Page 133 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Again, we see the importance of the pump family curve, with its
concentric ellipses of efficiency. It shows that in the beginning of
the operation, the pump is operating to the right of its BEP. As the
pressures rise in the system, the pump moves toward the left of the
REP. When the temperatures and pressures are reduced at the end of
the process, the pump migrates again on its curve to the right of the
BEP. During the entire operation, the pump is inside its primary or
secondary sweet zone.

Variable resistances
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The Hf and Hv represent the resistance losses in the system.
Specifically, the Hf represents the energy consumed or lost due to
friction in the system, and the Hv represents the energy consumed or
lost due to the velocity of the fluid moving through the system. The Hf
and the Hv are linked or connected because the Hv is used to calculate
the Hf. If there is no velocity in the fluid, then the fluid is not moving
through the pipes, and if the fluid is not moving through the pipes
there can be no friction between the fluid and the internal pipe walls. If
the resistance rises in the system, as in the case of a filter whose function
is to clog over time, the flow is reduced through the filter, and the
pressure or resistance rises. This means that the pump is moving toward
the left on its curve. The resistances can change in the short term, or in
the long term, with operations, with maintenance, or with a design
change. Let’s see how:

Short term resistance changes

The resistance in the pipes and system can change suddenly or in the
short term due to a design change, operation, or maintenance. For
example, many systems are designed incorporating variable speed
motors, VFDs, to control production in a plant. The resistance is
multiplied 4 times simply by doubling the velocity of the fluid in a pipe.
Sometimes, in an existing system, the engineer orders to install a new
control. For example, installing a flow meter into a pipe increases the
resistance and the pump moves on its curve. In an effort to improve the
final product, a production engineer orders a change to the screen
meshes in the filters. This changes the Hf and the Hv in the system and
the pump migrates on its curve.
In some plants, the operators have free reins to govern the flow in the
pipes by opening and closing flow control valves. Strangling a valve
reduces the flow and increases the resistance and pressure in the system