6.3.4 Use of thermal skirts and piston rings
To allow for differences in temperature where the inlet pipes enter the cylinder, the HP and IP pipes are attached to the outer casing through flanged thermal skirts (Figs 1.92 and 1.94). The skirt, which is bolted to the outer casing, reduces the conduction of heat from the inlet pipe to the casing and accommodates the differential expansion between the hot pipe and the cooler casing by elastic deformation.
It also carries the pipe forces into the outer casing; minimising the load on the inner. The outer casing is reinforced to minimise stresses and thermal discontinuities at the joint. Piston rings are used to seal the joint between the pipe and the inner casing; these allow for differential expansion in the radial direction between the casings and a small degree of transverse movement.
The piston rings may be of the conventional split type, housed in a groove, or unsplit rings in the 'stacked' ring design. The split type may be 'outspring-ing', housed in grooves in the pipe and springing out against the casing; or 'inspringing', which are housed between spacers in the casing and spring in to seal against the pipe. The 'stacked' design uses unsplit rings stacked in the gap between the pipe and the casing. They are not sprung against either surface but are a close tolerance against the pipe and the casing alternately (Fig 1.100). Several rings are used in series to reduce the pressure drop across each, and hence reduce steam leakage; however, there should not be too many rings, as a lightly loaded ring can vibrate and wear excessively. The split rings are made of a high temperature creep-resistant material, such as Stellite or a Nimonic alloy, in order to retain their springiness; the unsplit rings are made from a low alloy steel. Stellite-coated bearing surfaces or Stellite inserts in the casings reduce wear.
The exhaust steam connections are attached directly to the outer casing. When attached to the lower casing, where they are unlikely to need disconnecting, they are simply welded to stubs on the casing.