Finding Leaks in Your Vacuum Furnace – Part II

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In our first post in this series, we featured a video demonstrating how to perform a leak check with a helium mass spectrometer. However, there is also the rate of rise test, commonly known as the CDE (clean, dry, empty) linear leak rate. This comes later, when you want to know the rate of rise, or the true linear leak rate, of your vacuum furnace. To do this, you would:

  1. Pump the furnace down
  2. Let it sit for a predetermined amount of time (depending on the temperature of the furnace and the processes you are running)
  3. Stop the process cycle, which separates the pumps from the furnace
  4. Watch the rate of rise over a one-hour time period

However, this is an area where some people fail their mandated leak rate because they do not take into consideration that you do not necessarily obtain a true linear leak rate in a one-hour time sample since no one can truly ascertain linearity from one sample, you need multiple samples.

For example, let us say you require a leak rate of 5 microns per hour or less. If you pump the vacuum furnace down to 1 micron, shut it off and come back in an hour to find it has gone up to 8 microns then the leak rate is 7 microns/hr. If you look at just this sample, you would fail your mandated leak rate.

Looking at the same situation again, let us say you come back in one hour to find the leak rate is 7 microns/hr. Do you know if it is linear? No. Do you give up? No. You let it sit for another hour because, if it is truly linear, it will double (go up by 7 microns) in another hour. Then you could say it is linear and it fails. However if, in that hour, it goes from 8 to 10 microns instead of doubling, then you would divide 10 by two hours and you would get a leak rate of 5 microns/hr. Pass.

So many people give up prematurely and they have to either leak check the furnace again, run a cleanup cycle or call maintenance. As a result, they end up needlessly losing a solid 24 hours of production time because they prematurely got ahead of themselves and didn’t let the furnace sit for one more hour. By simply letting it sit another hour, you can confirm either:

  1. It was linear and truly leaking, or
  2. It was not linear yet but now it passes, and you are back in production in two hours rather than in 24 hours.

Vacuum Valve Troubleshooting

If your linear leak rate is linear and fails, this is an indication that you might need to perform a leak check. One of our most common Ask an Expert questions concerns finding leaks and their effect on the process and furnace. One of the first things you should do is conduct a conventional leak check with a helium mass spectrometer. Sometimes this is all you’ll need – you will find the leak and be back to production.

However, it is also important to consider your vacuum valves as an extension of leaks because they can also cause issues with the furnace. There is truly nothing worse than opening up your vacuum furnace after you have run a load and your parts are blue or something other than bright and shiny. Valves can leak between adjacent systems, which can then affect your leak rate, part colors and quality in your vacuum furnace.

Main Valve

To check the O-ring seal integrity of the main valve, you need to perform the following steps:

  1. Pump down the main vacuum chamber to 1,000 microns or less.
  2. Press the Process Cycle Stop button to stop the pumping of the furnace.
  3. Select the vacuum sensor tube that is reading the vacuum level of the diffusion pump. Normally, this will read somewhere in the range of 20 to 200 microns.
  4. While you are watching the vacuum level in the diffusion pump, press the Chamber Release button.
  5. As the inert gas enters the main vacuum chamber, there should be no upward change (loss in vacuum) in the diffusion pump’s vacuum reading.
  6. If the diffusion pump seems to be backfilling at the same rate as the main chamber, there is a leak at the main valve dish seal.

Roughing Valve

To check the seal integrity of the roughing valve, you need to perform the following steps:

  1. Pump down the main vacuum chamber to 1,000 microns or less.
  2. Press the Process Cycle Stop button to stop the pumping of the furnace.
  3. While the vacuum level in the main chamber begins to slowly climb, have someone stop the roughing pump and air release the roughing line.
  4. As air enters the roughing line, there should be no upward change (loss in vacuum) in the diffusion pump’s vacuum reading.
  5. If the chamber rapidly loses vacuum when you are venting the roughing lines, the roughing valve is leaking.

Foreline Valve

To check the seal integrity of the foreline valve, you need to perform the following steps:

  1. Pump down the main vacuum chamber to 1,000 microns or less.
  2. Press the Process Cycle Stop button to stop the pumping of the furnace.
  3. Select the vacuum sensor tube that is reading the vacuum level of the diffusion pump. Normally, this will read somewhere in the range of 20 to 200 microns.
  4. While you are watching the vacuum level in the diffusion pump, have someone turn off the roughing pump and air release the roughing lines.
  5. As air enters the roughing lines, there should be no upward change (loss in vacuum) in the diffusion pump’s vacuum reading.
  6. If the diffusion pump loses vacuum, the foreline valve is leaking.

Holding Valve

Typically, if the oil keeps disappearing from the holding pump and there are no signs of external oil leakage, the holding valve is leaking across its own seal, subsequently pulling oil backwards into the diffusion pump.

In Finding Leaks in Your Vacuum Furnace – Part III, we will discuss acceptable leak rates, as well as how to understand the different anomalies associated with leaks and perfect your methods for dealing with them.

 

4 Comments


  1. Hello, I’m from Slovenia and I have a problem on my Ipsen vacuum furnace.

    When diff pump opened main valve at 4×10-2, vacuum suddenly fall and diff pump main valve automatically closed. Then mehanicals pumps start to pump again vacuum to 4×10-2. It is constantly repeating. Sorry for my english, it’s basic. Thank you!

    Reply

    1. Hello,

      Thank you for reaching out to us. We have passed on your question and information to the appropriate Ipsen expert, and he should be in contact with you shortly to discuss this issue. If you need further assistance with a technical issue, you can also email TechnicalSupport@IpsenUSA.com or call our Aftermarket Support Helpline: 1-844-Go-Ipsen (International: +1-815-332-2530).

      Please let us know if we can assist you with anything else.

      Reply

      1. I have the same issue with my vacuum furnace HEQ 3836 s/n 2579. Can you tell me what could be wrong with the Vacuum furnace?

        Reply

        1. Hello Eduardo,

          Thank you for reaching out to us. Without knowing more details, there could be several things causing this issue – the main valve cylinder could be leaking when it retracts and/or there could be a leak in the poppet valve, a leak in the diffusion pump or a leak in the foreline.

          If there is a leak in the foreline, the furnace can still pump down to crossover, but when the main valve opens the chamber is exposed to the leak. This causes the pressure in the chamber to rise above crossover, thus causing the main valve to close. As such, the furnace pumps down to crossover again and the problem repeats.

          To help you pinpoint the exact source of your issue, we have passed your question and information on to the appropriate Ipsen expert, and he should be in contact with you shortly to discuss it further.

          If you need further assistance with a technical issue, you can also email TechnicalSupport@IpsenUSA.com or call our Aftermarket Support Helpline: 1-844-Go-Ipsen (International: +1-815-332-2530).

          Please let us know if we can assist you with anything else.

          Reply

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