Measuring Fluid Level at Subcritical to Supercritical Pressures in Once Through Boilers

摘要

Modern once through boilers have integral steam-water separation systems that are only needed for boiler startup however must remain in service throughout the boiler's entire operating range. Therefore the steam-water separation systems in supercritical boilers need to operate at both subcritical pressures during start up and low load to supercritical pressures approaching unit full load. The concept of "level" for steam water systems has traditionally been associated with subcritical operation at saturated steam conditions, where the meaning of "level" is truly the boundary of the two different phases that are present. "Level" for supercritical fluids is somewhat abstract since the definition of supercritical fluid indicates when vapor and liquid phases are indistinguishable. Yet the "level" signal provides important operational information and is used in automatic control as well as boiler protection logic. This paper discusses the term of "subcooled boundary" as an alternative for the term "level" and presents calculation methodologies including the transition zone from subcritical to supercritical.