Analysis of Feedwater Pump Suction Pressure Decay Under Instant Turbine Load Rejection

Abstract

As steam-electric generating plant unit sizes increase, and as the capacities of the feedwater pumps increase, the design provision of an adequate suction for the safe operation of the feedwater pumps becomes increasingly important, especially when the pumps take suction from a deaerator within the regenerative cycle. The design provision of sufficient suction head for feedwater pumps operating under a maximum and stable turbine load is rather simple. However, the design of an adequate suction head to meet an instant turbine load rejection from maximum load is rather complex. It deals with the rate of pressure decay within the deaerator, mass of condensate stored, size of suction pipes, rate of change of condensate temperatures entering the deaerator, and numerous other transient parameters. Previously published literature [1–5] offers approximate solutions which assumed a constant temperature of condensate entering the deaerator during transient opertion under load rejection. Such an assumption generally results in an unnecessarily conservative design. Particularly, it may not be economically justifiable for very large units to be designed on that basis. This paper presents a complete analysis of this transient operating condition for the purpose of designing an economic optimum feedwater pump suction system for safe and reliable operation. Varying temperatures of the condensate entering the deaerator during transient operation are considered. Mathematic derivations augmented by numerical examples are included, to facilitate system design.