How Diaphragm Pumps Handle Abrasive Materials and Ensure Self-Priming Efficiency
Positive Displacement Pumps are used for transferring fluids with high viscosity like fuels, food ingredients or chemicals. They are also commonly used for applications where precise metering is required.
They operate by alternating back and forth strokes of a piston, diaphragm or helical rotor. This cycle allows them to convey a fixed volume per shaft revolution.
Rotary Positive Displacement Pumps
A positive displacement pump draws a fixed amount of fluid in and forces it out through an outlet valve. These pumps are used for transporting liquids ranging in viscosity from those thicker than water to sludges and emulsions. They can be operated at high pressures, and are well-suited for applications requiring precise dosing. They are also preferred for applications involving fluids that contain solid particles or abrasives. Common types of rotary positive displacement pumps include piston, diaphragm, gear rotary, and screw pumps.
Because these pumps do not use impellers, they are not as susceptible to the problems that can affect centrifugal pumps, such as wear and cavitation. However, abrasive feed can still cause excessive wear on the components in some positive displacement pumps. This is especially true for rotary pumps that use pistons or plungers to trap and displace fluid. For this reason, abrasive feed should be avoided whenever possible.
Another issue with rotary positive displacement pumps is that they can produce pulsating discharge. This can lead to noise and vibration in the system as well as cavitation that can damage piping. However, this can be reduced by using multiple pump cylinders and pulsation dampers.
Another benefit of a rotary positive displacement pump is that it can often self-prime. This is due to the very small clearances that exist within the pump. However, care should be taken to ensure that the pump is not run dry for extended periods of time as this can reduce the efficiency and lifespan of the seal.
Reciprocating Positive Displacement Pumps
Using pistons within a cylinder, these pumps draw and pressurize fluid. As the piston moves back and forth it traps a liquid volume between the inlet and outlet valves, creating a differential pressure that overcomes the inlet valve to allow fluid to leave. Unlike centrifugal pumps that are sensitive to viscosity changes, positive displacement pumps maintain their flow rate independently of system pressure.
This ability to operate at consistent pressures makes these types of pumps ideal for applications requiring accurate metering and transfer, as well as abrasive or hazardous materials. Additionally, these pumps are self-priming, which reduces downtime and labor costs by eliminating the need for manual re-priming.
However, a drawback of these pumps is that they may continue to build pressure within the delivery pipework until something relieves the pressure, which could be the pump itself or the liner, leading to excessive vibration and noise in operation. In order to mitigate this, these pumps typically require accessories like pulsation dampeners in the pipework and discharge line to ensure safety and reliability. Moreover, the internal design of these pumps tends to make them more expensive and difficult to maintain than centrifugal options. Nonetheless, their capacity to handle corrosive or dangerous fluids, as well as their ability to consistently perform at low-pressure settings, offset these challenges. They are a great choice for high-viscosity applications in the pharmaceutical industry, chemical processing, and oil drilling industries.
Gear Pumps
Unlike diaphragm pumps, gears do not cause shear to the fluid. They are ideal for transferring shear sensitive liquids, such as emulsions, microbial cultures, and food products. Gear Pumps are also good for transferring liquids that are prone to changes in viscosity.
They are very compact and cost effective. They can be built from a variety of materials including stainless steel and have a high level of efficiency with 85% or more being achievable. They are reversible meaning they can operate in both directions to ensure the full contents of a hose are emptied. They are also self priming ensuring they do not require an external air supply. They are generally Atex rated (explosion proof) and can handle solvents.
The shafts are housed within sleeves bearing on each other with a recirculating polymer providing lubrication. The recirculating polymer is produced by the difference in pressure between the two gears. They can only run dry for a limited time and should be kept well lubricated to avoid gears galling, which can occur if the polymer melt is too hard or shear heat too high.
The gears rotate in opposite directions picking up the may bom ly tam truc ngang polymer and conveying it to exit around the outside of the meshed cogs. Lubrication grooves are incorporated to keep the gears lubricated. They can be single or double jacketed and fitted with different seal types – including mechanical, gland packing/stuffing, or magnetic coupling where no seal is present.
Diaphragm Pumps
The most flexible pump in the world, the Diaphragm Pump is easy to tote to wherever it’s needed – simply attach your air and liquid lines and you’re ready to work. Whether your application calls for low viscosity spraying, large solid handling or chemical and physical aggression these pumps can handle it.
The Diaphragm Pump has two air chambers that are supplied by compressed air whose alternating volume contraction and expansion creates the pumping action. A hermetic seal between the diaphragm, drive mechanism and compression chamber allows the pump to transfer, compress and evacuate a medium without requiring a lubricant.
During the suction stroke, air pressure is applied to the left diaphragm to change it from a flat to a convex shape, which opens the inlet check valve and draws fluid into the pump. Then, the pump shaft moves to the right, the right diaphragm changes from a concave to a convex shape and closes the outlet check valve while fluid is pumped out the discharge valve.
The air pressure is controlled by an input regulator. If the air pressure exceeds the discharge pressure the pumps will simply come to a stop. This prevents the pump from damaging itself or system piping. This type of high-pressure air driven pump can achieve an ultimate pressure of 30 psi, although the actual maximum attainable is lower because the diaphragm will break above this.
