Positive Displacement Pumps
A diaphragm pump is a positive displacement pump that uses a combination of the reciprocating action of a rubber or teflon diaphragm and suitable non-return check valves to pump a fluid. Sometimes this type of pump is also called membrane pump.
There are two main types of diaphragm pump:
In the first type, the diaphragm is sealed with one side in the fluid to be pumped, and the other in air or hydraulic fluid. The diaphragm is flexed, causing the volume of the pump chamber to increase and decrease. A pair of non-return check valves prevent reverse flow of the fluid.
The second type of diaphragm pump has one or more unsealed diaphragms with the fluid to be pumped on both sides. The diaphragm(s) again are flexed, causing the volume to change.
When the volume of a chamber of either type is increased (the diaphragm moving up), the pressure decreases, and fluid is drawn into the chamber. When the chamber pressure later increases from decreased volume (the diaphragm moving down), the fluid previously drawn in is forced out. Finally, the diaphragm moving up once again draws fluid into the chamber, completing the cycle. This action is similar to that of the cylinder in an internal combustion engine.
Applications
Diaphragm pumps may be low lift (flooded suction), low pressure pumps with low flow rates. They can handle sludges and slurries with a moderate amount of grit and solid content. Excessive solids cause blockages.
Diaphram pumps with teflon diaphrams, ball check valves, and hydraulic actuators are used to deliver precise volumes of chemical solutions at high pressures (as much as 5000 lbf/in) into industrial boilers or process vessels.
Diaphragm pumps can be used to make artificial hearts.
A progressive cavity pump, also known as a progressing cavity pump or eccentric screw pump, is a kind of pump which moves fluid by means of a cavity which progresses along the body of the pump. As the cavity moves, fluid is compressed and forced into the next stage; further rotation of the pump forces the fluid to flow through the various stages through the pump.
The rotor of the pump is a steel helix with a circular cross section, coated in a smooth hard surface, normally chromium. The rotor fits inside a pump body (the stator) which normally is a rubber lined steel tube. The rubber core of the stator contains two helical cavities (a double helix), each with twice the pitch of the rotor, which combine to produce lengthwise undulations with the same pitch as the rotor. The rotor is mounted eccentrically in the stator so that one side shares the axis of the stator and the other seals each of the stator cavities along a half-plane through the axis. As the rotor turns, it also orbits inside the stator, and the seal plane rotates around the stator, advancing the cavities to force their contents through the pump. Compare to a Archimedes’ screw and a helical pump.
Variants use different rotor shapes and rotor/stator pitch ratios.
While progressive cavity pumps offer long life and reliable service transporting thick or lumpy fluids, abrasive fluids will significantly shorten the life of the stator. A unique feature of the progressive cavity pump is the design of its stator. Common designs are the “Equal-walled” stator and the “Unequal walled” stator. The latter, being unequal in wall-thickness allows for larger-sized solids to pass through because of its ability to compress under pressure. The interface between rotor and stator is lubricated by the fluid being pumped, however if the pump is allowed to ‘run dry’ rapid deteriotation of the stator results. The term “run dry” is loosely related to the pump’s self-priming capabilities. This means the pump is able to run dry for a given period of time while it draws in the pumped medium.
Typical applications:
- Small sewage pumps
- Sewage sludge pumps
- Slurry pumping
- Heavy oil pumping
- Cement pumps
Positive displacement pumps from are able to handle low, medium and high viscosity media because of the non-contact pump element design, and can be found in virtually every industry where gentle, sanitary processing of viscous product is required.
Positive displacement pumps of conventional design, operate with no internal contacting parts in the pump head. The rotors are driven by a gear train in the pump gear gearbox providing accurate synchronisation or timing of the rotors. The rotors contra-rotate within the pump head carrying fluid through the pump, in the cavities formed between the dwell of the rotor and the interior of the rotorcase.
In hydraulic terms, the motion of the counter rotating rotors creates a partial vacuum that allows atmospheric pressure or other external pressures to force fluid into the pump chamber. As the rotors rotate an expanding cavity is formed which is filled with fluid. As the rotors separate, each dwell forms a cavity. The meshing of the rotor causes a diminishing cavity with the fluid being displaced into the outlet port.
The hygienic design, anti-corrosive stainless steel construction and smooth pumping action have long established these pumps in the food, beverage, dairy and pharmaceutical industries.
Features
- Gentle transfer of delicate suspended solids.
- Bi-directional operation.
- Compact size with high performance and low energy input.
- Ability to pump shear sensitive media.
- Easy maintenance.
A rotary vane pump is a positive displacement pump that consists of vanes mounted to a rotor that rotates inside of a cavity. In some cases these vanes can be variable length and/or tensioned to maintain contact with the walls as the pump rotates. The most simple vane pump is a circular rotor rotating inside of a larger circular cavity. The centers of these two circles are offset, causing eccentricity. Vanes are allowed to slide into and out of the rotor and seal on all edges, creating vane chambers that do the pumping work. On the intake side of the pump, the vane chambers are increasing in volume. These increasing volume vane chambers are filled with fluid forced in by the inlet pressure. Often this inlet pressure is nothing more than pressure from the atmosphere. On the discharge side of the pump, the vane chambers are decreasing in volume, forcing fluid out of the pump. The action of the vane drives out the same volume of fluid with each rotation. Multistage rotary vane vacuum pumps can attain pressures as low as 10-3 Torr.
Common uses of vane pumps include high pressure hydraulic pumps and automotive uses including power steering and automatic transmission pumps. Pumps for mid-range pressures include applications such as carbonators for fountain soft drink dispensers and espresso coffee machines.
They are also often used as vacuum pumps for providing braking assistance (through a braking booster) in diesel-engined vehicles. Furthermore, vane pumps can be used in low-vacuum applications including evacuating refrigerant lines in air conditioners, and laboratory freeze dryers, extensively in semiconductor low pressure chemical vapor deposition systems, and vacuum experiments in physics.
A peristaltic pump is a type of positive displacement pump used for pumping a variety of fluids. The fluid is contained within a flexible tube fitted inside a circular pump casing (though linear peristaltic pumps have been made). A rotor with a number of ‘rollers’, ‘shoes’ or ‘wipers’ attached to the external circumference compresses the flexible tube. As the rotor turns, the part of tube under compression closes (or ‘occludes’) thus forcing the fluid to be pumped to move through the tube. Additionally, as the tube opens to its natural state after the passing of the cam (‘restitution’) fluid flow is induced to the pump. This process is called peristalsis and is used in many biological systems such as the gastrointestinal tract.
Peristaltic pumps are typically used to pump clean or sterile fluids because the pump cannot contaminate the fluid, or to pump aggressive fluids because the fluid cannot contaminate the pump. Some common applications include pumping aggressive chemicals, high solids slurries and other materials where isolation of the product from the environment, and the environment from the product, are critical.
Higher pressure peristaltic pumps which can typically operate against up to 16 bar, typically use shoes and have casings filled with lubricant to prevent abrasion of the exterior of the pump tube and to aid in the dissipation of heat. Lower pressure peristaltic pumps, typically have dry casings and use rollers. High pressure peristaltic pumps typically use reinforced tubes, often called ‘hoses’, and the class of pump is often called a ‘hose pump’. Lower pressure peristaltic pumps typically use non-reinforced tubing, and the class of pump is sometimes called a ‘tube pump’ or ‘tubing pump’.
Because the only part of the pump in contact with the fluid being pumped is the interior of the tube, it is easy to sterilise and clean the inside surfaces of the pump. Furthermore, since there are no moving parts in contact with the fluid, peristaltic pumps are inexpensive to manufacture. Their lack of valves, seals and glands makes them comparatively inexpensive to maintain, and the use of a hose or tube makes for a relatively low-cost maintenance item compared to other pump types.
Typical applications
Dialysis machines
Open-heart bypass pump machines
Food manufacturing
Beverage dispensing
Pharmaceutical production
Sewage sludge
An axial piston pump is one where the pistons are arranged in a circular housing that is driven by an integral shaft that is, more or less, aligned with the pistons. One end of the cylinders, housing the pistons, fits closely to a body part that is stationary.
The device can operate as a pump or a hydraulic motor.
The body has two semi-circular ports that allow inlet of the operating fluid and exhaust of the fluid (be it air, oil or whatever).
The opposite end of the cylinder and piston assembly has the heads of the pistons that bear against a swash plate, a circular collar that can be set parallel to the top face of the body, or at an angle to it. When set at an angle the swash plate forces the pistons to reciprocate (move up and down) as their housing is rotated. The swash plate angle varies the piston’s stroke from zero (plate parallel to the body) to maximum (plate at maximum angle.
As each piston is drawn up its cylinder, to follow the up-slope of the swash plate, it draws fluid through the inlet slot in the body. As the piston’s housing continues to turn the pistons reach full stroke, for the swash plate setting, and then are forced into the housing, forcing the fluid out through the body slot and to the delivery hose from the pump.
If the swash plate is set parallel to the body’s face there is no movement of the pistons in their cylinders. Thus there is no output. Movement of the swash plate controls pump output from nothing to maximum volume. If the swash plate is arranged to adjust its angle according to output demand it can make the pump idle at no output when there is no flow demand in the system (swash plate parallel) and at maximum delivery when the swash plate is at the greatest angle.
In reality most systems use pressure as a control for this type of pump. The operating pressure reaches, say, 200 bar (2 MPa or 3000 psi) and the swash plate is driven towards zero angle (piston stroke nearly zero) and with the inherent leaks in the system allows the pump to stabilise at the delivery volume that maintains the set pressure. As demand increases the swash plate is moved to a greater angle, piston stroke increases and the volume of fluid increases, if the demand slackens the pressure will rise and the pumped volume diminishes as the pressure rises. At maximum system pressure the output is almost zero again.
If the fluid demand increases, beyond the capacity of the pump’s delivery, the system pressure will drop near to zero. The swash plate angle will remain at the maximum allowed and the pistons will operate at full stroke. This continues until system flow-demand eases and the pump’s capacity is greater than demand. As the pressure rises the swash-plate angle modulates to try to not exceed the maximum pressure while meeting the flow demand.
Designers have a number of problems to overcome in designing axial piston pumps. One is managing to be able to manufacture a pump with the fine tolerances necessary for efficient operation. The mating faces between the rotary piston-cylinder assembly and the stationary pump body have to be almost a perfect seal while the rotary part turns at, maybe, 3000 rpm. The pistons are usually less than half an inch (13 mm) in diameter with similar stroke lengths. Keeping the wall to piston seal tight means that very small clearances are involved and that materials have to be closely matched for similar coefficient of expansion.
The pistons have to be drawn outwards in their cylinder by some means. On small pumps this can be done by means of a spring inside the cylinder that forces the piston up the cylinder. Inlet fluid pressure can also be arranged so that the fluid pushes the pistons up the cylinder. Often a vane pump is located on the same drive shaft to provide this pressure and it also allows the pump assembly to draw fluid against some suction head from the reservoir, which is not an attribute of the unaided axial piston pump.
Another method of drawing pistons up the cylinder is to attach the cylinder heads to the surface of the swash plate. In that way the piston stroke is totally mechanical. However, the designer’s problem of lubricating the swash plate face (a sliding contact) is made even more difficult.
Internal lubrication of the pump is achieved by use of the operating fluid—normally called hydraulic fluid. Most hydraulic systems have a maximum operating temperature, limited by the fluid, of about 120 °C (250 °F) so that using that fluid as a lubricant brings its own problems. In this type of pump the leakage from the face between the cylinder housing and the body block is used to cool and lubricate the exterior of the rotating parts. The leakage is then carried off to the reservoir or to the inlet side of the pump again. Hydraulic fluid that has been used is always cooled and passed through micrometre-sized filters before recirculating through the pump.
Despite the problems indicated above this type of pump can contain most of the necessary circuit controls integrally (the swash-plate angle control) to regulate flow and pressure, be very reliable and allow the rest of the hydraulic system to be very simple and inexpensive.
Axial reciprocating motors are also used to power many machines. They operate on the same principal as described above except that the circulating fluid is provided under considerable pressure and the piston housing is caused to rotate and provide shaft power to another machine.