Document bBkq5bvp7oXpd91RN6rp8QXqy

A summarv of reports of experimental evidence and practical experience in the petroleum industry recites that no significant increase in fire safety is gained by the use of spark-resistant hand tools in the presence of gasoline and other hydrocarbon vapors However, some materials such as carbon disulfide, acetylene, and ethyl ether have very low ignition energy requirements For these, and similar materials, the use of special tools designed to minimize the danger of sparks in hazardous locations can be recog nized as a conservative safety measure Leather-faced, plastic, and wood tools are free from the friction-spark hazard Health Hazards Toxic effects Flammable liquids and their vapors may create health hazards from both surface con tact and inhalation of toxic vapors Irritation results from the solvent action of many flam mable liquids on the natural skin oils and tissue A toxic hazard of varying degree exists in practically all cases, depending on the con centration of the vapor See Chapter 42, "Elements of Industrial Toxicology " Some flammable vapors are heavier than air and will flow into pits, tank openings, con fined areas, and low places m which they con taminate the normal air and thus cause a toxic as well as explosive atmosphere Oxygen de ficiency occurs in closed containers, such as a tank which has been closed for a long time and in which rusting has consumed the oxy gen All containers should be aired and tested for toxic and flammable atmosphere before any entry Combustible gas indicators Unless tests prove otherwise, flammable and toxic mixtures should be assumed to be present m all tanks which have contained or have been exposed at any time to flammable liquids Tests for flammable vapor-air mix tures in tanks and other vessels may be made by making a chemical analysis of samples or by using a combustible gas indicator The most commonly used combustible gas indicator is an instrument operating on the principle that when a mixture of flammable vapor and air is passed over a heated electric filament, the resistance of the filament will be increased m direct proportion to the amount of combustible vapor present The imbalance of the Wheatstone bridge is read on the galvanometer, usually as the percent age of the lower flammable limit of a calibra tion substance such as hexane In a vapor concentration of less than the lower explosive limit (LEL), the needle will be on scale In a vapor concentration be tween LEL and the upper explosive limit (UEL), the needle will remain off scale above 100 per cent In a vapor concentration above the UEL, the needle will swing rapidly off scale and then drop back to zero Oxygen deficiency will produce unreliably low read ings A combustible gas indicator should be used only by experienced persons, and the opera tor should follow the manufacturer's instruc tions on balancing the unit For an accurate determination, the reading should be cor rected from the graph for the specific gas present in the atmosphere being tested Manufacturers supply correlation graphs for many combustible gases The operator can check it by placing sev eral drops of the flammable liquid in a small jar, shaking it a few times to permit evapora tion, and then holding the sample hose of the instrument m the mouth of the jar and press ing the aspirator bulb the required number of times If there is any question as to the in dicator's accuracy, it should be calibrated against a standard or returned to the manufac turer for checking The sampling hose should be kept where flammable liquid, steam, or water will not be drawn into the instrument Any of these sub stances would put it out of service Hot vapors also may affect the indicator if they condense inside Lead contamination can be encountered when sampling leaded gasolines and can be avoided by use of special high temperature filaments A tank, tested before being cleaned, may be found vapor-free However, if it contains sludge or scale, flammable liquid and vapor 'US Department of Commerce, Office of Tech nical Services SparkmgCharactenstics and Safety Hazards of Metalltc Materials Technical Report No NCF-T-1-57, PB 131131 1019