6.2.3 Methods of manufacture and construction - part 4
The provision for double tubeplates has been mandatory for UK power station contracts over the past years to eliminate any possibility of cooling water entering the steam space of the condenser. Each tubeplate interspace is drained to a low level drain vessel which is maintained at the condenser absolute pressure by means of a connection to the air pump suction line.
This equalisation of pressure on either side of the inner tubeplate ensures that no leakage occurs across it. Therefore, any leakage into the interspace will be cooling water, which is drained to waste. The tubeplate interspace can be hydraulically tested to a pressure of 0.7 bar during maintenance overhaul via 25 mm water connections on the tubeplate periphery. A vent is fitted at the top of the tubeplate interspace to ensure that it is full of water.
The formulae and rules governing the sizing of tubeplates have been based on the following standards and codes which have been progressively amended in line with tubeplate developments:
- BS1500 .
- BS1515 Parts 1 and 2 .
- ТЕМА Standards: Tubular Exchanger Manufacturers Association Standards, Fourth Edition 1959, Fifth Edition 1968 .
- ASME VIII Standards: American Society of Mechanical Engineers — Boiler and Pressure Vessel Code Section 8, Divisions 1 and 2 .
The wider use of computer techniques in the design of tubeplates is presently being researched. An example which has been used is a beam element program which is based on the theory of beams on elastic foundations, and analyses the effects of pressure and edge loading from the waterboxes.
The methods of manufacturing of the tubeplates vary. The steel (inner) tubeplate is often drilled first and used as the jig for drilling the non-ferrous (outer) tubeplate. Sometimes, however, inner and outer tube-plates are drilled together, even where they are of dissimilar materials, to ensure accurate tube-hole matching between the two tubeplates. The reason for using dissimilar materials is purely one of economy; however, as already mentioned, it is a requirement that non-ferrous materials are not to be used for the inner tubeplate of a PWR design. Bolt hole drilling of the tubeplate is carried out either by a jig or template to ensure matching with the waterbox and shell flange holes.
The joint between the inner steel tubeplate and the condenser shell is crucial and requires particular care as it cannot be easily renewed once tubing is complete. Figure 4.26 shows a typical detail of a bellows and flange joint. This joint is sealed with a neoprene-proofed fibre cloth and cord-filled dumb-bell gasket which is manufactured in one piece. The bolt holes in the tubeplate are jointed up with a water test groove, and a drilling from the hydraulic connection on the tubeplate periphery breaks into this groove, enabling the joint to be hydraulically tested prior to tubing. Access for tubeplate inspection may be gained from the waterbox, which is fitted after all tube fixings are complete. After complete erection, when all tube ends have been expanded and packed, the space between the tubeplates is filled with a fluorescent solution under pressure, and the tubeplates examined from both sides under ultraviolet light to prove the absence of leakage.