No fuel gas is “safe” because they are all meant to produce hot
flames, but the degree and kind of danger each presents is unique.
There have been several good safety points made so far in this
conversation but I will add a few I haven’t seen yet.
Acetylene -
First, acetylene is highly combustible compared to other gases. If
acetylene leaks it will combust in a mixture in air of anywhere from
2% to 80% and a static electricity spark is sufficient to ignite this
mixture.
By comparison, natural gas, propane and other similar gases have a
range of combustibility between 5% to 15%, meaning that in the case
of a leak there is a much lower range in which the leak will combust.
It also takes a hotter spark to ignite these gases.
The point is that a much smaller, by volume, acetylene leak is much
easier to ignite than other gases.
Second, handling the cylinder is much more important with acetylene
than other gases.
Acetylene is naturally unstable, meaning that it will explode
(violent decomposition is usually the term in the safety manuals) if
not handled properly. Improper handling can be as simple as dropping
the cylinder or knocking it over. In the safety literature this is
referred to as “mechanical shock”, i.e. the cylinder is struck or
jarred. While people may be careful when moving and changing
cylinders you still might have butterfingers or just be having a bad
day - dropping an acetylene cylinder can make it a really bad day.
Third, how acetylene is withdrawn from the cylinder can create
danger.
Acetylene should never be used at higher than 15 psi pressure
because to do so will cause the gas to “dissociate” from the acetone
in which it is dissolved too quickly and this can lead “violent
decomposition”, an explosion. This is why acetylene regulators have a
red field marked on them, yet I have seen people purposely set their
regulators in this red zone thinking if some pressure is good, more
should be better. And these were trained people who should know
better - the danger is in a novice who is not properly aware of the
unique danger acetylene cylinders represent.
The other thing about using acetylene is that you cannot withdraw
more than 1/7 of the cylinder’s contents per hour or again, “violent
decomposition” occurs. This becomes an issue when using a small
cylinder, say a B-tank and the tank is nearing empty. It becomes
easier to reach the 1/7th limit. The issue is not the volume of gas
in the cylinder but the rate at which it dissociates from the acetone
- do this too quickly and you have an explosion.
Propane -
People are more comfortable with propane because it is more familiar
- many use it to heat their homes in areas where there is no natural
gas service. However, the propane tank is kept some distance from the
house - nobody puts the propane tank in the basement!
Everyone is familiar with propane being heavier that air such that
leaks will sink and pool, and if there is a flame source like a pilot
light in the hot water heater, it will ignite.
The danger that is less often considered is the amount of energy
stored in a propane cylinder, or an acetylene tank for that matter.
Because propane is liquefied it is easier to put more energy in a
smaller container. Compare a B-tank of acetylene, which holds 59,000
BTUs of energy with a 20 lb propane tank which holds more than
432,000 BTUs - a lot more explosive power in a container about the
same size. To visualize what this means, one BTU is about the same
energy released by burning a single wooden kitchen match so consider
storing 59,000 or 432,000 kitchen matches in your studio and what
would happen if they ignited all at once!
A larger propane cylinder, say a 100 lb cylinder, holds more than
2-million BTUs and the power in this cylinder was demonstrated in
Chicago when such a cylinder exploded in a jeweler’s shop on the 6th
floor of a downtown office building. Nine people were injured, some
seriously, smoke filled the building and the windows were blown out.
http://findarticles.com/p/articles/mi_qn4155/is_20020402/ai_n12457092
Besides the danger of the cylinder itself, if a fire occurs there is
greater danger to responding firemen. In commercial and industrial
buildings fire companies make site visits and pre-plan where gas is
stored so if they respond to a fire they know what to expect. This
isn’t the case in a home studio with a cylinder in the basement -
consider your obligation to the local volunteer fire company.
When a propane cylinder explodes it does so in a manner referred to
as a BLEVE - Boiling Liquid Expanding Vapor Explosion. What happens
is that because propane is in liquid form the heat from a fire causes
it to evaporate in the tank and increase tank pressure. The pressure
rises faster than the relief valve can exhaust the gas and pressure
builds to a point where the cylinder ruptures. The explosion spreads
liquid propane over a wide area which itself then combusts, making
the fire worse. If you are old enough to recall Napalm, the effect is
somewhat similar.
Natural Gas -
Many people connect their torches directly to the city natural gas
line and while this solves the problem of having gas cylinders in the
building there is a unique danger here too.
In most places standard city gas service is too low a pressure,
often 1/4 psi, to allow a flashback arrestor to be used effectively.
Most flashback arrestors are designed to work with supply pressure of
at least 3/4 psi - if the pressure is less than that they completely
block gas flow. Jewelers don’t use them because they won’t get any
gas!
Without a flashback arrestor there is the potential for flame to
travel back through the torch, through the black pipe and to the gas
meter which explodes violently. There is flying shrapnel as the case
of the gas meter is destroyed and a significant gas leak is created.
This occurred several times in New York City last year, causing Con
Edison, the local gas utility, to do some investigation into ways to
prevent these explosions.
My company, G-TEC Natural Gas Systems, worked with Con Ed and
learned (1) while there are devices marketed as being designed
specifically to work with low pressure gas, none proved 100%
effective when tested by an independent laboratory and (2) standard
flashback arrestors, using 3/4 psi and more, which are available from
the well-known supply companies are 100% effective at preventing
flashbacks. The problem is getting sufficient gas pressure so the
flashback arrestor protects the gas line and does not block gas
flow.
The Plug -
Permit me a few words about the G-TEC Natural Gas Torch Booster,
which solves many of these problems. It is a CSA Certified device
that can boost low pressure gas pressure high enough for great torch
performance (most jewelers use 5 psi for brazing/soldering and 20 psi
for casting) and can be installed in home studios, shopping malls,
strip shopping centers and other places that may prohibit cylinder
gases. It provides high-pressure gas as it is used; there is no gas
storage. Natural gas itself, while dangerous like any gas, also has
advantages in being stable and lighter than air. By boosting gas
pressure above 3/4 psi it works with any standard flashback arrestor.
If you are interested you can find out more at www.safe-t-gas.com .
Ed Howard
G-TEC Natural Gas Systems
@ehoward