Some Principles of Modern Methods of Water Treatment for Steam Raising

Abstract

The paper describes some recent advances in knowledge of the physico-chemical principles on which the correct treatment of boiler feed water is based. Straub's work in America was confined mainly to the tannins, the use of which in boiler compounds is almost traditional. The effect of tannins in the boiler appears to be threefold: (1) the delay of the precipitation of calcium compounds by the action of heat or alkalies; (2) the reduction of the amount of deposit which may be due to a change in the crystal shape of calcium sulphate and calcium carbonate brought about by the presence of tannins; and (3) a marked effect in conjunction with alkali, in preventing corrosion by the formation of protective films. The principles laid down by R. E. Hall for the prevention of calcium sulphate scaling are restated. Recent work has led to a modification of the theory underlying Hall's recommendations, the significance of which is explained. A short description is given of Partridge's experiments on the growth of crystalline deposits on hot metal surfaces, but further work is necessary to explain why these deposits adhere to the metal. Recently published studies on the solubility of the calcium phosphate precipitated in the boiler, and the effects of sodium salts are described. Mention is also made of the formation of silicate scales and the use of sodium phosphate and caustic soda as preventatives. It is suggested that the work described in this paper emphasizes the fact that the prevention of scale in boilers is essentially a chemical problem, and that the success of the recommended methods of prevention depends on careful control at each stage of the feed water treatment.