Turbines are designed for some fixed operating conditions. When operated at those fixed condition, it is supposed to give maximum total efficiency. But off-design conditions occur several times due to system problem, environmental change, plant aging etc. Performance at those off-design conditions also affects economy of operation. Hence prediction of performance at off-design condition is a necessity.

First an 800MW steam turbine is designed for which design data is available. Available data matched with calculated data quiet satisfactorily. The same design procedure is adopted for a 210MW steam turbine. Design condition heat cycle data and some other available technical data were used for designing. Performance of the designed turbine at design condition was normal.

Three off-design conditions , varying steam flow rate, varying condenser pressure, and varying inlet steam parameters, were taken up for off-design performance study. Their effects were suitably incorporated to recalculate the performance of the same turbine at those conditions. Performance characteristics are obtained by varying one off-design condition at a time. All the off-design conditions were applied simultaneously taking from actual field condition and other performance parameters were calculated to compare with actual filed data.

Off-design performance predictions were found to be satisfactory.

Key words: off-design, performance, steam turbine, mass flow rate, condenser pressure ratio.