Page 134 - Know and Understand Centrifugal Pumps

The System Curve

in front of the valve. The pump moves away from the sweet zone of
efficiency.
In a maintenance fhction, working against the production clock,
someone changes a globe valve for a gate valve. A globe valve has
between 20 and 40 times more resistance than a gate valve. Someone
orders to exchange a bolted flanged long radius pipe elbow, with a
welded mitered 90" elbow. This affects the resistance in the system and
the pump on its curve.

The authors recommend that all plant personnel
including the engineers, operators, and mechanics receive training to recognize these
rapid, unexpected, quick changes in a system. Some of these changes can be
controlled within certain limits. Others must be avoided as standard plant procedure.
Almost no one in maintenance or reliability relates today's failed mechanical seal
with the inoffensive change in a pipe diameter six weeks ago. Engineers should train
their personnel to understand the result of these inadvertent changes. These rapid
changes in a system are the source of pump maintenance.

Long term resistance changes
~~ ~ ~~ ~ ~ ~~~
In the long term, filters and strainers become clogged: this is their
purpose. Minerals and scale start forming on the internal pipe walls and
this reduces the interior diameters on the pipe. A 4 inch pipe will
eventually become a 3.5 inch pipe. This moves the pump on its curve
because as the pipe diameter reduces, the velocity must increase to
maintain flow through a smaller orifice. The Hf and Hv increase by the
square of the velocity increase.
Also in the long term, the equipment loses its efficiency, and
replacement parts are substituted in a maintenance function. Also, the
plant goes through production expansions and contractions: new
equipment is added into the pipes. In short, the system and its
elevations and pressures, its resistances and velocities, are very dynamic.
The BEP of the pump is static.
What must be done is establish the maximum flow, and the minimum
flow, and implement controls. Regarding filters, you've got to establish
the flow and pressure (resistance) that corresponds to the new, clean
filter, and determine the flow and resistance that represents the dirty
filter and its moment for replacement. These points must be
predetermined. The visual graph of the system curve with its dynamic
resistances are seen in this example filtering and recirculating a liquid in
a tank. Consider the following graphs (Figures 8-18 and 8-19).