BHEL JOURNAL

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1. INTRODUCTION
An enhanced ecological awareness in the industrialised
countries prompted increased initiatives world over
to reduce CO2 emission levels in the power plants.
This is essentially achieved by improving the efficiency
of the plants. Figure-1 shows some possibilities of
increasing power plant efficiency [1]. In conventional


1.1 Potential Benefits
Material development work over the past two
decades has paved the way for large thermal power
plants to be built today with live steam temperatures
of 610°C, reheat temperatures of 625°C and
supercritical steam pressures. The likely potential for
reducing the heat rate by increasing the pressure and
temperature of the steam admitted to the turbine on
the basis of single and double reheating is shown in
Fig.4 [10]. At live steam conditions of 600°C and
300 bar with double reheating, for example, the heat

BHEL JOURNAL, September 2006 4
rate can be reduced by 8% compared with the heat
rate of today's standard power stations featuring
steam parameters of 540°C/180 bar and single
reheat.


1.2 Service Experience
Based on the operating data from supercritical
double reheat units in the range from 600-825 MW
plant sizes, Westinghouse and GEC reported [2] that
they achieved average availability of 80%. This is
higher than the average availability of 600-825 MW
units as a whole and is comparable to that of small
units. A VGB evaluation also shows that the
operational availability of their supercritical plants is
approximately the same as with sub-critical units [5].
Average forced outage rates for the period 1970-83
for ABB turbine operating with supercritical main
steam pressures in several countries in Europe was
only about 1% [8]. A 100 MW steam turbine has
been operating for several ten thousand hours in
USSR, as a test unit under steam conditions of 29.4
MPa and 630-650°C and reheat steam conditions of
3 MPa and 565°C. This unit was designed with
cooling of many elements of HP housing and the
operating experience shows that it is highly reliable
[9].

1.3 Present Trend
Encouraging operational history of the earlier
supercritical units, availability of more versatile
materials at a reasonable cost, progress in design
tools such as computer programmes, experience
gained in the designing and manufacture of large
steam turbines and the continuously increasing trend
of fuel cost, all together prompted several leading
power plant suppliers, in the recent past, to revive
their interest in units with advanced steam conditions.
To avoid any technical risk, the development
programmes have been planned in a phased manner.
Two such programmes were launched independently
on similar lines by the EPRI [9] and Toshiba
Corporation, Japan [7].