Positive Displacement Pumps are Different
July 25, 2016
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In the many differences that exist between centrifugal and positive displacement pumps (PD pumps), the one that has caused the most confusion has been the manner in which they each operate within the system.
The major advantage of a PD pump is that it can deliver consistent capacities. This is because the output is solely dependent on the basic design of the pump and the speed of its driving mechanism. From a practical standpoint, this means that the flow through the system can always be modified by changing one or both of these factors. For example, adjusting the length of the stroke or the reciprocating speed can change the output of a piston pump.
Such is not the case with the centrifugal pump. Centrifugal designs can only react to the pressure demand of the system and, if the back pressure on the pump changes, so too will the capacity of the pump. This can be disruptive for any process dependent on a specific flow rate and can also have significant ramifications on the stability, efficiency and reliability of the centrifugal pump. To varying degrees, PDs are suitable for handling highly viscous liquids. They also have the ability to handle liquids with a certain volume of entrained air and are thus, self-priming.
PD Pump Styles
Positive displacement pumps come in a variety of styles, all of which operate with a series of working cycles where each cycle encloses a certain volume of fluid and moves it mechanically through the pump into the system. Depending on the type of pump and the liquid being handled, this happens with little influence from the back pressure on the pump.
One of the oldest PDs is the piston pump, which uses a reciprocating piston or plunger to force liquid from the inlet side to the outlet side of the pump. As the movement of the plunger inside the cylinder creates pressure inside the pump, it is necessary to ensure that this kind of pump is never operated against any closed discharge valve. All discharge valves must be open before the pump is started to prevent any fast build-up of pressure that could damage the pump or the system.
A single diaphragm pump can be similar to the plunger pump except that a diaphragm instead of a plunger creates the reciprocating motion causing movement of the liquid through the pump. Larger models of this kind of pump are used to pump heavy sludges and debris-laden wastes from manholes and catch basins. Smaller models of the same basic design are used as chemical metering or proportioning pumps where a constant and specific amount of liquid is required.
A more common type of diaphragm pump is the air-operated double diaphragm pump (AODD), which uses pressurized air instead of a mechanical device to actuate the diaphragms. The AODD is basically two pumps in one where one, is on the suction cycle while the other is on the discharge cycle. The air valves alternately pressurize the inside of one diaphragm chamber and exhaust air from the other one.
This pump style does not require additional sealing devices. It can also be operated quite safely against a closed discharge valve as, under these conditions, the air pressure in the pump automatically balances out on each side of the diaphragms, thus stalling the pump.
The external rotary gear pump is a positive displacement pump wherein the unmeshing of the gears produces a partial vacuum to draw the liquid into the pump. The liquid is carried between the gear teeth and the casing to the opposite side of the pump. The subsequent meshing of the gears forces the liquid out through the discharge nozzle.
The direction of pump rotation determines which of the nozzles will be the inlet and the outlet. By reversing rotation, the function of the nozzles will be reversed and the pump will be able to pump “backwards.”
In the most common type, one gear is driven by the other. The driven gear usually runs in sleeve-type bearings that are located in the pump casing and surrounded by the pumped fluid. Consequently, these bearings and gears are dependent on the lubricating qualities of the pumped fluid. The lobe pump is a modification of the gear pump that operates on exactly the same principles, except there are usually only one, two or three “teeth” in each lobe. This arrangement permits the handling of larger solid particles and reduces the shear effect on the liquid.
Screw pumps are a special type of PD. In these units, the flow through the pumping elements is truly axial. The liquid is carried between screw threads on two or more rotors and is displaced axially as the screws rotate and mesh.
The meshing of the threads on the rotors and the close fit of the surrounding housing create one or more sets of moving seals in series between pump inlet and outlet. These sets of seals act as a labyrinth and provide the screw pump with its positive pressure capability. The successive sets of seals for fully enclosed cavities move continuously from inlet to outlet. These cavities trap liquid at the inlet and carry it along to the outlet, providing a smooth flow with minimal pulsations.
Progressive cavity pumps
The progressive cavity (PC) style has been referred to as a single-end, single-rotor type of screw pump where the pumping elements comprise a single rotor and stator. The stator usually has a double helical internal thread with a pitch twice that of the single helical stator. This results in two leads on the stator and one on the rotor. As the rotor rotates inside the stator, two cavities form at the suction end of the stator, with one cavity closing as the other opens. The cavities progress from one end of the stator to the other. The compressive fit between the rotor and stator creates seal lines where the two components contact. These seal lines keep the cavities separated as the cavities progress through the pump with each rotation of the rotor.
The elastomeric stator and stainless steel rotor allows the pump to handle some solid particles in suspension with a certain percentage of abrasives. The manner in which the rotor turns within the stator complicates the mechanical design of PC pumps. As the rotor turns in the stator, the center line of the rotor orbits about the center line of the stator.
This eccentric motion means the pump must be fitted with universal joints to transmit power from the concentric rotation of the drive shaft to the eccentrically rotating rotor. These joints must transmit torsional and thrust loads. Designs of this drive mechanism range from simple ball-and-pin mechanisms to heavy-duty sealed gear couplings.
The PC pump style provides a flow with relatively little pulsation. Since the longest path through the elements is a spiral, and not far from a straight line, the shear rates will also be low in comparison to those in other types of pumps.
When considering positive displacement pumps, it is worthwhile to note that the maximum pressure developed is limited only by the mechanical strength of the pump and/or system and/or the driving power available. The effect of that pressure can be controlled by a pressure relief or safety valve.
However, in some cases, the pressures that these pumps develop can pose a safety issue – something that is typically not a factor with centrifugal pumps. This safety aspect must be carefully analyzed during the system design as well as the operation of these pumps.
Ross Mackay specializes in helping companies increase their pump asset reliability and reduce operating and maintenance costs through pump training programs. He is the author of “The Practical Pumping Handbook”. He can be reached at 1-800-465-6260 or by visiting his website.