2.2 Steam chest materials
As stated in Section 2.1 of this chapter, the chests are manufactured from alloy steel. On current 660 MW machines operating at 565°C, the material is usually 0.5Cr Mo V steel, and on some recent machines operating at 538°C, the chest material is 2.25Cr steel.
However, on larger units operating at 565°C, it may be necessary to utilise higher-chrome ferritic steels with improved creep resistance in order to avoid unduly thick chest walls which would prejudice operational flexibility. Austenitic steels would provide a greater improvement in creep resistance but experience with these has shown that there would be problems of creep ratcheting.
The valves close onto removable-plate type seats, which can be replaced when worn or damaged. These seats are secured in place either by means of a screwed ring or by screws that are locked in place, and sometimes the seats are additionally prevented from rotating with radial dowels. A diffuser is generally provided; this is either integral with the valve seat or machined as part of the valve chest.
The mating faces of the valves and their seats are faced with Stellite in order to resist wear, mainly due to erosion by steam. Such wear is most likely to occur when the valve is cracked open: a jet of steam is then propelled at high velocity by the large pressure differential through the narrow port opening. The Stellite facing also protects against impact damage when the valves shut normally, or during frequent high speed test closures. Cushioning devices in the relay system or slow motion testing can be used to alleviate this.
The valve spindles are supported and guided by means of specially treated alloy steel sleeves in the valve covers, the materials of the spindles and the sleeves being selected both to avoid excessive oxide build-up between 30 000 h overhauls and to ensure the minimum wear of the components when subjected to the forces imposed on the valve from steam, vibration and the operating gear. The bushes are securely locked in place, the clearance between spindle and bush being suitable for the operating conditions of the valves. In order to prevent steam leakage to atmosphere, there is a gland steam leak-off sub-atmospheric connection from a chamber between the spindle and the bush to the turbine gland steam system. Steam leakage from emergency stop valves is also prevented by means of a Stellite back seat on the back of the valve head against the lower valve spindle bush.