Abstract
In recent years a number of failures have occurred in certain portions of boilers operating in the pressure range near 1400 psi. Many of these failures have occurred under conditions such that oxygen corrosion or scale could not be considered as the cause. Characteristically the tubes have shown internal attack along the top, generally with definite grooving and the production of black magnetic iron oxide.
The fundamental cause of this type of failure is steam blanketing. In a tube which is only slightly inclined to the horizontal, the steam inevitably tends to rise to the top and move along it. This introduces a considerable resistance to the flow of heat from the tube wall to the fluid, and causes the top of the tube to become more or less seriously overheated. Boiler water of normal alkalinity passing through the tube and touching the overheated surface is concentrated to an excessive degree, with resultant attack on the steel. As has been shown by Berl, the rate of oxidation of steel by water at high boiler temperatures is greatly accelerated by increase in concentration of sodium hydroxide.
Condensed case histories of occurrences of this type of failure in a number of high-pressure boilers are presented. These histories include failures in ash screens in dry-bottom furnaces, floor tubes in slag-bottom furnaces, and slag-screen and top-row tubes in straight-tube cross-drum boilers.
From certain of the case histories it is evident that the dissolution of the steel along the top of steam-blanketed tubes may be stopped or greatly retarded by eliminating caustic alkalinity from the boiler water. This, however, does not affect the fundamental cause, and may merely postpone trouble, as indicated by other case histories in which steam blanketing has resulted not in dissolution of the steel, but in serious cracking.
It is concluded that the best chance of obviating the difficulties which may result from steam blanketing is so to design the surfaces exposed to high rates of heat input that steam blanketing cannot occur, or where slightly inclined tubes must be used, to protect them so that the rate of heat input will be sufficiently low to avoid excessive overheating of the metal.
