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Refractory Brick in Marine Boilers Severe Operating Conditions—Selecting Brick of Good Grade and Quality—Care in Laying Wall—Reduction in Maintenance Costs given to the tendency of a brick to fracture due to thermal shock. When the changes in the tempera- ture in a furnace are rapid the ex- pansion on the face of the brick ex- posed to the fire is greater than the expansion further into the _ brick. This difference in the rates of ex- pansion and contraction (as the cycle is reversible) sets up strains in the brick which result in cracks or frac- ture. In a carefully layed wall the effects of spalling are first notitceable in the form of cracks on the face of the wall; while in a poorly layed wall, or in aggravated cases of spall- S PALLING is the term generally except in vicinity of spalling cracks. ing, corners of the brick may become completely broken off. Thermal shock may be induced by rapidly lighting off, or rapid cooling, | or by suddenly developed air leaks in the wall. Care in gradually warming up with a slow fire will reduce the tendency of a brick to spall. When a boiler is secured all openings into the furnace should be closed to pre- vent ingress of drafts of cold air. In oil fired boilers, rapidly cutting in or out large numbers of burners will produce unequal expansion with the This: is the second and concluding installment of Lieutenant Griffith’s ar- ticle on the selection, laying and care of fire brick in marine boilers. The first installment appeared in the March issue. 78 Part II By Lieut. C. A. Griffiths, U. S. Navy possibility of spalling cracks develop- ing. The selection of a fire brick that will resist spalling is very difficult as the property by which a brick will resist spalling is more or less ob- scure. The most probable factors which enter into the spalling resist- ant characteristics of a fire clay brick are those which are controllable dur- ing manufacture. After installation, the operation of the furnace and the operation of the burners in an oil fired boiler plays a most important part. Under steady steaming condi- tions spalling will not occur to such an extent as it will in fluctuating op- Fig. 4—Poor resistance to spalling. Good resistance to fusion and to shrinkage Wall well made up. Good grade of clay eration, because, during the former use, the brick becomes thoroughly soaked with heat, and the thermal Strains are reduced in proportion as soaking progresses. Adequate insula- tion backing the refractory wall will also result in a more uniform distri- bution of heat through the fire brick and reduce the tendency to develop spalling cracks. In the lower right hand side of Fig. 4 a number of cracks in the face of the brick, which otherwise had excellent resistance to fusion and good resistance to shrinkage, are a combination of shrinkage cracks and spalling cracks. The brick in this in- Stallation was used in an oil fired fur- nace up to a temperature of 3000 de- MARINE REview—April, 1931 gress Fahr. The wall was well layed with a good grade of clay, as can be observed in the left side of the figure, Spalling cracks developed when the fires were suddenly shut off and the furnace allowed to cool by the normal flow of air through the fire box. Fig. 5 illustrates the use of a poor grade of fire clay. This clay had a low softening temperature which caused it to fuse and run down the face of the brick. The bricks beneath the joints show signs of fusion which are possibly due to the fluxing action of the clay when it ran down over the lower brick. The _ brick, otherwise, shows good resistance to fusion, spall- ing and shrinkage. It is obvious that a boiler furnace wall made up with a clay of this character would fail to give satisfaction; the bonding prop- erty of the clay is poor and the wall, consequently, weakened; and, the probabilities of exceedingly large quantities of excess air entering the fire box through the cracks of the wall are great. Marine Boiler Operation Boiler operation afloat is quite prob- ably more rigorous than normal boil- er operation ashore because of the fluctuating demands made on the boiler. Once clear of port, the neces- sity for rapid changes of firing rates due to maneuvering the ship are no longer required and steady conditions comparable to those found ashore can be obtained. Certain factors in op- eration are beyond the control of the chief engineer while others can be controlled by him to a greater or less degree. It is with the controllable factors that the marine engineer is vitally interested. (a) A boiler steaming at the nor- mal load will, if properly designed, have a temperature balance in which the factors entering into it will in no case be excessive. However, when the boiler is operated at, say, 100 per cent overload, a greater quantity of fuel is burned, the boiler is being forced, and the temperatures in the fire box, uptakes, and stacks will all be decidedly greater. It is obvious that a brick wall in a forced boiler, subjected to the higher temperaturse, will receive more severe treatment than it would under normal rates of operation. The consequences of con- tinued steaming at forced rates with the resulting higher temperatures is more than liable to result in a rap