Page 95 - Root Cause Failure Analysis

86 Root Cause Failure Analysis

In any installation, the pump will operate at the point where its TDH equals the TSH.
When selecting a pump, it is hoped that the BEP is near the required flow where the
TDH equals TSH on the curve. If not, there will be some operating-cost penalty as a
result of the pump’s inefficiency. This often is unavoidable because pump selection is
determined by what is available commercially as opposed to selecting one that would
provide the best theoretical performance.
For the centrifugal pump illustrated in Figure 7-5, the BEP occurs at a flow of 500
gpm with 150 ft TDH. If the TSH were increased to 175 ft, however, the pump’s out-
put would decrease to 350 gpm. Conversely, a decrease in TSH would increase the
pump’s output. For example, a TSH of 100 ft would result in a discharge flow of
almost 670 gpm.

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 ser-
vice life decreases and the potential for premature failure of the pump or its compo-
nents 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 7-5.

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
operation. The heat that builds up within the pump can cause a catastrophic failure.
This operating condition, called shutofl, is a leading cause of premature 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, called
runout, also can accelerate wear and bring on premature failure.

Brake Horsepower
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 7-7 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 7-7 requires 22.3 horsepower at its BEP.
If the TSH required by the application increases from 150 ft to 175 ft, the horsepower
required by the pump will increase 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) x Specific Gravity x Total Dynamic Head (ft)
Brake Horsepower =
3960 x Efficiency