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The following 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 of certain terms that may be different
from their common usage - in such case the terms should be interpreted in the
context of the standards, or specifications, being referred to.
U - a commonly used symbol for peripheral speed, such as impeller eye
diameter speed or impeller diameter speed.
Ultimate stress - the maximum value of stress that a material can take without
losing its strength. See yield stress.
Ultrasonic examination - a method of non-destructive examination (NDE)
using the propagation of ultrasonic waves to detect any defect or non-conformity
of the part being examined, such as pump casing, cover, impeller, shaft, etc.,
based on an agreed acceptance criteria. The result of the NDE is recorded in an
ultrasonic examination report.
Unbalance - the uneven mass distribution on a rotating element that will cause
it to vibrate due to centrifugal force when the element is rotating. The magnitude
of vibration increases proportionately with unbalance mass and rotational
speed. Unbalance is the product of the unbalance mass multiplied by its
distance from the axis of rotation, or shaft centerline, and is commonly
measured in ounce-inches. See residual unbalance.
Unbalanced seal - a mechanical seal that is not hydraulically balanced to
minimize the heat developed at its seal face.
Unbalanced wear rings - a set of wear rings on an impeller whereby the front
ring has a different balance diameter (OD) from the back ring, as opposed to
having same OD dimension, for the purpose of controlling the amount of net
hydraulic axial thrust acting on the impeller. Also referred to as offset wear rings.
Undercut or undercutting - trimming the effective diameter of an impeller by
cutting its vanes only, and leaving its shroud uncut.
Underfile or underfiling - the removal, by hand-grinding, of some amount of
metal on the impeller vanes, either on the inlet side, or outlet side, for the
purpose of reducing the vane thickness and increasing the area opening
between two adjacent vanes, to change the performance of a pump.
Unfiltered vibration - vibration data that include the spectrum of different
frequencies under observation. By comparison, filtered vibration data are
unique to a discreet, or specific, frequency under observation.
Uniform flow - a flow where the liquid particles move along its line of flow with
constant velocity. See steady flow.
Upstaging - the addition of one or more impeller/s to previously de-staged
multistage pump to increase its differential head. (See definition of de-staging.)