Dear Mantony,

I am new to this forum. Please read  the following

a) In case of cup-in-shell type tanks (main tank sitting inside outer tank), there is a possibility of buoyancy on the inner tank bottom, specially when empty. So check the horizontality of the inner tank bottom shell before and after the hydrotest.  I do not think such horizontality (across the big bottom shell) is generally measured accurately, but still bulges are visible and recorded.

I am a witness to a save, when, as part of the pre-comm efforts, the manhole was cut on outer and inner vertical shells,  the outer annulus (annular vertical space between the two tanks) was being washed with water to clear the residual ammonia vapors, and by chance the water leaked into the bottom space of inner tank, it was the sudden bulge emerging from the inner shell bottom, that