Page 61 -

The Practical Pumping Handbook ........................................................................................... ~ ~

3.5.1.3 Closed loop system
A closed loop system is one in which the entire system is pressurized by
the pump. To achieve this, the pumpage is fully contained within a
series of pipes and pressurized process equipment all the way from the
pump discharge, through the system, and back to the pump inlet. In
such a layout, the Static Head in the system is effectively zero.

3.5.2 Friction head changes

3.5.2.1 System controls
A change in Friction Loss can be
caused by a variety of conditions
such as manual operation or
automated controls opening and
closing a different valving system.
This will result in the System
Curve adopting a different slope
that will pivot about its point of
origin at zero capacity.

3.5.2.2 Restrictions in piping
The same effect can also be Figure 3.11- System curve for friction
realized when the bore of the pipe changes
in the discharge side of the pump
reduces in size owing to some kind of buildup such as scaling, etc. Such
a buildup may also occur inside process equipment such as filters or
heat exchangers. These build-ups automatically reduce the bore of the
pipe and therefore increase the Friction Losses in that pipe.
It is worthwhile to compare the difference in effect between throttling
a valve and having scale build up in the pipe. The former will have an
immediate effect, while the scale build-up can frequently take years to
show some effect.

3.5.3 Recirculation systems
When a pump is required to operate under two distinctly different
operating conditions and the flow needs to be cut back occasionally to a
considerable extent, a recirculation line should be used. This allows the
system to continue to operate without loss of pump efficiency. It
redirects a percentage of the flow back to the suction source through
valves that are specially designed for this purpose.
One of the most important factors in a recirculation system is to ensure
that the liquid is recirculated back to the suction source of the pump. It
should not be piped back to the suction line immediately preceding the

40
m i!i::::i- ........ ....... ......