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Hydrogen Properties for Energy Research (HYPER) Lab How To

Cryogenic Seals using Indium

Finding a way to seal small, mobile molecules such as hydrogen and helium at cryogenic temperatures can be quite difficult. Most common seals break down at such cold temperatures, and even a tiny leak path can be catastrophic when working with flammable gasses and temperatures that can freeze the oxygen right out of the air. Luckily, we have wonder element 49: Indium. High purity indium has a lower melting point, and hardness than lead, making it malleable enough to be an effective sealing material. In addition, at high purities, indium readily pressure welds to itself, and bonds to other metals, glass, and ceramics.

Lab member Casey Evans installing a cryogenic indium seal.
Lab member Casey Evans installing a cryogenic indium seal.

In the HYPER lab, we’re set up to extrude 0.0625 inch (1.5875 mm) and 0.1 inch (2.54 mm) diameter wire, and therefore use this wire size most often. For most seals we create, this is a usable size, and allows us to standardize seal design and indium extrusion. Although much of our wire is recycled and re-extruded, we will occasionally buy new indium wire from Indium Wire Extrusion, currently priced at $150 /oz.

A good indium vacuum seal is designed to fill a small gap around the sealed volume with pure indium through an even application of pressure, usually using a bolted flange. Making things easier for us, Indium Wire Extrusion has some helpful guidelines for proper indium seal design. Indium Wire Extrusion recommends three types of indium seals:

  1. A semicircular indium seal. This seal is created by using a ball end mill to machine out a rounded ring in one side of the sealing flange. The other side of the sealing flange should be polished smooth to provide a good sealing surface. Recommended geometry is to use a ball end mill the same diameter as the wire you’re using and machine to a depth equal to half the wire diameter.
  2. A rectangular groove seal with mating surface. This seal is created by machining a rectangular step groove into one side of the flange, and a smaller rectangular step protrusion on the other side of the flange. This geometry has the advantage of not only providing a secure effective seal, but also self aligning the flanges so the seal occurs in the same place every time. Recommended geometry is a groove the same width as the the wire being used to seal and a gap area equal to ~80% of the wire cross sectional area. The protruded step should be chamfered to allow the indium to easily flow into the gap to the sides of the protrusion.
  3. Similar to the first sealing method, you can use a single seal on the flange, but with a square step groove. This can be machined with a standard flat nosed end mill. Again, the gap area of the seal should be 80% or slightly less of the cross sectional area of the wire.

Some other general suggestions when creating / using indium seals:

  1. While HYPER lab hasn’t done this extensively in the past, it can be very helpful to include a jack screw in the flange design. Indium seals after compression and cryo-cycling can be very difficult to pull apart and a jack screw will ensure that you can get the flange back apart for the next run!
  2. When creating the indium o-ring, cut the ends at as shallow an angle as possible. Ensure these ends are overlapping, so that one end of the indium wire will crush on the other to seal the ring together. A straight 90° cut will leave a gap between the ends of the indium wire in the ring, which can allow a leak path through the seal.

Cleaning Helium Compressors

The helium compressor that drives a cryocooler has to effectively reject the heat it’s removing from the helium stream to prevent itself from overheating, and keep the cryocooler cooling efficiently. In most cases, this means running a heat exchanger with a cooled water loop to keep everything cool. This can be very effective when you’re running high purity, clean water through the heat exchanger, but dirty, rusty, or impure water can reduce performance and foul the heat exchanger tubes. In the lab, we use a cooling loop independent from the building water paired with a water filter to help keep water as clean as possible in the loop. Unfortunately, our water pump in the loop had a cast iron casing and impeller, which began to rust and dirty the water. During the process of cleaning out the water and rebuilding the system, we’ve learned a few things about our cryocoolers and keeping them clean.

Sumitomo (SHI) Cryogenics:

The heat exchanger built into the SHI HE-4 cryocooler is a 1/2″ flattened copper tube, and pretty robust. It’s unlikely for anything to get stuck in the tube, although fouling on the surface is possible. Calling Sumitomo, they recommended reversing the water flow through the heat exchanger while the compressor is off to ensure the heat exchanger is free of obstruction. Running vinegar solution through the lines will help remove fouling.

Cryomech Compressors:

Similar to the Sunitomo cryocooler above, the Cryomech cryocoolers also use copper tube in their heat exchangers. The technical representative from Cryomech recommends reversing the flow on the heat exchanger and to flush the system with a standard strength Calcium, Lime, and Rust (CLR) Remover solution. In the event of sludge build up, a 50 wt. % solution of hydrogen peroxide (H2O2) is a viable option in extreme cases.

Cryo-cycling for better leak testing

One of the biggest issues that you will run into working with cryogenic fluid systems is finding and fixing leaks, especially those leaks that open up at low temperature. A helium leak detector and a bottle of soapy water does wonders for finding leaks at room temperature, but in order to get components down to working temperature in the cryostat you have to enclose the entire experiment in a vacuum chamber and cool everything down via cryocooler – not a very easy environment to isolate the location of a leak! An example of this is that in some cases (usually poor joints between multiple different materials), brazed joints will have no leakage at room temperature, but new leaks will open up after getting really cold. We have had this happen anywhere from 80 to 200 Kelvin, and as these temperatures are far below your typical room conditions, they’re very difficult to isolate. To try and speed up the process of checking if leaks would open, our recommended practice for all cold joints and fittings is therefore to test in liquid nitrogen (LN2) before installation on the system.

General Instructions for LN2 Leak Checking, based off ASTM E499/E499M – 11 Test Method A:

  • Cap the test specimen and connect to a helium bottle. Pressurize the test specimen with helium to the working pressure of the specimen.
  • Dip the test specimen into LN2 to cool it down to the LN2 temperature of approximately 77 K.
  • Sniff all fittings, welds, and solder joints with the mass spectrometer by passing the sniffer probe over likely leak points. Start at the bottom of the specimen and work your way up, holding the probe on or not more than 1mm from the surface. Do not move the probe faster than 20mm/s.
  • Continue sniffing in an orderly procedure from bottom to top. Identify any leaks so they can be remedied. Be aware that helium will rise, so a leak above a previously found leak may not actually exist. It is also important to be aware of the airflow in the room, as helium can be blown around the experiment and produce small “leaks” that don’t actually exist. When testing with a short line from the helium bottle, be aware that the regulator connection into the bottle often leaks at a higher rate than is acceptable for our cryo experiments, and can cause false alarms (This can usually be remedied with longer lines to move the specimen farther from the bottle and by keeping the specimen low, well below the bottle).
  • If any leaks are identified, take corrective action and perform this procedure again until leaks are no longer detected.

This is recommended for cryo-rated valves, small pressurized vessels, or any other cold equipment that you don’t mind completely submerging in LN2. It will definitely save yourself a lot of time by doing this every time!

Making a Cryogenic-compatible O-ring seal

One issue that I ran up against while re-designing CHEF for my thesis research was the connection point on the hydrogen liquefaction tanks. I decided to use VCR connectors because of their reliability at vacuum and low temperatures, this meant that a VCR connection directly to the tank would be easiest for design and build. As all VCR connections, off the shelf, are made of stainless steel, and the liquefier tanks were made from Aluminum (because of thermal concerns), welding a connector directly on was not an option. NPT connections were another possibility, but ultimately not chosen for fear of leakage at low temperatures. Luckily, Swagelok sells pass-through connectors, specifically a ‘Straight Thread O-ring Seal Male Connector’. Normally this would work well as the O-ring is what seals the connection, instead of the threads in a traditional NPT connection, however O-rings are usually made of synthetic rubber materials and do not work at temperatures much below room temperature (~298K). Because this needs to seal against liquid hydrogen (~20K), a rubber O-ring is not an option. We decided to try using indium as a stand-in O-ring. Indium is used in many cryogenic applications as a seal because of how well it conforms to whatever shape it is in, filling in tiny crevices and actually bonding to the metals, however it is usually used as a crush seal, meaning that there is no twisting involved in the sealing process.

After searching around I was unable to find much information on-line about an application of indium like this, only a comment made by Jake Leachman that he had seen a presentation on something similar to this at a cryogenics conference in the last year or two. So I decided to do what any engineer that’s faced with a new and untested idea does…just go for it!

For the seal I used:

1/16” Indium wire

SS-4-VCR-1-00032 ¼” Straight thread O-ring pass-through

Apiezon-N cryogenic high-vacuum grease

 

The top flange of the liquefier tanks had the correct size tapped hole machined into it. On each hole we had a 45 degree chamfer, 1/32” deep; we did this because we wanted somewhere for the indium to seal against instead of a sharp corner that might cut the indium instead of smear it.

To actually make the seal I had Casey Evans, our wonderful undergrad volunteer help me. Using gloves, we first cleaned all surfaces and indium with methanol and then acetone to first get rid of any oils on the surface and then any traces of the methanol left behind. The indium was cut to length by trial and error around the O-ring groove. We cut the end of the wire at about a 60 degree angle as shown below in the diagram. This is to ensure that as the pass-through is put in that it smashes and smears the indium seal together even more

Twisting indium seal.

After the indium wire has been cut, we coat the wire in a thin layer of Apiezon-N high vacuum grease in order to help cut down on the friction seen by the indium. We want the indium to seal, but at the same time we don’t want it to be smearing the entire time it is being crushed, just at the end to get the final seal. We now used a small flat object to help press the indium edges together to help them fuse together. At this point the indium should look like that shown in the two picture below. Notice on the second one where the edges have been pressed together.

IMAG0624IMG_0081

Now that all the other steps have been completed all that is left is putting it on the tanks! Originally we thought that tightening the seal down all the way would not be necessary, and actually possibly be detrimental to the capacity of the indium to seal if it were cut during the crushing. In reality we found that the seal did not seal easily, and was extremely finicky, opening up leaks when I was trying to put VCR connections on because of the twisting motions. Rubber O-rings are resilient   to small shifts in how tight the pass-through is because it is able to expand to the shape, however because indium is so malleable, it also will not expand and therefor will open up leaks if the pass-through is loosened at all. The first picture blow shows how the seal looked when not completely crushed, while the second shows when it is fully crushed in place. I found that tightening the pass-through till it wouldn’t turn easily worked well for sealing. The indium was literally squeezed out around the edges, and if one were to take the pass-through out, would see that this is true down into the threads as well! All in all, I found that these connections are actually quite reliable when tightened enough, I have four of them on my experiment and have had no leaks after tightening them enough; however I would warn against using this on connections that will be not be at least semi-permanent as there is risk each time to open up leaks when loosening and tightening the VCR pass-through.

IMG_0084(cropped)IMG_0111(cropped)

Conference Room Projector

Presentations are possible!

Want to plug in?

Here’s how:

The projector is controlled by the computer to the right of the projector.

Open Projector Control on the Desktop, if it isn’t already open.

Login information:

User: hyperUser

Password: hydrogen

Use the controls to turn on and operate the projector:

  • RGB1 is the conference room VGA cable
  • RGB2 is the design space VGA cable

Turn of the projector when you’re done.

Keep the whiteboard clean if you’ve used it!

Design Area Projectors

Presentations are Possible!

Want to plug in?

Here’s how:

Use the remote control for the projectors – either controls both projectors

The switch box selects input

Plug in the VGA

Turn off the projector when you’re done.

DI water change for cryo-cooler cooling loop

Follow these steps to change the DI water in the cryo-cooler cooling loop every 6 months, and the filter every 12 months. Remember: a happy cryo-cooler means a happy researcher!

To change the water:

  1. Open drain valve to allow water to drain into floor drain. Use a hose to make sure that water flows down drain. (warning: make sure water goes ONLY into a drain or water damage to lower rooms may occur)
    1. Allow line to drain till water is no longer continuously flowing (pump may be turned on to ensure all water is vacated)
  2. Close drain valve
  3. Change water filter while water is vacated from system
  4. Fill cooling water loop with de-ionized (DI) water from fill port on top of Jet Pump
    1. Pull DI water into system by putting a hose connected to supply side of pump into DI water holding container
    2. Allow return tube (normally connected into supply side of pump) to vent to DI water holding container until constant stream of water returns to ensure that air from the system has been removed.
    3. Turn off pump
  5. Reconnect all lines
  6. Run for 15 minutes to check for leaks
  7. Mark down maintenance date and actions performed on data sheet
Washington State University