Grooving. Grooving is probably the result of straining, springing, or buckling of the plates, aided by local corrosion or by the same forces which cause pitting. Straining of the plates may be due to insufficient or improper staying, thus causing the plates to spring back and forth as the steam pressure varies. This phenomenon is most commonly found in stationary boilers of the "Cornish" or "Lancashire" types appearing in the flat end-plates around the edge of the angle iron, or in the root of the angle iron. Too rigid staying of the ends by gussets or diagonal stays, or too great a difference in expansion between different parts, is almost sure to produce grooves.
Internal grooving may be caused as the direct result of excessive calking, which, by injuring the surface of the metal, exposes it to the corrosive action of the feed-water. It is to be expected that if strains which cause the plates to come and go are set up in the boiler— especially if the stresses can be concentrated along a definite line—a weakness will be developed there, and it will be a susceptible point for chemical attack. Sometimes grooving is so fine as to appear to be a mere crack. But the crack, although perhaps only 1/64 inch in width, may extend into the plate for a -considerable depth. Grooves are not readily detected, and if allowed to continue for any length of time are likely to produce serious results.
Prevention. The best way to prevent internal corrosion is to use water that has no corrosive effect on the plates. If internal corrosion has begun, a change of feed-water may prolong the life of the boiler, but in many instances it is cheaper to build a new boiler than frequently to change the water supply. Sometimes the introduction of a thicker plate at places where the water is found to be most active will be advisable; but, as these plates are stronger than the rest of the boiler, the strains will not be uniformly distributed, and stresses are likely to concentrate along the edge of this heavy plate, which will be a susceptible point for the formation of grooves.
The acidity of the feed-water may be neutralized by some alkaline substance, such as soda, before it enters the boiler. The amount of soda to be used varies with the acidity of the water; but it should always be used in the smallest possible quantity, as the soda is likely to produce priming in the boiler and will be injurious if there is much salt present. Vegetable oils should not be used for cylinder lubrication if the condensation is to be fed back to the boiler, as such oils contain acids which will always produce injurious effects. Mineral
oils alone should be used.
To allow for a general corrosion, 1/16 inch to 3/16 inch extra thickness of shell should be provided. All seams of a boiler should be tight, and no welded tubes should be used, as pitting and grooving are likely to occur in the vicinity of the weld. When not in use, no moist air should be allowed in the boiler. A boiler can be thoroughly dried out either by the application of heat or by placing in it lime, which will readily absorb the moisture.
The water fed to the boiler should be thoroughly filtered to remove as much grease as possible, for, although mineral oil is not likely to cause pitting, it has a serious effect in the formation of boiler scale.
Incrustation.The incrustation formed by the accumulation or the deposit of sediment in the feed water, is called scale or sludge. The solid matter in the feed-water may be precipitated by the rise in temperature, or left behind as the result of the evaporation of the water. These solids, unless blown out, are liable to become hardened on the inner surface of the boiler. A thin coating of scale in itself is beneficial, for it keeps the water from direct contact with the iron, and prevents corrosion and pitting; but the danger is that if a thin scale forms, a thicker one will form, and this heavy scale, being a poor conductor of heat, not only causes considerable waste of fuel, but allows the plates next the furnace to become overheated, with the result that they are likely to give way, and the boiler may collapse.
The amount of solid matter in solution is measured in grains per U. S. gallon. The quantity varies greatly in waters from different sources, but is seldom over 40 grains per gallon. It is not the quantity of matter in solution, but its nature, that determines the influence of feed-water. With proper attention to the boiler, the presence of a certain amount of carbonate or sulphate of soda would not be injurious ; while the same number of grains per gallon of salts of lime would cause serious trouble. Salts of lime (calcium), together with carbonate of magnesia, are the solids most frequently found, and are the most troublesome. Hard water contains considerable quantities of lime. So-called soft water has usually but little solid matter in suspension, but it may contain vegetable or organic impurities that will cause corrosion or pitting.