Page 130 - Know and Understand Centrifugal Pumps

The System Curve

The happy zone
~
Now we can see the importance of the concentric ellipses of efficiency
on the pump family curve. As much as possible we should find a pump
whose primary efficiency arc covers the needs of the system. Certainly
the needs of the system should fall within the second or third efficiency
arcs around the pump’s BEP. If the system’s needs require the pump to
consistently run too far to the left or right extremes on its curve, it may
be best to consider pumps in parallel, or series, or a combination of the
two, or some other arrangement, possibly a PD pump. We’ll see this
later.

As elevations change in the process of draining one tank and filling
another, the pump moves on its curve from one elevation extreme to
the other. If we’ve selected the right pump for the system, it will move
from one extreme of its happy zone, through the REP to the other
extreme.

This is the beginning of many problems with pumps. A pump is specified with the BEP
at one set of system coordinates. Then the system (the TDH) goes dynamic, changing,
and the pump moves on its curve away from its BEP out to one or the other extreme.
It is necessary to determine the maximum and minimum elevations in the system and
design the pump within these elevations. If the system continues to change on the
pump, you’ll either have to modify the system or modify or change the pump, unless
you really like to change bearings and seals.

Dynamic pressures

Let’s consider now a system with dynamic pressures and a constant
elevation. A classic example of this would be where a pump feeds a
sealed reactor vessel, or boiler. The fluid level in the reactor would be
more or less static in relation to the pump. The resistances in the
piping, the Hf and Hv, would be mostly static although they would go
up with flow. The Hp, pressure head would change with temperature.
Consider Figure 8-14.

The system curve, once again, is the visual graph of the four elements of
the TDH. The Hp is stacked on top of the Hs. The Hp changes with a
change in temperature in the reactor. If the reactor were cold, the Hp
would be minimum or zero. We’ll call this Hpl. When the tank and
fluid arc heated, the Hp rises to its maximum. This is represented as
Hp2 on the graph (Figure 8-15).