Positive displacement pumps are usually used to transfer fluids that have a high viscosity like fuels, chemicals or food ingredients. They are typically employed for applications which require precise measurement.
The piston, diaphragm, or helical-rotor is pushed back and forth in an arc. They are able to convey the same amount of liquid every time a shaft turns.
Rotary Positive Displacement Pumps
Positive displacement pumps pull a particular quantity of liquid into the pump and force it out through the outlet valve. These pumps can handle liquids of all viscosities and sizes, ranging from thicker than a drop of water, to emulsions or sludges. They can be operated with high pressures and are well-suited for applications that require precise dosage. They are also suited for fluids that contain hard particles or abrasives. The most common types of Rotary positive displacement pumps comprise piston, diaphragm, gear and screw pumps.
They are not as susceptible to problems such as cavitation and wear that can be experienced with centrifugal pumping systems. However, abrasive feeding can still cause excessive wear to the components of certain positive displacement pumps. This is especially the case for rotary pumps that utilize plungers or pistons to trap and displace fluid. Therefore, avoid abrasive feed whenever it is possible.
Another issue associated with the rotary positive displacement pumps is that they may produce pulsating discharge. This can cause noise and vibration in the system and cavitation, which could damage pipes. However, this can be minimized by using multiple pump cylinders and Pulsation dampers.
A rotary positive displacement pump may also self-prime. This is due to the small clearances inside the pump. It is essential to not allow the pump to run dry for long periods since this could lower its efficiency and decrease the life expectancy.
Pumps that Reciprocate Positive Displacement
These pumps pressurize and draw fluid by using pistons within the piston. The piston traps liquid between the valves for inlet and outlet as it travels in a forward and back. This creates a differential in pressure that overrides the valve at the inlet to allow fluid out. Positive displacement pumps don’t alter their flow rate depending on changes in viscosity. This is different from centrifugal pumps.
The ability to function at constant pressures make these pumps perfect for applications that require accurate metering and transfer, and also for abrasive or hazardous materials. The pumps can also self-prime eliminating the need to manually re-prime.
The disadvantage of these pumps, is that the pressure could build up within the pipework until the pressure is relieved. This can be caused by the liner or the pump. This causes excessive noise and vibration when running. To mitigate this problem, these pumps require accessories such as pulsation-dampeners for the discharge lines and pipework. These pumps are also difficult to maintain and cost more because of their design. They are able to handle potentially hazardous or corrosive fluids as well as their ability to operate consistently in low pressure settings offsets these challenges. These pumps are perfect for applications involving high viscosity in the oil drilling, pharmaceutical and chemical processing industries.
Gear Pumps
In contrast to diaphragm pumps and gears, they do not cause shear to the fluid. They are ideal for transferring fluids that are sensitive to shear, such as emulsions, microbiological cultures, and food products. Gear Pumps can also be used for transferring liquids that are prone to changes in viscosity.
These are compact and very cost-effective. They can be constructed from stainless steel or other materials. They have high efficiency levels of 85percent or more. Reversible, they are able to be operated in any direction to drain the entire contents of a tube. Self priming, they do not require external air. They are typically Atex rated (explosion resistant) and can handle solvents.
The shafts are encased in sleeves that bear on each other, and lubrication is provided via a recirculating fluid. Recirculating lubricant is produced by the pressure difference between the gears. They shouldn’t be allowed to get dry, and they need to be well lubricated so that they can prevent the gears from galling. This can occur when the melt of polymer is too hard, or if it is heated too much.
Gears rotate in opposite directions, absorbing polymer. The polymer is transferred to the cogs on the outside. Lubrication grooves are used to keep the gears well-lubricated. Single or double-jacketed, they can be fitted with various seals that include mechanical, gland packing/stuffing or magnetic coupling in the absence of a seal is in place.
Diaphragm pumps
Diaphragm Pumps are the most flexible tu dieu khien may bom around the globe. They can be easily transported to any location. Simply connect the air and liquid lines and you’re ready to go. If your project requires low viscosity spraying or large solid handling, chemical or physical aggression, these pumps are able to take care of it.
Diaphragm Pumps contain two chambers of compressed air that contract and expand in alternating volumes. This results in the effect of pumping. The pump can move, compress, and evacuate mediums without the use of lubricant.
During the suction, air pressure is used to transform the left diaphragm to convex. This opens up the inlet valve and draws fluid to the pump. The pump’s shaft moves to the left, and the diaphragm to the right shifts from a concave shape to convex, closing the outlet check valve when fluid is pumped through the discharge valve.
An input regulator controls the pressure of air. If the pressure of air exceeds the pressure at which it discharges, the pump will stop. This stops the pump from causing damage to the system pipework or itself. This high-pressure air driven pumps can achieve the pressure of 30 psi. However, the actual maximum is lower as the diaphragm ruptures above the pressure.