A series of upright boiler tubes periodically exposed to the abrasive action of a stream of blowing fluid cleaning medium. Vertically spaced shields mounted on the boiler tubes for protection from the fluid cleaning medium. A split ring casting located between the protective shields to maintain the boiler tubes in an aligned and fixed spaced relationship. The split ring casting is sized to overlap adjacent portions of the protective shields thereby covering any gaps exposing the boiler tubes to the action of the fluid cleaning medium.