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Guide to Compressed or Liquefied Gases in Laboratories

1.0 Introduction

The Guide to Compressed or Liquefied Gases in Laboratories complements the Northwestern University Policy on Compressed or Liquefied Gases in Laboratory and Laboratory Support Facilities. Where the Policy addresses roles and responsibilities, this Guide addresses specific procedures.

Hazards can result from improper handling of gas cylinders and high pressure equipment. A leaking cylinder could produce an atmosphere that is toxic, anesthetic, asphyxiating, or explosive; and in the event of a rapid escape, the cylinder becomes a randomly directed missile. The main purpose of properly handling compressed gases is, therefore, to prevent uncontrolled escape of the gas. 

A non-flammable inert compressed gas is defined by the Department of Transportation (DOT) as “any material or mixture which exerts in the packaging an absolute pressure of 280 kPa (40.6psia) or greater at 20C (68F).

At Northwestern University most laboratory gases are ordered through Procurement

2.0 General Precautions

  • Never drop cylinders or permit them to strike each other violently.
  • Do not expose cylinders to temperatures higher than 122°F (50° C). Some rupture disks on cylinders release at about 149°F (65° C).
  • Never tamper with pressure relief devices in valves or cylinders.
  • Before using cylinders, heed all label warnings and Safety Data Sheet warnings associated with the gas being used.
  • If a gas cylinder or dewar is in disrepair, affix a repair label and notify Procurement.

Repair labels are available in the Research Safety offices.

Cylinder Repair Label
2.1 Types of Compressed Gas Cylinders

There are three major groups of compressed gases stored in cylinders.

  • Liquefied gases are partially liquid at normal temperature and charge pressure. Examples: chlorine, propane, nitrous oxide.
  • Non-liquefied gases are entirely gaseous at normal temperatures regardless of charge pressure. Examples: argon, oxygen, nitrogen, hydrogen. The standard 5 foot gas cylinders supplied by gas vendors at a pressure of 2,200 – 2,400psi, contain an average of 250 cubic feet of gas at normal temperature.
  • Dissolved gases are dissolved in a liquid phase solvent. Dissolved gas cylinders are packed with an inert, porous filter saturated with the solvent which stabilizes the volatile gas. Acetylene is the only common dissolved gas.

Some gases, such as carbon dioxide, are commonly used in both a liquid and gas form. Cylinders designed for liquid phase dispensing have a siphon, or “dip”, tube.

2.2 Securing Cylinders

Secure the cylinder above its center of gravity (~2/3 up the cylinder). If the chain or belt is too low or too high, it will not hold the cylinder securely. Keep the cylinder valve-protection cap on when not in use. Secure cylinders by metal channels, nylon bench straps or chains.

2.3 Lecture Bottles

In addition to standard precautions, the following special rules apply to work with lecture bottles in the laboratory:

  • Lecture bottles do not have pressure-relief devices to prevent rupturing or a transport cap.
  • Unlike larger cylinders, lecture bottles all have identical valve threads, irrespective of the gas contained within.
  • If labels and valve tags do not agree or if there is any question as to the contents of a lecture bottle, return the unused bottle to the supplier or contact Research Safety. Whenever possible, purchase lecture bottles from suppliers who will accept the return of empty or partially empty bottles.
  • When transporting lecture bottles, use a cart and block the bottles to prevent rolling and falling.
2.4 Storage

Maximum allowable storage quantities vary depending on campus, building, floor, control area, fire rated design and type of gas. Only cylinders that are in use shall be kept in the laboratory. When the cylinder is not in use, close the main cylinder valve tightly and add the protective valve-protection cap.

Promptly remove the regulator from an empty cylinder, replace the valve-protective cap, and label the cylinder by using an “empty” tag or writing on the side of the cylinder with chalk. Never bleed cylinders completely empty; leave a slight pressure to keep contaminants out. Empty cylinders shall be promptly removed.

2.5 Return and Transport

Transport using a wheeled cylinder cart with the capped cylinder strapped to the cart. Do not move a gas cylinder that may have been exposed to fire or excessive heat. Notify Research Safety or Procurement.

2.6 Fittings and Connections

Threads on cylinder-valve outlet connections have been standardized by the Compressed Gas Association for specific gas types.

Gases valves

This prevents accidental mixing of incompatible gases from an interchange of connections.

Left hand fittings for fuel gases have a cut mark through the nut. Do not use cheaters or adapters that circumvent the CGA fittings.


Compression fittings require no pipe sealants. Follow the Pipe Sealants Guide.

Never lubricate, modify, force, or tamper with cylinder valves. Especially do not put oil or grease on the high-pressure side of a cylinder containing oxygen, chlorine, fluorine or another oxidizing agent. An auto ignition or explosion could result. Unlike larger cylinders, lecture bottles all have identical valve threads, irrespective of the gas contained within.

airgas illustration

3.0 Regulators

Acetylene (except small cylinders) 510
Air (non-medical grade, zero air) 590
Argon 580
Carbon dioxide (fitting requires flat washer) 320
Hydrogen 350
Hydrogen chloride 330
Hydrogen sulfide 330
Methane 350
Oxygen 540
Propane 510


Regulators and fittings are gas-type specific which limits interchange and adds safety. Special installation processes, not mentioned here, are used for toxic or high purity gases. Always make sure that the regulator and valve fittings are compatible.

To select the appropriate regulator:

  1. Determine the gas pressure needed
  2. Determine the maximum pressure the system may require
  3. Select a delivery pressure range so the required pressures are in the 25%-90% range of the regulator delivery pressure
  4. Check with the gas supplier about compatible connections and regulators

Check cylinder outlet and regulator inlet connections for debris or contamination before connecting. Some gases, such as carbon dioxide, require a gasket/washer. Ensure that a required gasket is in place before assembling the regulator onto the cylinder. 

  1. Tighten connecting nut with a smooth jaw wrench.
  2. Back out the pressure adjusting knob or key on the regulator.
  3. Open the cylinder valve just enough to indicate pressure on the regulator gauge (no more than one full turn).
  4. Check connections with a soap solution for leaks. Never use oil or grease on the regulator of a cylinder valve.

All compressed gas regulators should, at a minimum, be checked for external leakage and internal leakage (creep or crawl) regularly. Regulators should be removed from service at least every five years (more frequent in some cases) and returned to the manufacturer, or a competent agent to be inspected and/or refurbished as necessary. Regulators should also be tagged or labeled to identify the last date of inspection. Users should consult the manufacturer for specific procedures on how to check for external and internal leakage as well as the recommended frequency of the tests. Regulators are continuously exposed to high stresses due to cylinder pressures. In addition to that, the materials of construction are attacked internally by both mildly and severely corrosive gases. External corrosive environments can cause gauges and springs to corrode. Argon, helium and nitrogen regulators (CGA 580) will, under a given set of conditions, have a longer service life than regulators used for hydrogen chloride and hydrogen sulfide (CGA 330) simply because the gas service is more severe (corrosive).

The most common type of regulator failure is the internal leak, sometimes called creep or crawl. This can occur when the seat becomes damaged or displaced due to a foreign particle such as a metal chip or other material. When the seat cannot close completely, delivery pressure will not be maintained and regulator pressure cannot reach a state of equilibrium. Downstream or delivery pressure will continue to climb until the safety relief mechanism on the regulator is activated (usually a relief valve or a diaphragm burst hole). Checking for this type of failure is relatively easy if the device has a gauge that reads regulated pressure. The gauge pressure will start to rise above the set point and continue upward. This creates a potentially hazardous condition where any downstream equipment would be subjected to pressures beyond the rated limit. Regulators should be visually checked for this type of failure. Excessive flexing of metal regulator diaphragms can cause a radial crack, which allows gas to escape to the atmosphere through the vent hole in the bonnet. 

4.0 Piping and Manifolds

Piping, tubing, valves and fittings conveying hazardous materials shall be designed and installed in accordance with ASME B31 (Reference IFC 2012 – 5003.2.2). The system’s weakest component determines the overall pressure limit.

5.0 Toxic Gases

Purchase of diluted toxic gas, if feasible below a concentration known to be dangerous, will serve to reduce exposure risk.

Transport toxic gas cylinders with the valve-protection cap and the valve outlet cap on. Toxic-gas cylinders shall be stored in continuously mechanically ventilated enclosures with an extinguishing system (IFC 2012 – Section 6004.1.2). If the net toxic gas content exceeds one pound per cylinder no more than three cylinders of toxic gas are allowed per enclosure (gas cabinet). Flow-restricting orifices are recommended on cylinders of toxic gases. All portable tanks and cylinders must be marked to indicate the orifice (inches) on the certification tags and the vessel themselves.   Any new laboratory construction shall require vented gas cabinets for storage of highly toxic gases. Gas cylinder cabinets for toxic gases must have a fire extinguishing system (IFC 2012 – 6004.1.2.

Waste toxic gases shall be treated by absorption, wet or dry scrubbing, combustion, or condensation via refrigeration, before being vented to chemical fume hoods or other local exhaust arrangements. The safe venting of pressure-relief devices should be considered. (IFC 2012 – Section 6003.1.3 Treatment Systems)

If the physiological warning properties for the toxic or highly toxic gas(es) are above the permissible exposure level (PEL), an emergency alarm system is required (IFC 2012 – 6004.2.2.10); consult with Research Safety regarding this determination. Consult the emergency plan for the given lab area to determine the action expected during a leak situation. Contact Research Safety for information on selection, fit testing and training if respirators or Self Contained Breathing Apparatus (SCBA) have been provided. No one may use respirators on the job without prior medical approval, fit testing and training.

The visual alarm of a continuous leak detection system shall be blue at malfunction, yellow at the warning level and red at the danger level. A required leak detection system shall send all alarm signals to University Police.

6.0 Flammable Gases

The purchase of diluted flammable gas, if feasible below the explosive range, will serve to reduce an explosion risk.

Quantities of flammable gases allowable in Business Occupancy generally do not require installation of leak detection and emergency shutoff or excess flow control. (IFC 2012 – 5003.1.4)

Where leak detection is installed, the visual alarm of a continuous flammable gas detection system shall be yellow at the warning level and red at the danger level. A required continuous atmosphere detection system shall send all alarm signals to University Police as part of a mutually agreed upon emergency response plan.

Oxygen/fuel gas system for welding, brazing or glass blowing shall follow OSHA CFR 1910.153. This requires the use of Grade T welding hose color coded red/green for all oxy-gas welding applications and the installation of flash arresters on hydrogen and acetylene cylinders.

Flammable gas cylinders must be stored 20 feet away from oxidizers and oxygen gas cylinders or separated by a fire rated wall. When in use in an oxy/fuel system, no special separation is required.

6.1 Acetylene

The in-house transfer, handling, storage and utilization of acetylene in cylinders shall be in accordance with Compressed Gas Association Pamphlet G-1-2015. Acetylene cylinders have a porous filler material filled with acetone and dissolved acetylene. The cylinder must only be used in the upright position. If a cylinder has been handled in a non-upright position, do not use it until it has sat upright for at least 30 minutes.

Do not use tubing materials, such as copper and lead solder,  as they form explosive acetylides.

Never exceed the delivery pressure limit of 15psig indicated by the warning red line of an acetylene pressure gauge.

The use of an excess flow control valve is not recommended. Install a flash arrestor downstream from the regulator and check valves wherever backflow needs to be prevented.

6.2 Hydrogen

Individual hydrogen gas cylinders should contain less than 400scf. Larger hydrogen gas vessels may require a laboratory designed with explosion control and other safety measures. The installation of a flash arrestor is required, and the installation of an excess flow control valve is recommended. Stainless steel (316L grade) piping and tubing is generally used for hydrogen delivery.

6.3 Pyrophoric Gases-Silane

Gas vessels with more than 0.5 scf (14L) of silane content above 1.37% by volume are regulated by CGS G-13. The required engineering controls for silane are much more extensive than required for flammable gases. Silane design experts should be consulted. To determine allowable quantities, binary gas mixtures containing silane concentrations between 1.37%-4.5% by volume are considered flammable. Gas mixtures with silane gas above 4.5% by volume are considered pyrophoric.

7.0 Cryogenic Liquids and Liquefied Gases

The hazards of cryogenic liquids include fire or explosion, pressure buildup, embrittlement of structural materials, asphyxiation and destruction of living tissue on contact. Liquid helium, argon or nitrogen may displace air and create an atmosphere without sufficient oxygen. Portable cylinders of cryogens must only be stored in well ventilated areas. Storage of cryogenic liquids (i.e. liquid nitrogen) or liquefied gases (i.e. carbon dioxide) in cold rooms or other rooms without external ventilation is prohibited. Fire or explosion may occur when the liquid form of flammable gases, such as hydrogen, is used without proper management of the gaseous phase. Liquid oxygen may produce an enriched oxygen atmosphere, which increases the flammability of ordinary combustible materials. Enriched oxygen levels may also cause some nonflammable materials, such as carbon steel, to burn readily.

Contact with cryogenic liquids generally causes tissue freezing and frostbite. Even brief contacts may be intense and painful. Prolonged contact may result in blood clots. Appropriate protective clothing, gloves, and eye protection — preferably a face shield — shall be worn when cryogenic liquids are handled. Choice of personal protective equipment (PPE) should be carefully evaluated, as gloves not designed for use with cryogenic liquids can saturate and cause more extensive cold damage to the skin.

7.1 Dewars and Transfer Equipment

Use a phase separator or special filling funnel to prevent splashing and spilling when transferring liquid nitrogen into or from a dewar. The top of the funnel should be partly covered to reduce splashing. Use only small, easily handled dewars for pouring liquid. For the larger, heavier containers, use a cryogenic liquid withdrawal device to transfer liquid from one container to another. Be sure to follow instructions supplied with the withdrawal device. The receiving vessel must be raised so the delivery tube is immediately above the mouth of the vessel (i.e., the cryogenic liquid should never be allowed to fall through air to reach the receiving vessel). When a warm tube is inserted into liquid nitrogen, liquid will spout from the bottom of the tube due to gasification and rapid expansion of liquid inside the tube. Wooden or solid metal dipsticks are recommended; avoid using plastics that may become very brittle at cryogenic temperatures.

7.2 Monitoring for Oxygen Deficiency

Indoor areas where bulk inert gas systems are newly installed shall be continuously monitored with an atmosphere monitoring system. The system shall provide an audible and visual alarm (red light) when the oxygen level drops to 19.5%. The audible and visual alarm shall be located inside the area and immediately outside of all entrances to the indoor area. A blue indicator light shall indicate a detection system malfunction. A required atmosphere monitoring system shall send all alarm signals to University Police.

7.3 Minimum Ventilation Rate

Natural or mechanical ventilation shall be provided when bulk inert gas systems are installed in buildings, rooms, or any indoor confined area. Ventilation shall be provided throughout the space at the rate of not less than 1.0 cubic foot per minute per square foot of floor area determined by the area enclosed. (Reference: IFC2012 – 5004.3.1)

8.0 Restricted Commodity

Investigators must order Restricted Commodity using a NUFinancials requisition. The selected requisition category must be LAB-HAZARDOUS GASES RESTRICTED. Investigators must ensure that each person placing orders, whether department office staff or laboratory staff, understands these requirements. Research Safety must review and approve all requisitions for Restricted Commodity. Research Safety will not approve purchase orders for LAB-HAZARDOUS GASES RESTRICTED intended for use outside of authorized University facilities.

9.0 Quantity Limits on the Evanston Campus

The scope of this section generally applies to areas constructed for business occupancy (B). For quantity limits for all other occupancies, especially High Hazard Group (H), contact Research Safety.

The regulations for the Evanston Campus differ from the regulations on the Chicago Campus.

The building users on the Evanston Campus generally have to comply with the limits according to the International Building Code (IBC) and the International Fire Code (IFC). Groups of research laboratories within a building may be subdivided into fire control areas. For more specific information regarding laboratory design and fire control specification contact Facilities.

The total quantities of flammable or combustible liquids allowed in a fire control area (laboratory or suite of laboratories) are limited by the location in the building and the construction specifications.


2012 International Fire Code

Maximum Allowable Quantities in Storage per Building – Sprinklered Fire Business Occupancy Control Area







Cryogenic flamm 67 gal 90 gal 67 gal 45 gal 11 gal
Cryogenic Oxidizing 67 gal 90 gal 67 gal 45 gal 11 gal
Flammable gas (gaseous) 1,500 cubic feeta 2,000 cubic feeta 1,500 cubic feeta 1,000 cubic feeta 250 cubic feeta
Oxidizing gas (gaseous) 2,250 cubic feeta 3,000 cubic feeta 2,250 cubic feeta 1,500 cubic feeta 375 cubic feeta
Oxidizing gas (liquefied) 112 poundsa 150 poundsa 112 poundsa 75 poundsa 18.7 poundsa
Pyrophoric (needs approval) 37 cubic feeta 50 cubic feeta 37 cubic feeta 25 cubic feeta 6 cubic feeta
Corrosive 1,215 cubic feeta 1,620 cubic feeta 1,215 cubic feeta 810 cubic feeta 202 cubic feeta
Highly toxic (needs approval) 15 cubic feetb 20 cubic feetb 15 cubic feetb 10 cubic feetb 2 cubic feetb
Toxic 1,215 cubic feeta 1,620 cubic feeta 1,215 cubic feeta 810 cubic feeta 202 cubic feeta
a. Quantities shall be increased 100% when stored in approved cabinets, gas cabinets, or exhausted enclosures as specified by the International Fire Code.
b. Allowed only when stored in approved exhausted gas cabinets or exhausted enclosures.

Reference: International Fire Code (IFC) 2012 Excerpt from Tables 5003.1.1(1) and 5003.1.1(2).


Note that the McGaw/Olson building is classified as institutional occupancy NOT business occupancy, which limits the storage quantities in accordance with NFPA 99. Contact Research Safety for further information.


Maximum Size and Quantity Limitations for Compressed or Liquefied Gas Cylinders in Laboratories Located in Buildings Classified Business Occupancy

10 x 50 inches 9 x 30 inches 4 x 15 inches
6b 3 3a
a. Cylinders of all toxic gases shall be kept in a continuously mechanically ventilated hood or other continuously mechanically vented enclosure, with no more than 3 cylinders per enclosure.
b. In instructional laboratory work areas, the total number of cylinders shall be reduced to 3 maximumsized cylinders. Ten approximately 2″ × 12″ cylinders (lecture bottles) are allowed. In other than instructional laboratories, 25 lecture bottles are permitted.

Reference: NFPA 45


Bulk systems

  • Assembly of storage containers, pressure regulators, PRDs, vaporizers manifolds, and interconnecting piping that has a storage capacity of more than 20,000 scf (566m3) of compressed gas or cryogenic fluid including unconnected reserves regardless of storage pressure. A bulk system terminates at the point where gas at service pressure enters the supply line. (Reference: CGA P-18 2006).

Control Area

  • Each building’s floor has one or more control areas. This term is used in conjunction with Maximum Allowable Quantities and is further defined in the International Fire Code. A record of control area boundaries is maintained by Facilities.

Cylinder Parts

Cylinder parts drawing
  1. Cylinder valve-protection cap
  2. Valve hand wheel
  3. Retainer and valve stem packing nut
  4. Over pressure relief/safety device
  5. Valve outlet connection: Standardized CGA (Compressed Gas Association) outlet (i.e. No. 350 for hydrogen service) to prevent interchange of equipment with incompatible gases
  6. Cylinder collar
  7. Valve outlet cap
  8. DOT Identification, cylinder material, service pressure
  9. Cylinder serial number
  10. Month/Year hydrostatic test
  11. Other identification markings

Cylinder Dimensions

height chart for tanks

Emergency Alarm System

  • A system to provide indication and warning of emergency situations involving hazardous materials

Excess Flow Control

  • A fail-safe system or other approved means designed to shut off flow caused by a rupture in pressurized piping systems.


  • Under the Globally Harmonized System (GHS), chemical manufacturers, distributors, and importers are required to use labels that include the following: product identifier, signal word, pictogram, hazard statement, precautionary statement, and supplier information.
  • For example:
Gas tank label


Maximum Allowable Quantity (MAQ)

  • The term is further defined in the Appendix and by the International Fire Code. A record of MAQs per control zone is maintained by Research Safety.

Oxygen Deficient Atmosphere

  • An atmosphere in which the oxygen concentration is less than 19.5% by volume.

Permissible Exposure Limit (PEL)

  • The allowable OSHA limit for an air contaminant in which workers may be exposed day after day without adverse health effects

Standard Cubic Foot (scf)

  • Cubic foot of gas measured at 70°F (21.1 °C) and 14.7 psia (101.3 kPa abs).

Restricted Commodity

  • See the COMPRESSED OR LIQUEFIED GASES IN LABORATORIES policy for definition and list of gases

Toxic Gas and Highly Toxic Gas

  • A toxic gas is one with a median lethal concentration (LC50) of more than 200 and less than 2,000 parts per million by volume of gas or vapor when administered by inhalation for an hour (or less if death occurs within one hour) to albino rats weighing between 200 and 300 grams each. A highly toxic gas is characterized by a median LC50 of 200 ppm or less under the same conditions. (Reference: IFC 2012 – Chapter 2)


Airgas Pipe Fitting Sealants, 2018

Matheson, Safe Handling of Compressed Gases in the Laboratory and Plant, accessed from website 5/2015

Design and Safety Handbook for Specialty Gas Delivery Systems, Air Liquide America Specialty Gases 5/2015

Airgas Product Catalog, 2018

CGA P-1 – 2008 Safe Handling of Compressed Gases in Containers

CGA P-9 – 1992 The Inert Gases Argon, Nitrogen and Helium

CGA P-12 – 2009 Safe Handling of Cryogenic Liquids

CGA P-18 – 2006 Standard for Bulk Inert Gas Systems

CGA P-20 – 2009 Standard for classification of toxic gas mixtures

CGA G-1_13 – 2015 – Acetylene

CGA G-13 – 2016 Storage and Handling of Silane and Silane Mixtures

NFPA 45, Protection for Laboratories Using Chemicals, National Fire Protection Association, 1996.

International Fire Code – 2012