Pumps are used to move water or any other liquid from a lower elevation to
higher elevation. This is done by adding energy to the system by other means.
Pumps work by using mechanical forces to push the material, either by
physically lifting, or by the force of compression.
A positive displacement pump causes a liquid to move by
trapping a fixed amount of fluid and then forcing (displacing) that trapped
volume into the discharge pipe. Positive displacement pumps can be further
classified as either rotary-type or lobe pumps similar to oil pumps used in car
engines.
Reciprocating-type pumps use a piston and cylinder arrangement with suction
and discharge valves integrated into the pump. This type of pump was used
extensively in the early days of steam propulsion (19th century) . Though still used today, reciprocating pumps are
typically used for pumping highly viscous fluids including concrete and heavy
oils.
Centrifugal Pumps convert Mechanical energy into Hydraulic energy by centripetal
force on the liquid. Typically, a rotating impeller increases the velocity of
the fluid. The casing, or volute, of the pump then acts to convert this
increased velocity into an increase in pressure. So if the mechanical energy is
converted into a pressure head by centripetal force, the pump is classified as
centrifugal. Such pumps are found in virtually every industry, and in domestic
service in developed countries for washing machines, dishwashers, swimming
pools, and water supply.
A wide range of designs are available, with constant and variable speed
drives. Horizontal shafts are the most common. Single-stage pumps are usual in
the smaller ratings. Pumps with up to 11 stages are in service. A demanding
duty is boiler feed, and today's designs are typically 3 - 4 stage, with speeds
of up to 6000 revolutions per minute.
After motors, centrifugal pumps are arguably the most common machine, and
they are a significant user of energy. Running a constant speed pump throttled
causes energy waste. A condition monitoring test can detect this condition and
help size a smaller impeller, either new, or by machining the initial one, to
achieve great energy reduction.
Pumps also wear internally, at a rate varying with the liquid pumped,
materials of construction and operating regime. Again, condition monitoring can
be applied to detect and quantify the extent and rate of wear and also help
decide when overhaul is justified on an energy-saving basis.
