13.4.2 Steam-to-steam reheaters - part 4
Horizontal moisture separator reheaters
The overall size of the MSR vessels incorporating wire mesh separators was originally governed by the provision of sufficient area of horizontal mesh. It was most convenient to dispose the vessels horizontally and position the heater elements above the mesh.
The steam distribution system was below the mesh and the steam inlets were therefore positioned in the bottom of the vessel. The hot reheat connections were positioned on the top of the vessel.
With one vessel located each side of the LP cylinders at engine room floor level, the cold reheat pipework was routed from the HP turbine exhaust to the bottom of the MSR and the hot reheat pipework from the top of the vessel to the LP cylinders.
With the adoption of inclined chevron separators, it was no longer necessary to limit the steam distribution system to the lower section of the vessel. A typical schematic arrangement of a two-stage MSR is shown in Fig 2.106. The steam is introduced into the vessel through connections located at the end of the shell nearest the HP turbine.
This arrangement has the dual advantage of reducing both the length and complexity of cold reheat pipework and the associated pressure drops. The wet steam flows along the length of the vessel within the annulus formed by the shell and an internal support frame, before passing through the distribution plates and the separator elements. To achieve efficient operation, it is necessary to segregate the incoming cool steam from the reheated steam. A close-fitting shroud is therefore often provided to guide the steam from the reheater to the outlet connections. Where the hot shroud is in close proximity to the outer shell, it may require a thermal shield to prevent local heating and distortion of the shell. To cater for temperature differentials, which could cause unacceptable distortion, the earlier shroud designs were divided into a number of panels held in place by clamps, which permitted relative movement without significant leakage. In a Swedish unit, a combination of pressure differential, thermal distortion and discontinuity of shape was sufficient to cause a section of shroud to collapse away from the clamps. The section then vibrated in the steam flow and failed by fatigue. Shrouds are now designed to avoid similar failures.