8 Flange sealing system
8.1 Function and system layout
The cylinder casings of the turbine each consist of top and bottom halves; the top halves may be lifted clear for inspection and maintenance. The two halves of each casing are bolted together at the horizontal flange (see Fig 2.75).
During turbine starting, the relatively thin walls of the HP and IP casings tend to heat up more rapidly than the large masses of metal in the horizontal joint flanges. The function of the flange heating system is to provide steam heating (at appropriate conditions) to the HP and IP outer cylinder flanges, thereby re¬ducing the difference in temperature between each of the cylinder casings and their associated flanges and bolts. This prevents excessive thermal stressing and cylinder distortions.
There are two flange heating systems, one supplying the HP and the other supplying the IP. Although they are completely separate systems, both are similar in function and layout and a typical flange heating system is shown in Fig 2.76.
The flanges on the HP cylinder are supplied with steam from after the HP governing valves and the IP flanges are supplied with steam from after the IP governing valves.
Each cylinder flange heating supply line is provided with an isolating valve, a preset regulating valve or orifice and a permanent drain controlled by an orifice plate. The drain incorporates a dirt trap which pre¬vents a blockage of the hole in the orifice plate. Each supply line separates into two, feeding the right-hand flange and the left-hand flange.
Between adjacent flange bolt holes, there are al¬ternate top and bottom cross-drillings which provide passages enabling the hot steam to flow freely through the flange. This is shown diagrammatically in Fig 2.77.
The heating steam enters and leaves the flanges at several points by way of drillings from the flange outer surface into the bolt holes, the inlets and exits being suitably spaced to give even heating throughout the flanges. The annular spaces between the bolts and holes are swept throughout their length by the hot steam, thus preventing the accumulation of water in the recesses.
Steam leaves each flange from outlet pipes which converge into one pipe, fitted with a flow regulating orifice.
Subsequently, the combined flows from each side of the turbine are joined and led either to a suitable feedwater heater or to a drains receiver.