A positive displacement pump, also known as a PD pump, is a mechanical apparatus that moves a specific amount of liquid for each rotation or cycle it completes. The rate of liquid flow in a positive displacement pump is directly related to its speed and the number of cycles it completes within a set period.
Understanding the Working Principle of A Positive Displacement Pump
A Positive Displacement Pump operates by utilizing mechanisms such as a screw, blade, vane, lobe, gear, or diaphragm to create a chamber or cavity. This chamber temporarily stores the fluid, which is then pushed or pulled along the pipeline to its intended location through reciprocating or rotary motion.
API 676 Rotary Positive Displacement Pumps, specifically progressive cavity pumps, are applicable in the field of pumps.
Types of Positive Displacement Pumps:
There are several primary categories of Positive Displacement Pumps.
- A Progressive Cavity Pump, also known as a PC Pump
- Gear pumps, both internal and external
- Vane Pump, also known as an Impeller Pump
- Rotary lobe pumps
- Screw pump
- Peristaltic pump
The Progressive Cavity Pump consists of a metal rotor and a rubber stator housed together. The rotor’s rotational movement propels the fluid through the cavities, creating a screw-like effect. This type of pump is capable of handling solids effectively and operates at a constant flow with minimal pulsation. With a range of capacity from low to moderate, pressure from low to high, and a single seal, it offers reliable performance and minimal shearing of the fluid.
There are two different configurations in which gear pumps can be found.
- Internal gear, which has a moderate capacity, low to moderate pressure, max 14 Bar/ Spl 17 BarBar, no solids handling capability, one seal, and a constant flow
- External gears possess a moderate capacity and can handle moderate to high pressure, with a standard range of 20 Bars to a maximum of 250 bars. They consist of a single seal, cannot handle solids, and maintain a consistent flow.
The Vane Pump, also known as an Impeller Pump, possesses a moderate capacity and is designed to handle low pressure and moderate amounts of solids. It is equipped with a single seal and maintains a constant flow.
The Rotary Lobe Pump is known for its ability to handle moderate to high volumes, maintain a steady flow, and handle solids effectively. It operates at a moderate pressure level and features two to four seals, resulting in moderate pulsation.
The screw pump, also known as the multi-screw pump, comes in various versions that offer moderate to high capacity, high pressure capabilities. It is designed for lubricative liquids only and does not handle solids. This type of pump typically has one seal and provides a constant flow rate.
The air-operated diaphragm pump by Flux Pumps possesses a moderate to low capacity, moderate to low pressure, minimal efficiencies, lacks a seal, and exhibits high pulsation.
Peristaltic Pump (Hose pump) has moderate capacity, low pressure, good solids handling capability, low shear, no seal and high pulsation.
Main Uses of Positive Displacement Pumps
Positive Displacement pumps are commonly employed for fluids with a comparatively high thickness. They find use in scenarios that necessitate high precision, such as metering and dosing. Additionally, they are suitable for tasks that demand high pressure, like high-pressure washing. One example of their application is in Waste Water Treatment, as exemplified by the Netzsch Tornado Rotary Lobe Pump.
Distinguishing Positive Displacement Pumps from Centrifugal Pumps: An Overview
Positive Displacement Pumps offer several key advantages over Centrifugal Pumps in terms of handling highly viscous fluids and minimising power requirements. These PD pumps demonstrate excellent volumetric efficiency and require minimal driver power. Moreover, adjusting the flow rate is easily achievable through speed control, as it directly affects the flow rate. Unlike Centrifugal pumps, the sizing of the driver is less critical since a Positive Displacement Pump can deliver a consistent quantity of fluids regardless of system back pressure. Additionally, for sensitive fluids, these pumps ensure minimal shear action, promoting gentle handling.
When comparing the Positive Displacement Pump to the Centrifugal pump, it is important to consider the main drawbacks. One disadvantage is the potential for catastrophic damage if the pump runs dry, which can occur due to close clearances between parts or the interference fit between the rotor and stator in progressive cavity pumps. To mitigate this risk, all Positive Displacement pumps require a pressure relief valve to prevent pump or piping failure in case of accidents, closure of the delivery valve, or blockage in the piping.
Is Priming Necessary for A Positive Displacement Pump?
Positive Displacement Pumps can often prime themselves because of the minimal gaps present inside the pump. This characteristic aids in creating a vacuum that displaces the air through the pump until the liquid reaches it. It is important to be cautious when it comes to the suction line, specifically by installing a “gooseneck” that ensures there is a sufficient amount of liquid in the pump during the priming process. This precautionary measure helps prevent dry running and consequently avoids pump failure.
An Air Operated Diaphragm pump (AODD pump) possesses the ability to initiate suction without the need for liquid, although its lifting capability is restricted in the absence of fluid in the line.
The maximum suction lift in a metres water column (mwc) is 2.5 for dry conditions and 5.5 for wet conditions.
Can Positive Displacement Pumps Operate without Liquid?
Certain types of Positive Displacement Pumps, such as Air Operated Diaphragm pumps, do not require lubrication or have close clearances between parts. This allows them to operate without any fluids. Peristaltic pumps, on the other hand, can also run dry because their hoses are self-lubricated by the fluid they carry. However, it is not advisable to run other types of PD pumps without any fluid.
Is NPSH Necessary for Positive Displacement Pumps?
To ensure smooth and reliable pump operation without the risk of cavitation, every pump requires a specific Net Pressure Suction Head (NPSH). It is essential to design the system in a way that provides a sufficient margin between the Net Pressure Suction Head Available (NPSHA) and the Net Pressure Suction Head Required (NPSHR). This ensures that the pump operates efficiently and avoids any potential damage caused by cavitation.
How Can the Control of Discharge Pressure in Positive Displacement Pumps Be Explained?
Unlike a centrifugal pump, which produces pressure, a positive displacement pump does not produce pressure; it is the system itself that develops pressure from the pressure drop, which then creates back pressure, which largely depends on the flow rate through the system. The back pressure is also dependent on the pressure in the vessel at the point of discharge, i.e., whether there is a hydrogen blanket present or steam. The pressure is largely controlled by the pumping rate; therefore, pressure is controlled by varying the speed of the pump. In cases where the variable speed drive is not deployed, the system pressure will be controlled to a degree by the setting of the pressure relief valve.
Positive Displacement Metering Pumps
Positive Displacement Metering pumps are commonly employed in situations that demand high precision, such as dosing applications that necessitate pH control, for instance, in Waste Water Treatment plants or when filling lines require accurate measurements of fluid volumes dispensed into containers.