Chapter 38
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 2½ 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.
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 illustrated 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 classes of work, and the following is offered simply as a suggestion of what constitutes 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 | Circumference | | |__________________________|__________________________| | | | | | | | |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 |Circumference| 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 flanges, 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.
Flanges--For saturated steam, 160 pounds working pressure, all flanges for wrought-iron pipe should be cast-iron threaded. All high pressure threaded flanges should have the diameter thickness and drilling in accordance with the "manufacturer's standard" for "extra heavy" flanges. All low pressure flanges should have diameter, thickness and drilling in accordance with "manufacturer's standard" for "standard flanges."
The flanges on high pressure lines should be counterbored to receive pipe and prevent the threads from shouldering. The pipe should be screwed through the flange 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 flange to make it square with the axis of the pipe.
For pressures above 160 pounds, where superheated steam is used, all high pressure steam lines 4 inches and over should have solid rolled steel flanges and special upset lapped joints. In the manufacture of such joints, the ends of the pipe are heated and upset against the face of a holding mandrel conforming to the shape of the flange, the lapped portion of the pipe being flattened out against the face of the mandrel, the upsetting action maintaining the desired thickness of the lap. When cool, both sides of the lap are faced to form a uniform thickness and an even bearing against flange and gasket. The joint, therefore, is a strictly metal to metal joint, the flanges merely holding the lapped ends of the pipe against the gasket.
A special grade of soft flanging pipe is selected to prevent breaking. The bending action is a severe test of the pipe and if it withstands the bending process and the pressure tests, the reliability of the joint is assured. Such a joint is called a Van Stone joint, though many modifications and improvements have been made since the joint was originally introduced.
The diameter and thickness of such flanges should be special extra heavy. Such flanges should be turned to diameter, their fronts faced and the backs machined in lieu of spot facing.
In lines other than given for pressures over 150 pounds, all flanges for wrought-iron pipe should be threaded. All threaded flanges for high pressure superheated lines 3½ inches and under should be "semi-steel" extra heavy. Flanges for other than steam lines should be manufacturer's standard extra heavy.
Welded flanges are frequently used in place of those described with satisfactory results.
Fittings--For saturated steam under pressures up to 160 pounds, all fittings 3½ inches and under should be screwed. Fittings 4 inches and over should have flanged ends. Fittings for this pressure should be of cast iron and should have heavy leads and full taper threads. Flanged fittings in high pressure lines should be extra heavy, and in low pressure lines standard weight. Where possible in high pressure flanges and fittings, bolt surfaces should be spot faced to provide suitable bearing for bolt heads and nuts.
Fittings for superheated steam up to 70 degrees at pressures above 160 pounds are sometimes of cast iron.[78] For superheat above 70 degrees such fittings should be "steel castings" and in general these fittings are recommended for any degree of superheat. Fittings for other than high pressure work may be of cast iron, except where superheated steam is carried, where they should be of "wrought steel" or "hard metal". Fittings 3½ inches and under should be screwed, 4 inches and over flanged.
Flanges for pressures up to 160 pounds in pipes and fittings for low pressure lines, and any fittings for high pressure lines should have plain faces, smooth tool finish, scored with V-shaped grooves for rubber gaskets. High pressure line flanges should have raised faces, projecting the full available diameter inside the bolt holes. These faces should be similarly scored.
All pipe ½ inch and under should have ground joint unions suitable for the pressure required. Pipe ¾ inch and over should have cast-iron flanged unions. Unions are to be preferred to wrought-iron couplings wherever possible to facilitate dismantling.
Valves--For 150 pounds working pressure, saturated steam, all valves 2 inches and under may have screwed ends; 2½ inches and over should be flanged. All high pressure steam valves 6 inches and over should have suitable by-passes. All valves for use with superheated steam should be of special construction. For pressures above 160 pounds, where the superheat does not exceed 70 degrees, valve bodies, caps and yokes are sometimes made of cast iron, though ordinarily semi-steel will give better satisfaction. The spindles of such valves should be of bronze and there should be special necks with condensing chambers to prevent the superheated steam from blowing through the packing. For pressures over 160 pounds and degrees of superheat above 70, all valves 3 inches and over should have valve bodies, caps and yokes of steel castings. Spindles should be of some non-corrosive metal, such as "monel metal". Seat rings should be removable of the same non-corrosive metal as should the spindle seats and plug faces.
All salt water valves should have bronze spindles, sleeves and packing seats.
The suggestions as to flanges for different classes of service made on page 311 hold as well for valve flanges, except that such flanges are not scored.
Automatic stop and check valves are coming into general use with boilers and such use is compulsory under the boiler regulations of certain communities. Where used, they should be preferably placed directly on the boiler nozzle. Where two or more boilers are on one line, in addition to the valve at the boiler, whether this be an automatic valve or a gate valve, there should be an additional gate valve on each boiler branch at the main steam header.
Relief valves should be furnished at the discharge side of each feed pump and on the discharge side of each feed heater of the closed type.
Feed Lines--Feed lines should in all instances be made of extra strong pipe due to the corrosive action of hot feed water. While it has been suggested above that cast-iron threaded flanges should be used in such lines, due to the sudden expansion of such pipe in certain instances cast-iron threaded flanges crack before they become thoroughly heated and expand, and for this reason cast-steel threaded flanges will give more satisfactory results. In some instances, wrought-steel and Van Stone joints have been used in feed lines and this undoubtedly is better practice than the use of cast-steel threaded work, though the additional cost is not warranted in all stations.
Feed valves should always be of the globe pattern. A gate valve cannot be closely regulated and often clatters owing to the pulsations of the feed pump.
Gaskets--For steam and water lines where the pressure does not exceed 160 pounds, wire insertion rubber gaskets 1/16 inch thick will be found to give good service. For low pressure lines, canvas insertion black rubber gaskets are ordinarily used. For oil lines special gaskets are necessary.
For pressure above 160 pounds carrying superheated steam, corrugated steel gaskets extending the full available diameter inside of the bolt holes give good satisfaction. For high pressure water lines wire inserted rubber gaskets are used, and for low pressure flanged joints canvas inserted rubber gaskets.
Size of Steam Lines--The factors affecting the proper size of steam lines are the radiation from such lines and the velocity of steam within them. As the size of the steam line increases, there will be an increase in the radiation.[79] As the size decreases, the steam velocity and the pressure drop for a given quantity of steam naturally increases.
There is a marked tendency in modern practice toward higher steam velocities, particularly in the case of superheated steam. It was formerly considered good practice to limit this velocity to 6000 feet per minute but this figure is to-day considered low.