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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.
Radial flow impeller - a type of impeller whose discharge vanes, and
discharge flow, are in radial direction or perpendicular from the pump shaft.
Radial flow pump - pump in which the head is developed principally by action
of centrifugal force; the liquid enters the impeller axially at the hub and flows
radially to the periphery.
Radial split pump - a pump whose casing split, or whose principal joint, is
perpendicular to the shaft centerline. The complete casing assembly is usually
referred to as the case and cover. It is also known as vertically split pump but
the use of this term should be avoided because it can be confused for a vertical
pump. A radially split case pump can be either a horizontal or a vertical pump.
Radial thrust - a thrust that acts in the radial direction, or perpendicular to the
pump shaft; a radial thrust can be due to hydraulic thrust, static thrust, or a
combination of both.
Rated point - a pump guarantee point.
Rated operating point - the point at which the pump performance is
guaranteed by the pump vendor subject to the performance tolerance of the
applicable standard or user specification. The pump may, or may not, actually
operate at the rated condition. The point at which the pump operates most of
the time is called the normal operating point.
RCA – see Root Cause Analysis.
Relative density - Ratio of a liquid density to water density at 39.2 degrees
Fahrenheit (4 degrees Centigrade). Also known as specific gravity.
Residual unbalance - the amount of mass unbalance remaining in a rotor after
balancing the rotor on a balancing machine - it is commonly expressed in ounce-
inches. Residual unbalance causes a rotor to vibrate due to centrifugal force
when in operation. The magnitude of vibration increases proportionately by the
square of the increase in rotor speed. Thus, the rotor speed is taken into
account when calculating the amount of acceptable residual unbalance from the
equation, U = K x W / Nwhere:U - residual unbalance, per plane, in ounce-
inchesK - constant ranging from 4 to 8 depending on balance qualityN - rotor
speed, in revolutions per minute (RPM)Use K=4 for balance equal to ISO 1940-1
grade G1, and K=8 for ISO Grade G2.5.
Rigid rotor - a rotor that does not operate about 70% of its critical speed and
maintains its balance at any speed below 70% of its critical speed.
Rigid shaft - see rigid rotor
Root Cause Analysis – an analytical tool or procedure used to identify the root
cause of a failure or problem based on the symptoms, and the potential solution;
cause and effect analysis. The purpose of an RCA is not to point finger or blame
an individual for the failure, but to find a solution to prevent the failure or
problem from recurring in the future. Usually involved a mapping (or fishbone)
technique or sequence
Rotor - the assembly of all the rotating elements of a pump such as the shaft,
the impeller, wear rings, shaft sleeves, bearings, etc., including the pump half co