Positive-Displacement Pumps
Positive-displacement pumps are another category of pumps.
Types of positive-displacement pumps are reciprocating, metering, and rotary
pumps. Positive-displacement pumps operate by forcing a fixed volume of
fluid from the inlet pressure section of the pump into the discharge zone
of the pump. These pumps generally tend to be larger than equal-capacity
dynamic pumps. Positive-displacement pumps frequently are used in hydraulic
systems at pressures ranging up to 5000 psi. A principal advantage of hydraulic
power is the high power density (power per unit weight) that can be achieved.
They also provide a fixed displacement per revolution and, within mechanical
limitations, infinite pressure to move fluids.
In a reciprocating pump, a volume of liquid is drawn into
the cylinder through the suction valve on the intake stroke and is discharged
under positive pressure through the outlet valves on the discharge stroke.
The discharge from a reciprocating pump is pulsating and changes changes
only when the speed of the pump is changed. This is because the intake
is always a constant volume. Often an air chamber is connected on the discharge
side of the pump to provide a more even flow by evening out the pressure
surges. Reciprocating pumps are often used for sludge and slurry.
One construction style of a reciprocating pump is
the direct-acting steam pump. These consist of a steam cylinder end in
line with a liquid cylinder end, with a straight rod conection between
the steam piston and the pump piston or plunger. These pistons are double
acting which means that each side pumps on every stroke.
Another construction style is the power pump which
convert rotary motion to low speed reciprocating motio using a speed reducing
gear. The power pump can be either single or double-acting. A single-acting
design discharges liquid only on one side of the piston or plunger. Only
one suction and one discharge stroke per revolution of the crankshaft can
occur. The double-acting design takes suction and discharges on both sides
of the piston resulting in two suctions and discharges per crankshaft revolution.
Power pumps are generally very efficient and can develop high pressures.
These pumps do however tend to be expensive.
Metering pumps provide precision control of very low flow
rates. Flow rates are generally less than 1/2 gallon per minute. They are
usually used to control additives to the main flow stream. They are also
called proportioning orcontrolled-volume pumps. Metering pumps are available
in either a diaphragm or packed plunger style, and are designed for clean
service and dirty liquid can easily clog the valves and nozzle connections.
A rotary pump traps fluid in its closed casing and discharges
a smooth flow. They can handle almost any liquid that does not contain
hard and abrasive solids, including viscous liquids. They are also simple
in design and efficient in handling flow conditions that are usually considered
to low for economic application of centrifuges. Types of rotary pumps include
cam-and-piston, internal-gear, lobular, screw, and vane pumps. Gear pumps
are found in home heating systems in which the burners are fired by oil.
Rotary pumps find wide use for viscous liquids. When pumping highly viscous
fluids, rotary pumps must be operated at reduced speeds because at higher
speeds the liquid cannot flow into the casing fast enough to fill it. Unlike
a centrifugal pump, the rotary design will deliver a capacity that is not
greatly affected by pressure variations on either the suction or discharge
ends. In services where large changes in pressure are anticipated, the
rotary design should be considered.
Browse through the Moyno technical bulletins to see how the
rotor turns inside the casing. This is called a "progressing cavity". This
pump handles solids beautifully. It is said that they can pump strawberries
with little damage to each berry.
