In our previous post, we discussed common causes of leaks. However, the semantics of performing leak checks and finding the leak rate is often very similar to the age-old question, “Which came first: the chicken or the egg?” Or, in this case, “Which comes first: the leak check or the leak rate?”
While many often debate the answer to this question, it is more important to understand the basics if one is to truly get a grasp on this subject. To start, before you ever run a furnace, you should perform a leak rate check and know your cold linear leak rate. Why is this important? Because it lets you know before you run your furnace if there are any leaks you need to deal with. It is also essential to know the linear leak rate of your furnace so, if it ever gets too high, you can recognize that you should not be running the furnace – you should be finding the leaks and fixing them.
Acceptable Leak Rates
However, knowing your cold linear leak rate is only one small component of the overall picture. People will often ask, “What is the acceptable leak rate for my furnace?” There are two ways to look at this question. First, you should consider the acceptable leak rate for the furnace. Second – and sometimes more importantly – you should consider the acceptable leak rate for the parts you are processing.
When working with Ipsen furnaces, it is important to know that all of our furnaces with all-metal hot zones ship at a linear leak rate of two microns/hr. or less, and our furnaces with graphite hot zones ship at a linear leak rate of five microns/hr. or less. The reason for the difference between the leak rates is due to the hygroscopic nature of the hot zone and its ability to maintain gases and water vapors; as a result, it is slightly harder to get accurate leak rates on a graphite furnace. However, these leak rates are acceptable for the furnace; in fact, they are our ISO Standard for Pre-Shipment Qualifications.
Sometimes, though, a customer asks, “But my furnace is old, it’s dirty and it has carbon buildup. I can’t keep leak rates like that all the time, I run low-quality production. Give me a real number I can run at and not damage my furnace.”
Well, if you want to look at just the furnace, you could consider 15 microns/hr. (anything under 15 microns/hr. is acceptable for most commercial vacuum heat treating).* However, in other instances where customers are going to run titanium and super high alloys, the parts are now driving the specifications. This is because to run titanium you might need a leak rate of three microns or less in a furnace with a graphite hot zone (even though our furnace ships at five microns). In essence: your production often directs or mandates what your leak rates can be. Other times, with less critical parts, the furnace mandates what your leak rates can be.
Why is this the case? Well, consider what happens if you run the furnace at too high of a leak rate. It consumes itself – just like a lightbulb. For example, there is a vacuum inside the lightbulb with a filament. When you crack the lightbulb’s glass, air gets in and hits the hot filament, vaporizing it. That is exactly what happens to your hot zone. If too much air is going inside, the hot zone starts to consume itself and vaporize. Which is why, if you have an excessive leak rate, you can actually destroy a furnace in a matter of months.
Understanding Leak Anomalies
In addition to acceptable leak rates, there is often confusion surrounding the process of leak checking because, all too frequently, we say, “Oh, go leak check the furnace … did you leak check it … are you leak checking it, etc.” However, leak checking is an art. It is not something you learn in a textbook; you don’t just go to Google and find it. It takes years of practice, years of understanding cause-and-effect relationships to do leak checking. No one comes in, reads a manual on leak checks and perfects it right away. It is about understanding all of the different anomalies associated with leak checking.
For instance, you have to remember helium is lighter than air.
So, if you are leak checking a big furnace, and you know helium is lighter than air, then, where would you start leak checking the furnace?
You would start from the top because helium floats away. You must also be cognizant of your environment when leak checking. If the wind is blowing in your face, you wouldn’t start on a 25-foot-long furnace with the wind blowing down its length; you would start at the end where the wind is blowing towards so, as you leak check, the wind blows the helium across areas that have already been checked.
Of course, there is more to leak checking than that. For instance, you need to consider if you have multi-pump or single-pump units, how to cycle pumps, when to have pumps on, when to have pumps off, etc. For example, do you have one diffusion pump on your furnace? Do you have two? Or, do you have four? While four pumps would give you a much better vacuum, is that how you should end your final leak check – with four pumps on? No, you shut three off and just use one. The reason for this is that, when a leak detector is hooked to a pump and you have a little leak, you are asking the helium to go through all of these pumps and get to the final pump. However, there may not be enough helium to reach the pump where you hooked up the leak detector. This is because it is going off in pumps one, two and three, but it never made it to pump four. As a result, you need to shut off pumps one, two and three so if there are any leaks in the furnace, the only route of escape is the pump you are monitoring.
Make sure to keep any eye out for our next post, Finding Leaks in Your Vacuum Furnace – Part IV, where we will discuss 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.
*NOTE: While 15 microns might be tolerable for most furnaces and the furnace will not be damaged, 15 microns is not a number Ipsen condones. Please be cautious and follow the specifications outlined in your specific furnace manual.