In 1946 a detailed study was made of the possibility of applying a special pressure-tight steel casing to the boilers for two projected 150,000 kw capability units, for the purpose of permitting operation with forced draft only. Factors influencing the decision to proceed with this type of construction are reviewed in detail. Operating histories, as affected by the pressurized construction, are reviewed for these two units and for four subsequent additional units of similar design. The principal difficulties which were encountered are enumerated and measures which were taken to overcome these difficulties are discussed. Static-pressure operating levels, auxiliary power, and thermal efficiency comparisons are given for pressure operation versus suction operation.