Page 286 - Intro Predictive Maintenance

Operating Dynamics Analysis 277

For the centrifugal pump illustrated in Figure 13–3, the BEP occurs at a flow of 500
gallons per minute with 150 feet TDH. If the TSH were increased to 175 feet, however,
the pump’s output would decrease to 350 gallons per minute. Conversely, a decrease
in TSH would increase the pump’s output. For example, a TSH of 100 feet would
result in a discharge flow of almost 670 gallons per minute.

From an operating dynamic standpoint, a centrifugal pump becomes more and more
unstable as the hydraulic point moves away from the BEP. As a result, the normal
service life decreases and the potential for premature failure of the pump or its com-
ponents increases. A centrifugal pump should not be operated outside the efficiency
range shown by the bands on its hydraulic curve, or 65 percent for the example shown
in Figure 13–3.

If the pump is operated to the left of the minimum recommended efficiency point, it
may not discharge enough liquid to dissipate the heat generated by the pumping oper-
ation. This can result in a heat buildup within the pump that can result in catastrophic
failure. This operating condition, which is called shut-off, is a leading cause of pre-
mature pump failure.

When the pump operates to the right of the last recommended efficiency point, it tends
to overspeed and become extremely unstable. This operating condition, which is called
run-out, can also result in accelerated wear and premature failure.

Brake horsepower (BHP) refers to the amount of motor horsepower required for
proper pump operation. The hydraulic curve for each type of centrifugal pump reflects
its performance (i.e., flow and head) at various BHPs. Figure 13–5 is an example of
a simplified hydraulic curve that includes the BHP parameter.

Note the diagonal lines that indicate the BHP required for various process conditions.
For example, the pump illustrated in Figure 13–2 requires 22.3 horsepower at its BEP.
If the TSH required by the application increases from 150 feet to 175 feet, the horse-
power required by the pump increases to 24.6. Conversely, when the TSH decreases,
the required horsepower also decreases.

The brake horsepower required by a centrifugal pump can be easily calculated by:

(
Flow GPM) ¥ Specific Gravity ¥ Total Dynamic Head Feet)
(
Brake Horsepower =
3960 ¥ Efficiency
With two exceptions, the certified hydraulic curve for any centrifugal pump provides
the data required by calculating the actual brake horsepower. Those exceptions are
specific gravity and TDH.

Specific gravity must be determined for the specific liquid being pumped. For
example, water has a specific gravity of 1.0. Most other clear liquids have a specific
gravity of less than 1.0. Slurries and other liquids that contain solids or are highly