Aside from the physical properties by which a fire brick is judged, it is sometimes customary to require a chemical a.n.a.lysis of the brick. Such an a.n.a.lysis is only necessary as determining the amount of total basic fluxes (K_{2}O, Na_{2}O, CaO, MgO and FeO). These fluxes are ordinarily combined into one expression, indicated by the symbol RO. This total becomes important only above 0.2 molecular equivalent as expressed in ceramic empirical formulae, and this limit should not be exceeded.[75]
From the nature of fire brick, their value can only be considered from a relative standpoint. Generally speaking, what are known as first-grade fire brick may be divided into three cla.s.ses, suitable for various conditions of operation, as follows:
Cla.s.s A. For stoker-fired furnaces where high overloads are to be expected or where other extreme conditions of service are apt to occur.
Cla.s.s B. For ordinary stoker settings where there will be no excessive overloads required from the boiler or any hand-fired furnaces where the rates of driving will be high for such practice.
Cla.s.s C. For ordinary hand-fired settings where the presumption is that the boilers will not be overloaded except at rare intervals and for short periods only.
Table 61 gives the characteristics of these three cla.s.ses according to the features determining the quality. This table indicates that the hardness of the brick in general increases with the poorer qualities.
Provided the hardness is sufficient to enable the brick to withstand its load, additional hardness is a detriment rather than an advantage.
TABLE 61
APPROXIMATE CLa.s.sIFICATION OF FIRE BRICK
________________________________________________________________________ | | | | | | Characteristics | Cla.s.s A | Cla.s.s B | Cla.s.s C | |_____________________|________________|________________|________________| | | | | | | Fuse Point, Degrees | Safe at Degrees| Safe at Degrees| Safe at Degrees| | Fahrenheit | 3200-3300 | 2900-3200 | 2900-3000 | | | | | | | Compression Pounds | 6500-7500 | 7500-11,000 | 8500-15,000 | | | | | | | Hardness Relative | 1-2 | 2-4 | 4-6 | | | | | | | Size of Nodules | Medium | Medium to |Medium to Large | | | | Medium Large | | | | | | | | Ratio of Nodules | High | Medium to High | Medium Low | | | | | to Medium | |_____________________|________________|________________|________________|
An approximate determination of the quality of a fire brick may be made from the appearance of a fracture. Where such a fracture is open, clean, white and flinty, the brick in all probability is of a good quality. If this fracture has the fine uniform texture of bread, the brick is probably poor.
In considering the heavy duty of brick in boiler furnaces, experience shows that arches are the only part that ordinarily give trouble. These fail from the following causes:
Bad workmanship in laying up of brick. This feature is treated below.
The tendency of a brick to become plastic at a temperature below the fusing point. The limits of allowable plastic temperature have already been pointed out.
Spalling. This action occurs on the inner ends of combustion arches where they are swept by gases at a high velocity at the full furnace temperature. The most troublesome spalling arises through cold air striking the heated brickwork. Failure from this cause is becoming rare, due to the large increase in number of stoker installations in which rapid temperature changes are to a great degree eliminated. Furthermore, there are a number of brick on the market practically free from such defects and where a new brick is considered, it can be tried out and if the defect exists, can be readily detected and the brick discarded.
Failures of arches from the expansive power of brick are also rare, due to the fact that there are a number of brick in which the expansion is well within the allowable limits and the ease with which such defects may be determined before a brick is used.
Failures through chemical disintegration. Failure through this cause is found only occasionally in brick containing a high percentage of iron oxide.
With the grade of brick selected best suited to the service of the boiler to be set, the other factor affecting the life of the setting is the laying. It is probable that more setting difficulties arise from the improper workmanship in the laying up of brick than from poor material, and to insure a setting which will remain tight it is necessary that the masonry work be done most carefully. This is particularly true where the boiler is of such a type as to require combustion arches in the furnace.
Red brick should be laid in a thoroughly mixed mortar composed of one volume of Portland cement, 3 volumes of unslacked lime and 16 volumes of clear sharp sand. Not less than 2 bushels of lime should be used in the laying up of 1000 brick. Each brick should be thoroughly embedded and all joints filled. Where red brick and fire brick are both used in the same wall, they should be carried up at the same time and thoroughly bonded to each other.
All fire brick should be dry when used and protected from moisture until used. Each brick should be dipped in a thin fire clay wash, "rubbed and shoved" into place, and tapped with a wooden mallet until it touches the brick next below it. It must be recognized that fire clay is not a cement and that it has little or no holding power. Its action is that of a filler rather than a binder and no fire-clay wash should be used which has a consistency sufficient to permit the use of a trowel.
All fire-brick linings should be laid up four courses of headers and one stretcher. Furnace center walls should be entirely of fire brick. If the center of such walls are built of red brick, they will melt down and cause the failure of the wall as a whole.
Fire-brick arches should be constructed of selected brick which are smooth, straight and uniform. The frames on which such arches are built, called arch centers, should be constructed of batten strips not over 2 inches wide. The brick should be laid on these centers in courses, not in rings, each joint being broken with a bond equal to the length of half a brick. Each course should be first tried in place dry, and checked with a straight edge to insure a uniform thickness of joint between courses. Each brick should be dipped on one side and two edges only and tapped into place with a mallet. Wedge brick courses should be used only where necessary to keep the bottom faces of the straight brick course in even contact with the centers. When such contact cannot be exactly secured by the use of wedge brick, the straight brick should lean away from the center of the arch rather than toward it. When the arch is approximately two-thirds completed, a trial ring should be laid to determine whether the key course will fit. When some cutting is necessary to secure such a fit, it should be done on the two adjacent courses on the side of the brick away from the key. It is necessary that the keying course be a true fit from top to bottom, and after it has been dipped and driven it should not extend below the surface of the arch, but preferably should have its lower ledge one-quarter inch above this surface. After fitting, the keys should be dipped, replaced loosely, and the whole course driven uniformly into place by means of a heavy hammer and a piece of wood extending the full length of the keying course. Such a driving in of this course should raise the arch as a whole from the center. The center should be so constructed that it may be dropped free of the arch when the key course is in place and removed from the furnace without being burned out.
[Ill.u.s.tration: A Typical Steel Casing for a Babc.o.c.k & Wilc.o.x Boiler Built by The Babc.o.c.k & Wilc.o.x Co.]
Care of Brickwork--Before a boiler is placed in service, it is essential that the brickwork setting be thoroughly and properly dried, or otherwise the setting will invariably crack. The best method of starting such a process is to block open the boiler damper and the ashpit doors as soon as the brickwork is completed and in this way maintain a free circulation of air through the setting. If possible, such preliminary drying should be continued for several days before any fire is placed in the furnace. When ready for the drying out fire, wood should be used at the start in a light fire which may be gradually built up as the walls become warm. After the walls have become thoroughly heated, coal may be fired and the boiler placed in service.
As already stated, the life of a boiler setting is dependent to a large extent upon the material entering into its construction and the care with which such material is laid. A third and equally important factor in the determining of such life is the care given to the maintaining of the setting in good condition after the boiler is placed in operation.
This feature is discussed more fully in the chapter dealing with general boiler room management.
Steel Casings--In the chapter dealing with the losses operating against high efficiencies as indicated by the heat balance, it has been shown that a considerable portion of such losses is due to radiation and to air infiltration into the boiler setting. These losses have been variously estimated from 2 to 10 per cent, depending upon the condition of the setting and the amount of radiation surface, the latter in turn being dependent upon the size of the boiler used. In the modern efforts after the highest obtainable plant efficiencies much has been done to reduce such losses by the use of an insulated steel casing covering the brickwork. In an average size boiler unit the use of such casing, when properly installed, will reduce radiation losses from one to two per cent., over what can be accomplished with the best brick setting without such casing and, in addition, prevent the loss due to the infiltration of air, which may amount to an additional five per cent., as compared with brick settings that are not maintained in good order. Steel plate, or steel plate backed by asbestos mill-board, while acting as a preventative against the infiltration of air through the boiler setting, is not as effective from the standpoint of decreasing radiation losses as a casing properly insulated from the brick portion of the setting by magnesia block and asbestos mill-board. A casing which has been found to give excellent results in eliminating air leakage and in the reduction of radiation losses is clearly ill.u.s.trated on page 306.
Many attempts have been made to use some material other than brick for boiler settings but up to the present nothing has been found that may be considered successful or which will give as satisfactory service under severe conditions as properly laid brickwork.
BOILER ROOM PIPING
In the design of a steam plant, the piping system should receive the most careful consideration. Aside from the constructive details, good practice in which is fairly well established, the important factors are the size of the piping to be employed and the methods utilized in avoiding difficulties from the presence in the system of water of condensation and the means employed toward reducing radiation losses.
Engineering opinion varies considerably on the question of material of pipes and fittings for different cla.s.ses of work, and the following is offered simply as a suggestion of what const.i.tutes good representative practice.
All pipe should be of wrought iron or soft steel. Pipe at present is made in "standard", "extra strong"[76] and "double extra strong"
weights. Until recently, a fourth weight approximately 10 per cent lighter than standard and known as "Merchants" was built but the use of this pipe has largely gone out of practice. Pipe sizes, unless otherwise stated, are given in terms of nominal internal diameter. Table 62 gives the dimensions and some general data on standard and extra strong wrought-iron pipe.
TABLE 62
DIMENSIONS OF STANDARD AND EXTRA STRONG[76]
WROUGHT-IRON AND STEEL PIPE
_______________________________________________________________ | | | | | | Diameter | Circ.u.mference | | |__________________________|__________________________| | | | | | | | |External| Internal |External| Internal | | |Standard|_________________|Standard|_________________| | | and | | | and | | | | Nominal | Extra |Standard| Extra | Extra |Standard| Extra | | Size | Strong | | Strong | Strong | | Strong | |_________|________|________|________|________|________|________| | | | | | | | | | 1/8 | .405 | .269 | .215 | 1.272 | .848 | .675 | | 1/4 | .540 | .364 | .302 | 1.696 | 1.144 | .949 | | 3/8 | .675 | .493 | .423 | 2.121 | 1.552 | 1.329 | | 1/2 | .840 | .622 | .546 | 2.639 | 1.957 | 1.715 | | 3/4 | 1.050 | .824 | .742 | 3.299 | 2.589 | 2.331 | | 1 | 1.315 | 1.049 | .957 | 4.131 | 3.292 | 3.007 | | 1-1/4 | 1.660 | 1.380 | 1.278 | 5.215 | 4.335 | 4.015 | | 1-1/2 | 1.900 | 1.610 | 1.500 | 5.969 | 5.061 | 4.712 | | 2 | 2.375 | 2.067 | 1.939 | 7.461 | 6.494 | 6.092 | | 2-1/2 | 2.875 | 2.469 | 2.323 | 9.032 | 7.753 | 7.298 | | 3 | 3.500 | 3.068 | 2.900 | 10.996 | 9.636 | 9.111 | | 3-1/2 | 4.000 | 3.548 | 3.364 | 12.566 | 11.146 | 10.568 | | 4 | 4.500 | 4.026 | 3.826 | 14.137 | 12.648 | 12.020 | | 4-1/2 | 5.000 | 4.506 | 4.290 | 15.708 | 14.162 | 13.477 | | 5 | 5.563 | 5.047 | 4.813 | 17.477 | 15.849 | 15.121 | | 6 | 6.625 | 6.065 | 5.761 | 20.813 | 19.054 | 18.099 | | 7 | 7.625 | 7.023 | 6.625 | 23.955 | 22.063 | 20.813 | | 8 | 8.625 | 7.981 | 7.625 | 27.096 | 25.076 | 23.955 | | 9 | 9.625 | 8.941 | 8.625 | 30.238 | 28.089 | 27.096 | | 10 | 10.750 | 10.020 | 9.750 | 33.772 | 31.477 | 30.631 | | 11 | 11.750 | 11.000 | 10.750 | 36.914 | 34.558 | 33.772 | | 12 | 12.750 | 12.000 | 11.750 | 40.055 | 37.700 | 36.914 | |_________|________|________|________|________|________|________|
__________________________________________________________ | | | | | | | | Length | | | | Internal | of | Nominal Weight | | | Transverse |Pipe in | Pounds per | | | Area |Feet per| Foot | | |_____________________| Square |_________________| | | | |Foot of | | | | Nominal | Standard | Extra |External|Standard| Extra | | Size | | Strong |Surface | | Strong | |_________|__________|__________|________|________|________| | | | | | | | | 1/8 | .0573 | .0363 | 9.440 | .244 | .314 | | 1/4 | .1041 | .0716 | 7.075 | .424 | .535 | | 3/8 | .1917 | .1405 | 5.657 | .567 | .738 | | 1/2 | .3048 | .2341 | 4.547 | .850 | 1.087 | | 3/4 | .5333 | .4324 | 3.637 | 1.130 | 1.473 | | 1 | .8626 | .7193 | 2.904 | 1.678 | 2.171 | | 1-1/4 | 1.496 | 1.287 | 2.301 | 2.272 | 2.996 | | 1-1/2 | 2.038 | 1.767 | 2.010 | 2.717 | 3.631 | | 2 | 3.356 | 2.953 | 1.608 | 3.652 | 5.022 | | 2-1/2 | 4.784 | 4.238 | 1.328 | 5.793 | 7.661 | | 3 | 7.388 | 6.605 | 1.091 | 7.575 | 10.252 | | 3-1/2 | 9.887 | 8.888 | .955 | 9.109 | 12.505 | | 4 | 12.730 | 11.497 | .849 | 10.790 | 14.983 | | 4-1/2 | 15.961 | 14.454 | .764 | 12.538 | 17.611 | | 5 | 19.990 | 18.194 | .687 | 14.617 | 20.778 | | 6 | 28.888 | 26.067 | .577 | 18.974 | 28.573 | | 7 | 38.738 | 34.472 | .501 | 23.544 | 38.048 | | 8 | 50.040 | 45.664 | .443 | 28.544 | 43.388 | | 9 | 62.776 | 58.426 | .397 | 33.907 | 48.728 | | 10 | 78.839 | 74.662 | .355 | 40.483 | 54.735 | | 11 | 95.033 | 90.763 | .325 | 45.557 | 60.075 | | 12 | 113.098 | 108.43 | .299 | 49.562 | 65.415 | |_________|__________|__________|________|________|________|
Dimensions are nominal and except where noted are in inches.
In connection with pipe sizes, Table 63, giving certain tube data may be found to be of service.
TABLE 63
TUBE DATA, STANDARD OPEN HEARTH OR LAP WELDED STEEL TUBES
+-----+--+----+-----+------+------+------+------+-------+-------+-------+ |S E D|B | T | I D |Circ.u.mference| Transverse |Square |Length |Nominal| |i x i|. | h | n i | | Area | Feet |in Feet|Weight | |z t a|W | i | t a | |Square Inches| of | per |Pounds | |e e m|. | c | e m +------+------+------+------+ Exter |Square | per | | r e| | k | r e |Exter-|Inter-|Exter-|Inter-| -nal |Foot of| Foot | | n t|G | n | n t | nal | nal | nal | nal |Surface| Exter | | | a e|a | e | a e | | | | | per | -nal | | | l r|u | s | l r | | | | |Foot of|Surface| | | |g | s | | | | | |Length | | | | |e | | | | | | | | | | +-----+--+----+-----+------+------+------+------+-------+-------+-------+ |1-1/2|10|.134|1.232| 4.712| 3.870|1.7671|1.1921| .392 | 2.546 | 1.955 | |1-1/2| 9|.148|1.204| 4.712| 3.782|1.7671|1.1385| .392 | 2.546 | 2.137 | |1-1/2| 8|.165|1.170| 4.712| 3.676|1.7671|1.0751| .392 | 2.546 | 2.353 | | 2 |10|.134|1.732| 6.283| 5.441|3.1416|2.3560| .523 | 1.909 | 2.670 | | 2 | 9|.148|1.704| 6.283| 5.353|3.1416|2.2778| .523 | 1.909 | 2.927 | | 2 | 8|.165|1.670| 6.283| 5.246|3.1416|2.1904| .523 | 1.909 | 3.234 | |3-1/4|11|.120|3.010|10.210| 9.456|8.2958|7.1157| .850 | 1.175 | 4.011 | |3-1/4|10|.134|2.982|10.210| 9.368|8.2958|6.9840| .850 | 1.175 | 4.459 | |3-1/4| 9|.148|2.954|10.210| 9.280|8.2958|6.8535| .850 | 1.175 | 4.903 | | 4 |10|.134|3.732|12.566|11.724|12.566|10.939| 1.047 | .954 | 5.532 | | 4 | 9|.148|3.704|12.566|11.636|12.566|10.775| 1.047 | .954 | 6.000 | | 4 | 8|.165|3.670|12.566|11.530|12.566|10.578| 1.047 | .954 | 6.758 | +-----+--+----+-----+------+------+------+------+-------+-------+-------+
Dimensions are nominal and except where noted are in inches.
Pipe Material and Thickness--For saturated steam pressures not exceeding 160 pounds, all pipe over 14 inches should be 3/8 inch thick O. D. pipe.
All other pipe should be standard full weight, except high pressure feed[77] and blow-off lines, which should be extra strong.
For pressures above 150 pounds up to 200 pounds with superheated steam, all high pressure feed and blow-off lines, high pressure steam lines having threaded f.l.a.n.g.es, and straight runs and bends of high pressure steam lines 6 inches and under having Van Stone joints should be extra strong. All piping 7 inches and over having Van Stone joints should be full weight soft flanging pipe of special quality. Pipe 14 inches and over should be 3/8 inch thick O. D. pipe. All pipes for these pressures not specified above should be full weight pipe.
f.l.a.n.g.es--For saturated steam, 160 pounds working pressure, all f.l.a.n.g.es for wrought-iron pipe should be cast-iron threaded. All high pressure threaded f.l.a.n.g.es should have the diameter thickness and drilling in accordance with the "manufacturer"s standard" for "extra heavy" f.l.a.n.g.es.
All low pressure f.l.a.n.g.es should have diameter, thickness and drilling in accordance with "manufacturer"s standard" for "standard f.l.a.n.g.es."
The f.l.a.n.g.es on high pressure lines should be counterbored to receive pipe and prevent the threads from shouldering. The pipe should be screwed through the f.l.a.n.g.e at least 1/16 inch, placed in machine and after facing off the end one smooth cut should be taken over the face of the f.l.a.n.g.e to make it square with the axis of the pipe.
[Ill.u.s.tration: 2000 Horse-power Installation of Babc.o.c.k & Wilc.o.x Boilers and Superheaters, Equipped with Babc.o.c.k & Wilc.o.x Chain Grate Stokers at the Kentucky Electric Co., Louisville, Ky.]