Acoustical wall and ceiling systems are widely used in schools for sound dampening. A variety of products are available including modular wall panels (textile and metal-covered), suspended ceiling tiles (t-bar ceilings), and surface mounted ceiling and wall panels. Ceiling tile (usually in a T-bar ceiling) is the most common ceiling finish in schools. Because of the large ceiling surface area, the likelihood of its being disturbed during modifications/renovations, and its contact with HVAC systems, it is an important product to consider for air quality and materials efficiency.
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Generally, the source of radon gas is from the ground beneath the school. It flows into the school through cracks and openings when the air pressure inside the school is lower than the air pressure outdoors. Typical cracks and openings include the joint where the floor meets the wall, expansion joints in the floor, openings in the floor for the passage of pipes and wires, and hollow masonry walls that penetrate the floor. Since it cannot be determined before construction whether radon will be a problem, it is cost-advantageous in high radon potential counties to take the following precautions:
I2-3. Tanks D-4-F and D-12-F were opened to D-54-F by failure of about 100 rivets along the seam just below the armor, (Photo I-3), and to each other through a 2-inch crack in a seam of bulkhead 134 which was badly wrinkled. Swash plates and floors, frames 132 through 137, were crumpled, bulkhead 138 was slightly buckled and No. 2 torpedo bulkhead between tanks D-4-F and D-12-F and tank D-6-F was deflected but not opened. The original shell plating at the bottom of D-6-F was dished in and a riveted seam opened. Tanks C-34-V, D-2-V and D-10-F, between torpedo bulkheads 4 and 5, were opened to the sea through leaky seams in the shell. Deformation of bulkheads and stiffeners occurred as far inboard as the port bulkhead of after generator room, D-5-E. No panting of damaged structure developed.
A mixture of silica sand or ganister, fire-clay, and water can also be used when available. This mixture contains 85to 95 percent of silica sand or ganister; the rest is fireclay. The exact percentage of silica sand or ganister and fire-clay are determined by how workable a mix is desired. More clay gives a more workable and sticky mixture but increases the amount of shrinkage when the lining is dried. With all types of refractory mixes, only enough water should be added to make the mixture workable. An excess of water, although making the mixture easier to handle, causes more shrinkage and cracks in the lining when it is dried. For low shrinkage and fewer cracks, use small amounts of water and clay.Before ramming a lining in the ladle, arrangements must be made for venting the lining during drying. This is done by drilling 3/16 inch or 1/4 inch holes through the sides and bottom of the ladle shell on 3 to 4-inch centers. If this is not done, drying will take too long. Numerous injuries to personnel have resulted from the use of improperly dried ladles. When moisture is pocketed under molten metal, a large volume of water vapor is rapidly formed and the metal is blown out of the ladle with explosive force. In addition to this, even slight traces of moisture in ladle linings will cause porosity and casting unsoundness. The most practical way to determine when a ladle is dry is to apply heat until steam flows from the vent holes, and then continue to apply heat until this flow stops completely.With a properly vented ladle shell, the lining is then rammed in place. It is best to use a wood or metal core to form the inside of the ladle. The form can be made with taper and allowance for lining thickness. After the bottom of the ladle is rammed into place, the form is centered with wedges and the sides of the lining rammed. A harder and more dense lining can be made and the water kept to a minimum when 140 a form is used. Also, the job is a lot easier than trying to ram a lining against vertical walls. When ramming a lining in layers, be sure to roughen the top of each layer before ramming in the next layer. After the lining has been rammed, the form is rapped lightly to loosen it and then drawn from the ladle. To make drawing of the form easier, and to keep the form from absorbing water from the mix, it should be covered with a thin layer of grease or with aluminum foil. If aluminum foil is used, it is peeled from the lining after the core has been removed. Care should be exercised so that aluminum foil does not fold and cause a deep crack in the lining. The ramming of the lining must be very hard and uniform. If the lining shows a tendency to crack into layers when the form is withdrawn, each layer was not roughened enough before the next was rammed in. Such a ladle can be dangerous to use.The thickness of the lining varies with the metal to be handled and with the size of the ladle. For example, a ladle for the pouring of steel requires a heavier lining than one for cast iron, bronze, or aluminum because steel, at the high temperatures required, attacks the lining material much more rapidly than any of the other metal s.A lining for a ladle to hold and pour 75 pounds of steel will have a thickness of about 1 inch on the bottom and 1 inch on the sides at the bottom and will taper to about 3/4 inch at the top of the ladle. This thickness is also satisfactory for any of the other metals. For the lower-melting-point metals, the main consideration in determining the thickness of lining is proper insulation in order to prevent chilling of the molten metal and to avoid damage to the ladle shells as a result of overheating.Drying of a new or patched lining is an operation that can cause a lot of trouble if it is not done properly. A new lining or patch should be heated gently at first to get rid of most of the water without blowing a hole in the lining or cracking it because of steam pressure. Aboard ship, this can best be done by first drying the ladle in a core oven and then completing the drying with a torch. The torch should be positioned, with respect to the ladle, so as to insure complete combustion of the gas and delivery of maximum heat to the lining. If a new lining is heated too fast at the start, the water travels back to the shell and makes that part of the lining weak and soggy. After the lining or patch is thoroughly dried slowly, the temperature can be safely increased to as high as obtainable. It is desirable to maintain a new lining at red heat for several hours before using it. If a slowly heated new lining cracks, the ramming mixture contained too much clay or water. ANYONE WHO POURS METAL FROM A LADLE WITH A DAMP LINING OR DAMP PATCH CAN EXPECT TO FIND UNPLANNED HOLES IN HIS CASTING.Pouring lips of ladles are a frequent source of trouble because they are often patched and then proper drying of the patch is overlooked. A wet patch on a pouring lip will put gas in the metal and cause blowholes in the castings.Patching of a ladle is best done when the lining is cold. All adhering slag and metal must be removed in the area to be patched. If possible, undercut the old lining to help hold the patch. Brush loose dirt from the old lining and wet the lining thoroughly. Patch large holes with the same mixture used for lining. Patch small holes and cracks with a mixture of four parts clean sand and one part fireclay. DRY A PATCH THE SAME AS A NEW LINING.In figure 190 are shown the various steps in the lining of a teapot ladle. Part (a) is a cutaway view of the ladle shell. The bottom is rammed, the forms set in place, and refractory rammed on the side. Many times, a heavy-duty refractory brick is placed in the bottom of any ladle to take the force of the molten metal stream when the ladle is filled. This reduces erosion of the ladle bottom. Part (b) shows the sides partly rammed after the forms are set. The completed lining with forms still in place is shown in (c). Order of withdrawing the forms is shown in (d).Ladles for steel are commonly used only once per lining because of the fluxing action of the metal at the high temperatures. In an emergency, if great care is used in skimming slag, ladles may be used twice for steel, but it is not good practice. This does not hold true for the other metals, however, and the ladles may be used for many heats. Care should be taken to remove all metal and slag after each use, but it is impossible to remove all of the debris. Therefore, ladles should be used for only one metal. A separate ladle should be used for each metal or the metal will become contaminated and unfit for use.POURING THE MOLDThe placing of weights and clamps on a mold is only a minor operation in the making of a casting but one that will produce defects if not properly done. Weights are used to prevent the force of the molten metal from lifting the cope as it fills the mold, thereby producing a swelled casting or a runout. The position of the weight on the mold should be determined and the weight placed on the mold gently, without any movement of the weight across the top of the cope. 141 Any such movement can cause the cope to break or force sand into the gating system or open risers.Clamps serve the same purpose as weights and are used to clamp the cope and drag together when the casting is poured in the flask. In placing the clamps, a wood wedge is usually used to tighten the clamp down on the flask. Before placing the clamp, the area next to the clamp should be cleaned of excess sand to prevent mold damage when the clamp is set. The wedge should be placed between the clamp and the top edge of the flask, the clamp brought up snug, and then tightened by driving the wedge. Care must be taken to hit the wedge and not the clamp or flask.Although clamps and weights are used for the same purpose, clamps are much more dependable. It is too easy to underestimate the lifting power of the metal and to use too few weights. On the other hand, too many weights can crush a mold.With the ladle thoroughly dried, preheated to a red heat, and securely in the bail, molten metal from the furnace is tapped into it. Filling the ladle to its brim is unwise from the standpoint of safety and for the production of good castings. Filling the ladle to its brim should be done only when absolutely necessary, and then extreme caution should be exercised in handling the ladle and pouring.If the ladle is filled to about 3/4 of its capacity, metal will not flow over the lip until the ladle is inclined to an angle of approximately 60 from the horizontal. This permits a good control of the stream, making it possible to keep the ladle quite low and thus, keep the height of fall of the metal low. This lessens mold erosion, entrapment of air, formation of oxides, and metal spills.Figure 191 shows the proper method of pouring, while figure 192 shows poor pouring technique. In the good pouring technique, notice that the lip of the ladle is as close as possible to the mold.Slag on the metal should be skimmed carefully prior to and during pouring. If a steel or iron slag is too fluid to be skimmed properly, dry silica sand should be spread across the surface of the molten metal to thicken the slag. Dry metal rods or special metal skimmers should be used for skimming or stirring metal. Wood skimmers or stirrers should never be used because the wood contains moisture which often produces unsoundness in castings. SPEED OF POURINGThe pouring basin should be filled quickly to prevent nonmetallics and slag from entering the mold cavity and must be kept full. In order to do this, the ladle stream must be controlled carefully. Once pouring has started, it must continue without interruption until the mold is filled. One allowable exception is to stop pouring through the sprue when the metal has filled 1/3 of a top riser. The riser is then filled last with hot metal to improve feeding. This exception applies only to top risers. With side risers, the mold might not be full when metal is seen in the riser.The use of a pouring basin and plug to get more uniform pouring is shown in figure 193. Part (a) shows the basin ready to receive the metal. In (b) the basin is partially filled. When the basin is properly filled, the plug is withdrawn as in (c). The use of a pouring basin permits better control of the metal entering the gating system. Another variation of this method is to put a thin sheet of the metal being poured over the sprue opening. It will melt out when the basin is filled with hot metal. Keep the basin full of metal at all times.When pouring a metal that forms dross (especially aluminum, aluminum bronze, or magnesium), every effort must be made to avoid turbulent entry of the metal into the mold. It is particularly important in such cases that the lip of the ladle be as close to the pouring basin as possible. The sprue must be filled quickly and kept full so that the tendency for dross and entrapped air to enter the mold will be at a minimum. Here again the pouring basin and plug can be used to advantage. The use of skim gates or perforated cores placed in the sprue or pouring basin (as shown in figures 194 and 195) aids in removing dross from the metal and preventing its entrance into the casting cavity. Agitation of the molten metal while it is being transported to the mold also increases dross formation and gas absorption.POURING TEMPERATUREClose control of pouring temperatures is essential to the consistent production of good castings. An immersion pyrometer and an optical pyrometer are furnished for temperature determination. Because of the high temperature involved, the immersion pyrometer is not used for iron or steel. The Chromel-Alumel immersion thermocouples are limited to temperatures of 2,500F. The optical instrument is impractical for nonferrous metals because their pouring temperatures are too low. 142 The operation of an immersion pyrometer is a simple matter. The instruments are usually of two types; a self-contained unit or the unit in which the immersion unit is connected by wires to the reading instrument. Before use, the pyrometer should be checked to make sure that the immersion part of the instrument is clean. Lead-wire pyrometers should be checked for any breaks or loose connections in the wire. When taking a reading, the immersion tip should be submerged in the molten metal to a depth of approximately 3 inches and moved slowly back and forth or in a circle. After the temperature reaches a fairly steady reading, it should be recorded. The immersion tip should then be withdrawn from the melt. Immersion pyrometers should be handled with care and periodic inspections made for proper upkeep. Whenever possible, the instruments should be checked and calibrated for good operation.The optical pyrometer operates by matching the intensity of light from the molten metal with that of a standard light source within the instrument. Exact operating procedures are available with the instruments. Generally, the field of vision will be uniform, as shown in figure 196a, when the instrument is set at the proper temperature. If the instrument is set too high, the inner circle of the field will be brighter as in (b). If the inner circle is darker, the instrument is set at too low a reading. This instrument should be handled with care and given periodic checks and calibrations for proper operation.Excessive pouring temperatures (that is, temperatures above those required for the proper filling of the mold) result in excessive oxide or dross formation, segregation, rough and dirty casting surface, unnecessarily high liquid shrinkage, coarse-grain metal structure, and increased danger of cavities, tears and porosity. Figure 197 shows the increase in grain size that resulted with increased pouring temperatures for a copper-base alloy. Notice that the high pouring temperature resulted in a very coarse grain structure.If the pouring temperatures are too low, entrapped gas and dross, misrun castings, or castings with surface laps (cold shuts) are likely to result. Proper pouring temperatures for a given metal vary with the casting size, design, and desired rate of pouring. For this reason, the pouring ranges given below should be taken as a general guide only:MetalPouring-TemperatureRange Steel2850F. to 2950F.Gray iron2300F. to 2600F.Aluminum1250F. to 1400F.Manganese bronze1875F. to 1975F.Compositions G & M2000F. to 2200F.In general, thin-walled castings are poured on the high side of the range and thick-walled castings on the low side.SUMMARYThe important factors in pouring a casting are summarized as follows:1. Ladle equipment must be kept in good repair.2. All ladle linings must be rammed uniformly hard and be of the proper thickness.3. All ladles must be thoroughly dried and at a red heat for some time before use with the high melting point alloys.4. Ladles should not be filled to more than 3/4 of their capacity.5. Metal should be skimmed free of all slag or dross before pouring.6. In pouring, the ladle should be as close to the pouring cup or sprue as possible.7. Once pouring has started, the stream should not be interrupted. A steady rate of pouring should be used and the sprue should be kept full at all times.8. The metal should be poured at the correct temperature, neither too high nor too low. 143 Figure 186. Lip-pouring ladle.
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