Whenever a gas is introduced into a brazing furnace for a brazing operation, I’m always very concerned about the dew point of that gas because dew point represents moisture in the gas and moisture represents the presence of oxygen. This applies to regular atmosphere brazing furnaces and also to vacuum furnaces, into which a gas is introduced either for quenching or for partial-pressure brazing.

A sometimes overlooked aspect of the entire furnace brazing process is the piping used to bring in a gaseous atmosphere to the furnace. The piping may originate outside the brazing shop’s walls, starting at a large bank of gas cylinders, or from a large liquid-containing tank sitting on a concrete pad out in the yard.

In any event, the gaseous atmosphere, which begins its journey from the tank or gas cylinder out in the yard, is in the “driest” condition (lowest dew point) it will see in its journey to the brazing furnace. The piped atmosphere can pick up significant amounts of moisture and other contaminants throughout the entire length of the piping system used to bring it to the furnace. It is very important that the entire piping system be as clean and leak-tight as possible to ensure that the gas used in the brazing furnace is as dry as possible (must be -60°F/-50°C or drier for effective brazing).

To achieve this, the gas piping used should be:

          1. Certified

          2. All joints should be soldered, brazed, welded or O-ring sealed

          3. Run piping underground if possible

          4. Insulate piping if possible

 

1. Certified gas piping

It is not acceptable to merely go down to the hardware store and purchase standard copper or steel piping for this purpose. It should be ordered as piping that is certified for use in piping gaseous atmospheres. Unfortunately, when piping is manufactured and supplied from many different locations around the world, there are pipes/tubes being manufactured and sold with unsound walls, causing leaks. Such poor-quality piping is completely unacceptable for use. The only way to ensure sound material is, in my opinion, to buy certified gas piping.

2. All joints should be soldered, brazed, welded or O-ring sealed

A number of brazing shops have run piping from their gas tanks using standard threaded plumbing-type fittings to make their pipe connections. Unfortunately, such connections, even if “sealed” using Teflon tape or “dope,” will open up with time and cause gross-leaks of air (thus oxygen) into the system, even when the piping is pressurized.

Oxygen molecules see any gaseous piping system as a “vacuum” and want to enter that piping system to help “equalize” the amount of oxygen in the pipes with the amount of oxygen on the outside of the pipes. To us, the pressure inside the pipe may have significance since we can feel the pressure if it is directed at us. But to atoms and molecules of oxygen, since they are so tiny, the pressure that we feel is meaningless. All the atom or molecule of oxygen knows is that there is no oxygen in the piped gas, and it wants to equalize it. So, it goes right in through any leak path and will contaminate that gas in its effort to equalize the amount of oxygen inside the pipe with the amount of oxygen on the outside of it.

Internal pressure of the gas inside the pipe has no real meaning at all to the molecules of oxygen. As an example, it is not uncommon for gas bottles such as nitrogen cylinders at the gas-dealer’s shop to be leak-tested prior to delivery of the cylinders, even though those bottles have been pressurized to about 3,000 psi. However, if that supplier does a bubble test (e.g., by spraying a soapy solution onto the top seal “spud” of the cylinder) prior to delivery of the cylinders, and if a bubble develops at that location, then that cylinder has been contaminated by outside gases. Yes, atoms of contaminating gases have migrated upstream against thousands of pounds of internal pressure in that tank because it saw the inside of that tank as a “vacuum” to that particular type of atom (such as oxygen getting into a bottle of argon gas).

We will conclude our discussion in Part 2.