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Anvils in America - THE anvil book.

Blacksmithing and metalworking questions answered.

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Basic Welding Gas Information

  • Acetylene 2,548°C (4,618°F) with air.

  • Acetylene C2H2 burns at 5900°F w/ Oxygen, 1483 Btu/cuft, 25ft/sec.

  • Acetylene may be drawn at a maximum rate of 1/7 the cylinder capacity per hour.

  • Propane C3H8 burns at 5650°F w/ Oxygen, 2600 Btu/cuft, 12ft/sec.

  • Propane flame burns at 2950°F in free air. 21,600 Btu/lb.

  • Hydrogen (in air) 1900°C (3452°F)

  • Oxy-hydrogen 2420°C (4388°F)

  • Oxy-coal gas blowpipe 2200°C (4992°F)
  • Even though the propane flame has almost twice the heating value of as acetylene its speed of burning is approximately one half that of acetylene. Propane produces about half the heating value per unit of time (if everything else is equal) as acetylene. Most propane welding tips are the multiple orifice type to provide better flame conditions due to the slow flame propagation.

  • Oxy-acetylene flame burns at the rate of approximately 25 feet per second (* Note). To have a torch operate successfully the gas velocity must be equal to this burning velocity. Otherwise the flame enters the torch tip and causes flashback. Rosebud tips drawing large quantities of gas require higher pressures and larger hose diameters to maintain the minimum operating velocities to the torch tip.


    When building a gas forge, the flame front velocity must be maintained to keep the flame out of the burner. In an atmospheric forge this requires a properly designed venturi and a nozzle to raise the gas/air velocity above the flame front velocity. In a fan driven forge the fan must produce enough air (volume and pressure) to maintain this velocity. Reducing the outlet slightly (a nozzle) can help prevent flash back by raising the velocity. The size of the fire chamber and resulting back pressure require a balance between it and the burner. Anti-flashback screens put in carburettors and explosion proof nautical devices prevent the flame front from passing through by creating barrier of both slightly higher velocity at the screen and a low velocity oxidized gas barrier near one surface.


    High temperature chemistry is not my field but here are some obsevations:
    The glowing walls of the forge create a condition where nomal "combustion" as we think of it is replaced by a "catalized thermal reaction" or "energized thermal reaction". The glowing walls of the forge create a condition where flame front is no longer a factor (thats AFTER you get a good yellow glow). . .

* Note, it had been previously stated here that the burning rate of the Oxy-acetylene was half the speed of sound. This was an error in my source material or my assumptions. In a closed space where explosive detonation occurs gases do burn at or above the speed of sound.

Flame front velocity = normal flame propagation speed.


The pressure available from a propane bottle is dependent on the VAPOR PRESSURE of the "propane". The vapor pressure of the "propane" is in direct relation to the TEMPERATURE OF THE LIQUID "PROPANE" in the bottle. As well as the actual mix of gasses in the "propane".

Not all "LP gas" or "propane" are the same. There are other gasses mixed in that can affect the vapor pressure and thus the pressure you can get out AT A GIVEN TEMPERATURE OF THE LIQUID PROPANE IN THE BOTTLE. Propylene, Butanes, Methane, etc. can be mixed with propane in LPG. (Liquid Petroleum Gas)

The liquid propane in the bottle cools because you are using some in gas form and evaporation of the liquid propane to gas requires heat energy. The heat energy for evaporation in a tank comes from the liquid propane. As the propane cools from your using some, the temperature is lowered and as the temperature is lowered, less can evaporate. Unless.... the tank wall is warm and replaces the heat in the liquid that is being "used" when evaporating the liquid to gas. Small bottles cool down faster for a given flow rate because they cannot add heat to the cooled liquid as well as a bigger bottle. Less surface area in contact with the liquid.

When the LIQUID IN THE TANK is 110 degrees F, the pressure in the tank will be about 200 psig. When the liquid is 60 F the pressure is 90 psig. When the liquid is 0 degrees, you get about 20 psi and when the liquid is about 40 degrees below zero, you will get NO pressure.

So if you want pressure, you need to keep the liquid in the tank warm. By warming the tank safely. Or using a bigger tank that is not empty as has been said.

Tipping or shaking a propane tank that is not full puts more of the liquid in contact with the tank wall and warms the liquid some, thus giving higher vapor pressure. DO NOT tip a tank over and get liquid propane in the line. If the liquid does not vaporize before it gets to the burner orifice, you will have BIG FLAME like VIC said.

Some new tank valves are more flow restrictive. They are not pressure restrictive but the pressure at the valve outlet will go down if you try to use too much through the valve. This is PRESSURE DROP DUE TO FLOW through the valve. Pressure available to the burner is only controlled by the temperature of the liquid in the tank and the pressure drop due to flow between the gas in the tank and the burner. If it says made in China on the valve, it is likely to give less flow before pressure drops off. If the valve says Rego or Sherwood, it is more likely to give higher flow. This is not an exact rule, but is as good as you are going to get unless you know the btu per hour rating of the valve at a given outlet pressure.

As John Odom said, Regulators are sized by btu's per hour also. Just like the tank valves. The pressure output is not the same as flow output. You can have a very small flow capacity regulator that puts out high pressure as has been said. Outlet pressure will drop below the setting if you exceed the flow capacity of the valve.

Raising the pressure output on your regulator gives you more flow since your gas piping between the regulator and the burner has a given pressure drop for a given flow rate. Raising the pressure at the inlet of your piping allows more gas FLOW to go through your piping to the burner and still give enough pressure at the burner for the burner to function. If th eregulator outlet gage says 20 psig, you DO NOT have 20 psig at the burner if the burner is running. You have 20 psig minus the pressure drop in your piping.

The confusion generated by using pressure readings to control or define FLOW is exactly why I continue to recommend a needle valve FLOW CONTROL instead of a regulator. In addition, neglecting chemical energy, the only energy available to mix the gas and air in a non blower burner is pressure energy. The pressure energy in the fuel gas. Using a regulator to reduce the pressure available to the burner reduces the energy available to mix the gas and air. So using a regulator far upstream of your burner is shooting yourself in the foot. But it is what you have been told and what is "easier". Using a regulator is NOT better. For those not paying attention to their forge, a regulator and lower gas pressure is marginally safer. Less pressure in a given piping system neans less gas flow in the event of a leak.

A regulator does reduce the pressure that is at it's inlet to a preset pressure at it's outlet. Assuming of course that the inlet pressure available is above the outlet setpoint. However, a regulator should be thought of as an AUTOMATIC FLOW CONTROL VALVE. The handle sets a spring pressure. That spring pressure acts on a diaphragm. On the other side of the diaphragm is the outlet gas pressure. The valve is connected to the diaphragm. When the outlet pressure drops, because you are flowing more gas, the spring pushes more on the diaphragm since the gas is not balancing it. When the spring pushes the diaphragm, the valve opens more and more gas flows which raises the pressure in the outlet which raises the pressure on the diaphragm which then closes the valve if the outlet pressure setting is met. A regulator is a flow control device that robs pressure energy from the gas. That pressure energy can be used by a good non blower burner to mix the gas and air better. Thus allowing your forge to run hotter, better, with less scaling and more efficiently.

You can achieve the same function as a regulator by using a properly sized and designed hand valve and not lose the pressure energy. AND it's easier for a one or two burner forge.

Everyone who brags about how low a pressure their burner can run at is doing a disservice. They should instead be saying how low a GAS FLOW RATE their burner runs smoothly at. There would be far less confusion if Flow instead of pressure were understood. Gas has energy content per unit volume. So many BTU's per cubic foot. Not "so many BTU's per PSI".

John Odom deserves mention in teaching me some of this propane gas mix info in the past.

The more you learn, the better you can do. But if this is not making sense, PLEASE Stick with what others have told you and you are familiar with.

I sure hope this helps somebody because it took long enough to type! Grin.

- Tony - Friday, 01/07/05 01:13:40 EST

1/7th Rule Acetylene is disolved in liquid acetone. Its release is similar to boiling a cryogenic liquid requiring the absorption of heat to maintain the pressure equilibrium. If you exceed the draw rate the liquid cools until it will no longer evaporate. On the way it cools the container and we observe condensation and frost forming on the container surface (especially propane bottles). Thus we say it has "frozen up".

To further the problem in acetylene cylinders you have the explosion prevention pumice foam fill that prevents circulation of the acetone, thus heat absorption is very slow. In propane cylinders it is common to see frost form on them but it is rare on acetylene cylinders. Generally acetylene cylinders just quit working without warning.

For brief periods cylinders will deliver much more than the 1/7th draw rate. The 1/7th rule is such that you can continuously draw gas from the cylinder until it is empty. At the draw rate you theoretically have 7 hours of fuel at a nominal temperature (I'd guess 70°F or 21°C). So in cold conditions the rule may be a 1/10th rule or in very hot conditions a 1/5th rule. . .

1/7 Rule multiplier @ 70°F
Factor Full Half Qtr.
1 7 hrs. 3.5 hr. 105 min.
2 105 53 27
3 46 23 12
4 26 13 7
5 17 9 4
6 12 6 3
7 9 4 2
T = 7hr. / k²
Lets assume straight line relationships and do a little simple ratio math. At double the draw rate you would theoretically empty the cylinder in 3.5 hours. However, lack of heat absorption prevents this. How soon? I would guess that at double the draw rate your useful time will be reduced by half so 1.75 hours (105 minutes). At triple the draw rate the non-adjusted time would be 2.3 hours and third that would be 46 minutes. This is rough proportioning but it seems to reflect the reality I have experienced using large rose buds with acetylene cylinders. They easily draw acetylene so fast that the cylinder is only good for the 4-5 minutes it takes to adjust the torch and get setup to do what you want to do.

I am sure there is someone out there that can calculate all this using the gas rules, thermal dynamics, and the published charts. . . But I think I am pretty close.

- guru - Sunday, 03/26/06 09:45:42 EST

Acetylene is dissolved in acetone in th porous filling of the tank. It is NOT stored as a compressed gas, because if that is done it can/will explode at any pressure above 15 psig. If you draw more than the 1/7 of the cylinder size per hour, acetone from the tank is drawn out and may damage the regulator, hoses and seals. The flame will sputter. If too much acetone is removed, the tank can explode when subjected to rough handling after use. Acetylene is a dangerous material and needs to be handled with respect.

- John Odom - Sunday, 03/26/06 09:26:38 EST
Deflagration -
A flame front propagating through a flammable gas or vapor at a velocity less than the speed of sound in that gas or vapor.
Detonation -
(Also "Stable Detonation") A flame front propagating through a flammable gas or vapor at a velocity equal to the speed of sound in that gas or vapor. * URL outdated.
Explosion dynamics

The pressure wave developing during an explosion and, therefore, the effect of an explosion, strongly depend on the velocity at which the flame propagates. Compared with flame propagation in a mixture at rest or in a mixture at laminar flow, the flame in a turbulent mixture is accelerated due to the wrinkling of the flame front. As long as the propagation velocity remains below sound velocity, the combustion process is specified as a deflagration. Under certain conditions, for example in the case of flame propagation in a pipe of sufficient length, turbulence is generated in front of the flame by the blast effect of the combustion. Consequently - in a feed-back process - the flame can be accelerated to such a degree that transition to detonation takes place.The detonation is characterised by a combustion wave coupled to a shock wave propagating at supersonic speed (about 2000 m/s). Extremely high mechanical impulse loading then occurs in the direction in which the detonation propagates.

Deflagrations and detonations are classed under the general term explosions. The safe design of plants and the protection of plants by means of flame arresting devices require an intimate knowledge of the explosion phenomena. The Bundesanstalt made important contributions in this field, in particular as regards turbulent flame propagation and deflagration to detonation transition.

Physikalisch-Technische Bundesanstalt

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