Steels_ Metallurgy and Applications, Third Edition

Stainless steels 365

6.3 cm. At higher temperatures, greater wall thicknesses would be required or
else stronger steels would be selected. These would include 9% Cr-Mo-V-Nb
(T91) and 12% Cr-Mo-V (X20) which could be used in thinner sections, thereby
reducing the cost of the pipework. At the super-critical Drakelow 'C' station,
operating at 599~ bar, both Type 316 and Esshete 1250 have been used for
steam pipes.

Other components

The steam drum in the steam-water circuit between the evaporator and super-

heater is fabricated from C-Mn steel plates and presents no problems in material

selection.

The economizer tubing, situated near the exhaust end of the boiler, operates

in a gas temperature of 400~ and again a C-Mn steel is adequate for these

conditions.

Feedwater pipes are made in C-Mn-Nb steel (BS 3602 Grade 490 Nb) and

operate at temperatures up to 250~ Previously, a 0.4% C-Mn steel was used

for this purpose but BS 3602 Grade 490 Nb (0.23% C max.) is more weldable

and has a higher proof strength than the traditional medium-carbon steel.

Co-extruded tubing

As indicated earlier, C-Mn or low-alloy steels, clad with a stainless steel or

superalloys, are used in parts of the boiler where metal loss is excessive due

to high-temperature corrosion. The experience of these materials in UK power

stations is the subject of a detailed paper by Flatley et al. 35

The tubes are manufactured by conventional hot extrusion and in essence

comprise a corrosion-resistant outer layer over a conventional, load-beating inner

layer. Co-extruded tubing involving Type 310 (25% Cr, 2% Ni)/carbon steel was

first introduced into the furnace wall of a 500 MW boiler in 1974 and gave

a corrosion benefit of three to ten times, depending upon the severity of the

location. This practice has now become widespread in UK coal-fired stations. The

tube dimensions have varied from 76.2 mm OD • 8.3 mm total wall thickness

(4.3 mm outer 310 on 4 mm C steel) to 50.8 turn OD • 6 mm total wall (3 mm

outer on 3 mm inner). 35

In the superheater and reheater stages of the boiler, the combination of mate-

rials has generally involved Type 310 heat-resistant steel over Esshete 1250, the

high rupture strength steel. Co-extruded tubing of this type has been installed

in various power stations in the UK, providing a benefit of 2.5 or more over

monobloc Esshete 1250. 35 To avoid the possibility of intergranular attack due

to the migration of carbon at the service temperature, later installations have

incorporated niobium-stabilized, Type 310 material.

Flue gas desulphurization equipment

Prior to the privatization of the electricity industry, the UK Government

announced plans for the installation of flue gas desulphurization (FGD) equipment