FMDS0759 Factory Mutual Data Sheet 7-59

FM Global Property Loss Prevention Data Sheets

7-59 September 1977 Revised May 2000 Page 1 of 11

INERTING AND PURGING OF TANKS, PROCESS VESSELS, AND EQUIPMENT

Table of Contents Page 1.0 SCOPE SCOPE ................................................................................................................................................... 2 1.1 Changes .................... .............................. .................... .................... .................... .................... .................... .................... .................... .................... ................... ................... ................... ......... 2 2.0 LOSS PREVENTION RECOMMENDAT RECOMMENDATIONS IONS ....................................................................................... 2 2.1 Introduction ................... ............................. .................... .................... .................... .................... .................... .................... .................... .................... .................... .................... ............... ..... 2 2.1.1 Sources of Inert or Purge Gas .................... .............................. .................... .................... .................... .................... .................... .................... .............. .... 2 2.1.2 Purging .................... .............................. .................... .................... .................... .................... .................... .................... .................... .................... .................... .................... .......... 3 2.1.3 Inerting .................... .............................. .................... ................... ................... .................... .................... .................... .................... .................... .................... .................... ............ .. 3 2.2 Operation and Maintenance .................... ............................. ................... .................... .................... .................... .................... .................... .................... ................... ......... 3 3.0 SUPPORT FOR RECOMMENDATIONS RECOMMENDATIONS ............................................................................................... 3 3.1 Applications .................... .............................. .................... .................... .................... .................... .................... .................... .................... .................... ................... ................... .............. .... 3 3.2 Inert Gas Sources .................... .............................. .................... .................... .................... .................... .................... ................... ................... .................... .................... .............. .... 3 3.3 Methods of Application ................... ............................. .................... .................... .................... .................... .................... .................... .................... .................... ................. ....... 8 3.3.1 Batch Purging Methods .................... .............................. .................... .................... .................... ................... ................... .................... .................... ................ ...... 8 3.3.1.1 Calculations ................... ............................. .................... .................... .................... .................... .................... .................... .................... .................... ............ .. 9 3.3.1.2 Examples .................... .............................. .................... .................... .................... .................... ................... ................... .................... .................... ................ ...... 9 3.3.2 Continuous Inerting Methods ................... ............................. .................... .................... .................... .................... ................... ................... ................ ...... 10 3.3.2.1 Fixed Rate Application. .................... .............................. .................... .................... .................... .................... .................... .................... ............ .. 10 3.3.2.2 Variable-Rate Variable-Rate or Demand Application. ................... ............................. .................... .................... .................... ................... ......... 10 3.3.2.3 Peak Demand for Continuous Inerting ................... ............................. .................... .................... .................... ................... ......... 10 4.0 REFERE REFERENCES NCES ...................................................................................................................................... 11 4.1 FM Global ................... ............................. .................... .................... .................... .................... .................... .................... .................... .................... .................... .................... ............... ..... 11 4.2 NFPA NFPA Standards ................... ............................. .................... .................... .................... .................... .................... .................... .................... .................... .................... ............... ..... 11 APPENDIX APPEND IX A GLOSS GLOSSARY ARY OF TERMS ..................................................................................................... 11 APPENDIX APPEND IX B DOCU DOCUMENT MENT REVISION HISTORY HISTORY ..................................................................................... 11

List of Tables Table 1. Maximum Maximum Recom Recommende mended d Percen Percentage tage of Oxyge Oxygen n to Prevent Ignition Ignition of Flammable Gases and Vapors Using Nitrogen and Carbon Dioxide for Inerting ................... .......................... ....... 5 Table 2. Maximu Maximum m Recom Recommende mended d Percen Percentage tage of Oxyge Oxygen n to Prevent Ignition Ignition of Combustible Dusts Using Carbon Dioxide as the Atmospheric Diluent .......................................... 6 Table 3. Maximu Maximum m Recom Recommende mended d Percen Percentages tages of Oxygen to Preven Preventt Ignition of Variou arious s Metal and Metal Hydride Dusts in Inert Atmospheres ...................................................................................... 8

©2000 Factory Mutual I nsurance Company. All rights reserved. No part of this document may be reproduced, stored in a retrieval system, or transmitted, in whole or in part, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without written permission of Factory Mutual Insurance Company.

7-59

Inerting and Purging

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1.0 SCOPE This data sheet outlines procedures for preventing explosions by the use of inert gas in enclosed or semienclosed spaces such as tanks, process vessels, or other process equipment containing flammable vapors or gases, or combustible dusts. This data sheet does not apply to purging of enclosures for electrical equipment in hazardous locations (see Data Sheet 5-1, Electrical Equipment in Hazardous Locations , for further information). In this data sheet, inert gases are those that do not react significantly with either oxygen or flammables. 1.1 Changes May 2000. This document has been reorganized to provide a consistent format. 2.0 LOSS PREVENTION RECOMMENDATIONS 2.1 Introduction 2.1.1 Sources of Inert or Purge Gas 2.1.1.1 The purging or inerting gas should be obtained from a dependable source, capable of supplying the required rate and total quantity of gas. This is done in order to maintain the desired degree of oxygen deficiency or level of flammable gases or vapors within the enclosure protected. Where the equipment to be protected is of critical importance, and the supply is obtained only from storage tanks or gasholders, provisions should be made to maintain the supply while recharging the tank or tanks. If gas generators are used without storage tanks or gasholders, their combined capacity should be such that loss of a single unit will not reduce the supply below peak demand requirements. 2.1.1.2 Pipe, valves, and fittings should be of materials suitable for the intended operating pressures and temperatures, and anticipated corrosion or vibration. Cast iron should not be used. 2.1.1.3 Moisture traps should be provided at low points as necessary, and piping should drain toward them. Blowdown connections and protection against freezing should be provided, as necessary. 2.1.1.4 Strainers, screens or filters should be installed where the inert or purge gas system has pressure regulators or flow control valves which are critically important for proper regulation in order to meet peak demands. These will keep rust and scale from entering the control devices. 2.1.1.5 Where two or more subdivisions in an installation are protected, back pressure control valves should be provided, to limit the flow in any branch. This will prevent excessive flow of gas due to failure or malfunction in one part of the distribution system from reducing the supply to other protected subdivisions below a predetermined amount. A manual shutoff valve should be provided for each subdivision of the main distribution system. 2.1.1.6 A check valve or other suitable device should be provided at each connection between a point-ofuse for purge or inert gas and the main distribution system. This will prevent contamination of the system through a drop of purge gas pressure or excessive pressure in the purged unit. An additional check valve or other suitable device should be provided on the discharge side of the gas supply source. 2.1.1.7 Cross-connections between the purge or inert gas distribution system and any other utility system, such as compressed air, should not be permitted. Emergency connections to individual inerted units should be by a temporary hose connection rather than permanent piping, and located downstream of the check valve at the point of use. 2.1.1.8 The purge or inert gas system should be electrically bonded and grounded to minimize static electricity. 2.1.1.9 Facilities for introducing purge or inert gas into the enclosure to be protected should be arranged to ensure effective distribution. Multiple inlets may be necessary. High velocity gas streams should be avoided, to prevent agitation of the product and generation of static electricity.

©2000 Factory Mutual Insurance Company. All rights reserved.

Inerting and Purging FM Global Property Loss Prevention Data Sheets

7-59 Page 3

2.1.2 Purging 2.1.2.1 A pressure gauge or manometer should be provided at the purge gas inlet and outlet connections to indicate conditions within the equipment being purged. Positive or negative pressure in excess of safe limits should be avoided. 2.1.2.2 In sweep-through purging, a slight positive pressure should be maintained. 2.1.2.3 Combustible gas indicators and oxygen analyzers should be used to monitor the purging operation. When purging to remove combustibles, the maximum flammable vapor concentration should be less than 25% of the lower explosive limit (in air). After purging is completed and air introduced, ventilation should maintain the atmosphere in the enclosure below 25% of the lower explosive limit. If vapors exceed safe levels, inert gas should be introduced, and purging resumed. 2.1.3 Inerting 2.1.3.1 Instrumentation should be provided to monitor the quantity and quality of the inert gas supply. a) Appropriate oxygen concentration instrumentation should be provided, arranged to sound an alarm at predetermined levels, and shut down or provide corrective action when unsafe conditions are approached. Samples should be taken from as many points in the distribution system as necessary, including from within the protected equipment, to ensure that the desired oxygen concentration is maintained throughout the entire process. b) Temperature, pressure, and flow measuring devices should be provided, as well as other appropriate instrumentation. When conditions are critical, these measurements should be recorded continuously. The devices should be provided with alarms and interlocks to initiate corrective action. 2.2 Operation and Maintenance 2.2.1 All systems should be thoroughly inspected and tested for proper operation following an established schedule and procedure. Any troubles or impairments should be corrected at once. 3.0 SUPPORT FOR RECOMMENDATIONS 3.1 Applications The primary function of the inert gas is to prevent the formation of explosive vapor-air mixtures in enclosed spaces. It may be used to purge tanks or process vessels prior to repair by displacing flammable vapors before air is introduced, and to purge air out before vapors are reintroduced. It also may be used to blanket flammable products in storage tanks or reactors, and prevent fires or the formation of explosive mixtures in drying ovens and glove boxes. Inert gas may be used to prevent dust explosions by keeping the oxygen concentration below that necessary to propagate an explosion. Equipment such as mixers, pulverizers, grinders, conveying systems, dust collectors, and storage bins may be protected in this way. Inerting also may be used to prevent spontaneous heating of materials or to safeguard operations that must be conducted above the ignition temperature of the material. 3.2 Inert Gas Sources Carbon dioxide or nitrogen in cylinders or bulk tanks is commonly used where the systems to be protected are small, and loss through leakage is slight. They may be the most practical supply even for large installations where no oxygen can be tolerated. Inert gas generators are a common means of obtaining inert gas. These burn hydrocarbon fuels such as oil or natural gas, generating 9 to 12% carbon dioxide, some carbon monoxide, and about 85% nitrogen as products of combustion. Up to 8% oxygen may remain with fuel oil firing, and usually less than 1% with natural gas. The composition may vary, depending upon the operation of the equipment. Coolers and scrubbers are needed to remove particles of burning soot, and to lower the temperature of the gas. Scrubbing or other conditioning is used to remove material that may contaminate the process.


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Products of combustion from process furnaces or boilers are often used as a source of inert gas. Cooling and scrubbing are usually needed to remove contaminants. Processing may be needed to get rid of excess oxygen under some firing conditions. Exhaust gases from internal combustion engines may be similarly used. Steam used for purging or inerting must be supplied at a rate sufficient to maintain the vessel temperature at 160°F (71°C) or higher. Care must be taken that condensation of steam by cooling does not draw in atmospheric oxygen, or collapse the vessel by implosion. Other means of obtaining inert gas are by catalytic combustion of ammonia and by using a package unit, recovering nitrogen from the air by liquification. Which gas is most suitable for a specific application depends on a number of factors. Besides being inert from a fire or explosion standpoint, it must be nonreactive with the materials being processed, and the materials of construction. The gas also must be free of moisture, if moisture causes excessive corrosion or contaminates the product. It must be reliably available at adequate volume and pressure to supply peak demands. The amount of inert gas required, and rate of application will depend upon the amount of oxygen in the gas and maximum permissible percentage of oxygen below which ignition will not occur. This varies with the flammable or combustible materials involved and the inerting medium, as shown in Tables 1, 2, and 3.



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Table 1. Maximum Recommended Percentage of Oxygen to Prevent Ignition of Flammable Gases and Vapors Using Nitrogen and Carbon Dioxide for Inerting

Acetone Benzene (Benzol) Butadiene Butane Butene-1 Carbon Disulfide Carbon Monoxide Cyclopropane Dimethylbutane Ethane Ether (Diethyl) Ethyl Alcohol Ethylene Gasoline Hexane Hydrogen Hydrogen Sulfide Isobutane Isopentane JP-1 Fuel JP-3 Fuel JP-4 Fuel Kerosene Methane Methyl Alcohol Natural Gas (Pittsburgh) Neopentane n-Heptane Pentane Propane Propylene

N 2 -Air Maximum Recommended O 2 Percent 11 9 8 9.5 9 4 4.5 9 9.5 9 8.5 8.5 8 9 9.5 4 6 9.5 9.5 8.5 9.5 9 9 9.5 8 9.5 10 9 9 9 9

CO2 -Air Maximum Recommended O 2 Percent 12.5 11 10.5 11.5 11 6.5 5 11 11.5 11.0 10.5 10.5 9 11 11.5 5 9 12 11.5 11 11 11 11 11.5 11 11 12 11 11.5 11 11

1. Data were obtained from publications of the U.S. Bureau of Mines. 2. Data were determined by laboratory experiments conducted at atmospheric temperature and pressure. Vapor-air inert-gas samples were placed in explosion tubes and exposed to a small electric spark or open flame. 3. The maximum recommended oxygen concentration should be reduced by 17% f or every 180°F (100°C) temperature increase above 32°F (0°C) up to 392°F (200°C). Above that temperature, the oxygen concentration should be reduced a further 12% for each 180°F (100°C) increase. For significant increases in pressure aboveatmospheric, a lower oxygen concentration willlikely be needed. The influenceis unpredictable, so tests or other specific information should be sought for the conditions involved.


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Table 2. Maximum Recommended Percentage of Oxygen to Prevent Ignition of Combustible Dusts Using Carbon Dioxide as the Atmospheric Diluent Dust

Maximum Recommended Oxygen Concentration, Percent Agricultural Clover seed 9 Coffee 11 Cornstarch 5 Dextrin 8 Lycopodium 7 Soy flour 9 Starch 6 Sucrose 8 Chemicals Ethylene diamine tetra acetic acid 7 Isatoic anhydride 7 Methionine 9 Ortazol 13 Phenothiazine 11 Phosphorous pentasulfide 6 Salicylic acid 11 Sodium ligno sulfonate 11 Steric acid and metal stearates 8 Carbonaceous Charcoal 11 Coal, bituminous 11 Coal, sub-bituminous 9 Lignite 9 Miscellaneous Cellulose 7 Lactalbumin 7 Paper 7 Pitch 5 Sewage sludge 8 Sulfur 6 Wood flour 10 Plastic Ingredients Azelaic acid 8 Bisphenol A 6 Casein, rennet 11 Hexamethylenetetramine 8 Isophthalic acid 8 Paraformaldehyde 6 Pentaerthritol 8 Phthalic anhydride 8 Polymer glyoxyl hydrate 6 Terephthalic acid 9 Plastics-Special Resins and Molding Compounds Coumarone-indene resin 8 Lignin 11 Phenol, chlorinated 10 Pinewood residue 7 Rosin, DK 8 Rubber, hard 9 Shellac 8 Sodium resinate 8



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Dust Maximum Recommended Oxygen Concentration, Percent Plastics-Thermoplastic Resins and Molding Compounds Acetal resin 5 Acrylonitrile polymer 7 Butadiene-styrene 7 Carboxymethyl cellulose 10 Cellulose acetate 5 Cellulose triacetate 6 Cellulose acetate butyrate 8 Ethyl cellulose 5 Methyl cellulose 7 Methyl methacrylate 5 Nylon polymer 7 Polycarbonate 9 Polyethylene 6 Polystyrene 8 Polyvinyl acetate 11 Polyvinyl butyral 8 Plastics-Thermosetting Resins and Molding Compounds Allyl alcohol 7 Dimethyl isophthalate 7 Dimethyl terephthalate 6 Epoxy 6 Melamine formaldehyde 11 Polyethylene terephthalate 7 Urea formaldehyde 10 1. Data are from U.S. Bureau of Mines Report of Investigations 6543. The data were based on laboratory experiments conducted at room temperature and pressure. Results were adjusted for strong ignition sources such as open flames. 2. The maximum permissible concentration of oxygen to prevent ignition by spark, when using nitrogen as the atmospheric diluent, can be calculated by a ‘‘rule of thumb’’ formula: On = (1.3) Oc – 4.5 Where ‘‘On’’ is the maximum recommended oxygen concentration using nitrogen as the atmospheric diluent. ‘‘Oc’’ is the maximum recommended oxygen concentration using carbon dioxide as the atmospheric diluent.


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Table 3. Maximum Recommended Percentages of Oxygen to Prevent Ignition of Various Metal and Metal Hydride Dusts in Inert Atmospheres Maximum Recommended Oxygen Percent Carbon Dioxide- Nitrogen-Air Air 2 7 16 – 0* –* 16 17 13 – 10 – 11 – 0* 2* 0* 5 14 – 12 11 0* 2 6 5 15 –* 0* 4* 13 10 0* 1* 0* 2* 13 – 9 9 0* 0* 8 8

Aluminum (atomized) (Al) Antimony (Sb) Dowmetal Ferrosilicon Ferrotitanium Iron, Carbonyl (Fe) Iron, Hydrogen reduced (Fe) Magnesium (Mg) Magnesium Aluminum (Mg-Al) Manganese (Mn) Silicon (Si) Thorium (Th) Thorium Hydride (Th H2) Tin (Sn) Titanium (Ti) Titanium Hydride (Ti H2) Uranium (U) Uranium Hydride (UH3) Vanadium (Va) Zinc (Zn) Zirconium (Zr) Zirconium Hydride (Zr H2)

Argon-Air – – – – – – – – – – – 2 4 – 4 8 2 2 – – 4 6

Helium-Air 10 – 3 – – – – 3 6 – – 5 5 – 7 8.5 2.5 4 – – 5 8.5

Note: Data were obtained by laboratory experiments conducted at atmospheric temperature and pressure. An electric spark was the ignition source. An (*) means that these metal dusts in piles or layers will ignite or glow in atmospheres of these inert gases and dust clouds of Zr, Th, U, and UH3 also ignited in CO 2.

3.3 Methods of Application Several methods may be used to form and maintain a noncombustible atmosphere in an enclosure. These include batch methods applicable to ‘‘one time’’ or occasional use, as in purging equipment during shutdown, and ‘‘continuous’’ inerting methods intended to ensure safe conditions during normal operations. 3.3.1 Batch Purging Methods These include siphon, vacuum, pressure, and sweep-through purging. Siphon Purging Equipment may be purged by filling with liquid (product or water), then introducing purge gas into the vapor space as the liquid is drained. The volume of purge gas required will be equal to the volume of the vessel, and the rate of application will correspond to the rate of draining. This method may not be suitable if the liquid is above its flash point because of evaporation into the space. Vacuum Purging Equipment that normally operates at reduced pressure (or in which it is practical to develop reduced pressure) may be purged during shutdown by breaking the vacuum with purge gas. If the initial pressure is not low enough to ensure the desired low concentration of oxygen or flammable gas or vapor, it may be necessary to re-evacuate and repeat the process. The amount of purge gas needed will be determined by the number of applications required to develop the desired concentration. Pressure Purging Enclosures may be purged by introducing purge gas under pressure and, after the gas has mixed, venting the enclosure to the atmosphere. More than one pressure cycle may be necessary to reduce the concentration of oxygen, flammable gas, or vapor to the desired level.



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Sweep-Through Purging This process involves introducing a purge gas into the equipment at one opening, and letting the enclosure contents escape to the atmosphere at another, sweeping out residual vapor or gas. The quantity of purge gas required will depend on the physical arrangement. A single pipe line can be effectively purged with only a little more than one volume of purge gas. However, vessels will require quantities of purge gas much in excess of their volume. If the system is complex, involving side branches through which circulation cannot be established, this method may be impractical, and pressure or vacuum purging might be necessary. 3.3.1.1 Calculations Calculations For siphon, vacuum, or pressure purging, the quantity of inert gas required may be calculated as follows: Volume of inert gas required for one inerting cycle: V2 =

(P2 – P 1) V1 P2

Oxygen content of enclosure after one inerting cycle, using an oxygen free inert gas: O2 = O1

P1 P2

Oxygen content of the enclosure after one inerting cycle, using an inert gas containing oxygen: O2 = O1

P1 P2

+ O3

(P2 – P 1) P2

where: P1 = Pressure in enclosure (absolute) prior to introduction on inert gas. P2 = Pressure in enclosure (absolute) after introduction of inert gas. V1 = total volume of enclosure. V2 = volume of inert gas required, measured at pressure, P 2. O1 = proportion of oxygen in enclosure at start of inerting cycle. O2 = proportion of oxygen in enclosure at end of inerting cycle. O3 = proportion of oxygen in inert gas. The units can be english or metric, providing P 1 and P2, and V1 and V2 are measured in the same units. 3.3.1.2 Examples Example No. 1. (Vacuum purging) Assume a 1000 gallon vessel operating under an absolute pressure of 5 psi. P1 = 5 psia P2 = 14.7 psia V1 = 1000 gal = 1000/7.48 = 133.7 ft 3 V2 =

14.7 – 5 (133.7) = 88.2 standard ft 3 inert gas required for one cycle. 14.7

If the vessel originally contained air (0.209 parts oxygen) and the inert gas contains no oxygen, the oxygen content after one inerting cycle would be: O2 = 0.209

(5) = .071 or 7.1% oxygen. (14.7) ©2000 Factory Mutual Insurance Company. All rights reserved.

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Example No. 2. (Pressure purging) Assume a 4.5 m3 vessel operating at atmospheric pressure, which can be pressurized to two atmospheres (absolute) with inert gas containing 2 percent oxygen. The vessel is originally filled with air. Volume of inert gas required for each cycle: P1 = 1 atmosphere P2 = 2 atmospheres V1 = 4.5 m3 V2 =

(2 – 1) (4.5) = 2.25 m3 at 2 atmospheres pressure or 2.25 × 2 = 4.5 standard m 3 2

Proportion of oxygen after one cycle: O2 = 0.209

(1) (2 – 1) + 0.02 = .104 + .01 = .114 = 11.4% (2) (2)

Proportion of oxygen after the second cycle: O2 = 0.114

(1) (2 – 1) + 0.02 = 0.057 + 0.01 = 0.067 = 6.7% (2) (2)

3.3.2 Continuous Inerting Methods These include fixed-rate application and variable-rate or demand application. 3.3.2.1 Fixed Rate Application. This method involves the continuous introduction of inerting gas into the enclosure at a constant rate, and a corresponding release of inert gas and whatever gas, vapor, or dust has been picked up in the equipment. The rate must be sufficient to supply the peak requirement in order that complete protection may be provided. Advantages are simplicity, lack of dependence on devices such as pressure regulators, and possible reduced maintenance. Disadvantages: 1. Where the space contains a volatile liquid, a continuous loss of product will occur, due to constant sweeping of the vapor space by the inert gas. 2. A large quantity of inert gas is used, because it is supplied whether needed or not. 3. There may be disposal problems (toxic and other effects) for the mixture that is continuously released. 3.3.2.2 Variable-Rate or Demand Application. This method involves the introduction of inerting gas into an enclosure at a variable rate, dependent on demand. This is based usually on maintaining within the protected enclosure a pressure slightly above that of the surrounding atmosphere. An advantage is that inert gas is supplied only when actually needed, reducing the total quantity required, loss of product, and disposal problems. A disadvantage is dependence on the functioning of flow control valves actuated by very low pressure differentials, which are sometimes difficult to maintain. 3.3.2.3 Peak Demand for Continuous Inerting Normally, the peak demand for continuous inerting is controlled by the maximum rate of liquid withdrawal and temperature change. For a vessel containing a liquid, the inert gas demand for liquid withdrawal will be the capacity of the largest pump that can be used to withdraw liquid, or the maximum possible gravity outflow rate, whichever is greater. ©2000 Factory Mutual Insurance Company. All rights reserved.


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The maximum demand from temperature change will occur in outdoor tanks operating at near atmospheric pressure as a result of sudden cooling by a summer thunderstorm. The rate of inert gas supply necessary to prevent vessel pressure from falling significantly below atmospheric pressure can be calculated as follows: a) For tanks over 800,000 gal (3000 m 3) capacity, 2 ft 3 of inert gas/hour/ft 2 (0.6 m3 /hour/m2) of total shell and roof area. b) For smaller tanks, 1 ft3 of gas/hour/40 gal of tank capacity (1 m 3 inert gas/hour/5.3 m 3 of tank capacity), or the rate corresponding to a mean rate of change of the vapor space temperature of 100°F (55°C) per hour. The rates for temperature change and liquid withdrawal must be added together. In some equipment, such as pulverizers, the rate of inert gas supply necessary to exclude air may be dominated by leakage, and temperature change can be ignored. 4.0 REFERENCES 4.1 FM Global Data Sheet 5-1, Electrical Equipment in Hazardous Locations . 4.2 NFPA Standards Inerting and Purging are covered by NFPA No. 69, Explosion Prevention Systems. There are no conflicts with that standard. APPENDIX A GLOSSARY OF TERMS Inert gas: in this data sheet, these are gases that do not react significantly with either oxygen or flammables. Inerting: the long term maintenance of an inert atmosphere in an enclosed space. Purging: the short term introduction of an inert gas to permit the transition from above the upper explosive limit to below the lower explosive limit, or vice versa, without passing through the explosive range. APPENDIX B DOCUMENT REVISION HISTORY December 1977 — new document issued.

FM Engr. Comm. September 1977


FMDS0759 Factory Mutual Data Sheet 7-59

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