Compressed Gases
Compressed gases are defined as materials that fit into any of the following categories.
1. Any material or mixture in a container with an absolute pressure exceeding 40 psi (276 kPa)
at 21oC (70oF).
2. Any material or mixture in a container with an absolute pressure exceeding 104 psi (717 kPa)
at 54oC (130oF).
3. Any liquid flammable material having an absolute vapor pressure exceeding 40 psi (275 kPa)
at 38oC (100oF).
These are often grouped by similar properties. Nonliquefied gases are gases with low boiling points.
These include common materials such as nitrogen, oxygen, and helium. Some of these are present
in liquid form at "cryogenic" temperatures, and are used in the liquefied state. These are known as
cryogenic fluids. Liquefied gases have higher boiling points, and include such common materials as
propane, carbon dioxide, and chlorine. Acetylene is the only widely used dissolved gas. It is supplied
in an acetone solution.
Hazards and General Precautions
Compressed gases often represent multiple hazards. These gases are under tremendous pressure, and
can produce unexpected propulsion in improperly controlled situations. Additionally, these gases may
quickly disperse into the lab atmosphere. This may produce a danger of asphyxiation or toxic effects
(for some gases). Cryogenic gases present unique hazardous properties, which include pressure
buildup, embrittlement of materials in contact, frostbite, and asphyxiation. Additionally, oxidizers or
flammable gases present severe fire or explosion hazards in cryogenic form.
Proper use of compressed gases requires that you be familiar with procedures for their safe handling.
All departmental employees are required to view the film Working With Compressed Gas as part of
initial safety training. Additional "one-on-one" training should be provided for those transporting or
using compressed gases for the first time (or until proficient in the required skills).
The following general precautions apply to all compressed gases.
1. Never drop cylinders or allow them to strike other objects with force.
2. Leave the cylinder cap on until the cylinder has been secured and is ready for use.
3. Use an appropriate cart to transport cylinders. Cylinder should be secured to the cart to
move. Do not attempt to roll, drag, or slide cylinders.
4. Do not tamper with safety devices on cylinders, regulators, valves, or associated systems.
Do not subject compressed gas cylinders to temperatures exceeding 52oC (125oF). Never
5.
allow a flame to contact a compressed gas cylinder.
6. Use compressed gases only in well ventilated areas. Use a fume hood when handling toxic,
corrosive, or flammable gases.
1
A:\HAZ9604.WPD, 8/24/01
7. Use the smallest cylinder size available that is adequate for your application. This is especially
important when working with toxic materials.
8. When discharging gas into a liquid, use a trap or check valve to prevent liquid from backing
into regulator or cylinder.
9. Refer to the label information and MSDS before handling any compressed gas. Know the
specific hazards of the material and obtain proper personal protective equipment before
proceeding with use.
10. Cylinders stored outside for an extended time present an inviting environment for wasps and
similar insects. Use extra caution when removing a cap from a cylinder that you suspect has
been stored outdoors.
11. Turn off the cylinder valve for cylinders not in use.
Some hazards associated with specific gases are listed in Table 1. Many of these are used at MSU.
An explanation of the categories is given below.
Oxidant
These gases will initiate and support combustion. When using oxidants, no sources of ignition should
be present. Oxidants should be stored away from combustible materials. Oil, grease, and other
combustibles should never be used on oxidant cylinders, regulators, or other fittings.
Inert
Inert gases are inherently unreactive. However, in a confined area, an inert gas can displace air and
cause asphyxiation. Inert gases should be used only in well ventilated areas.
Corrosive
Corrosive gases may cause damage to equipment and may cause injury to persons exposed. These
gases should be used in a fume hood. Proper selection of materials used to handle these gases will
reduce the associated hazards. Some anhydrous gases display corrosive properties only in the
presence of air or water.
Flammable
Flammable gases, mixed with air or other oxidants, will burn or explode upon ignition. When using
flammable gases, no sources of ignition should be present. Flammable gases should be stored away
from oxidants. Table 1 lists the limits of flammability for some common gases. Within the range
listed, gas/air mixtures may be ignited (at ambient temperature and atmospheric pressure). Mixtures
above or below the range listed do not ignite. Follow general handling precautions for using
flammable materials when working with these gases. Remember that concentrations of these gases
may quickly reach limits of flammability.
2
A:\HAZ9604.WPD, 8/24/01
Bond and ground cylinders, lines, and equipment in service with flammable gases. Usually, this is
accomplished by using metal (conductive) lines to interconnect the system. Use of plastic lines or
tubing without separate bonding could result in static charge generation.
Because of the extreme ignition characteristics of acetylene, it is supplied dissolved in acetone. To
avoid the presence of acetone in the gas exiting the cylinder, don't allow the tank pressure to fall
below about 50-75 psi. Acetylene should never be used at pressures exceeding 15 psi. Acetylene
may form explosive compounds with copper, silver, and mercury. Avoid contact with these materials
in systems using acetylene. Acetylene cylinders should remain upright at all times, and should be
opened only about 1?turns in use.
Toxic
Several gases listed are toxic. Significant health effects or death may result from exposure. Although
some gases may be detected visually or by odor, others present no warning of their presence (for
instance, carbon monoxide). Toxic gases should be handled in an operating fume hood. The values
listed are exposure limits established by OSHA, NIOSH, or other agencies. The values are expressed
as TWA (time weighted average - maximum exposure allowed over a 10-hour day, 40-hour week),
Ceiling (exposure not to be exceeded any time), or STEL (short term exposure limit - maximum
exposure over a fifteen-minute period.)
Cryogenic
Access to cryogenic materials is restricted to personnel with proper training in their proper handling.
Materials of construction used to contain cryogenic fluids must withstand extremely cold
temperatures without embrittlement or other damage. Containers and systems must have pressure
relief mechanisms to avoid dangerous buildup of pressure. Cryogenic fluids in contact with skin or
eyes will produce tissue damage very quickly. When handling or transferring cryogenic fluids, wear
splash goggles and impervious insulated gloves that can be removed easily. In addition, the use of
a face shield is recommended. In procedures where splash or spray of the fluid is possible, wear an
impervious apron or coat, trousers without cuffs, and high topped shoes. Pot holders should be
available for some operations.
3
A:\HAZ9604.WPD, 8/24/01
Table 1. Properties of Common Compressed Gases
Flammable Toxic
(limits of (exposure Valve
flammability, limits, Outlet,
Name Formula M.W. CAS # Oxidant Inert Corrosive vol %) ppm) CGA Comments
Acetylene C2H2 26.04 74-86-2 2.5-100 2500 300 1?turns max. on cylinder valve. Do not use copper
Ceiling (or brass >65% Cu) tubing or fittings. 15 psig max.
Air Components, mole %: 78.09 N2, 590
20.95 O2, 0.93 Ar, 0.03 CO2
Argon Ar 39.95 7440-37-1 X 580
Carbon Dioxide CO2 44.01 124-38-9 X 5000 320
TWA
Carbon Monoxide CO 28.01 630-08-0 12.5-74 35 TWA 350
Chlorine Cl2 70.91 7782-50-5 X X 0.5 660
Ceiling
Helium He 4.00 7440-59-7 X 580 May be supplied as a cryogenic fluid.
Hydrogen H2 2.02 1333-74-0 4.0-75 350
Nitrogen N2 28.01 7727-37-9 X 580 May be supplied as a cryogenic fluid.
46.01 10102-44-0 X X 1 STEL 660
Nitrogen Dioxide/ NO2/
Nitrogen Tetroxide N2O4
92.01
Nitrous Oxide N2O 44.01 10024-97-2 X 25 TWA 326
Oxygen O2 32.00 7782-44-7 X 540
Propane C3H8 44.10 74-98-6 2.1-9.5 1000 510
TWA
Sulfur Hexafluoride SF6 146.07 2551-62-4 X 1000 590
TWA
4
A:\HAZ9604.WPD, 8/24/01
Cylinder Storage
Secure all compressed gas cylinders. Since a single cylinder strap will not always hold a cylinder
upright in an earthquake, a dual restraint is advised. The primary strap or chain should support the
cylinder about a third of the distance down from the top. A secondary support near the cylinder base
provides best seismic protection. Cylinder caps should be on all cylinders not in use.
Cylinder storage must be segregated based on cylinder contents. A separate storage facility must be
available for empty and full cylinders. Flammable gases may not be stored with oxidants. We have
elected to provide separate storage for flammable gases. Oxidizing gases are stored with other inert
gases.
Cylinder Exchange
1. Do not drain the cylinder "dry." Leave about 25-100 psi of gas. A truly "empty" cylinder will
allow ambient air to leak in, contaminating the next tankful.
2. Close the main valve, usually by turning clockwise from above. It should not take all your
strength to close the valve properly. Bleed off any excess pressure; both gauges of the
regulator should read zero. Close the second stage of the regulator (usually turn the T-handle
counterclockwise).
3. Remove the regulator with an adjustable wrench. Vise-Grips are not recommended, as they
damage the nut. The direction you need to turn depends on the gas tank. Argon, helium, and
nitrogen turn "right," while hydrogen and acetylene turn "left." To check, you may look
closely at the part of the thread you can see.
4. Cap the tank and label it with an "EMPTY" tag. You may now remove the tank from its
clamp on the bench. Place it in a proper cart for transporting to the storage area. Be sure to
chain the tank into the cart; even an empty tank is a hazard if it is dropped.
5. Chain the empty tank into its place in the storage area. Never leave the tanks unchained.
6. Pick up a full tank and fill in the sign-out sheet (usually inside the storage area door). Again,
chain the tank into place on the cart. If for some reason the tank has no cap, be sure to put
one on it before moving the tank. When a full, uncapped tank falls, the valve can break off.
If this happens, it may go through a brick wall, since the gas exits with a thrust of over one
ton!
7. Take the tank to the workplace and strap it into place. Never work with an unsecured tank
of gas. You'll need to jockey the tank around so the outlet points in the proper direction for
the regulator, before you strap it in.
5
A:\HAZ9604.WPD, 8/24/01
8. After the tank is strapped in place, remove the cap. At this point, it is a good idea to "crack"
the valve slightly (appropriate only for inert gases). This will blow out any dirt or dust in the
outlet that would otherwise clog the regulator.
9. Replace the regulator. A regulator does NOT need Teflon tape at the tank. The seal is
between the end of the brass "nipple" of the regulator and the inlet seat on the tank. The
thread does NOT seal the joint. If the joint between regulator and tank leaks after being
tightened properly, the inlet nipple probably needs to be replaced, or the inside of the valve
is dirty. Take it off, wipe out the valve seat, and try again.
If you hold the regulator aligned properly with the valve, you can "spin" the nut almost tight
with your fingers. Then use the adjustable wrench to snug it down. Use one hand, not two,
and just a bit of muscle. A good snug fit is all you should need.
10. Open the tank valve. A half turn or a full turn is enough. You should see at least 2000 psi
on the first stage gauge. If not, the regulator may be faulty. Remove it and crack the valve
a bit. If gas comes rushing out under high pressure, the regulator is bad. If not, someone
may have returned an empty tank to the "Full" rack.
11. Set the second stage pressure by turning the T-handle clockwise, slowly. Estimated pressure
requirements for common applications are: about 50 psi for gas chromatographs, 35 psi for
compressed air or nitrous oxide (atomic absorption), 10 psi for acetylene (also for AA).
Listen for leaks. Note that static pressure will be higher than flowing pressure. That is, once
you open the valve to your instrument, the gauge pressure will probably drop. If you need
a particular pressure, you may have to screw in the T-handle a bit more.
Regulators and Fittings
Label each regulator with the gas it is used for, and never use it for any other gas. Don't attempt to
adapt one type of regulator to another type of tank, or you may risk an explosion. This is especially
true for hydrogen regulators, which tend to adsorb hydrogen gas. If they're switched to an oxygen
tank, a violent reaction can occur that blows off the regulator and tank valve. To prevent this sort
of mishap, different types of gas use different types of regulators, and they're not normally
interchangeable. These are specified by the Compressed Gas Association and are normally expressed
as "CGA" numbers. Table 1 includes standard regulator specifications for common gases used at
Murray State. Never oil any compressed gas regulator.
When you are running gas lines and need to use fittings, be sure that the fittings are properly mated.
Basic types of fittings are: pipe thread (usually slightly tapered); compression (hardware store);
compression (Swagelok or other commercial type); ISO (like pipe thread, but straight instead of
tapered). Male and female threads should fit together easily. If they jam after one turn, they may not
be the same type of fitting. For instance, 1/4" and 6 mm compression fittings look the same, but they
are different enough to damage the threads if you try to mix them.
6
A:\HAZ9604.WPD, 8/24/01
Teflon tape is only needed for pipe threads or similar tapered threads and should not be used on
compression fittings. A fitting that will not seal without tape should be replaced. Read the
instructions that came with the fittings and use them properly. A properly installed Swagelok fitting
can be finger-tightened to stand up to 100 psi. If you think there is a leak, use soapy water to locate
it. A little extra tightening may be all you need.
7
A:\HAZ9604.WPD, 8/24/01
|