Based on field data

the drum urgent offlet explosion of a subcritical pressure boiler is investigated in detail. The cause

developing of the explosion and the strange phenomenon of two broken holes on the same tube are interpreted. It is pointed out that the cause leading to the explosion is the thin wall of the tube. The wall of the tube becomes thinner and thinner subject to its inner fluid erosion. Until the time the explosion occurred

it is thick enough to endure the inner fluid pressure.Also pointed out is that the designing safety factor of the erosion and corrosion of the tube thickness seemed to be too small.