Document e7mejG6RJKNR85qeV0KMY6a4

6 1955 National Softly Congress by the active support of all levels of man agement, should produce the optimum in safety. In mathematics, there is a basic principle that if you take any number, however small, and divide h by two and then in succession diride the answer by two, you will never reach zero. I submit that if the dividing principle is continued long enough you will get close enough for all practical purposes. Let ns think of this job of ours in some what the same fashion. Recognizing the frailties of human nature, the limitations of material things, we cannot hope to reduce our accident occurrences to zero. Let it be sufficient for us to know that, by the ever-lasting continuation of our progress in the field of control, we, too, can continue the process until we can get close enough for all practical purposes. Application of Safety Control Devices to Fuel Gases in Industry By F. B. BEVELHEIMER Supvr.. Fuel & Utilities, Steel Div., Ford Motor Co., Dearborn, Mirh The uses of gaseous fuels in industry* are many and diverse. The range of applications varies from small furnaces that hat solder ing irons to large furnaces that heat ten ton steel ingots; and from ovens in the foundry' that dry sand to ovens in the as sembly* plant that dry* the finishes of new automobiles. Several different gases are used; among these are coke oven gas. natural gas, pro pane and acetylene. These gases differ some what in chemical composition and have different burning characteristics. There is one thing that they* all have in common. They will bum, and, under certain condi tions, they will become an explosive hazard. In the past, the handling of gases was rather haphazard. Gas was piped to an oven or furnace through one hand-operated valve. Lighting a furnace safely was purely a mat ter oi experience. Accidents happened. To prevail explosions, and to make lighting and operating safe and foolproof, safety controls were devised. For the safety of personnel and for pro tection against fire or explosion, careful con sideration has been given to the hazards of each project It bos been said that the best safety apparatus is a safety-conscious man. This statement may be true, but no man can be a substitute for the constant attention which properly designed automatic safety apparatus will provide. The final test of safety apparatus is that it fails safe in the event of the interruption of the safety se quence or the development of unsafe condi tions in operation. Partial protection is more dangerous than no protection at all. Insuf ficient, poorly installed.' or poorly maintained equipment could cause operators to lapse into carelessness by supposed safety not actually present. Safety controls should be understood to mean automatic supervising devices of suffi cient number, properly constructed and so arranged that they will maintain the re quired conditions of safety for the purpose of preventing the development of any fire or explosive hazard. One type of equipment is a paint-drying oven. If flammable vapors are permitted to ac cumulate within an oven enclosure, their concentration may come within the explosive range: experience has shown that such an accumulation in the presence of an ignition source may be ignited and exploded, or re sult in a serious fire. A study of industrial heating equipment shows that it is unwise to depend for safety on elimination of a course of Ignition. The only reasonable solution is to introduce a sufficient quantity of air into the oven in order to dilute any possible flammable vaporair mixture to a safe concentration below the lower explosion limit. Another type of hazard which can be created is an explosive fuel mixture w*hich necessitates other types of safety controls. Fuel Safety Controls. These are devices much as safety* shut-off valves, combustion Metals Industry 7 safeguards, pressure switches, temperature, at approximately atmosphere pressure; and and over-temperature controls. These are below 1200F. It is used for the processing interlocked and arranged in control circuits of materials and finishes. The safety equip to minimize the possibility of dangerous ment may be itemized as follows: accumulation of explosive air fuel mixture in a heating system operating at 1200*F. and below. 1. Purging timer. This times the pre-ventilization of the oven. It is not consid ered necessary to purge small enclosures Safety shut-off valves are normally dosed if they have an interlock to insure that (closed when de-energisedJ automatic valves installed in the fuel piping to shut off the all doors are open, and blowers are oper ating before the gas can be admitted to fuel in the event of unsafe conditions or the burners. during shut-down periods. The safety shut off valve may be considered -as the key unit of all the safety* controls used to guard against the explosion or fire hazards which could result from accidental interruption to the various services or operations, such as flame failure, failure of fuel pressure, fail ure of combustion air pressure, failure of 2. Flame protective device. This may be either the flame rectification type or the infra-red photo electric type. 5. Safety-shut-off valve. 4. Pilot gas solenoid. 5. Normally closed valve. Power-operated type for coke oven gas. exhaust or recirculating fans, excessive tem 6. Normally* open valve, same as above. The perature or power failure. Safety* shut-off normally closed valve is installed in the valves should be installed in such manner system between the safety* shut-off valve that they cannot be by-passed or blocked in and the burners. The normally open the open position. A safety-shut-off valve valve is installed in a vent line between should be used for safety shut-off service the safety shut-off valve and the nor only, and not subject to the wear and tear mally dosed valve. of a temperature control valve. 7. Air pressure swatch. Combustion (flame faHurc) safeguards are devices w*hich detect the presence or absence of flame. In the event of flame failure, they* will automatically shut off the supply of fuel, and if so arranged, they will also sound an alarm. Pressure switches mamtain an electric cir cuit when the pressure in fuel or combustion air supply line is above the proper minimurn setting for correct combustion. The circuit is interrupted if the pressure drops below the' minimum setting. The safety shut-off valve controlled by pressure switches does not automatically reopen when pressure is restored to normal. All safety shut-off calves must be reset manually. This is important. Temperature control devices are instru ments for automatically controlling the heat of the furnace or oven. Over-temperature controls shut off the fuel supply if a safe operatingtemperature is exceeded. To these controls, we should add timing devices and other power operated valves. A typical example is the system of con trols used for drying ovens and draw fur naces. A drying oven or draw furnace may 8. Gas pressure switch. These controls are arranged in an inter locking system that assures that each event in lighting a furnace shall happen in proper sequence. After the purging cycle has be gun. nothing can happen until the timer indicates that the purge period is completed. Then the pilot may be lighted. When the pilot burner is lighted, the main burners may be lighted. The safety shut-off valve, the key unit of the system, can be opened only if all of the following exist: 1. Presence of pilot flame is established through the flame protective device. 2. Gas pressure within the range estab lished by pressure switches is available. 3. Compressed air used for temperature control and dampers is available as re quired. 4. Temperature of oven or furnace is below the point established by the over tem perature control. The safety shut-off valve will automati cally dose if any* of the following occur: 1. Pilot flame is extinguished. be described as a heated enclosure operating 2. Power failure has occurred.