As mentioned in last months article, all vacuum furnaces will leak air into their vacuum chambers over time, causing the pressure in the vacuum chamber to go back up. Thus the term “leak-up rate” is used to determine just how fast that vacuum chamber is allowing air back in. Of course, air represents the presence of oxygen entering the furnace, which is not good for brazing. The air will also contain moisture, which then becomes another source of oxygen. Moisture itself can become an adherent problem on the metal surfaces being brazed, with adsorbed moisture often being very difficult to drive off the metal surface (can be quite a problem in aluminum vacuum brazing).
So the leak-up rate of any given vacuum brazing furnace must be controlled to very low levels, often in the range of 2-to-10 microns (millitorr) per hour allowed leakage into the chamber, in order to assure that good brazing can occur.
Therefore, one of the main duties of vacuum brazing personnel is to control the vacuum-furnace leakage by monitoring the leak-up rate of the vacuum furnace on a regular basis, and then, if an unacceptable leak-up rate is measured, to then find the source of the leak, and to seal that leak.
The usual way to find any leak in the vacuum furnace is via the use of a helium leak-detector. There are two (2) methods that can be used, namely, the helium-sniffer technique, or the helium-spray technique. In a helium-sniffer technique, the vacuum furnace is filled with helium, and a helium-sniffer wand is used at each of the furnace joints to see if any helium can be detected coming out of the furnace. This does not work for many vacuum brazing furnaces since it requires the furnace to be pressurized, and many furnaces are not capable of handling positive pressures in their chambers, only vacuums.
The helium-spray technique does just the opposite, in that the vacuum furnace is pumped down to its operating vacuum level, and then helium is sprayed onto all external furnace joints and connections, using a helium-gas probe such as is shown in Fig. 1. The helium detector (shown in Fig. 2) which is connected to the furnace then monitors any helium being sucked into the vacuum furnace through any external leak-path. For brazing operations, I strongly recommend that only the helium-spray technique be used.
Leak detection via helium-spray technique.
The equipment for this process consists of a portable instrument package, usually on a wheeled cart, including a bottle of helium gas, gas flow regulator, and an attached hose with a helium-spray nozzle (similar to that shown in Fig. 1), along with a helium mass-spectrometer instrument with its own small vacuum pumping system. Please note that the mass-spectrometer instrument used is one that only detects helium.
A photo of a typical portable helium leak-detection unit is shown here in Fig. 2.
Very simply put, the vacuum furnace is pumped down to operating vacuum level, and while it is pumping, it will obviously be drawing outside air into the vacuum-furnace hot zone chamber through whatever small leak may be present in any of the seals/gaskets, connections, etc., attached to the vacuum furnace. Helium-gas from the hose is then sprayed around each of the potential leak sites, usually starting with the furnace door, since the door is the most common source of leaks into the furnace. When the tiny helium atoms encounter a small leak path through which air is being sucked into the furnace, the helium gas will also quickly enter the furnace at that point. While the leak-test operator is slowly moving the helium-wand around each of the fittings, the mass-spectrometer is constantly sampling the gas in the vacuum chamber through the test port provided on the side of the chamber. The helium mass-spec unit has it’s own pump drawing out gas molecules from inside the chamber. When helium enters through the small leak path into the furnace, it will quickly expand in volume (since there is a vacuum in the chamber), and will rapidly be pulled from the chamber by the mass-spec’s own vacuum pump. When any atoms of helium enter it, an alarm will automatically be emitted (sound, horn, lights, etc.).
This time delay between the entrance of helium through the leak into the furnace and the exit of that helium atom into the mass-spec unit and the sounding of the alarm is very short, and depending on the size of the vacuum furnace, is usually measured from fractions of just one second, up to a few seconds! Yes, it happens very fast.
When that alarm sound is heard, the leak-detection operator will stop and very slowly move the helium probe around the seal/gasket area at which he/she was working when the alarm sounded, until the exact location of the leak around the periphery is located. This is found by listening to the intensity of the alarm, which will get loader or softer as one moves closer to, or farther away from, the actual leak point.
Once the leak point is found, then the furnace maintenance personnel can correct that leak, perhaps by taking the seal apart, thoroughly cleaning the joint area between the two connections and putting in a new gasket, etc. When the leak is found along the edge of the door to the vacuum furnace, it is usually because the large O-ring seal in the door has developed a crack, and must be replaced. Sometimes, however, if the leak is found in any of the other furnace connections, maintenance personnel will merely put some kind of sticky vacuum-sealer over the discovered leak and hope that that will be sufficient to correct the problem, rather than actually trying to repair the leak. I’ve personally witnessed gasketed joints on vacuum furnaces that have been thickly coated with a vacuum-sealing compound, indicating repeated applications of that sealer over the years, rather than having the personnel take the joint apart and fix it properly.
As good as the helium leak detector test is, I’ve only heard of one method that is cheaper and easier at locating a vacuum-furnace leak. This technique involves first filling the vacuum chamber with sugar, closing the door of the furnace, and then releasing an army of ants around the furnace. The ants will quickly find the entrance point of the leak, but then you’re left with a messy cleanup of the sugar and a hefty pest-control bill to rid the plant of the ants. So, I guess we’ll stick with using the helium leak detector instead. (No, this did NOT actually happen. It’s merely a joke!☺Have to have some humor in our technical lives!).
NEXT MONTH: In next month’s blog-article I’ll discuss the concept of virtual leaks in a vacuum brazing furnace, and how that may impact on your brazing.
Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Dan can be reached via e-mail at firstname.lastname@example.org, and his website can be visited at: http://www.kaybrazing.com/
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