Page 285 - Intro Predictive Maintenance

276 An Introduction to Predictive Maintenance

15 HP

65% 70% 75%
20 HP
200 BEP
80%
20 HP
75% 15 HP
80%
Total Dynamc Head (Feet) 100 65%
150
70%

50

100 200 300 400 500 600 700 800 1000

Figure 13–5 Brake horsepower needs to change with process parameters.

under specific operating conditions. For example, a pump with a discharge pressure
of 100psig and a positive pressure of 10psig at the suction will have a TDH of
90psig.

Most pump hydraulic curves define pressure to be TDH rather than actual discharge
pressure. This consideration is important when evaluating pump problems. For
example, a variation in suction pressure has a measurable impact on both discharge
pressure and volume. Figure 13–3 is a simplified hydraulic curve for a single-stage
centrifugal pump. The vertical axis is TDH, and the horizontal axis is discharge
volume or flow.

The best operating point for any centrifugal pump is called the best efficiency point
(BEP). This is the point on the curve where the pump delivers the best combination
of pressure and flow. In addition, the BEP defines the point that provides the most
stable pump operation with the lowest power consumption and longest maintenance-
free service life.

In any installation, the pump will always 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 it is not, some operating-cost penalty will
result from the pump’s inefficiency. This is often unavoidable because pump selection
is determined by choosing from what is available commercially as opposed to select-
ing one that would provide the best theoretical performance.