Establish Critical Point Margin
Figure 2 shows a plot of the actual measured pressure at the BFW pump suction and the DA vapor pressure for a typical steam power plant. The difference is the available NPSHa at the pump inlet. Also note that during the DA pressure transient, the actual pressure at pump suction decreases as the DA vapor pressure decays for almost a full minute. However, due to the time it takes for this hot-water slug in the pump suction line to pass, the pump suction vapor pressure does not decrease until after the "critical point," corresponding to the BFW pump suction pipe residence time where the NPSHa is at a minimum. If the value of NPSHa at this point (critical point margin) falls below the pump NPSHr, the system design is inadequate and the pump could be damaged due to cavitation of the fluid inside the pump during the transient.
2. Good correlation with theory. The proposed simplified calculations to determine NPSH in the deaerator system give a close, conservative estimate of the more analytic approach described in the reference. Source: Bechtel Power
Options for Adding NPSH to the System
The main BFW pumps are generally large, high-energy pumps needing large amounts of NPSHr. One solution would be to raise the DA to a higher elevation to increase the NPSHa. This solution is normally not practical or cost-effective. Another approach is to install a low-speed, low-NPSH booster pump upstream of the BFW pump. The booster pump discharge pressure then provides the added NPSH required by the BFW pump. In addition, the same NPSH analysis must be made on the booster pump. The only difference is that in the case of the booster pump arrangement, the critical point and the critical point margin need to be evaluated at the booster pump suction as well as the BFW pump suction.
Additional Transient Condition
An additional transient condition that the system designer must consider occurs during a "hot start." In this situation, steam flash (water-steam mixture) can occur at the pump suction and cause cavitation damage to the pump internals. However, the mechanism causing steam flash is slightly different than what was discussed earlier.
On a plant trip, the DA pressure drops and the water temperature inside the DA drops. However, the pump and suction piping near the pump remain at a higher temperature due to the mass of the metal. As a result, when the pump is operated on a hot restart of the plant, steam flash and cavitation are likely to occur at the pump suction.
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