Understanding positive displacement pumps
Positive Displacement Pumps
In order to better understand positive displacement pumps it’s important to understand how they differ from centrifugal pumps - especially the way in which each operates within a system. This articles highlights some of those differences and describes each type of positive displacement pump.
Positive displacement pumps are used in a range of applications from services where a specific amount of liquid is required to be moved into a system to heavier duty services where the liquid is simply to viscous to be handled by a centrifugal pump.
Positive displacement pumps operate with a series of working cycles. 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.
While the maximum pressure developed is only limited by the mechanical strength of the pump and system and the driving power available, the effect of that pressure can be controlled by a pressure relief or safety valve.
A major advantage of the positive displacement pump is its ability to deliver consistent capacities because the output is solely dependent on the basic design of the pump and the speed of its driving mechanism. This means that if the liquid is not moving through the system at the required flow rate, it can always be corrected by changing one or both of these factors.
Such is not the case with the centrifugal pump. Its design can only react to the pressure demand of the system, and if the back pressure on the pump changes so 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, positive displacement pumps are suitable for handling highly viscous liquids. They are also self-priming and therefore have the ability to handle liquids with a certain volume of entrained air.
The Piston Pump
The oldest and best known positive displacement pump is the piston pump. It uses a piston or plunger to force liquid from the inlet side to the outlet side of the pump.
As the piston moves upwards it will reduce the pressure in the pump body. This causes the pressure in the suction line to open the suction valve and permit the liquid to flow into the pump. In the same way, the higher pressure in the discharge line keeps the discharge valve closed. This is called the “suction cycle.”
When the plunger moves downwards, it increases the pressure in the body, closing the suction valve and opening the discharge valve to force the liquid out of the pump. This is called the “discharge cycle.”
The movement of the plunger inside the pump body creates pressure inside the pump. Because of this, you must ensure that a piston pump is never operated against a closed discharge valve. All discharge valves must be open before the pump is started to prevent a quick increase of pressure that could damage the pump or the system.
The Diaphragm Pump
A single diaphragm pump is similar to the plunger pump, except that the up and down motion causing movement of the liquid through the pump is created by a diaphragm instead of a plunger.
Larger models of this kind of pump are used to pump heavy sludge and debris-laden wastes from manholes and catch basins.
Smaller models of single diaphragm pumps are used as chemical metering or proportioning pumps where a constant and specific amount of liquid is required.
The Air Operated Double Diaphragm Pump
A more common type of diaphragm pump is the air-operated double diaphragm pump. It uses pressurized air to activate the diaphragms instead of a mechanical device. This is basically two pumps in one -- 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.
The Rotary Gear Pump
The external rotary gear pump is a positive displacement pump where 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 meshing of the gears forces the liquid into the outlet line.
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’.
Where one gear is driven by the other, the driven gear usually runs in sleeve type bearings. The bearings and shaft journal 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 type shown in Fig. 3, is referred to as an ‘External Gear Pump’ and have all the gear rotors cut externally. On the ‘Internal Gear Pump’, one rotor with externally cut gears runs in the bore, and meshes with a second internally cut gear.
Other types are available where the gears have no metallic contact with each other. In such cases, both rotors are driven by synchronized gears separated from the pump chamber. As both shafts pass through the pump casing, two sets of seals are required. The absence of metallic contact between the surfaces of the rotors and the casing, means that wear on these parts is insignificant. The only wear that occurs is due to friction with the pumped fluid.
The Lobe Rotor Pump operates on exactly the same principles, except there are usually only 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 rotary positive displacement pump in which 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.
The Progressive Cavity Pump
This pump has been referred to as a single-end, single rotor type of screw pump where the pumping elements comprise a single rotor and a 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. One cavity closing as the other opens and the cavities progress from one end of the stator to the other.
The result is 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.
The compressive fit between the rotor and stator creates seal lines where the two components contact. The seal lines keep the cavities separated as the cavities move through the pump with each rotation of the rotor.
The elastomeric stator and stainless steel rotor allow the pump to handle large solid particles in suspension and a certain percentage of abrasives.
The manner in which the rotor turns within the stator complicates the mechanical design of progressive cavity pumps. As the rotor turns in the stator, the centerline of the rotor orbits about the centerline 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.
Depending on the size of the pump, designs of this drive mechanism range from simple ball-and-pin mechanisms to heavy-duty sealed gear couplings.