|Four-tube superheater awaiting attention|
Superheaters have to be certified as passing an even more demanding test than the boiler itself. The boiler requires a 1.5 times working pressure test (412.5 psi) whereas the superheater needs to withstand 2 times working pressure (550 psi). These figures reflect the fact that Sentinels had about the highest working pressure of any steam loco on British Railways.
There is yet the boiler inspector's formal test by Peter Hawkins for certification before the superheater can be installed in the boiler (and one or two (hundred) other things in the meantime!). It's all progress!