Page 282 - Intro Predictive Maintenance

Operating Dynamics Analysis 273

Multistage pumps that use opposed impellers are much more stable and can tolerate
a broader range of process variables than those with an inline configuration. In the
opposed-impeller design, sets of impellers are mounted back-to-back on the shaft. As
a result, the other cancels the thrust or axial force generated by one of the pairs. This
design approach virtually eliminates axial forces. As a result, the pump does not
require a massive thrust-bearing or balancing piston to fix the axial position of the
shaft and rotating element.

Because the axial forces are balanced, this type of pump is much more tolerant of
changes in flow and differential pressure than the inline design; however, it is not
immune to process instability or to the transient forces caused by frequent radical
changes in the operating envelope.

Factors that Determine Performance
Centrifugal pump performance is primarily controlled by two variables: suction con-
ditions and total system pressure or head requirement. Total system pressure consist
of the total vertical lift or elevation change, friction losses in the piping, and flow
restrictions caused by the process. Other variables affecting performance include the
pump’s hydraulic curve and brake horsepower.

Suction Conditions. Factors affecting suction conditions are the net positive suction
head, suction volume, and entrained air or gas. Suction pressure, called net positive
suction head (NPSH), is one of the major factors governing pump performance. The
variables affecting suction head are shown in Figure 13–2.

Centrifugal pumps must have a minimum amount of consistent and constant positive
pressure at the eye of the impeller. If this suction pressure is not available, the pump
will be unable to transfer liquid. The suction supply can be open and below the pump’s
centerline, but the atmospheric pressure must be greater than the pressure required to
lift the liquid to the impeller eye and to provide the minimum NPSH required for
proper pump operation.

At sea level, atmospheric pressure generates a pressure of 14.7 pounds per square inch
(psi) to the surface of the supply liquid. This pressure minus vapor pressure, friction
loss, velocity head, and static lift must be enough to provide the minimum NPSH
requirements of the pump. These requirements vary with the volume of liquid trans-
ferred by the pump.

Most pump curves provide the minimum NPSH required for various flow conditions.
This information, which is usually labeled NPSH R , is generally presented as a rising
curve located near the bottom of the hydraulic curve. The data are usually expressed
in “feet of head” rather than psi.

The pump’s supply system must provide a consistent volume of single-phase liquid
equal to or greater than the volume delivered by the pump. To accomplish this, the