In our previous post on inert gas leaks, we discussed two common methods for performing final charging of the inert gas system – manual purging and evacuation/refilling of the system. However, it is also important to consider the external and internal causes and effects of inert gas leaks in relation to the vacuum furnace.
To start, let us consider this question – what happens to the furnace when there are inert gas leaks in the system?
Typically speaking, when inert gas is leaking in from the inert gas system or from valving on the furnace, the furnace could exhibit poor pumping, poor leak rates, poor ultimate vacuum, poor ability to pump against normal outgassing loads, backstreaming, etc.
However, the key is – if it’s an inert gas leak – the furnace and parts are usually clean because inert gas does not contain oxygen. In short, you can have all of these negative side effects occurring, but the parts still look good – which is generally indicative of some sort of internal gas leak.
As a result, people will often think that if they are leaking inert gas – such as argon or nitrogen – from their backfill system into the furnace, it will not affect the cleanliness of their process. The only side effects they expect to see are poor pumping, poor leak rates, poor ultimate vacuum, etc.
Now, while it’s a true statement that inert gas, in and of itself, will not affect the cleanliness of their process, people can still get discolored loads.
To best answer this question, it is important to understand that discoloration can come from a variety of things – process materials, air leaks in the furnace, pumping system issues, inert gas leaks and more. For example, discolored loads in the gold and blue spectrum are generally indicative of an air leak.
Oftentimes, a furnace user will leak check a furnace and correct any leaks they find. Then, when they check the furnace’s leak rate, it is phenomenal. So they declare the furnace leak-free. But they still end up getting discolored loads out of the furnace.
The reason why: there might be a leak in the inert gas system’s supply to the furnace.
Oh, but we thought discoloration doesn’t occur with inert gas leaks?
Well – while the inert gas, itself, doesn’t directly contain oxygen – when the gas flows in, it can pull (aspirate) air into the gas stream and, subsequently, the furnace, yielding contaminated parts. In the end, these little leaks end up accumulating and aspirating air into the furnace when the gas flows in; the air then mixes with the inert gas, thus reducing the percentage of cleanliness and offsetting the dew point.
So you have a leak in the inert gas system and are getting discolored parts, but everything is checking out fine (e.g., the leak rate, pumpdown, etc.). You leak check the lines between the furnace and the backfill reservoir tank, but you still get discolored parts. What do you do next?
Go outside and leak check the evaporator stand. Oftentimes, because it isn’t easily visible, people forget that it is part of the inert gas system. After all, out of sight, out of mind. However, this portion of the inert gas system is equally important and also needs to be checked for leaks. Overall, it is essential to remember that the tolerance of leaks on the entire system – from the liquid storage system out to the furnace – must be zero.
In our next post in the series, Finding Leaks in Your Vacuum Furnace – Part X, we will discuss how to determine if the furnace or the pumping system is the cause of the leak.