POWER PLANT STEAM GENERATION
Steam Generators (Boilers). For collateral reading and further detailed
information, see (1) Steam/Its Generation and Use, by Babcock & Wilcox, 1978 and (2)
Combustion/Fossil Power Systems, by Combustion Engineering, Inc, 1981.
Steam Pressures and Temperatures
Rated Pressure and Temperature. The boiler shall be specified for the maximum
operating steam pressure required at the superheater outlet for operation of the turbine
turbine throttle valve inlet plus the main steam line pressure drop (between the
superheater outlet and turbine throttle valve inlet at the maximum continuous rating of
the boiler) rounded to the next higher unit of 5 psi (34 kPa gage). Based on the
safety valve pressure settings in accordance with the ASME Boiler and Pressure Vessel
The boiler shall be specified for the maximum steam temperature required at
the superheater outlet for operation of the turbine generator. The specified
temperature is equal to the sum of the operating temperature at the turbine throttle
valve inlet plus the main steam temperature drop (between the superheater outlet and
turbine throttle valve inlet) with the sum rounded out to the next higher unit of 5
Maximum Allowable Working Pressure. The maximum allowable working pressure
(MAWP) of a boiler is an absolute limit of pressure in psig at which a boiler is
permitted to operate. The ASME Boiler and Pressure Vessel Code states that no boiler
shall be operated at a pressure higher than the MAWP except when the safety valve or
Safety Valves and Safety Relief Valves. In accordance with the rules of the
ASME Boiler and Pressure Vessel Code, one or more safety valves on the boiler shall be
set at or below the MAWP. If additional safety valves are used, the highest pressure
setting shall not exceed the MAWP by more than 3 percent. The capacity of all safety
valves or safety relief valves for each boiler shall be such that the valves will
discharge all the steam that can be generated without allowing the pressure to rise more
than 6 percent above the highest pressure at which any valve is set and in no case
higher than 6 percent above the MAWP.
Normal Operating Pressure. In order to avoid excessive use and wear of safety
or safety relief valves, the maximum boiler operating pressure in the boiler steam drum
or at the superheater outlet is usually not greater than 95 percent of the lowest set
pressure of the relief valves at these points. This allows operation of the boiler
below the blowdown range of the safety valves, which is usually 3 to 4 percent of the
Natural Gas Firing. For natural gas characteristics and application, see
NAVFAC MIL-HDBK-1003/6, Sections 5 and 9.
Fuel Oil Firing. For fuel oil characteristics, application, handling,
storage, and burning, see MIL-HDBK-1003/6, Sections 5 and 9.
Coal Firing. For characteristics, application, handling, and
coal, see MIL-HDBK-1003/6.
Definitions of Boiler and Stoker Criteria.
Stoker Grate Burning Rate. Burning rate is the higher heating value (in Btu)
of the type of coal used; multiplied by the number of pounds of coal burned per hour to
obtain the rated boiler capacity; divided by the total active burning area, in square
feet, of the stoker grate. The maximum values shown are based on the assumption that
furnace walls are water cooled, that there is adequate furnace volume, and that the most
desirable type of coal for the unit is used; in the absence of these conditions, values