I/EC
Safety
Combustible Gas Detectors N e w chemicals and exotic fuels the demand equipment
for
special
to spot
and storage
increase
handling
hazards
by J. E. Zatek, Mine Safety Appliances Co.
INSTRUMENTATION TO DETECT combustible gases is playing a vital role in providing information to industry about safety techniques which are necessary in dealing with its products. The complex nature of modern production processes has created a need for continual sampling and analysis for the presence of combusti-
Editor's Note: The Lammot Du Pont Safety awards, highest honors available for improvement in chemical manufacturing safety, were presented during the 89th Annual Meeting of the Manufacturing Chemists' Association at White Sulphur Springs, W. Va. The awards are presented each year to the two companies which show the greatest percentage reduction in their average disabling injury frequency rate for the past two years, compared to their rate for the immediately preceding three years. Awards are divided into two categories: 2 million man-hours and over, and less than 2 million manhours. Chemical and Metallurgical Div., General Electric Co. was the winner of the first category with a 6 1 % reduction. Escambia Chemical Corp. won the second category with a commendable 79 f ( reduction in frequency.
bles in air as a safeguard against explosive and toxic hazards. Instruments today have eliminated much of the guesswork in detecting the presence of flammable gases or vapors, and have made it possible to determine much lower concentrations of these combustibles than was formerly possible. Today's compact and simplified combustible gas indicators have a wide field of application for measuring hazards created by the presence of flammable gases in mixtures with air. Operation of Detectors
T h e common safety testing device operates on the catalytic behavior of a heated platinum filament. T h e platinum detector filament is heated to a suitable operating temperature by passing an electric current through it. Upon coming into contact with samples containing flammable gases, combustion on the filament surface raises the temperature in proportion to the quantity of combustibles in the sample, replacing the detector filament in one arm of a standard Wheatstone bridge circuit, which provides means for measuring the electrical resistance change, it is possible to measure not only the presence of a combustible, but its explosibility as well. T h e substantially linear deflection of the meter for gas concentrations is limited to the range of 0 to 100% of the lower explosive limit (LEL) of the combustible being tested. For mixtures that are within the explosive range, the meter needle will be deflected to the right of the scale, above the 100 reading, and
remain there. For mixtures that are too rich to burn - i.e., above the upper explosive limit (UEL), the meter needle will be momentarily deflected to the right of the scale, and then will return to some intermediate point, or below zero, because there is insufficient oxygen in the sample for complete combustion. Testing Devices and Applications
Probably the most widely used safety testing device approved both by the Underwriters and Factory Mutual Laboratories is the general purpose detector, factory calibrated on a specific combustible which is representative of substantially all other gases or vapors. These indicators have universal applicability for literally "seeing" the presence of any combustible in air without the need of positive identification of the unknown flammable material, since at the lower explosive limit, the heat of combustion of most hydrocarbons is approximately the same. A very common application of this instrument is in the detection of explosive gases in underground utility systems such as gas mains, electric and telephone ducts, and sewer drains. In examining these atmospheres, frequently it is necessary to determine whether the combustible contaminant is a fuel gas caused by a leakage from a gas transmission line or a petroleum vapor, such as gasoline, which may have originated from an industrial or domestic source. A charcoal filter is available which may be used with this instrument to VOL. 53, N O . 8
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SAFETY distinguish between condensable hydrocarbon vapors and natural or manufactured illuminating gas. These filters absorb organic vapors that may be present, so that differences in the readings with and without the charcoal filters are an indication that either petroleum vapor or natural gas or both are present in the sample. A special instrument has been designed which differentiates electrically between natural gas and petroleum vapors, through the use of a selector switch. When the switch of such an instrument is in the normal position, the detector filament is at a temperature sufficiently hot to burn natural gas, petroleum vapors, or any other combustible present in the sample. I n the alternate setting of the switch, the filament temperature is reduced below the point at which natural gas is burned. Thus, the difference in readings can readily establish if the test atmosphere contains natural gas, those combustibles which have a low ignition temperature, such as petroleum vapors, or both. Two styles of combustible gas indicators have been designed which can be calibrated to measure specific combustible gases. O n e instrument variation is provided with adjustable calibration controls, and can measure five different gases or vapors in the range 0 - 1 0 0 % L E L for each. A second type has a dual scale multiple calibration of 0 to 2 5 % and 0 to 1 0 0 % of the lower explosive limit. T h e selector switch on the panel of these special version devices changes the metering circuit from one calibration to another, so that per cent L E L measurements can be m a d e directly, thus eliminating the need for calibration curves or charts.
filter cavity of the standard instruments. These filters produce a chemical reaction with the tetraethyllead vapors to produce a more volatile product of combustion, thereby preventing contamination of the catalytic platinum filament. In all instruments of the hot wire type, flashback arrestors are mounted in the flow system at the inlet and outlet of the detector housing, to prevent the propagation of a flame outside the filament chamber in the event that an explosive mixture of gas is encountered. For those instances where it is desired to test atmospheres which may contain mixtures of acetylene or hydrogen with oxygen in excess of that normally found in air, indicators are equipped with specially designed heavy duty flashback arrrestors capable of confining explosive mixtures within the combustion chamber. Another instrument has recently been employed by industrial hygiéniste to measure toxic as well as explosive hazards. Since the threshold limit value for many gases and vapors represents a very small fraction of the lower explosive limit concentration, a highly sensitive combustible gas indicator is utilized. These models are equipped with a dual scale meter graduated from 0 to 1 0 0 % and 0 to 1 0 % of the lower explosive limit. T o determine the explosive concentration of a variety of different combustibles, calibration curves are furnished for correlating meter readings for individual gases. Readings are taken on the 0 - 1 0 % range to measure toxic concentrations of those gases and vapors which may be flammable. A "reference c h a r t " is provided to convert these meter readings directly to parts per million.
When a hot wire type combustible gas indicator is used for testing vapors of leaded gasoline, a combustion product of the tetraethyllead is deposited on the filament unit, reducing its catalytic activity. T o circumvent this poisoning effect, a special version of the standard instrument was designed for gasoline service. Manufacturers boosted the voltage across the detector to maintain a sufficiently high filament temperature to prevent contamination. An alternate method of preventing such poisoning is with an "inhibitor filter," available for insertion in the
So far, we have indicated that the scale range of these instruments is limited to 0 to 1 0 0 % of the lower explosive limit. If leaking gas in buried pipe systems is to be located, a means to increase the basic range of the instrument must be employed. Until the past few years, this has generally been accomplished by the use of a dilution tube, dilution valve, or range multiplier, all of which essentially cause the dilution of the sample in a ratio of 10:1 or 2 0 : 1 , thus producing a new mixture which can be indicated on the scale of the instrument. Recently, a new in-
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strument was created to perform the j o b more conveniently. Essentially, this detector is two instruments built in a common case, with one circuit the same as that used in the instruments previously described, and the other, a thermal conductivity circuit to measure gas concentrations above the lower explosive limit. A dual scale meter is graduated from 0 to 100% of the lower explosive limit, and 0 to 1 0 0 % gas by volume. Such instruments are normally calibrated on natural gas and have found considerable usage in utility companies for the initial detection of low gas concentrations and for tracing high concentrations of escaping gas at its source. Calibration Kits
For many years, there was a need for a field method to verify the performance of hot wire instruments upon which the health and safety of workers depend. Today, manufacturers of vapor testers have made available convenient calibration checking devices which are small, lightweight, and engineered for simplicity of use in the field. These test kits provide a means ot preparing a known concentration of a gas-air mixture for the purpose of checking the accuracy of the combustible gas indicators. Larger quantities of test gases are also available in compressed gas cylinders at high pressures. Limitations in Use
T h e instrument will only respond to those combustibles which are drawn through the sampling system. If the flash point of a material is higher than the normal ambient testing temperature, a relatively low concentration will be indicated. Should a closed vessel holding, such a contaminant be subsequently heated, as by welding or cutting, the vapor concentration will increase and the atmosphere of this container, which originally showed only a low concentration of vapor, may now be explosive. When testing atmospheres in drying ovens or other places where the temperature is abnormally high, some difficulty may be encountered when testing solvents (such as naphthas) having a relatively high boiling
point. Vapors in such samples may condense in the sampling line or indicator flow system operating at temperatures below those of the oven, thus giving a false indication of safety. I n some instances, con densation can be prevented by heating the sampling line a n d in strument to a temperature equivalent to or above that of the space to be tested. The combustible gas indicators have been designed for testing flam mable gases a n d vapors in air and are, therefore, not applicable for measuring combustibles in steam or inert atmospheres, d u e to the a b sence of oxygen necessary to support combustion on the filament unit. Similarly, these instruments will not indicate the presence of explosive or combustible mists or sprays, such as lubrication oils, or explosive dusts, such as grain or coal dust. Although trace quantities of halogenated hydrocarbons a n d acid gases, such as hydrogen chloride or hydrogen sulfide, will not seri ously affect the performance of a hot wire instrument, significant con centrations of these compounds, when tested continuously, will cause corrosion of the filaments and neces sitate their replacement at more frequent intervals than would lie expected. Furthermore, their prod ucts of combustion will rapidly desensitize the filament units causing incorrect response of these gases on the meter scale. Since it has been determined that minute concentrations of silicone vapors, even one or two parts per million, can rapidly poison the cata lytic activity of a platinum filament, use of a hot wire combustible gas indicator is not recommended where these vapors may be present. Effective and proper use of instru ments to detect explosive a n d toxic concentrations of combustibles re quires that the operator know their limitations.
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. . . use α
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Of A m e r i c a n C a n , C o m p a n y CHEMICAL SALES DEPARTMENT Registered U. S. trademark MENASHA, WISCONSIN MARATHON · A D i v i s i o n of A m e r i c a n Can Co. C H E M I C A L S A L E S DEPT. · M E N A S H A , W I S . Please send Information File N o . J-81 Q . Samples Π of MARASPERSE for use in
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VOL. 53, NO. 8 ·
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