Protection of Boiler Feed Pump Against Transient Suction Pressure Decay

In a previous paper [1], the analysis of pump suction pressure decay utilizing the varying condensate temperature approach was first introduced. Although the paper provided power plant engineers with an accurate analytical method for determining the magnitude of feedwater pump suction pressure decay, no protective method to prevent or alleviate the pressure decay under instant turbine load rejection was mentioned. As unit size has rapidly increased in recent years, the power plant deaerator requires not only a larger storage capacity but also must be elevated higher to protect the boiler feed pumps against suction pressure decay under load rejection. Since this trend is expected to continue, the cost of supporting the power plant deaerator may soon become prohibitive. Several protective methods have been previously introduced. However, the evaluation of the methods was invariably based on the constant-condensate-temperature approach, which is not suitable for central station regenerative cycle units. This paper reevaluates those methods in general and recommends the on-off type deaerator bypass system as the most effective and economical method. The use of this protective method will not only reduce deaerator storage capacity to the absolute minimum as required solely for surge, but will also largely preclude the provision of additional static head for instant load reduction. As a result, enormous savings can be expected both from the deaerator itself and its supporting structures. The magnitude of savings in supporting structures alone may amount to more than a half million dollars for a 750 MW fossile unit. Based on the varying condensate temperature approach, mathematical equations expressing the deaerator pressure decay as well as design parameters required for designing the protective system have been derived. The paper also presents a detailed description of the recommended protective system and discusses some advantages of this system over the others. Finally, a few engineering examples are included to illustrate the application in system design.