1022 PREVENTION OF TOXIC CHEMICAL RELEASE Purging of process vessels, tanks and piping before startup and after shutdown is imperative for pilot plants and large- scale plants in the process industries or wherever combusti- ble gases and vapors are handled. Gases most often used for purging are nitrogen, carbon dioxide, or gases derived from the combustion of hydrocarbons. The reader is referred to the sections on Vapor and Gaseous Pollutant Fundamentals and to Fossil Fuel Cleaning for unit operations and incinera- tion procedures involved in hazardous gas removal. To show how the purging procedure can be “mapped” a typical purg- ing chart will be constructed and the salient points explained (see Appendix). Inert gases such as nitrogen have the property of not only depressing or narrowing the explosive range of a combus- tible gas or vapor, but also of preventing the formation of explosive mixtures with air when these inert gases are mixed in suitable proportions with either air or with the combus- tible gas or with an explosive mixture of both. By displacing or mixing air contained in a vessel, tank or piping system to be placed into gas service, with a suitable amount of an inert gas such as nitrogen, a combustible gas may subsequently be introduced without the formation of an explosive mix- ture. Similarly, by displacing or mixing the combustible with a suitable amount of nitrogen, air may later be introduced without causing an explosive mixture to develop. During the purging procedure constant sampling of contents must be pursued using standard accepted methods of chemical analysis. GAS FLAMMABILITY LIMITS A flammable mixture of a gas, such as acetylene and air, may be diluted with one of the constituents (acetylene or air) until it no longer is flammable. The limit of flammability due to dilution is the borderline composition: a slight change in the direction will support burning, while in the other direction combustion cannot be supported and maintained. At the ends of these two extremes there are well defined limits within which self-propagation of flame will take place on ignition. These are known as “upper” and “lower” limits as defined in terms of the percentage by volume of com- bustible gas present in a mixture of the gas and air. Table 1 below lists these limits for some of the more common gases and vapors for conditions of atmospheric pressure and temperature. Within these limits, the combustible gas and air mix- ture liberates sufficient energy to continue to propagate flame from one mixture layer to the other. Mixtures above the upper limit may burn on contact with external air, since these layers are formed in the zone where gases mix. Certain conditions effect a shift in the two limits, either increasing or decreasing the spread between them as we will note later. These conditions include: ignition source, ignition inten- sity, direction of flame propagation (upward, downward, or across), size and shape of container, vessel, or piping orien- tation, temperature, pressure and humidity in the containing vessel, oxygen content and turbulence. EXPLOSIONS When a chemical reaction is accompanied by the libera- tion of heat, as the reaction progresses, it is followed by an increase in the amount of heat, which in turn helps to accel- erate the reaction. Thus the two advance together, both in highly intimate connection and mutually helpful, until the entire mass has been heated and chemically converted. When a burning substance, a match for instance, is placed in contact with the extreme outer limit of an explosive mix- TABLE 1 Per cent gas or vapor in mixture—flammability limits Gas or vapor Lower Upper Gas or vapor Lower Upper Acetylene 2.5 80.0 Carbon monoxide 12.5 74.2 Acetone 2.6 12.8 Hydrogen 4.0 74.2 Ethane 3.2 12.5 Ethyl ether 1.9 48.5 Propane 2.4 9.5 Carburetted 6.4 47.7 Butane 1.9 8.4 Hydrogen sulfide 4.3 45.5 Benzene 1.4 8.8 Methyl alcohol 6.7 36.5 Gasoline 1.5 6.2 Coal gas 3.9 29.9 Turpentine 0.8 — Ethylene 2.8 28.6 Ammonia 16.0 27.0 Pennsylvania 4.9 14.1 Ethyl alcohol 3.3 19.0 Natural gas — — Methane 5.0 15.0 Reference: G. W. Jones, Chemical review Vol. 22, 1938. pp. 1–26. C016_011_r03.indd 1022C016_011_r03.indd 1022 11/18/2005 1:12:55 PM11/18/2005 1:12:55 P © 2006 by Taylor & Francis Group, LLC PREVENTION OF TOXIC CHEMICAL RELEASE 1023 ture of gas and air, ignition takes place at the point of contact, i.e. incites chemical reaction. Combustion proceeds from the outside of the mixture towards the center, and thereby forms a plane of combustion which divides the gaseous mixture into two parts. On the one side are the highly heated products of combustion, and on the other, is the still unconsumed gas mixture. The velocity at which this plane advances in different for each gaseous mixture, and depends both on the compo- sition of the mixture and on the pressure to which it is sub- jected. The higher the velocity of propagation the greater the rise in temperature, and this latter, in turn, directly influ- ences gas expansion and the products of combustion, which thereby exert such a high pressure on their environment that any opposing medium, vessels, tanks, piping, walls, etc. is ruptured. APPLICABLE EXPLOSIBILITY DATA Not all explosibility data reported in the literature are appli- cable to purging problems. From the safety standpoint, it is desirable to select the widest explosive limits for the purge operation. In addition, an ample safety factor be applied, especially to the lower explosive limit. For acetylene-air mixtures, at atmospheric pressure and temperature, the published and accepted values for the lower explosive limit (LEL) and upper explosive limit (UEL) are 2.5 and 80.0% acetylene in air respectively. These are the widest limits recording. See Table 1. In every case involving combustible gas handling or processing, constant awareness of the inherent dangers to life and limb as well as property is imperative. In the case of acetylene, it is a well known fact that it has unstable characteristic at any pressure and whether or not a decom- position would take place depends on the intensity of the initial source of ignition. Higher pressures have the effect of lowering the initial energy necessary to initiate a decomposition. ACETYLENE—CASE STUDY Acetylene is inherently an unstable gas at any pressure and whether or not a decomposition would take place depends on the intensity of the initial source of ignition. Higher pres- sures have the effect of lowering the initial energy necessary to start a decomposition. Higher initial pressures will also result in an increase in the ratio of the maximum pressure developed in the decomposition to the initial pressure. As for acetylene-air mixtures, pretty much the same behavior is manifested. With decreasing ignition energies, the initial pressures must be correspondingly increased so as to bring the total gas mixture volume to decomposition. The size, shape and orientation of a process vessel as well as its material of construction have a profound effect on the ignition limits of combustible gas mixtures. The same applies to acetylene decompositions. It is a matter of heat balance during the combustion process. Even the relative position of the source of ignition. The widest explosive range is obtained for vertical cylindrical vessels or tanks when the ignition source is located at its base. The presence of water vapor and high humidity acts as a diluent and an inert gas. This effect is typical for all combus- tible gas–air mixtures. As for the effect on explosion limits, the ratio of vessel surface to cross-sectional area from a cooling or heat bal- ance point of view, long slender vessels (high ratio of surface to sectional area) narrow the limits of explosibility. It’s all a matter of the rate at which heat is removed from the gas mix- ture inside the vessel. As for the temperature effect, higher temperatures induce convection currents within the vessel and increase turbulence and widen the limits. Thus, only tests of actual operating setups can tell the effects on the widening or narrowing of the explosibility range. Then, once established, the LEL and UEL determined experimentally can be used to develop a purging graph here- inafter discussed and developed. EFFECT OF INERT GAS The effect of an inert gas on explosibility of combustible gases in air can be shown graphically on Figure 1 for acetylene– air mixtures. Once the conditions for actual operating condi- tions and configuration are determined experimentally, the graph can be set up as we shall see. As nitrogen is added to mixtures of the gas and air within the explosive range, a series of new mixtures are formed each of which has a different UEL and LEL than the pre- ceding mixture and the explosive range is narrowed along definite lines of demarcation. As the air and combustible gas contents are reduced by the addition of nitrogen, the line of LELs and the line of UELs converge and meet at a point, i.e., D. Here the range has degenerated to zero. No mixture of acetylene, air and nitrogen which contains less oxygen than the lowest point on the line LDU (point D ) is explosive in itself, but all mixtures within the areas bound by LUD are within the limits of explosibility and are therefore explosive. Again, in Figure 1, line ADE is drawn tangent to LDU at D. Any mixture represented by point X to the right of line DE and below the upper explosive limits DU is not explosive in itself. However, on dilution of that mixture with air, a new series of mixtures will be formed having compositions fall- ing along line X A which passes through the explosive area. Similarly, any mixture represented by a point to the left of line LD and DE will not form explosive mixtures on further dilution with air. Note line LD is not vertical but swings to the right as falls. APPLICATION OF GRAPH At startup when placing gas equipment into service purg- ing from air to inert gas, the object is to reduce the oxygen C016_011_r03.indd 1023C016_011_r03.indd 1023 11/18/2005 1:12:55 PM11/18/2005 1:12:55 P © 2006 by Taylor & Francis Group, LLC 1024 PREVENTION OF TOXIC CHEMICAL RELEASE 100 90 80 70 60 50 40 30 20 10 0 z 100 90 80 70 60 50 40 30 20 10 0 Z Y X 100 90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100 20 18 16 14 12 10 8 6 4 2 0 F D B E 21 97.5 % Y START (LEL) A Combustible gas (Acetylene per cent by Volume) Inert gas (Nitrogen) per cent by Volume (Taking Equipment out of Service) X START (LEL) Line of LEL’S C 20% Lower Limit 2.5 % C 2 H 2 pure and dry 97.5% air dry 0% N 2 inert added Line of flammability limits from Fig.1 C 2 H 2 and air only zero inerting nitrogen Explosion (2) range narrowed on addition of nitrogen Line of UEL’s Upper Limit 80% C 2 H 2 pure and dry 20% Air dry 0% N 2 inert added 2.5 4.8 Inert Gas (Nitrogen) per cent by Volume (Putting Equipment into Service) Air per cent by Volume (Equivalents) Oxygen equivalent in air per cent by Volume (1) FIGURE 1 Acetylene-Air-Nitrogen purging chart (atmospheric temperature and pressure). C016_011_r03.indd 1024C016_011_r03.indd 1024 11/18/2005 1:12:55 PM11/18/2005 1:12:55 P © 2006 by Taylor & Francis Group, LLC PREVENTION OF TOXIC CHEMICAL RELEASE 1025 content of the air inside the equipment to a point that acety- lene (or other gas) may be subsequently introduced with- out forming an explosive mixture. From Figure 1 it may be observed that the safe condition will be reached when the oxygen content within the vessel or equipment has been reduced by the introduction of nitrogen to below approxi- mately 6.6% by volume, point F where FC drawn tangent to LDU at D intersects the vertical axis AB. Any mixture of this combustible gas, nitrogen and air having an oxygen content represented by a point below this line FC may be diluted with the combustible gas without forming an explo- sive mixture (see Appendix). WHEN SHUTTING DOWN Likewise, when shutting down or withdrawing equipment containing acetylene from service or when purging from combustible to inert gas, the object is to reduce combustible gas content to such a point that the air may subsequently be introduced without fear of forming an explosive mixture. Figure 1 shows that a safe condition will be reached when the combustible gas content within the equipment or piping has been reduced to below 4.8% by volume, denoted by point E. On occasions the mixtures of acetylene, nitrogen and air are to be diluted with a further quantity of inert gas so as to render them nonexplosive or incapable of forming explosive mixtures on later addition of air. Object here is to add suf- ficient nitrogen to convert the existing composition of the mixture to some composition corresponding to a point to the left of LD and DE. If we assume that the mixture is again denoted by point X , then on addition of nitrogen, successive mixtures will be formed along XB. Point where XB crosses DE denotes the safe endpoint composition. Then on the fur- ther addition of air (intentional or accidental) the line of mix- tures will fall to the left of LDE. In practice, it is desirable to decrease the proportions of oxygen and combustible gas below the determined values to provide a factor of safety. To prevent explosion in chemical processing, it is necessary to keep the air–gas or–vapor or mixture of the gases, air and nitrogen, below the LEL within the equipment most likely to be affected. An established safe practice is to keep the mixture well blended with air or inert gas so that calculated or experimental values never reach more than 25% of the LEL or other low limit as determined from Table 1 in temperatures below 250°F. Placing system into service or out of service should be done when system has been depressurized to atmospheric pressure. DEPRESSURIZING TO ATMOSPHERE More often than not, combustible-gas releases to atmosphere do not contain air. When handling acetylene in its pure state a maximum pressure of 15 psig is usually used. Thus on release to atmosphere large quantities of the gas may be emitted through pressure relief valves to flares or high stacks. Then again the gas may be returned to gas holder storage. If released to atmosphere, inert gas purging should be provided. An excellent reference on inert-gas purges of stacks on pressure releasing has been published * ; for process equipment see review. 12 SAMPLING OF MIXTURE CONTENTS During startup or shutdown of systems, sampling of mix- tures within equipment should be conducted downstream of the point of injection of nitrogen to ensure proper mixing of gases before sampling. Taking samples from stratified streams should be avoided. For piping systems sampling should be taken downstream of elbows and open valves. In anticipation of this elbows and turns as well as valves should be designed into the system originally. In actual testing it has been observed that form 6 to 10 volume changes are neces- sary to purge equipment or piping systems. Sampling should begin after this purge. Purging to a point below the LEL may be done using an explosimeter † that has been properly calibrated for the gas–air–nitrogen mixture. Laboratory testing procedures may also be used where samples are washed † with acetone and the volume of acetylene absorbed deducted from the entire sample to give volume of the remainder gases as air or nitrogen. Spark-resisting tools should be used to prevent sparking and possible ignition of combustible gas. Work shoes with rubber soles and so steel studs should be used. In addition, continuous sampling of the workroom atmosphere should be conducted using explosimeters built for that purpose. INERT GAS SUPPLY Where rigid purity of inerting gas is not a requirement, the use of inert gases obtained from the combustion of hydrocar- bon fuels may be used. This decision is one to be based on experience and processing needs. Gas analysis can now be accurately measured and con- trolled in tenths of a percent range. These inert gas genera- tors are provided with rugged instrumentation and controls for hazardous environments as found in desert and offshore installations. The analysis can even be accurately measured down to the ppm range where instrumentation maintenance is present. The same caliber of technician is needed whether or not the nitrogen source is cryogenic or hydrocarbon. Such inert gas generators are currently supplied with automatic trimmer control which analyzes the discharge gas and automatically controls the air-to-fuel ratio. For critical installations they are even equipped with inexpensive auto- matic vent and alarm systems. For more information and their technical manual, write to C.M. Kemp Manufacturing Company, Glen Burnie, Maryland 21061. † Explosimeters are available from Mine Safety Appliances Company, Pittsburgh, PA 15235 and Davis Emergency Equipment Company, Newark, New Jersey. C016_011_r03.indd 1025C016_011_r03.indd 1025 11/18/2005 1:12:55 PM11/18/2005 1:12:55 P © 2006 by Taylor & Francis Group, LLC 1026 PREVENTION OF TOXIC CHEMICAL RELEASE Skid-mounted units for applications in remote loca- tions, or where onsite cryogenic facilities are too large to be cost-effective, are available. These units provide nitro- gen at purities of 97 to 99.5%. They operate by pressure- swing absorption, in which a bed of activated carbon absorbs oxygen from air at high pressure and desorbs it at reduced pressure. The nitrogen produced by this cyclic process con- tains 5 ppm of water and CO 2 . 400 to 20,000 cfh of nitro- gen can be supplied, at a pressure of 100 psig. Write Airco Industrial Gases, Murray Hill, New Jersey. Nitrogen for purging may also be supplied from cas- cade systems or directly from “bottle” trailers under pres- sure, normally 2000 psig. When drawn from large cascade systems or reservoirs the nitrogen is piped into a “running tank.” The “run” tank is then pressurized and its contents are expanded through pressure regulators as desired for purging. When pressurized trailers are used, the high pressure gas is regulated down to the desired pressure for purging. ACKNOWLEDGMENTS This article appeared in its original form in Chemical Engineering, December 9, 1980, pp. 65–68. It has now been redone to provide more useful detail in the designation and use of the chart. A special section on checking points and areas on the graph has been included. This author grate- fully acknowledges with thanks the permission by Chemical Engineering to update and republish. APPENDIX STEPS IN SETTING UP PURGING CHART 1) Draw right triangle ABC whose size is to accom- modate ordinate and abscissa scales. 2) Fill in both ordinate and abscissa scales for the combustible of concern. 3) From Table 1 superimpose the explosive range of the combustible gas of concern on line AC, using the abscissa scale values. 4) Using the experimentally determined value equiv- alent of point, E , strike that value on line BC. 5) Draw line AE. 6) Draw lines LD and UD. 7) Draw line FDC. 8) Establish any point X below line FDC. 9) Draw lines AX and BX as shown. 10) Complete the chart by adding in the notes and other embellishments. Draw chart carefully so that the various points can be determined with accuracy. And remember to use a safety factor in applying the chart figures, i.e. 25% of the chart values (points E and F) when taking equipment out of ser- vice and when putting equipment into service, respectively. PURGING CHART DESIGNATIONS AND USE Point D ϭ No mixture of acetylene, air, and nitrogen which contains less than 6.4% oxygen is explosive. LDU ϭ All mixtures within area of triangle LDU are explosive. Point X ϭ Any mixture such as denoted by point X is not explosive. Line XA ϭ On dilution of mixture denoted by point X with air, new mixtures along XA are formed. LDE ϭ Any point to the left of LDE will not form explosive mixtures on dilution with air. FDC ϭ Any mixture of acetylene, nitrogen and air rep- resented by a point below FDC may be diluted with acetylene without forming an explosive mixture when placing equipment into service. Point F ϭ fe point reached after air displaced with nitro- gen and ready to add acetylene (6.6% oxygen) when placing equipment into service. Point E ϭ Safe point reached after acetylene displaced with nitrogen and ready to add air (4.8% acetylene). This is the situation when taking equipment out of service. CHECKING POINT AND AREAS ON GRAPH Purpose is to become better acquainted with the graph and to check various points. UEL From UEL on AC read down vertically to 80% acety- lene on BC. Then read horizontally left to YY and read 20% air by volume, with Zero inert nitrogen. Then by calculation and since composition of air is 21% oxygen and 79% nitro- gen by volume: O 2 ϭ 4.2%; N 2 ϭ 15.8%. LEL Likewise: 2.5% acetylene and 97.5% air and zero inerting nitrogen. In the air: 20.48% O 2 and 77.02 N 2 %. Point 1 This lies within area bound by LDU and should be explosive. Now from point 1 drop vertically and read 30% acetylene on BC. Move horizontally left to YY and read air per cent ϭ 4.6. Then by calculation, O 2 content in air ϭ 9.66% and nitrogen content ϭ 36.34%. By difference inert- ing nitrogen is 70–46 ϭ 24% by volume. Since 9.66% O 2 is greater than 6.4%, the mixture is explosive. Point X Should not be explosive. From X drop vertically and read 21.5% acetylene as before. Move horizontally left and read 19% air on YY. Then by calculation find O 2 in air ϭ 3.99% and N 2 in air ϭ 15.01%, leaving 59.5% inerting nitro- gen by volume. Since O 2 ϭ 3.99% and is less than critical value of 6.4%, mixture is not explosive. Point 2 Should be explosive. Acetylene ϭ 30%, air ϭ 70%. O 2 ϭ 14.7% and nitrogen ϭ 53.3%. Zero inerting nitrogen. Now since O 2 is greater than 6.4%, mixture is explosive. C016_011_r03.indd 1026C016_011_r03.indd 1026 11/18/2005 1:12:56 PM11/18/2005 1:12:56 P © 2006 by Taylor & Francis Group, LLC PREVENTION OF TOXIC CHEMICAL RELEASE 1027 X A line and UEL line intersect Acetylene ϭ 19%, air ϭ 29%, O 2 ϭ 6.09%, 52% inserting nitrogen. This is a borderline case and safely assume nitrogen is explosive. Dilute with acetylene of nitrogen to render mixture non-explosive. XB On dilution of mixture X with nitrogen, new mixture will fall along XB and remain non-explosive. In this process, per cent O 2 will continue to drop below 6.4%. Dilution with acetylene would effect the same results. When shutting down Add N 2 until acetylene reaches 25% of experimentally determined lower limit. Thus, 0.25 ϫ 4.8 ϭ 1.2%. When starting up Add N 2 until O 2 reaches 0.25 ϫ 6.4 ϭ 1.6%. When purging with any gas allow from 6 to 10 volume space changes before testing for concentrations. The Design Institute for Emergency Relief Systems (DIERS) Users Group, which is an affiliate of the American Institute of Chemical Engineers, has developed some methodology to design emergency relief systems. 8 The DIERS study was very extensive and complicated. It involved significant devel- opments and applications of complex theories and experi- ments. Some aspects were reaction kinetics under runaway conditions and multiphase critical flashing flow for viscous and nonviscous systems. A number of DIERS users have attempted to simplify the DIERS technology. 9,10,11 REFERENCES 1. Prevention of Acetylene–Air Explosions by Addition of Carbon Dioxide or Nitrogen. Dr. W. Gliewitzky, Berlin Autogene Metall bearbeitung, 1940, No. 1, 2–5. 2. Bureau of Mines Bulletin 279 (1938). Limits of Inflammability of Gases and Vapors. H.F. Coward and G. W. Jones. 3. Theoretical and Practical Considerations in Purging Practices. S. S. Tomkins, A.G.A. Proceedings (1934), pp. 799–828. 4. Precautions in High Pressure Acetylene Work. B.I.O.S. Final Report No. 1162. Item No. 22, London—H.M. Stationary Office. S.O. Code No. 51–1275–62. Tests conducted during years 1940, 1941 and 1944. 5. Industrial Explosion Prevention and Protection, Bodurtha, Frank T., Engineering Department, E.I. du Pont de Nemours & Company, Inc., 1st Ed. 1980, McGraw-Hill Book Company, New York City. 6. Kemp Gas Generator Technical Manual, The C.M. Kemp Manufac- turing Company, Engineered Gas Systems Division, Glen Burnie, Maryland 21061. 7. Gascope Combustible Gas Indicator, Models 60 and 62, Mine Safety Appliances Company, 600 Penn Center Boulevard, Pittsburgh, PA 15235. 8. H.G. Fisher. DIERS Research Program on Emergency Relief Systems. Chem. Engr. Prog., 81 (8), 33–36 (August 1985). 9. H.K. Fauske, G.H. Clare, and M.J. Creed. Laboratory Tool for Characterizing Chemical Systems. Proceedings of the International Symposium on Runaway Reactions, Cambridge, MA, March 7–9, 1989. Center for Chemical Process Safety/AIChE, New York, 1989, pp. 372–394. 10. J.A. Noronha, R.J. Seyler, and A.J. Torres, Simplified Chemical and Equipment Screening for Emergency Venting Safety Reviews Based on the DIERS Technology. Proceedings of the International Symposium on Runaway Reactions, Cambridge, MA, March 7–9, 1989. Center for Chemical Process Safety/AIChE, New York, 1989, pp. 660–680. 11. D.P. Mason. Highlights of FM Inspection Guidelines on Emergency Relief Systems. Proceedings of the International Symposium on Run- away Reactions, Cambridge, MA, March 7–9, 1989. Center for Chemi- cal Process Safety/AIChE, New York, 1989, pp. 722–750. 12. Chatvathi, Kris, and Richard Siwek. Suppression Systems Mitigate Explosions, Chem. Eng. Prog. Vol. 92, March 1996. JOHN D. CONSTANCE (DECEASED) Consultant, Cliffside Park, N.J. C016_011_r03.indd 1027C016_011_r03.indd 1027 11/18/2005 1:12:56 PM11/18/2005 1:12:56 P © 2006 by Taylor & Francis Group, LLC . 1022 PREVENTION OF TOXIC CHEMICAL RELEASE Purging of process vessels, tanks and piping before startup and after shutdown is imperative for pilot plants and large- scale plants in. terms of the percentage by volume of com- bustible gas present in a mixture of the gas and air. Table 1 below lists these limits for some of the more common gases and vapors for conditions of. Francis Group, LLC PREVENTION OF TOXIC CHEMICAL RELEASE 1025 content of the air inside the equipment to a point that acety- lene (or other gas) may be subsequently introduced with- out forming an