Find the NPSH margin
The deaerator is installed at some elevation above the BFW pump to provide the NPSH required by the pump. By definition, the NPSHr is the total suction head over and above the vapor pressure of the liquid pumped.
The DA elevation minus the dynamic losses in the BFW suction piping between the DA and the BFW pump equals the NPSH available (NPSHa) to the pump. The difference between the value of the NPSHa and that required (NPSHr) by the pump gives the NPSH margin.
The NPSH margin or the NPSH margin ratio (NPSHa/NPSHr) is an important factor in ensuring adequate service life of the pump and minimizing noise, vibration, cavitation, and seal damage. The NPSH margin requirement increases as the suction energy level (for example, high suction specific speed, high peripheral velocity of impeller, and the like) of the pump increases. In the case of the BFW pump, this ratio could be in the range of 1.8 to 2.5. These margins are typically based on steady-state operation.
In addition, the NPSH margin improves the ability of the BFW pump to handle a DA pressure transient. Once a design is determined to have an adequate NPSH margin, the next step is to determine if the NPSH margin is adequate during a pressure transient.
Expect Deaerator Pressure Decay
Immediately after a steam turbine generator trip, turbine extraction steam is no longer available to the deaerator, resulting in decay of the DA pressure. Also during a sudden steam turbine generator load reduction, the extraction steam pressure decreases until the extraction stage supplying the DA can no longer maintain DA pressure. This also results in DA pressure decay as the lower-temperature condensate continues to enter the DA, cooling the stored feedwater.
The decrease in DA pressure causes some of the water in the DA storage tank to flash to steam until saturation pressure is reached at the new DA pressure. The water in the BFW pump suction line has a static head exerted on it by the level in the DA storage tank, preventing it from flashing immediately. Therefore, the water in the suction line can be considered as a slug of hot fluid that must be moved through the pump in some finite amount of time. In other words, the pump will not perceive a decrease in vapor pressure (or a decrease in water temperature) until the entire slug of hot water has passed through the pump.
During the passage of the hot-water slug, the combination of high vapor pressure at the pump suction along with a decrease in pump suction pressure (due to DA pressure decay), results in a "critical point" at which the suction pressure may drop below the minimum required pressure (that is, the vapor pressure of the hot-water slug plus the pressure equivalent of the NPSHr). This low suction pressure could result in cavitation damage to the pump internals due to insufficient net positive suction head.
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