6.2.3 Methods of manufacture and construction - part 2
Methods used in the manufacture and assembly of the individual major components which form a completed transverse underslung condenser will now be outlined under the headings listed earlier.
General structural design
The transverse underslung (bridge) condenser design (Fig 4.25) turns the top half of the condenser into a stiff bridge structure incorporating the LP cylinders and bearings, the turbine weight being carried by the bridge to the foundation piers at either end. The condenser is supported upon coil springs, and to avoid the weight of the condenser or its contents being transmitted to the turbine exhaust casing, the waterboxes and tubes are filled with water and the springs adjusted to their normal operating position by levelling bolts before welding the main exhaust joint.
The tubenest is arranged so that the tubes are transverse to the turbine axis. The operational weight of the integral bridge structure exceeds 2200 tonnes. The shell is stiffened against the vacuum load using plates and ribs which are optimised for strength and economics of fabrication.
Depending on the number of passes, transverse underslung condensers may have inlet, outlet and return waterboxes. In every case the waterbox dimensions are sufficient to allow easy access for inspection via bolted and hinged manhole covers. The major design objectives to be realised in their construction, in addition to access, are those of pressure and vacuum loading, and support.
The static and dynamic forces generated by the flowing cooling water are taken by the waterbox structure (Fig. 4.23), which is shaped to promote a smooth flow of water through the tubeplate and tubes, and prevent erosion at the inlet. Flange rotation is prevented by selecting plate sizes of adequate stiffness or, alternatively, the plates are stiffened by external reinforcement, usually in the form of ribs.
The waterbox flanges are jig or template drilled to ensure matching with the tubeplate and shell flange. Earlier waterboxes were generally protected against corrosion in service by cathodic protection, with protection bosses provided to receive anodes. It is now normal practice to offer protection against iron oxide scale by means of a rubberised coat, applied to grit-blasted surfaces prior to dispatch to site.