Equipment and Design
August 1952
I t is often difficult to determine the most effective safety factor to use in designing equipment when a human factor or an act of God is part of the risk
L
month we discussed some unusual factors of safety. This month, on the same theme, we are going to discuss some factors of safety that are difficult to specify. Every engineer realizes the importance and universal need of some factor of safety in every kind of engineering work. Often these are very simple calculations which one does automatically. I n other cases, however, although the need is fully recogni~ed,specifying the proper, effective, and correct type of safety factor is a very difficult problem. AST
Pressure cooker hazards
To illustrate this point, we are retreating from the usual scope of technical papers into the domestic realm of the kitchen. A great many people today cook vegetables, meats, and other foods in cast-aluminum pots called pressure cookers. There are many different designs for this equipment, and all are supposed to be safe for use by young and old, male and female. We have been impressed with the ingenuity of the devices used to ensure safety in operating pressure cookers. Nevertheless, we have found that the safety factor in all designs has not been sufficiently correct in type and effectiveness to prevent very bad accidents in some cases. I n operation the best designed pressure cooker in our opinion works like this: The food is placed in the cooker with a minimum amount of water and then a cover with a breech-block type of closure is placed on the cooker and sealed tight with a rubber gasket by turning the cover about loo, in relation to the body of the cooker, tightening the cover against the gasket. The cover diameter is as large as the body of the cooker with an area of 44 square inches. There is a rupture disk molded into the cover which is never disturbed. I n addition, there is an opening that is closed by inserting a heavy weight with a matching seat, which acts by gravity as a foolproof safety August 1952
valve and develops a pressure of 15 pounds per square inch gage. I n operation in the kitchen, this device should be safe for use by all over 5 years of age. If the underside of the safety valve hole becomes clogged by overloading with spinach, cabbage, or broccoli the rupture disk is not affected and would relieve excess pressure. I n normal use, the pressure inside the cooker permits the food to be cooked a t a temperature of 250' F., the temperature ,which must be handled when the cooking is finished. It is very evident that if a woman opens the cover (with 15 pounds of pressure inside) by the leverage she can exert on the handles, she is certain to be badly burned-and sad to relate this happens. Bad accidents have occurred, burning the flesh from the wrist to the elbow on one or both arms. The proper way to open the cooker is to take the entire assembly by the handks off the stove to the sink and run cold water over the cover and down the sides for perhaps 4 or 5 seconds; then the safety valve is removed without danger. The last few ounces of pressure within the vessel are thus relieved, and the cover can be opened. When people try to lift the safety valve before cooling with cold water, although not as serious as opening the cover under pressure, the mental error is immediately translated into a penalty. These seem like simple instructions, but people have been hurt because they could reverse the order of operations. It is a t once apparent that human forces are involved which are entirely without the realm of engineering, truly up to the individual. In all such cases where it is difficult to select the correct factor of safety, one must outguess the capacity of the human mind for doing things ignorantly or absolutely backwards. The kind of safety factor to put on a vessel of this sort to be used widely by a large number of people in everyday service presents a problem which the
equipment manufacturer must face. The proper answer has not yet been found. It is still possible to be badly burned using a pressure cooker, and this same condition exists in many other fields. Construction in earthquake areas Because of the stimulus of two worldwide wars, there is now great chemical activity in countries which previously were on a rather simple level of living. Many of these new countries bring us into earthquake territory similar to that on our West Coast. These areas have been carefully charted, and the chemical industry has found it advantageous to expand into parts of them. This brings up the question of formulas, specifications, and interpretation of satisfactory earthquake safety factors. At the present time such factors are not too well known or widely used. Engineers are often given instructions to use an earthquake factor of 0.1, 0.2, or 0.3. These factors are most frequently used in the building code for factories, office buildings, and dwellings. In such application, the earthquake factor is a simple increase in design values to resist stresses produced by lateral forces. These stresses are calculated as the effect of a force applied horizontally a t each floor or roof level above the foundations. Such forces are proportional to the total dead load except for warehouses, in which this force is often proportional to the total dead load plus 50y0 of the total vertical designed live load. Further, the force is assumed to come from any horizontal direction. The earthquake factor is applied to these lateral forces. It is apparent also that, by design, all bracing systems, both horizontal and vertical shall transmit all forces to the resisting members and (Continued on page 74 A )
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
73 A
Pyrofiex constructed H C l cooling tower.
PYROFLEX CONSTRUCTIONS
shall be of sufficient strength and detail to resist the horizontal forces provided for. Bracing should be located symmetrically about the center of mass of the building, or the building should be designed for the calculated rotational forces about the vertical axis. This horizontal force formula is simple and is expressed as F = CW where F equals the horizontal force in pounds, and W equals the total dead load. This is figured tributary to the point under consideration, except, as mentioned, for warehouses. Machinery or other fixed concentrated loads should be considered as part of the dead load; this is discussed later in more detail. C equals a numerical constant and is given several interpretations. For floors, roofs, columns, and bracing in any story of a building as a whole, C is equal t o the fraction
Typ i c a I I n s tu I la ti o n s This engineered functional equipment combines the positive corrosion protection of Pyroflex skeet liming with the rugged streagth of acid-proof masonry. However, Knight ases many other types of materials defiending on service. ~
EQUlPMENT
T I M E IN SERVICE
OPERATING CONDITIONS Service
I
Absorption Tower Chlorinator Acid Mist Extractor Cooling lower
I .-
Drying Tower Pickling Tank Storage Tank
SO8 Gas, H,S04 and Water
1
Chlorine Benzol and Chlorobenzol H,S04 Mist, €IC1 Gas and Water Conc. H,SO, and HC1 Gas
4 yrs.
I
50'F
6 yrs.
1100°F
10 yrs.
Autoclaves Absorbers Acid Boilers Concentrators Distillation Towers Gas Coolers
230'F ~
3~1s.
250°F
iConc.H,SO,and Chlorine Gas 30% H,SOI 33% HC1 and Traces Organic Solvents
1
85'F
1
5 yrs.
I
I
8 yrs.
7yrs.
Gas Washers Acid Storage Tanks Plating Tanks Scrubbers Sumps Tanks
When inquiring about Pyroflex Constructions, please specify type of equipment in which you are interested as well as service conditions involved. Bulletin No. 2-L will be sent on request.
74 A
where ilr equals the number of stories or the number of stories contributing load above the floor under consideration. For bearing walls, partitions, curtain walls, enclosure walls, and freestanding masonry walls over 6 feet in height, Cis equal t o 0.2 with a minimum of 5 pounds per square foot. For cantilever parapets and other cantilever walls, except retaining walls, C is equal t o unity. For towers, tanks (including contents), chimneys, smokestacks, and penthouses, when connected to or a part of a building, C is equal to 0.2. I n case the wind load would produce higher stresses than these figures would ordinarily give, the wind load shall be used in lieu of the forces determined by these formulas. It is a good rule, that stresses in materials shall not exceed by more than one third the allowable working stresses permitted by the codes in local use, except that rivets may be stressed the same in tension as is allowed in shear. The allowable sheer in reinforced concrete walls 6 inches or more in thickness shall not exceed 0.05 of the ultimate compressive strength of the concrete in any location definitely known to be earthquake country. Ordinarily, lime mortar is not used in any construction in such areas. A factor of safety of four times the usual is used for veneer ties securing masonry to an inner wall. I n earthquake country it is also necessary to (Continued on page 76 A )
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 44, No. 8
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August 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
I MS-8
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WASTE-WATER PROBLEM? Hardinge CLARIFIERS may solve it. 0 I n industrial areas, Hardinge Clarifiers have proven their worth in the recovery of valuable suspended solids from industrial waste water, frequently leaving the effluent clear enough for reuse in the process. Also used in the treatment of industrial water supply. 0 Hardinge Clarifier systems are relieving stream pollution in many localities. Available with steel, tile, concrete, or wood tanks, Hardinge Circular Clarifiers are usually equip ed with cut-flight scrapers which remove settled solids rapidly through a central discharge. Can g e used in conjunction with chemical treatment and flocculation. State your water treatment problem and write for Bulletin 35-C-12.
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0 Affracfive, Smoofh Finish
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N o w available in pipe covering and block insulation, Mundet 85% Magnesia permits new heat-saving efficiency. Precision manufacturing on the latest type of automatic equipment insures uniform standards. Extra durability is built into the insulation. It does not "powder", settle or disintegrate. It is unaffected by either steam or water leakage. It maintains an attractive, smooth finish. Precision pipe fit is assured, with no spaces left for the escape of heat. You benefit from the most modern manufacturing facilities for the production of heat insulation. Write for specification information and recommendations. Mundet Cork Corporation, Insulation Division, 7101 Tonnelle Avenue, North Bergen, N. J.
-MUNDET
apply certain factors of safety and even modifications in design to many types of chemical equipment. Let us take filter presses, for example, of the plate-andframe type. These should be secured to the floor in the usual way, but' in addition the earthquake factor requires a brace on the end frames near the outer corner leg and about 18 inches from the floor. This brace goes to the floor a t an angle of 45' in each direction-that is, the brace is a t 45' to the center line, a t 45' to the face of the end plate, a t 45" to the sides or edges of each of the plates, and a t 45' to the floor. The braces are shaped pieces of heavy tubing set in actual socket castings or bearing blocks, both on the press and on the floor, the latter sockets being secured to the floor with four (at least) very strong bolts. A brace of this type is located a t each one of the four corners of the filter press. I n a caustic chlorine installation, the familiar pattern of the cell house consists of cells approximately cubical in form, from 3 to 5 feet on each side, arranged in long rows, each cell connected electrically in series with copper conductors. There is always a certain amount of movement due to thermal expansion and vibration between cells, and in a long row of 50 or more cells, the expansion of each cell alone means that the metal conductors from cell to cell have t'o absorb an appreciable change in length. This ordinarily would not be serious except that the electrodes leading into the cell usually go through the cover in an insulated joint which is not too strong or rugged. Additional stresses produce distortion, and cracks affect the insulation. I n earthqua,ke country, the usual solid bus bars are replaced with flat braided copper strips 3/'g inch thick by 4 inches wide, connected between the electrodes at six or eight cell intervals. A better method is to use solid copper connectors and give each one a generous bend, and in addition brace each cell, making it more difficult to shift it horizontally on the foundations. This combination adds to the expense but will resist a very pronounced shake many times. Correspondence concerning this column will be forwarded promptly if addressed t o the author, YGEditor, INDUSTRIAL AND ENGINEERINQ CHEMISTRY, 1155-lBth St., N . W , Washington6, D. C.
i
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 8
