2.7 Loop pipework
The steam chests are connected to the turbine through loop pipes. On present machines these are arranged to be relatively short, in order to reduce the quantity of entrained steam between the governing valves and the turbine when the valves are shut.
On earlier (500 MW) machines, where the chests are firmly anchored to the supporting steelwork or the concrete foundations, the loop pipes are long and flexible in order to allow the turbine and the pipes to expand freely without undue forces/moments on the turbine cylinder.
All loop pipe joints with the steam chests are welded, as are all other loop pipe joints, except where dismantling flanges (see below) are provided. The loop pipes enter the cylinders, preferably radially, at the top and bottom of the cylinders. On many current machines, the loop pipes from each side combine to form one inlet into each half of the cylinder by means of a tee-piece connection. The loop pipes into the top half cylinder are provided with dismantling flanges for maintenance purposes, used when the cylinder is opened up. These flanges have a gasket-type joint that has a design life of 30 000 h between replacements. It is important that adequate provision for drainage of the loop pipes is made, to ensure that there is no risk of water entering the turbine during start-up. Where the pipes or the tee-pieces enter the double-shell cylinder, provision is made on the pipe connecting to the cylinder for a thermal sleeve which secures the pipe to the outer cylinder. The loop pipe passes to the inner cylinder and the expansion between the pipe and cylinder is allowed for by a sliding joint on the outside of the pipe, using either stacked continuous rings or split piston rings. A more detailed description of this type of joint is given in Chapter 1, Section 3.