Positive displacement pumps, which lift a given volume for each cycle of operation, can be divided into two main classes, reciprocating and rotary. Reciprocating pumps include piston, plunger, and diaphragm types; rotary pumps include gear, lobe, screw, vane, and cam pumps.
The plunger pump is the oldest type in common use. Piston and plunger pumps consist of a cylinder in which a piston or plunger moves back and forth. In plunger pumps the plunger moves through a stationary packed seal and is pushed into the fluid, while in piston pumps the packed seal is carried on the piston that pushes the fluid out of the cylinder. As the piston moves outward, the volume available in the cylinder increases, and fluid enters through the one-way inlet valve. As the piston moves inward, the volume available in the cylinder decreases, the pressure of the fluid increases, and the fluid is forced out through the outlet valve. The pumping rate varies from zero at the point at which the piston changes direction to a maximum when the piston is approximately halfway through its stroke. The variation in pumping rate can be reduced by using both sides of the piston to pump fluid. Pumps of this type are called double acting. Fluctuations in pumping rate can be further reduced by using more than one cylinder.
Overall pumping rates of piston pumps may be varied by changing either the reciprocating speed of the piston rod or the stroke length of the piston. The piston may be driven directly by steam, compressed air, or hydraulic oil or through a mechanical linkage or cam that transforms the rotary motion of a drive wheel to a reciprocating motion of the piston rod.
Piston and plunger pumps are expensive, but they are extremely reliable and durable. Piston pumps are known to have been running without repair or replacement for more than 100 years.
The action of a diaphragm pump is similar to that of a piston pump in which the piston is replaced by a pulsating flexible diaphragm. This overcomes the disadvantage of having piston packings in contact with the fluid being pumped. As in the case of piston pumps, fluid enters and leaves the pump through check valves. The diaphragm may be actuated mechanically by a piston directly attached to the diaphragm or by a fluid such as compressed air or oil.
Diaphragm pumps deliver a pulsating output of liquids or gases or a mixture of both. They are useful for pumping liquids that contain solid particles and for pumping expensive, toxic, or corrosive chemicals where leaks through packing cannot be tolerated.
Diaphragm pumps can be run dry for an extended period of time. Furthermore, the pumping rate of most such pumps can be changed during operation.
The most common type of gear pump is illustrated in . One of the gears is driven and the other runs free. A partial vacuum, created by the unmeshing of the rotating gears, draws fluid into the pump. This fluid is then transferred to the other side of the pump between the rotating gear teeth and the fixed casing. As the rotating gears mesh together, they generate an increase in pressure that forces the fluid into the outlet line. A gear pump can discharge fluid in either direction, depending on the direction of the gear rotation.
An internal gear pump is shown in . The driven gear is a rotor with internally cut teeth, which mesh with the teeth of an externally cut idler gear, set off-centre from the rotor. The crescent part of the fixed casing divides the fluid flow between the idler gear and the rotor. Gear pumps can pump liquids containing vapours or gases. Since they depend on the liquid pumped to lubricate the internal moving parts, they are not suitable for pumping gases. They deliver a constant output with negligible pulsations for a given rotor speed. Erosion and corrosion lead to an increase in the amount of liquid slipping back through the pump. Since gear pumps are subject to clogging, they are not suitable for pumping liquids containing solid particles. Since they do not need check valves, however, they can be used to pump very viscous liquids.
Lobe pumps resemble external gear pumps, but have rotors with two, three, or four lobes in place of gears; the two rotors are both driven. Lobe pumps have a more pulsating output than external gear pumps and are less subject to wear. Lobe-type compressors are also used to pump gases; each rotor has two lobes.
In a screw pump, a helical screw rotor revolves in a fixed casing that is shaped so that cavities formed at the intake move toward the discharge as the screw rotates. As a cavity forms, a partial vacuum is created, which draws fluid into the pump. This fluid is then transferred to the other side of the pump inside the progressing cavity. The shape of the fixed casing is such that at the discharge end of the pump the cavity closes, generating an increase in pressure that forces the fluid into the outlet line.
Screw pumps can pump liquids containing vapours or solid particles. They deliver a steady output with negligible pulsations for a given rotor speed. Since screw pumps do not need inlet and outlet check valves, they can be used to pump very viscous liquids. Although screw pumps are bulky, heavy, and expensive, they are robust, slow to wear, and have an exceptionally long life.
A sliding vane pump is illustrated in . The rotor is mounted off-centre. Rectangular vanes are positioned at regular intervals around the curved surface of the rotor. Each vane is free to move in a slot. The centrifugal force from rotation throws the vanes outward to form a seal against the fixed casing. As the rotor revolves, a partial vacuum is created at the suction side of the pump, drawing in fluid. This fluid is then transferred to the other side of the pump in the space between the rotor and the fixed casing. At the discharge side, the available volume is decreased, and the resultant increase in pressure forces the fluid into the outlet line; the pumping rate can be varied by changing the degree of eccentricity of the rotor. Vane pumps do not need inlet and outlet check valves; they can pump liquids containing vapours or gases but are not suitable for pumping liquids containing solid particles. Vane-type compressors are used to pump gases.
Vane pumps deliver a constant output with negligible pulsations for a given rotor speed. They are robust, and their vanes, easily replaced, are self-compensating for wear. Pumping capacity is not affected until the vanes are badly worn.