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These definitions are intended to give basic understanding of the terms
commonly used in the pump industry. Some industry standards, or customer
specifications, may have specific meaning for certain terms that may be different
from their common usage - in such instance the terms should be interpreted in
the specific context of their usage.
Face-to-face - When referring to impellers, a face-to-face arrangement is when
two adjacent impellers, in multistage pump, are arranged so that the inlet of the
impellers are facing toward each other. When referring to ball bearings, face-to-
face arrangement is when two adjacent bearings are arranged so that the face,
or low shoulder of the of the outer rings, are next to each other. The bearing
contact angles converge inward to the shaft. See definition of back-to back.
Factoring - A method of modeling or predicting the performance of a new pump
size using a size factor of a known model of a different pump size but both of
which are of the same specific speed. (Also referred to as size factoring.)
False brinelling - In ball bearing, it refers to the removal of material from
bearing elements by the fidgeting action between the balls and races due to
mechanical vibration when the bearing is not rotating, such as the occurrence of
dents during handling or transportation. False brinelling can cause a bearing to
Filter – A metal screen installed upstream of the pump suction through which a
pumped liquid is passed through to remove or strain solid particles in the liquid.
(Also referred to as suction strainer.)
Fire pump - A pump specifically designed and built for fire protection service
and meets the specifications set forth in a governing code, or standard. In North
America the code is published by the National Fire Protection Association in
NFPA 20, and the fire pumps are tested or listed by a third-party agency such
as Underwriters Laboratories, Inc. or FM Global. The use of third-party listed or
approved fire pumps are usually required not only by building codes, or local
regulations, but also by property insurance companies as well. A fire pump is the
main pump unit connected in parallel with a jockey pump in a fire sprinkler
system. When a sprinkler opens in the event of a fire the pressure drop in the
sprinkler system is detected by the automatic controller and the fire pump is
activated. See jockey pump.
First critical speed - ISO Standard 13709 and API Standard 610 define critical
speed as the shaft rotational speed at which the rotor-bearing-support system is
in a state of resonance. The lowest resonant frequency that produces the first
mode shape is called the first critical speed; the next higher frequency is called
the second critical speed, and so on. The second, or third, critical speed is not a
multiple of the first critical speed - the numbers have no correlation.
Flexible-coupled pump - a pump directly coupled to its driver through a
flexible coupling. The pump and driver each has its own shaft. See close-
Flexible rotor - a rotor which operate near, or above, its critical speed, and
when balanced at speed below its critical may lose its balance when it operates
above its critical speed.
Flexible shaft - see flexible rotor.
Floating throat bushing - A tight clearance bushing that can move radially to
compensate for shaft deflection or shaft whip. The bushing is a plain circular
ring, usually made of carbon and is lightly pressed by a spring against the
bottom wall of the stuffing box. Because the bushing can slide radially against
the wall to adjust to the shaft deflection, it can have a tighter clearance than a
fixed throat bushing. A typical range is 0.002" to 0.004" clearance.
Floating throttle bushing - same definition as floating throat bushing. The
term throat bushing usually refers to the bushing on the low pressure stuffing
box, and the term throttle bushing usually refers to the bushing on the high
pressure stuffing box. Also, a throttle bushing usually has a tighter clearance
than a throat bushing because it is subjected to a higher differential pressure.
Fluting - a minor form of electrolysis resulting from leakage of small electrical
current that passes through a bearing that, over a period of time, is accelerated
by the combined effect of vibration and lubricant degeneration due to vibration.
Fluting causes a bearing to become noisy.
Foot valve - a type of non-return valve, or a one-way valve, installed at the end
of a vertical suction pipe of a pump operating under suction lift to keep the pump
primed, or to prevent it from losing its prime.
Forces and Moments -
Four quadrant curves -
Free air - the air content of a pumped liquid due to air leakage from the
atmosphere, such as leakage at the suction piping, auxiliary piping, stuffing box,
etc., as opposed to dissolved or entrained air. Free air causes more severe
degradation of pump performance than dissolved or entrained air. According to
a major pump company (A395-EL) a 2% free air by volume will cause a 10% loss
in pump capacity and 4% free air will result in a 43.5% loss in capacity.
Free-wheeling – a condition whereby a pump is rotating slowly, in its normal
direction of rotation, with its driver de-energized and the rotation is solely
caused by the suction pressure of the liquid acting on the impeller to rotate the
rotor. This is to be differentiated from a coast-down condition. See coast-down.
Froth pump -