The term Best Efficiency Point, or BEP, refers to the flow rate where the
efficiency is highest on the pump performance curve at its maximum impeller
diameter. If a reference pump curve were plotted at a smaller impeller cut
diameter its flow rate with the highest efficiency, at that diameter, had to be
extrapolated to the equivalent flow rate at maximum impeller diameter by using
the
affinity laws to determine its BEP. (See Affinity Laws)

In most Best Efficiency Point (BEP) evaluation it is the numeric value of flow rate
at BEP that is more important than the numeric value of efficiency at BEP,
unless one is using efficiency at BEP to determine the optimum
specific speed
(NS) that will yield the highest efficiency in making a hydraulic pump selection for
a particular set of operating conditions. (See
Specific Speed)

The BEP (its numeric efficiency value and its location along the x-axis of a pump
curve) is determined by the hydraulic combination of impeller and volute in the
pump; it does not necessarily correspond to the design flow of the  particular
impeller or volute. The same impeller will yield different BEP points when used in
combination with different volutes; similarly the same volute, when used with
different impellers, will yield different BEP points.

[The term
volute is used here for simplicity but it can also refer to a diffuser.]

Factors affecting BEP

The numeric value of flow at BEP is not a hard number but is just its very close
approximation because BEP may deviate easily from its expected value even in
pumps with consistent historical test results.  There are several factors that may
cause BEP to deviate lower, or higher, in number:

  1. [Differences in pa.....]

  2. Differences in impeller material will result in variations in casting shrinkage
and surface finish. Variations in impeller “BA”, among other impeller dimensions,
will cause the BEP to shift in the pump curve.

  3. Differences in casing material will also result in variations in casting
shrinkage and surface finish. Variations in volute throat area, among other
volute dimensions, will also cause the BEP to shift.

  4.Variations in machining tolerance, clearances, fits, and assembly.

  5. Differences or changes in test procedure, test set-up, and calibration of test
instruments.

  6. [Because it.....]

  7. [The maximum.....]

Applications of BEP

The Best Efficiency Point (BEP) is an important parameter used in evaluating
the hydraulic characteristic of centrifugal pumps and defines its operation.

The BEP is an important parameter because it is used to set limits to the pump
operation.

  1. It is typical to cutoff or stop the end-of-curve at 120% to 130% of BEP - the
most common practice is at 125% of BEP. The cutoff prevents the potential of
oversizing the driver HP.

  2. Pump parameters such as specific speed (Ns), suction specific speed (Nss),
hydrodynamic size (Z), and viscosity correction factors, etc., are calculated from
data obtained at BEP such as flow, head, NPSHR, etc.

  3. It is common practice to determine a pump's recommended minimum
continuous stable flow (MCSF), allowable operating range (AOR), and preferred
operating range (POR) as percentages of BEP.

  4. The radial thrust factor, K, used in thrust calculations is based, partly, on
the rated flow’s relative position to BEP. The hydraulic thrust loads and shaft
deflection are lowest when a pump operates at, or close to, BEP

  5. [Many users...]

  6. [The pump.....]


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Best Efficiency Point (BEP)
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