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Unbalance is the term used for a mass distribution in rotating bodies that is not rotationally symmetric.

To avoid unbalance in centrifugal pumps, the pump's rotating assembly is carefully balanced mechanically to DIN EN ISO 21940-11 ("Mechanical vibration − Balance quality requirements for rotors in a constant (rigid) state") prior to assembly. This ensures that the geometric mass distribution is corrected in such a way that the rotation axis of the pump rotor is a so-called free axis (no free inertia effects during rotation).

A distinction is made between dynamic and static balancing. In static balancing, the pump rotor's mass gravity centre is placed as accurately as possible in the rotation axis of the pump shaft, and, consequently, the resultant force of the centrifugal forces (not to be confused with the resultant moment of force) disappears. This is also called "single-plane balancing" and is usually applied when the axial dimension of the rotating mass is substantially smaller than its diameter.
In dynamic balancing on balancing machines, the pump rotor's rotation axis becomes the principal axis of inertia. The consequence is that not only the resultant force but also the resultant static moment of the centrifugal forces disappears. This is called "two-plane balancing".

A pump rotor which has been carefully balanced mechanically before commissioning may develop symptoms of unbalance during operation. Rotating at the frequency of the rotational speed, this unbalance impairs the smooth running of the centrifugal pump.

Mechanical unbalance is a result of asymmetrical wear or asymmetrical mass accumulation due to incrustations, while hydraulic unbalance is a result of asymmetrically acting hydraulic forces and moments on each vane or on each vane channel (see Vibration). 

All forms of unbalance may arise simultaneously, in groups or individually.

Under-filing is the retrospective machining of impeller vanes on centrifugal pumps  with low specific speeds for the purpose of slightly increasing the head at the design point. Under-filing the concave side of backward curved impeller vanes results in a slightly steeper vane outlet angle. The H/Q-curve is shifted upward, increasing the head by approx. 3 % at the flow rate at BEP (Q_opt) depending on the type of impeller and the manner by which it is under-filed. In some cases, under-filing also slightly improves efficiency.
See Fig. 1 Under-filing

 

Under-filing represents a special form of impeller trimming.

A unit in conjunction with a numerical value describes the quantitative value of a physical dimension. Units are defined and expressed in a unit system (see also SI) as:

physical dimension = numerical value · unit
e.g. length l = 1.83 m

The SI units are binding in Germany as statutory units for official and commercial transactions. There are seven SI base units, from which further units are derived. see Fig. 1 Unit

A complete list of statutory units is contained in the brochure entitled "Gesetzliche Einheiten im Pumpenbau" (statutory units in pump manufacture) by the German Engineering Federation (VDMA).

See in Appendix, Fig. 2 Unit(opens in a new tab)

The H/Q curve (see Characteristic curve) of a centrifugal pump is called unstable in a given flow rate range, if its slope in this range is positive. Under certain operating conditions, e.g. during parallel operation centrifugal pumps with unstable H/Q curve may cause operating problems (see Transient flow).

A term which is also used to describe the pump power output.