Diaphragms are located in the turbine cylinder as close as practical to the horizontal and vertical axes of the machine, in order to reduce to a minimum the effects of differential expansion. Particularly when the machine is small, the loss in output arising from the internal clearances can be significant.
It is for this reason that the practice developed of supporting and guiding the diaphragms in the inner cylinder by keys in each half. This method of location, which is known as kinematic support, is generally used on current HP and IP turbines of the disc and diaphragm type, and on the early stages of LP turbines.
The details of a kinematic support system are influenced by the method of manufacture. As the adjacent surfaces of the cylinder and diaphragm are circular, the diaphragm has to be supported on keys fitted into milled recesses in both cylinder and diaphragm, as close to the horizontal joint as practical. Transverse location of the diaphragm is achieved by keys on the vertical centreline fitted into milled key-ways. Adequate clearances are provided on fitting the keys to ensure freedom of movement for differential expansion. To avoid steam leakage between the two halves of the diaphragm, the top half sits on the bottom half, and keys at the half-joint are arranged in line with, and transverse to, the machine centreline. This locates the top half-diaphragm axially and radially with the bottom half-diaphragm, and provides an effective steam seal. The keys on the bottom half horizontal joint have to be sized to take the weight of the complete diaphragm.
To enable the top half-diaphragm to be retained in the top half cylinder when it is raised, stop-plates are provided at the horizontal joint on each side. The diaphragm is located axially against a machined recess in the cylinder which offers frictional resistance to the expansion of the diaphragm relative to the cylinder. When sizing the support keys, it is usual to take into account the torque reaction on the diaphragm.
All keys have provision for adjustment, so that correct alignment of the diaphragms can be achieved.
A typical arrangement of diaphragm support and location keys is shown in Fig 1.89.