ES&H Manual

Flammable Gas Supplement




Chapter 1 – Storage/Use of Flammable Gases




1.0            Introduction


The use of flammable gases in physics experiments presents a unique type of installation, requiring special considerations.  In many cases, mixing of gases is involved.  Large volumes of gases may be present; thus even small leaks or ruptures of thin windows may cause incursions into the flammable concentration region with a large inventory to support fire.  Some flammable gases may be stored in the liquid state, increasing the inventory.  Electrical equipment is an integral part of such installations and can thus provide an ignition source if such a system is improperly designed, fabricated, or operated.  The purpose of this procedure is to mitigate the hazards associated with storage and use of flammable gases.


2.0            Scope


This standard applies to activities using flammable gases, whether part of approved experiments taking beam or in testing labs and shops on the Jefferson Lab site.  This standard specifically excludes liquid hydrogen target systems, gases used as fuels, gases used for welding, burning and brazing procedures.  See ES&H Manual Chapter 6122 Welding, Brazing, Cutting, and Grinding Safety and ES&H Manual Chapter 6150 Compressed, Liquefied, and Solidified Gas Program.  If the amount of flammable gas stored at any single location exceeds 10,000 pounds, the requirements of Occupational Safety and Health Administration (OSHA) Code of Federal Regulations (CFR) 29 part 1910 “Occupational Safety and Health Standards,” shall also be followed.


Flammable Gases may be diluted with inert gas to the point where the mixture is not flammable.  This standard does not apply to facilities using only non-flammable mixtures if they are pre-mixed and verified when obtained.  If a malfunction of an on-site mixing apparatus could cause a mixture to become flammable, the standard does apply to the worst case scenario.  The flammability of a mixture can be determined by referencing United States Department of the Interior Bureau of Mines Bulletins 503 and 627.  Contact the Physics Division ESH&Q Group for copies of these bulletins or the reference section in the Jefferson Lab library.


3.0            Classification of Gas Storage and Usage Facilities


3.1              An analysis of the gas system shall be conducted using the flowchart shown in Figure 1.  The analysis is based on the energy in the gas available for a fire, and classifies gas systems in order of increasing risk.  The flowchart in Figure 1 utilizes the limits on quantities of hydrogen gas.  These quantities must be adjusted for other gases using the heat of combustion as described in Flammable Gas Supplement Chapter 7 Heats of Combustion.  The results of the analysis indicate the Gas System Class as described in Table 1.


Table 1: Gas System Classes

Class 0

risk of small local flash fire

Class I

risk of a local fire

Class II

risk of a general fire


Examples of gas systems analysis are given in Flammable Gas Supplement Chapter 2 Examples of Gas System Class Determination.


3.2              The upper limit for a Class 0 gas system is the combustion energy in 0.6 kg of hydrogen.  The equivalent quantities of some typical flammable gases are given in Table 2.  This equivalence is scaled according to the heat of combustion of the gas.


Table 2:  Maximum Quantities of Flammable Gases Meeting Criterion for a Class 0 System


Mass (kg)

Volume (SCF)



















Dimethyl Ether




3.3              Installations may be subdivided into separable parts if restrictions exist to keep the gas flow from one part to another from exceeding ten times the normal flow rate.  Normally, the separable parts will be in physically separate rooms.  The gas system Class shall be determined for each installation or separable part thereof using Figure 1.


3.4              Gas in cylinders connected to a piping system is included in the quantity calculation.  Nearby stored gas may be excluded from the quantity calculations if the stored quantity is less than the amounts listed in Table 2 for Class 0 or the storage is located outdoors.

3.5              A written analysis of the Gas System Class is preferred; if the Gas System Class determination has not been documented in a written analysis, then Class II will be assigned by default.


Figure 1:  Flow Chart for Risk Classification


Before beginning to follow this flowchart, the following determination is needed:  evaluate the total gas inventory in terms of the hydrogen equivalent Q using the heat of combustion as a scaling parameter.  The system may be separable into parts each with its own risk classification.  For enclosed storage facilities, the value of Q is determined by the total gas present.  For outdoor storage facilities, the value of Q is determined by the largest single container or manifold system.  For systems in which the gas is not in storage, the inventory is the sum of the mass in all containers, including detectors, piping, and manifolds.




  1. At box 4, ventilation should be considered for accidental venting of gas including “worst case” scenarios such as the rupture of one or more detectors with discharge of their inventory.  See text for consideration of “steady state” leaks.  For storage facilities, this should consider the worst case such as the dumping of one full gas container.  In some systems a credible accident may have to include the dumping of several cylinders manifolded together.
  2. Ignition sources including arcing devices, electrical equipment not rated for Class 1 Division 2 locations per National Fire Protection Association (NFPA) 70, National Electric Code (NEC) 2002 edition Article 500-Hazardous (Classified) Locations, Classes I, II, III, Divisions 1 and 2, welding, grinding, open flames, etc.


4.0            Procedures for Approval


4.1              Class 0

The gas system class analysis shall be reviewed by the Physics Division Experimental ESH&Q Review Committee (E2RC) or by an independent reviewer appointed by the committee, in which case a copy of the independent review shall be sent to the E2RC.  Approval by the Physics Division Safety Officer is required before the introduction of flammable gas into a system.


4.2              Classes I and II

Procedures are the same as for Class 0, with the addition that if an independent reviewer or review panel is appointed, the E2RC must formally concur with the report of that review.  Approval by the Division Associate Director is required before the introduction of flammable gas into a system.


Nothing in this document is intended to prevent the use of methods or materials of equivalent or superior quality to those prescribed below.  The E2RC will review documented requests for equivalency.


5.0            Requirements for Flammable Gas Installations


5.1              Class 0 Installations:

5.1.1        The area shall be posted “Danger-Flammable Gases, No Ignition Sources” using standard signs available from the Jefferson Lab Safety Lab (in Building 35 or by contacting the Physics Division ESH&Q Staff).  A list of responsible persons with their phone numbers shall also be posted.

5.1.2        Combustibles and ignition sources shall be minimized within 10 feet or three meters of gas handling equipment, piping or apparatus.

5.1.3        A pressure regulator appropriate for the gas and its environment shall be used.

5.1.4        An orifice, excess flow valve or other fixed means of limiting the flow to no higher than ten times the maximum operational flow rate shall be installed.

5.1.5        All gas cylinders shall be secured.  Cylinders not in use shall be capped.  Empty cylinders shall be removed at the earliest convenient date or on a predetermined, regular schedule coordinated with vendor delivery of new cylinders.  See also ES&H Manual Chapter 6150 Compressed, Liquefied, and Solidified Gas Program.

5.1.6        Enclosed volumes containing piping or equipment shall be incapable of becoming pressurized.  For example, chest freezers shall not have latching doors.  Electrical devices enclosing or enclosed within these volumes shall be listed for use in Class 1, Division 2 locations per NEC Article 500 or otherwise be documented and approved as non-sparking devices.

5.1.7        Leaks from experimental devices such as drift chambers shall be measured and documented prior to initial operation (with nonflammable gas, if possible).  Leakage above seven liters/hour from any one chamber shall be mitigated.  Recheck for leaks after major repairs or modifications, and at least every twelve months.  Leakage exceeding 20% of the lower explosive limit at a distance over five centimeters from an identified “point” leak shall be repaired.

5.1.8        Ventilation above one air change per hour shall be maintained in areas using or storing flammable gas if normal operational flow rates are less than 5 Standard Cubic Liters Per Minute (SCLM).  This ventilation may be accomplished by mechanical or natural ventilation.  For natural ventilation a room vent with a minimum of 1/2 square foot of free area shall be provided per 1000 cubic feet of room volume.  If normal operational flow rates are greater than or equal to 5 SCLM, supervised mechanical ventilation in accordance with Section 7-2.2.1(a) of NFPA 58 Liquefied Petroleum Gas Code, 2004 edition shall be provided (not applicable to outdoor Jefferson Lab storage).

5.1.9        Welding permits (Fire Hazard Work Permit) Reference ES&H Manual 6122 Appendix T2 Fire Protection: Hot Work Permits shall not be issued for areas within 35 feet or ten meters of the equipment containing flammable gas unless approved in advance by the responsible Division/Section head or designee.


5.2              Class I Installations are subject to the following additional requirements:

5.2.1        The system, including vessels, chambers, supply and vent piping, and exhaust points shall be labeled “flammable gas.”

5.2.2        Piping requirements: Exceptions to this paragraph are permitted adjacent to experimental apparatus where needed for flexibility, electrical isolation, repairs or because of congestion.  This exception is limited to within five meters of the normal operating position.        Piping and fittings shall be protected from mechanical damage.        Piping shall be rated for the expected temperature and pressure.        Supply piping shall be metallic.        Piping shall be supported in a substantial and workmanlike manner.        Piping shall not be installed inside cable trays with electrical conductors.

5.2.3        Joints shall be made by welding, brazing, pipe thread, or commercial fittings appropriately installed.  Custom-made fittings required by detector design shall provide secure connections.

5.2.4        The entire piping system shall be pneumatically tested for leaks at approximately 0.9 times the relief pressure before operating the system.  Any piping with relief valve settings above 150 psig shall be tested at 1.25 times the relief pressure.

5.2.5        Bubblers, flow meters, and other instruments shall be securely mounted and protected from possible breakage.

5.2.6        Provisions shall be made to purge the entire system with an inert gas.  If vacuum pumps are used for this, they shall be listed for flammable gas service.

5.2.7        Pressure relief devices shall be provided to limit the pressure to the maximum working pressure in various parts of the system.  In the case of low pressure equipment, dedicated bubblers may be used as relief devices.  Common exhaust piping (where the flammable gas vent is shared with exhaust vents for other systems) shall not be used if equipment overpressure from any combination of devices sharing the exhaust could result due to built up back pressure.

5.2.8        Relief devices in flammable gas service with a capacity over two standard liters per minute shall be vented outdoors.  The exhaust locations shall be chosen to minimize fire hazards and shall not be within 10 feet or three meters of an air intake.  Vents shall be protected from clogging by debris, snow, or ice.

5.2.9        Flammable gas detectors shall be installed near equipment installations, mixing stations, and in storage sheds (the measures in this requirement are not necessary for outdoor storage applications):        A low level alarm no higher than 10% of the lower explosive limit (LEL) shall sound a local alarm and be used to initiate corrective action according to a plan included in the documentation of system operating procedures.        A high level alarm no higher than 25% of the lower explosive limit (LEL) shall summon the Newport News Fire Department through the Jefferson Lab fire alarm network.  This high level alarm shall also automatically shut off the supply of flammable gas and turn off power to potential ignition sources within 10 feet or three meters of operative gas usage apparatus.        “Crash buttons” shall be provided to accomplish the shutdowns described above.  These devices shall be conveniently located, and one shall be adjacent to the fire alarm panel, if present.  Crash buttons should shut off all flammable gas sources which could conceivably be confused by unfamiliar person in a state of panic.  Crash buttons shall be labeled “Gas System and Experiment Power Shutdown.”  They shall be located on the Building Evacuation Plan Maps.        Automatic restart of flammable gas systems and power sources shall not be allowed after a high level alarm.  This restriction is intended to require a safety assessment of the situation.  In case of an alarm follow the local emergency plan.

5.2.10    Visual indication of the actual use of flammable gas shall be provided at both the storage location and at the experimental apparatus.  Such lights shall be controlled automatically and shall indicate actual “gas on” and “gas off” status in real time.  Flammable gas alarm status shall also be displayed at the locations of these warning lights.

5.2.11    Possible Oxygen Deficiency Hazards shall be addressed according to ES&H Manual Chapter 6540 Oxygen Deficiency Hazard (ODH) Control Program.  The hazard shall be considered for each building or room using or storing flammable or inert gas.

5.2.12    The following documentation shall be provided to the E2RC and a copy kept at the system site.       A general description including the types of gases to be used.       An accurate piping and instrument diagram with symbols per ISA S5.1 (Instrument Society of America), including the normal set point of regulators.       An instrument and valve summary.       A plan view of the installation including the locations of flammable gas detector heads with their elevations marked.       Procedures for normal and abnormal operations including purging, start-up, gas bottle changes, mixing, leak detection, tests, alarms, shutdown, emergency situations, and ventilation.       Documentation and/or test results demonstrating the adequacy of the pressure-relief system.       A call list, including home telephone numbers and available pagers, of personnel familiar with the operation of the system.       A summary of leak-test measurements.

5.2.13    The Physics Division ESH&Q group shall be notified of actual gas start-up and system shutdown.

5.2.14    The Physics Division ESH&Q group shall be notified before using any types of gas not found in the stockroom, and a copy of the SDS for the new gas shall be provided to both the Physics Division ESH&Q group and to the Jefferson Lab SDS Coordinator.  Reference ES&H Manual Chapter 6610 Chemical Hygiene and ES&H Manual Chapter 6610 Appendix T1 Safety Data Sheets


5.3              Class II Installations are subject to the following additional requirements:

5.3.1        Storage and processing enclosures shall be constructed, where practical, to comply with the guidelines of Chapter 7 Liquefied Petroleum-Gas Liquid Transfer of NFPA-58 (see Flammable Gas Supplement Chapter 3 Standard for the Storage and Handling of Liquefied Petroleum Gases) While this document is specifically applicable only to LP gas storage facilities, it is a useful guide.  Exceptions may be made with the written approval of the E2RC.


5.3.2        In addition:        All storage enclosures shall be maintained free of standing water and/or ice to prevent falls of personnel handling gas system components.        Adequate hardware for securing all cylinders used or stored shall be available.        Windows in gas sheds shall be wire glass set in metal frames with a fixed sash.        Enclosures near areas of vehicle access shall be protected with bumper posts.        The use of gas system enclosures to store oxidizers or gases used as fuels shall be prohibited.  These enclosures shall not be used to store items not relevant to the gas system.        Electrical installation shall comply with NEC Article 500, Hazardous (Classified) Locations.  The classification guidelines are provided in Flammable Gas Supplement Chapter 4 Electrical Classification Guidelines and Flammable Gas Supplement Chapter 5 Electrical Installation.        There shall be provisions for the ventilation of such enclosures per NFPA-58 (Flammable Gas Supplement Chapter 3 Standard for the Storage and Handling of Liquefied Petroleum Gases).  Mechanical ventilation failure shall be alarmed.

5.3.3        The use of line-regulators downstream of cylinder regulators is strongly encouraged.

5.3.4        Fire sprinklers shall be installed in accordance with NFPA 13 Standard for the Installation of Sprinkler Systems, 2002 edition, to protect any adjoining or enclosing buildings from a fire in the gas storage facility.  Sprinklers shall not be installed within the gas storage facility itself since it is not desirable to quench a gas fire with the leak still present.  (Note that this is an exception to the general rules for sprinkler installation at Jefferson Lab.)


5.4              Outdoor Installations are subject to the following requirements in addition to applicable Class 0, 1, or 2 requirements outlined above.

5.4.1        Outdoor installations shall meet all applicable sections of NFPA 55 Standard for the Storage, Use, and Handling of Compressed Gases and Cryogenic Fluids in Portable and Stationary Containers, Cylinders, and Tanks, 2003 Edition and 58 for outdoor storage, use, and handling of liquefied gasses.  In particular, designers of outdoor installations are cautioned to pay attention to the requirements of these standards for issues such as perimeter construction, roof construction, protection of stored gas from vehicle damage, weather protection, site terrain, etc., and to design the installation in a manner that separates adjacent storage tanks by at least the minimum allowed value based on the size of the tanks.

6.0            Revision Summary


Revision 1.1 – 06/16/16 – Replaced Material Safety Data Sheets (MSDS) with Safety Data Sheets (SDS)

Periodic Review – 05/06/16 – No changes per TPOC











Flammable Gas - Physics

Jack Segal





This document is controlled as an on line file.  It may be printed but the print copy is not a controlled document.  It is the user’s responsibility to ensure that the document is the same revision as the current on line file.  This copy was printed on 6/16/2016.