In our previous post, we discussed the importance of leak checking the furnace when it is at temperature, as well as the difference between a hot leak check and a hot leak rate check. However, when it comes to leak checking the furnace, it is also important to consider the inert gas system.
When working with inert gas leaks, it is imperative to first put everything into perspective. Let’s start by reviewing the essential parts of the furnace system.
Generally, a liquid nitrogen or liquid argon storage system is located outside the facility. Argon and nitrogen are the two fundamental gases used in heat treatment. The liquid from the outside storage system flows through an evaporator in the evaporator stand, which changes the argon or nitrogen from a liquid state to a gas state. As the gas emerges from the evaporator it is at high pressure; it then goes through a regulator, which reduces the high pressure argon or nitrogen gas to a suitable pressure before it fills up the backfill reservoir tank. Finally, the gas is fed from the backfill reservoir tank into the furnace – which is in vacuum – for backfilling and/or partial pressuring.
So why is this important?
The reason we went over the different components that comprise the backfill system is because one of the most often misunderstood things about inert gas leaks is that people don’t realize the tolerance of leaks on the entire system – from the liquid storage system out to the furnace – must be zero.
To further understand why this is the case, let’s go through a little scenario first …
Let us say the backfill reservoir tank has an internal pressure of 200 pounds per square inch (PSI). Then there are the valves at the furnace – the backfill valve and the partial pressure valve. When those valves are closed, you have 200 PSI in the backfill tank and in the line running between it and the valves on the furnace.
In addition, the furnace is running its production in a vacuum of -30 inches of mercury (inHg). When you finish processing the load in the furnace, the furnace needs to be backfilled in order to turn the cooling system on and cool down the parts. That’s when you need the gas, because in a vacuum there is no convection (i.e., no temperature rate of rise). So you need to backfill the furnace from -30 inHg to -5 inHg, all the way up to 20 bar, depending on what kind of furnace it is.
The thing is, though, if there’s a little leak and there’s 200 PSI in the system – but it’s before the point of backfilling – then there is no air getting in whatsoever. But when you open the backfill or partial pressure valve, the gas starts flowing into the furnace to displace the -30 inHg (i.e., -1 atmosphere). So you backfill the furnace from vacuum up to -5 inHg (i.e., positive pressure), all the way up to 20 bar; however, when the valves are open and gas is flowing into the furnace you get an effect that is called aspiration, or gas entrainment.
Essentially, aspiration or gas entrainment is when all of those harmless little leaks that were in your system and used to be an outbound or positive pressure leak have now changed direction because the gas is flowing very quickly into the furnace. As a result, air is being pulled in through those leaks and mixing with the flow of inert gas going into your furnace, which then causes discoloration, braze problems and more; essentially, all of the nasty things you do not want in your vacuum furnace or process are now in there.
In an effort to prevent discoloration, many furnace owners and operators are adding a hygrometer, also known as a dew point analyzer, to their furnace. Doing so allows them to monitor the moisture content in the backfill and partial pressure gases before the parts are exposed to the gases.
The main point of this post, though, is that you can have any kind of pressure leaks on the backfill system of the furnace, the partial pressure system, the argon/nitrogen tank, the evaporator, the lines to, before and after it, etc. while the gas is stagnant. However, as soon as gas starts flowing into the furnace, these “harmless” leaks start aspirating air into the furnace. This is why it is paramount that the tolerance of leaks in your backfill system equals zero percent.
Make sure to watch for the next post in our series, Finding Leaks in Your Vacuum Furnace – Part VI, as we continue to take a closer, more in-depth look at inert gas leaks, specifically the steps you would take to find inert gas leaks and correct them, as well as how they can affect your process.