Steels_ Metallurgy and Applications, Third Edition

Stainless steels 363

Evaporator

Reheater

Super heater [ I.____...._J'~~ Turbine

'! Condenser

i ~176176 ..............

Figure 4.39 Water-steam flow in coal-fired boiler

superheater and the water being directed to the feedwater circuit. On reaching a
temperature of 540-565~ and a pressure of 170 bar in the superheater, the steam
is conveyed from the boiler to the turbine via headers, steam pipes and valves.
The headers are very large pressure vessels, with many stub tube connections,
and act as collection units for steam from the various tubes in the superheater. A
typical unit in a large power station would measure 500 mm diameter x 40 mm
wall thickness x 12 m long.
As indicated in Figure 4.39, part of the steam leaving the turbine is diverted
to the reheater section of the boiler. At this stage, the steam has lost a substantial
amount of pressure but, after reheating, the steam is then passed again into the
turbine via a header system.
The conditions described above relate to traditional coal or oil-fired stations
but in recent years, the combined cycle gas turbine (CCGT) has been introduced
in which the maximum steam temperature is only of the order of 400-450~

Steel selection

From the foregoing remarks, it can be appreciated that boiler tube materials are

subjected to a wide range of temperatures and operating conditions and economic

selection dictates that a variety of steel grades is used to satisfy these conditions.

However, one statistic worthy of note is that in the 660 MW boilers at Drax,

there are 480 km of tubing with 30 000 butt and fillet welds.

Evaporator tubing

As indicated earlier, the fire-side temperature of evaporator tubing is generally

of the order of 450"C and C-Mn steel, typically BS 3059 Grade 440, is used in

this region. However, in areas of the furnace wall where the operating conditions