To determine operating cost changes as a function of time, it is necessary to be able to determine state-line efficiencies and flow quantities with some level of confidence that the values are sufficiently accurate to be used for making maintenance and repair decisions.

The expansion efficiency of a steam path, if the total expansion is in the superheat region, can be determined with considerable accuracy by means of an enthalpy drop test using high quality instrumentation to measure steam pressure and temperature. (Cotton 1993) Such a test will establish steam conditions at entry to and exit from a section. A more difficult parameter to establish is the expansion efficiency in a low-pressure section, and only by a detailed audit can this be determined. Even after such an audit, the results leave some degree of uncertainty as to the exact value, and for an audit to be conducted the unit needs to be open for a detailed examination. However, the results of an audit can be used with some level of confidence. But in an operating unit, it is generally the experience of station personnel that is relied upon to determine, or estimate, the low-pressure section condition, normally from past experience and observed changes in operating parameters.

The flow quantities to each section and the quantities being extracted are other parameters that are difficult to establish with any degree of accuracy, but if calculated values taken from the heat balance are used, an estimate...