Air Operated Piston Pump
An air operated piston pump is a type of positive displacement fluid transfer pump. The alternating supply of compressed air causes the piston to reciprocate up and down, drawing in and expelling the fluid.
The varying ratio of pressure and delivery rate allows for optimal performance in various applications. Matching the pump to the system requires determining which type of fluid is being pumped, its viscosity and its flow requirement.
Compressed Air
Compressed air is used to power industrial tools and facilitate key factory processes. This process involves squeezing a larger volume into a smaller one, resulting in the creation of pressurized air (for industry, 7 bar is the standard).
Within an air operated piston pump, this compressed air is utilized to drive a piston/diaphragm around a cylinder and transfer fluids. The air distribution system, which consists of control valves and air chambers, regulates the flow of compressed air into specific areas of the pump to initiate and control reciprocating motion.
Piston pumps come in a variety of ratios, air operated piston pump ranging from 1:1 all the way up to 100:1. The higher the ratio, the greater the pressure generated by the pump. Regardless of the ratio however, each pump will generate the same force on the piston when it is in its ‘intake stroke’. This is because the effective difference in area of the air side of the piston and the fluid side of the piston remains constant.
Piston pumps operate with no complex transmission, allowing them to have the lowest possible noise level and eliminating the issue of pulsation in the output air. They are also able to operate in the harshest environments due to their sealed configuration, making them a reliable solution for heavy applications such as food production and automotive industries.
Intake Stroke
The piston’s down movement in the intake stroke causes the exhaust port to be uncovered, letting the fresh charge that was compressed in the crankcase flow into the cylinder. This is followed by the power stroke, with the increased pressure from exothermic combustion pushing down on the piston. As the piston approaches BDC, the exhaust blowdown begins to force the combustion products out of the cylinder through the open exhaust ports.
During the expansion stroke, a new supply of air is brought into the cylinder through the inlet valve. The piston then retracts, pushing the fluid in the pump chamber backward through a discharge valve. This is repeated continuously, generating a continuous flow of fluid that can be directed to any desired location or process.
To control the timing of IVO and EVC, an NI-PXIe-6358 card with LabVIEW software is used to send commands to an ARM® CortexTM-M3 microcontroller. A photo interrupter sensor is used to monitor the position of the piston and generate digital signals that are subsequently processed by the controller. Pressure sensors (Kistler 6052C) and charge amplifiers (Kistler 5018A) were selected to measure the pressure inside the cylinder head. A linear sensor was also included to track the piston displacement and generate encoder signals. These were then transmitted to the programmable controller, which drives the actuation mechanism.
Compression Stroke
When the air distribution system directs compressed air to the cylinder, it pushes against the piston’s diaphragm, causing it to move. This downward stroke creates a vacuum, sucking fluid into the pump through an inlet valve. This suction phase is what makes piston pumps ideal for handling a broad range of viscosities and slurries.
As the piston moves down during the intake stroke, it also closes an exhaust valve and a discharge valve. This ensures minimal leakage and efficient pressure buildup.
On the upward stroke, the piston’s motion compresses the cylinder. This cyclic operation is what allows piston pumps to generate such high pressures, which are often much higher than vane and gear pump pressure ratings.
As the piston approaches BDC, the power stroke begins. At the end of this cycle, inertia causes the piston to return to its original position. In the process, it expels the pressurized fluid out through an open discharge valve. As it leaves the cylinder, air operated piston pump factory the fluid is transferred to the piping system to perform its desired industrial application. The accumulated pressure generated during the compression stroke is powerful enough to overcome any resistance in the piping system. This can include things like length of pipe, piping diameter, and the presence of regulating valves. Piston pumps are also able to deliver the same consistent flow rate and pressure throughout the operating cycle, regardless of the fluid’s viscosity or the system’s conditions.
Discharge Stroke
During the discharge stroke the piston or plunger moves away from the cylinder’s suction port, decreasing the pressure within the cylinder. This allows the cylinder to fill with liquid from the suction line. When the piston or plunger moves toward the cylinder’s discharge port, increasing the pressure within the cylinder forces the discharge valve open, expelling the fluid out of the pump into the delivery pipe.
The air used to power this positive displacement pump is supplied through an air vessel (generally a metal tube-shaped container with a separating diaphragm) which is installed very close to the suction valve and the delivery valve. The lower half of the air vessel is filled with water and the upper part with compressed air. As the positive displacement pump cycle is completed, the air vessel and the delivery valve are alternately emptied and charged by the reciprocating movement of the piston or plunger inside the cylinder.
The pulsation of the pump discharge can be reduced with a properly placed pulsation dampener in the suction line and/or an air-filled accumulator in the discharge line. A pulsation dampener in the discharge line is especially useful for situations where unwanted effects such as piping vibration and water hammer are a concern. The pulsations can also be absorbed by using multiple pumps with the same displacement.