relief valves are often required to prevent damage to the pump. These valves release excess pressure when the system reaches a preset limit, thereby protecting the pump from potential damage.

Conclusion

In summary, positive displacement pumps are a vital tool in many industrial applications, providing consistent flow rates, high efficiency with viscous fluids, and the ability to handle a wide range of fluid properties. Despite their limitations, such as reduced efficiency at higher pressures and the need for relief valves, they remain a reliable choice for many industries.