13.4.2 Steam-to-steam reheaters - part 6
Early designs of reheater in the UK, using hairpin tubes in carbon steel, suffered extensively from erosion of the inside of the tubes, particularly at the hairpin bends, and the impact of high velocity steam from leaks in these tubes caused consequential damage to adjacent tubes. Major retubing with erosion resistant material was necessary in certain reheaters, with restrictions on internal steam flow and hence performance. Some reheaters had to be scrapped completely.
Some early American MSR designs used cupro-nickel tubes but, at elevated operating temperatures, this material produced high levels of copper in the reactor feedwater. Since carbon steel tubes suffered from erosion and had to be protected against rusting during shutdown periods, 18/8 stainless steels were provided in some continental units. This option was expensive, particularly when compared to carbon steel, and occasionally suffered from cracking caused by salts which leaked out of the condenser. Seawater could be carried around the feed circuit and since the reheater is the only significant drying unit in a wet steam cycle, sodium chloride was ultimately deposited in the reheater.
The reheater tubing in present machines is now 18% ferritic stainless steel. This has a greater tolerance both to the deposition of salts and to general corrosive and erosive conditions. Since tubeplates continue to be made from carbon steel, the tubeplate surface is normally clad with a layer of Inconel to facilitate welding of the tubes to the tubeplate. Both the tube-plate cladding process and tube welding are followed by full NOT procedures.
Present designs of reheater are sized using heat transfer and pressure drop data obtained from tests on existing units for both the internal and external surfaces of reheater tubes under operational conditions. These tests have enabled higher operating velocities to be used without impairing the integrity of the tube wall. Higher velocities lead to enhanced heat transfer coefficients and consequently permit a more compact and economic MSR design.
Current reheater tube banks are based on hairpin tubes arranged in rectangular-shaped nests as in earlier designs. In earlier machines, the limited length of tube available from the manufacturers resulted in multiple-nest arrangements and central headers. Since tube manufacturers can now provide longer tubes, headers can be placed at the ends of the vessel, making maintenance and withdrawal of tubenests more convenient.
Sufficient space must, of course, be allowed at both ends of the vessel to permit tubenest replacement: this should be a rare occurrence and might not be necessary in the life of the plant. Leaking tubes detected by increased heating steam flow would normally be taken out of service by plugging.
The tubes are supported by drilled plates spaced so that under the most extreme conditions of operation there are no harmful vibrations. The distance between tube supports is such as to limit any vibrationally-induced stresses and the effect of the gas flow across the tube to acceptable levels.
The natural frequency and mode shape of the tube, as supported in the nest, are calculated to avoid the primary modes. It is not possible to eliminate all forms of tube vibration, as successively higher modes occur at small frequency intervals and the actual frequency is influenced by factors such as alignment of the tube support plates and the amount of clearance in the support plate holes, both of which are variable over the operational life of the unit. In order to avoid binding of the tubes in their support plates, which caused tube failure in some MSR designs, it is essential that the plates maintain their alignment when the tubenest is subjected to operational temperature gradients.