What is Positive Displacement Pump & How Does it Works?
A positive displacement pump moves a fluid by a mechanism like pistons, screws, gears, rollers, diaphragms or vanes. These pumps cyclicly close a fixed volume and moving it mechanically through the system. This kind of pump are categorize based on the mechanism used to pump the fluid, and it can have a variety of application based on its mechanism. Here you can find positive displacement pumps in different categories. At Linquip, our goal is to help you select and purchase the best pump possible for your application, and we are here to help you on all stages of design, purchase, and maintenance. Feel free to browse our comprehensive list of positive displacement pumps, positive displacement pump' manufacturers, and service providers, or if you feel overwhelmed, just contact us and our experts will help you on with every stage of your project.
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What Is Positive Displacement Pump?
Positive displacement pumps force a fluid to move by trapping a fixed volume and pushing (displacing) it out of the discharge pipe. Cyclic pumping can be accomplished with pistons, gears, screws, rollers, diaphragms, or vane mechanisms.
There are positive displacement pumps that use expanding cavities on the suction side and decreasing cavities on the discharge side. When the suction side cavity expands, liquid flows into the pump, while when the discharge side cavity collapses, liquid flows out. During each cycle, the volume remains constant.
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Working Principle of a Positive Displacement Pump
A positive displacement pump is a mechanical device that uses a screw, a blade, a vane, gear, or a diaphragm to operate. The pump creates a chamber or cavity between the pumping elements, and the fluid is temporarily stored in the cavity and is transported by reciprocating or rotating motion along the pipe to reach its destination. There are a wide variety of pump designs, but most of them fall into two categories: reciprocating and rotary.
Reciprocating Positive Displacement Pump
Reciprocating Positive Displacement pumps function by back and forth movement (strokes) of a piston, plunger, or diaphragm performed repeatedly. These cycles are known as reciprocation.
Piston pumps work by creating a vacuum, opening an inlet valve, closing the outlet valve, and sucking fluid into the piston chamber (the suction phase). By reversing the piston motion, the inlet valve, under pressure, will be closed, and the outlet valve will open, discharging the fluid contained in the piston chamber (the compression phase). A simple example is a bicycle pump.
There can also be double-acting piston pumps with inlet and outlet valves on either side of the piston. The piston is suctioned on one side while it is compressed on the other. Radial pistons are employed in industrial applications, where they offer a greater degree of complexity.
Plunger pumps work similarly. A piston pump moves fluid based on its cylinder volume, while a plunger pump moves fluid based on its plunger size. Maintaining the pumping action and avoiding leaks depends on the seal around the piston or plunger. Plunger pump seals are generally easier to maintain due to the fact that they are stationary at the top of the pump cylinder, unlike seals of the piston pumps, which move up and down repeatedly inside the pump chamber.
As opposed to pistons and plungers, a diaphragm pump uses a flexible membrane to move fluids. Expanding the diaphragm increases the volume of the pumping chamber and draws fluid into the pump. Diaphragm compression decreases the volume and causes fluid to be expelled. Diaphragm pumps are ideal for pumping hazardous fluids because they are hermetically sealed systems.
In reciprocating pumps, the cyclical action of the piston creates pulses in the discharge, with fluid acceleration during the compression phase and fluid slowdown during the suction phase. The vibrations caused by this can cause damage to the installation, so damping or smoothing is often employed. It is also possible to minimize pulsing by using two or more pistons, plungers, or diaphragms, with one in compression and the other in suction.
Because reciprocating pumps are repeatable and predictable, they are ideal for dosing or metering applications that require accuracy. It is possible to provide a measured amount of pumped fluid by adjusting the stroke rate or length.
Rotary Reciprocating Pump
The action of rotating cogs or gears is used in rotary positive displacement pumps to transfer fluids, rather than the backward and forward motion of reciprocating pumps. The rotating element creates a liquid seal around the pump casing and induces suction at the pump inlet. The fluid is drawn into the pump and enclosed within the teeth of its rotating cogs or gears until it reaches the discharge. Gear pumps are the simplest rotary positive displacement pumps. There are generally two main types of gear pumps: external and internal.
An external gear pump comprises two shafts with interlocking gears (either or both shafts may be driven). The gears rotate, trapping the fluid between their teeth and moving it from the inlet to the outlet around the casing. The fluid does not pass back through the center, between the gears, as they are interlocked. Due to a tight tolerance between the gears and the casing, the pump develops suction at the inlet, preventing liquid from leaking back from the discharge side.
Oils and other liquids with high viscosity can be pumped with gear pumps that need to be lubricated by the fluid they pump. Because of this, gear pumps should not be run dry. Since the gears and casing have close tolerances, fluids containing entrained solids or abrasive fluids may cause these pumps to wear rapidly.
The lobe pump and vane pump are two designs similar to the gear pump.
The rotating elements of the lobe pump are lobes. This design eliminates wear, contamination, and fluid shear by preventing the lobes from coming into contact with one another during the pumping process. Vane pumps consist of a set of moveable vanes (spring-loaded, under hydraulic pressure, or flexible) installed in an off-center rotor. Vanes maintain a tight seal against the casing wall, forcing trapped fluid into the discharge port.
Another class of rotary pumps transfers fluid along the screw axis using one or more meshed screws. This pump uses the same principle as an Archimedes screw, which has been used for irrigation for thousands of years.
Function of a Positive Displacement Pump
With positive displacement pumps, contrary to centrifugal pumps, a given flow rate may be achieved regardless of discharge pressure for a given speed (rpm). Therefore, positive displacement pumps facilitate constant flow. There is, however, a slight increase in internal leakage with increasing pressure, which prevents a truly constant flow rate.
Positive displacement pumps do not have shutoff heads like centrifugal pumps, so they cannot operate against a closed valve on the discharge side. When a positive displacement pump operates against a closed discharge valve, it produces flow, and the pressure in the discharge line rises until the discharge line bursts, the pump is seriously damaged, or both.
Therefore, a relief or safety valve is required on the discharge side of the positive displacement pump. Both internal and external relief valves can be used. The pump manufacturer normally provides internal safety valves or relief valves. Internal valves are usually used only for safety purposes. The use of an external relief valve in the discharge line, and a return line to the suction line or supply tank, increases safety for both people and equipment.
Usage of Positive Displacement Pumps
Positive Displacement pumps are normally suited for fluids with a high viscosity, such as oil, paints, resins, and foodstuffs. They can be used for metering or dosing applications requiring high accuracy. Also, they can be used to wash with high pressures. Another application is waste water treatment.
Common Applications of Positive Displacement Pumps
There are numerous industrial applications for positive displacement pumps depending upon their type. Some of them are listed below:
- With water, you can use plunger pumps and piston pumps for high-pressure washing. These pumps can also be used in oil extraction and paint spraying.
- Diaphragm pumps are used for metering or dispensing, spraying and cleaning, water treatment, paints, and applications with corrosive liquids.
- Gear pumps are used for pumping high viscosity fluids in chemical, petrochemical, and food industries.
- The lobe pumps are commonly used in chemical, food, pharmaceutical, sanitary, and biotechnology industries.
- Oil production, fuel transfer, and irrigation all use screw pumps.
- Fluids with low viscosity can be handled by vane pumps, which can also be used to transport fuel and to load and unload vehicles.
FAQ about Positive Displacement Pumps
- What Is a Positive Displacement Pump?
A positive displacement pump maintains a constant flow at a fixed speed, regardless of pressure changes. Fluid is moved by the pump when it traps a fixed volume and forces it through the discharge pipe.
- What Are Positive Displacement Pumps Used for?
Positive displacement pumps are used when high viscosity fluids at high pressure need to be handled. Because its efficiency does not change depending on pressure, it can also handle relatively low flows. This pump is suitable for more difficult circumstances where dynamic pumps may fail.