Positive Displacement Pump
As the name suggests, positive displacement pumps make use of solid object to displace the liquid in the pump. The object may be in the form of gear teeth, piston, plunger, screw threads, vanes or some other special shapes. All the pumps have some movement imparted to the displacement object. The liquid is thus forced to move in a certain direction, sometimes with the help of non-return valves.
Positive displacement pumps are usually used to pump oil. The lubricating property of oil tends to lessen the wear-down of the displacement moving parts. If the rubbing parts are made of special self-lubricating materials like graphite, Teflon, or some elastomers then the pump can be used for pumping water.
The solid displacement object can either move in a reciprocating manner or a rotary manner. The pumps are called by the mode of operation. Pumps are called piston pumps, plunger pumps, gear pumps, screw pumps, lobe pumps, vane pumps, and so on. The liquid to be pumped is literally pushed aside by the displacement object.
Some pumps need to have inlet and outlet non-return valves fitted to work. Examples of this are the piston pump, plunger pumps and diaphragm pumps. Others like gears, screws, lobe, and vanes rotating at high speed need not employ valves to work.
One characteristic of positive displacement pumps is the fact that the pressure on the discharge side of the pump can build up and increase to a very high value if it is not released. A pressure relief valve is often fitted at the discharge side of the pump to prevent damage to the pump housing due to over pressure.
The common positive displacement pumps are briefly described here:
Rotary pumps are positive displacement pumps using rotary action for pumping. Some of the pumps are:
Each of these pumps has a driver gear and a driven gear that are meshed together at the gear teeth. The driver gear shaft is rotated by the prime mover, usually a motor. The driven gear is rotated freely by the rotation of the driver gear. Some of the oil that is being pumped is led to the bush bearings of the gear shafts for lubrication. The contact surfaces of the gears are always submerged in the same oil and are thus lubricated.
Helical gear pumps have the gear teeth cut at an angle instead of axially. They tend to smoothen the flow of the oil in the pump.
The flow of the oil follows the helical rotation of the screw. The oil is literally screwed up in the same direction as the screw shaft. There are mono screw pumps, double screw pumps and triple screw pumps being manufactured. For double and triple screws, timing gears are attached to the ends of the shafts so as to maintain the spacing between the screw threads. The screws are arranged so as not to touch one another. The same oil that is being pumped lubricates the timing gears and bearings.
Lobe pumps are similar to gear pumps. The difference is in the shape of the gears. They are not actually gears, but are specially shaped and rounded objects called lobes. If their shape is such that one lobe is not able to rotate the other, then the lobes does not function like gears anymore. In this case, timing gears at the end of the shafts are placed to maintain the spacing of the lobes, just like in the screw pumps.
Vane pumps make use of spring loaded movable plates contained in a rotating shaft rotor. The housing to which the plates rub against is usually made either off centered from the rotor, or with an elliptical shape. As the oil is moved along the chamber enclosed between each vane, the volume of the oil becomes smaller due to the change in shape of the housing. The oil is thus compressed, and is released to the discharge end of the pump.
These belong to a class of pump using reciprocating action. The pumps are:
Pistons are moved inside a cylinder in a reciprocating manner to pump the liquid. Piston rings are used to seal the piston top from the bottom part. The bottom part of the piston contains the mechanism to convert rotary motion of the motor, through a crankshaft to become reciprocating motion. The piston top, piston rings, and the cylinder make up the pump housing for pumping the liquid. These pumps have to make use of non-return valves at the suction and discharge in order to work.
These are almost similar to piston pumps. Instead of using pistons and piston rings, they make use of finely machined plungers of very small clearances in order to seal the liquid to be pumped. The plungers are highly polished and made relatively long so that only very little liquid can escape through the clearances. The reciprocating action may be brought about using off centered crankshafts for the pumping out stroke and spring for the return stroke.
Special Types of Pumps
For want of a better name of principle of operation some pumps cannot fit into any of the above types of pumps. The pumps listed here are unique.
The peristaltic pump moves liquid just like the action of the intestines in the body. A flexible rubber tube that is able to flatten when pressed and revert back to a round tube when released is the main component of this pump. A rotating mechanism driven by motor is designed to allow a tube filled with water to be squeezed from one end to the other. The portion of the tube being squeezed travels along the tube in the direction of the flow.
Diaphragm pumps use rubber diaphragms instead of pistons or plungers. The round diaphragms fixed at the edges, are pushed and pulled in a reciprocating motion at the center. One side of each diaphragm contains the liquid to be pumped, while the other side contains the reciprocating mechanism. The liquid side contains non-return valves at both the suction and discharge ends.
This type of pump does not have rubbing movement in the liquid like in the piston pump. It is the effect of the flexible diaphragm that causes the reciprocating pumping effect on the liquid. Because of this, the pump can be used to pump water rather than oil. Indeed oil may be detrimental to the rubber diaphragm.
The driver mechanism for this type of pump is usually compressed air. Using a slide valve and lost motion mechanism, the sliding plunger is designed to change direction once it reaches the end of its stroke. The inlet pressure of the compressed air adjusts the pumping speed. The sliding mechanism is either self-lubricated, or by oil mist in the compressed air.
Usually no relief valve is fitted although it is a positive displacement pump. This is because when the pressure of the fluid to be pumped exceeds the compressed air pressure, the sliding plunger no longer moves.