Need advice and hints for working with helium

[Majorana]

TL;DR

Looking for components and materials suitable for working in contact with helium at (relatively) high temperature and pressure.

Hello everybody,
we are setting up the prototype of an equipment, that will be filled with pressurized helium. There is a small external heat exchanger, and the prototype has also manometers and thermometers inserted at various stations through the walls of the pressure case. The working temperature of helium will be higher than 350°C (623 K), with pressures up to about 30 Bar (3 MPa) and possibily higher.

Now, I have learned that helium, just like hydrogen, has some interesting peculiarities. Working with helium is definitely NOT like working with air. Helium - I read - has the capacity to seep where air could not (hydrogen is the same). Manufacturers of compressors warn that a compressor originally designed to work with air would NOT work with helium, because all the seals, gaskets, greases etc used with air, are not leakproof with helium. The same holds for all the materials and components fitted on the prototype case: valves, manometers, gaskets, sealing greases or tapes used on threaded connections, the external heat exchanger, and so on. The usual flexible tubes, with the safety steel wire mesh outer sleeve and inner pressure tube made of synthetic (polymer) rubber, are another big question: will hot helium escape through the material (possibly degrading its strenght at the same time)??... The fact that helium is hot in our application will probably make everything worse (the folks at NASA have decades of experience with helium, but they use to work at cryogenic temperatures, as far as I know, where helium behaves quite differently than at 350+°C, so I don't know whether any useful info might come from there).

There is a well-known effect with hydrogen: the embrittlement of several types of steel when exposed to an hydrogen atmosphere ("hydrogen embrittlement"), due to the ability of hydrogen to seep into the reticular structure of steel. I don't know if the same danger exists with helium. So far, a search for the term "helium embrittlement" returned nothing, but maybe that somebody out there has more actual information than a stupid search engine... I remember that many years ago, a famous (in my country) laser physicist and engineer explained me that, in HeNe laser tubes, that usually have a hollow cylindrical aluminium cathode, when the laser is NOT in use, the helium in the mixture tends to be slowly absorbed into the aluminium cathode over time. Later, when the tube is powered and heats up to working temperature, the cathode expels the helium again. That is the proof that, even when cold and at very low pressure like in an unpowered HeNe laser tube, helium *can* actually seep into some metals (aluminium, at the very least) and possibly other materials.

Basically, what I need is a series of advices, hints, suggestions, and information about equipment sources, in order to identify the most appropriate materials, components and equipment to work with helium at high temperature and pressure.

● Compressor - A compressor is needed in order to fill the equipment case at the required pressure. This is the only piece of equipment that does not need to withstand high temperatures (the filling is done only once, with ambient temp helium coming from a cylinder). There is more than a manufacturer of helium compressors, of course, but the problem is that the machines I found so far are huge, enormous industrial compressors, that deliver pressures and especially flows far exceeding our needs (they are intended to fill hundreds of high-pressure helium cylinders per day). We need a small helium compressor, able to exceed the 30 Bar final pressure required in the equipment case, but giving just a small flow, something similar in size and performances (pressure apart) to a domestic refrigerator compressor, or the compressors used with airbrushes in hobby modeling. I think that some small laboratory helium compressor could perhaps do the job, but it's not easy to find, also because I don't know if there are manufacturers specialized in lab helium equipment.

● Valves - Valves are required at the connection ports to evacuate air from the pressure case, and to fill it with helium after purging. On the web I found a few warnings against using valves not specifically designed to work with helium (like most ball valves) because that gas would slowly make its way through the valve, even if perfectly closed and airtight. As of this writing I could not find any ads about valves designed for hot helium. My question is: if I can't find any valve specifically advertised to work with hot helium, what type of valve should I look for, that works reliably under those conditions?

● Forbidden materials - Is there any material(s) that for any reason should NOT stay in contact with hot, pressurized helium?

● Sealing materials - Where there are threaded connections (purge/filling ports, instrumentation ports, to/from tubes towards the external heat exchanger etc.) I am not sure about what type of sealing materials should be used, or NOT used. Is the usual white Teflon tape for threaded connections good with hot helium? Or is there any specific grease or sealant to be used instead?

● Gaskets - Same as above. Are high-temperature FKM (Viton) gaskets adequate for working with hot helium, or should I look for some specific materials?...

Thank you very much in advance to everybody for any information or suggestion you can provide!

 

[.Scott]

I search on "helium permeation rates by temperature". It looks like about anything will allow Helium permeation and at 350C, and the Arrhenius-type response to rising temperature could be a major problem if your system needs to hold the gas for days at a time.

 

[Majorana]

I search on "helium permeation rates by temperature". It looks like about anything will allow Helium permeation and at 350C, and the Arrhenius-type response to rising temperature could be a major problem if your system needs to hold the gas for days at a time.

Hello Scott! Thank you so much! This thing of helium permeation through materials is the proverbial pain in the Hydrogen with steel, for example, seems to do worse things at room temperature: "Hydrogen embrittlement is maximised at around room temperature in steels, and most metals are relatively immune to hydrogen embrittlement at temperatures above 150 °C." [source: The Welding Institute]. Do you think that things with helium actually go the other way (worst at high temperature)? Of course it must be possible to hold pressurized helium in a cylinder, or pressure vessel with connection lines. Maybe helium permeation cannot go beyond a certain point (sort of "saturation") after which the vessel/tube material does not allow helium absorption any longer?... Just speculating. Especially, I cannot find any information that proves - or disproves - any alteration of material strenght (like the embrittlement with hydrogen) after prolonged exposure to helium: if it permeates the material and ends up included in its structure, after all, a change in mechanical characteristics is not completely implausible...

if your system needs to hold the gas for days at a time

It should not be days... it should be months, at the very least, or years even better... like the fluid in a home refrigerator, that can work for decades without servicing.

 

[.Scott]

Especially in contrast to Hydrogen, Helium is very inert - it is the most inert element.
It's two electrons exactly fill the 1s2 shell.
So, I would not expect it to have much of a chemical effect on anything.

But that doesn't mean it is harmless. Most famously, Elon Musk has had conniptions keeping his Helium COPV from exploding - blaming the first one on sabotage before finally figuring out what was going on. But the problem related to the cryogenics.

Helium is also tiny - with a diameter of about 62 picometers. In contrast to bulk metals, aluminum atoms keep 286pm from each other and iron atoms are about 290pm. I imagine those Helium atoms sliding through unused space in the metal lattice - with the metals acting as much like filters as solid barriers.

Graphene will hold together at 600C and it may be a solution. The University of Manchester, England published this articles comparing graphene to other materials. Quoting from that article:

To put this into perspective, one-atom-thick carbon is less permeable to gases than a one-kilometre-thick wall of glass”, explains Geim.

I think any system you put together for containing Helium at 350C for years will need to be preceded with prototypes. You may find that you'll be writing the book on this one.

Best of luck!

 

[Majorana]

Especially in contrast to Hydrogen, Helium is very inert - it is the most inert element.
It's two electrons exactly fill the 1s2 shell.
So, I would not expect it to have much of a chemical effect on anything.

But that doesn't mean it is harmless. Most famously, Elon Musk has had conniptions keeping his Helium COPV from exploding - blaming the first one on sabotage before finally figuring out what was going on. But the problem related to the cryogenics.

Helium is also tiny - with a diameter of about 62 picometers. In contrast to bulk metals, aluminum atoms keep 286pm from each other and iron atoms are about 290pm. I imagine those Helium atoms sliding through unused space in the metal lattice - with the metals acting as much like filters as solid barriers.

Graphene will hold together at 600C and it may be a solution. The University of Manchester, England published this articles comparing graphene to other materials. Quoting from that article:

To put this into perspective, one-atom-thick carbon is less permeable to gases than a one-kilometre-thick wall of glass”, explains Geim.

I think any system you put together for containing Helium at 350C for years will need to be preceded with prototypes. You may find that you'll be writing the book on this one.

Best of luck!

Thank you again Scott! Mmm... I don't think I'm going to write any book, at least not about pressure vessels for high temperature helium! The question in its entirety is multifaceted, that's very clear.

I tend to rule out using graphene because of the costs related to it, and the complexity of the machinery required. On the other hand, the metals used for the pressure vessel (case) and all the thingamajigs inside are stainless steel, copper and aluminium, that for pure chance are the metals that, according to the liturature I found to date, behave better with hydrogen for what concerns gas permeation/diffusion. I am inclined to believe that, for the same temperature and pressure conditions, hydrogen is a more "evading" gas than helium, so helium will probably tend to stay confined better than hydrogen (in vessels and tubing of the same material).

Actually, my main problem - and the focus of my post - is about the accessories, not the pressure vessel itself. For example, the valves. I must use commercially available valves, I cannot design - much less manufacture - a custom graphene-lined valve... The same goes for the connection ports that are needed to hook up the instrumentation (thermometers, manometers, various probes): I don't know what are the best sealing materials to achieve the best helium-tight coupling. And don't mention the compressor: I'm blowing my brains on finding that one...

 

[jrmichler]

Swagelok is a manufacturer of top quality tube fittings. Check them out: https://products.swagelok.com/en/all-products/fittings/tube-fittings-adapters/c/154?clp=true. They have a contact page with a Technical Product Information category.

 

[DaveE]

Could you use the valves temporarily to seal the vessel long enough to remove the plumbing and replace it with a mechanical seal, like a flange. Then let the valve leak, it won't matter.

 

[Majorana]

Could you use the valves temporarily to seal the vessel long enough to remove the plumbing and replace it with a mechanical seal, like a flange. Then let the valve leak, it won't matter.

View attachment 369180

Dave, that's really one clever idea! A valve on the purge/fill port(s), and a permanent stopper, for example a threaded/bolted cap, to effectively seal the passage when the valve must stay closed! Of course, the cap must have some kind of gasket or sealing material on the contact surface, unless it's welded in place (very impractical). That reminds me that all the ports on the pressure vessel, where the valves or the instrumentation are attached, must have some very effective sealant on the threaded connections. I suspect the good old Teflon tape used by plumbers is not good enough here...

 

[Majorana]

Swagelok is a manufacturer of top quality tube fittings. Check them out: https://products.swagelok.com/en/all-products/fittings/tube-fittings-adapters/c/154?clp=true. They have a contact page with a Technical Product Information category.

Thank you, I will check with them! Of course, a manufacturer on my side of the swimming pool (Europe) would be preferable for practical reasons, but in case I would buy just everywhere.
I have an impression, that the market offers a wider range of valves and materials intended for gaseous hydrogen, than for helium. Am I right?... Maybe because hydrogen is more common than helium in the industry. As a general criterion, do you think that a valve, gasket etc. designed to work with hydrogen can be effectively used with helium, given that helium is not reactive and presumably a bit less prone to permeate solid materials than hydrogen? In this case I could look for my components in hydrogen hardware catalogues, it might be easier...

 

[DaveE]

That reminds me that all the ports on the pressure vessel, where the valves or the instrumentation are attached, must have some very effective sealant on the threaded connections. I suspect the good old Teflon tape used by plumbers is not good enough here...

Maybe a flange with a soft metal gasket, like Indium?
IDK, maybe it would melt (157^oC). Maybe you would want it to melt, like solder?
The simple mechanics of squeezing a seal between two flat plates seems more reliable than screw threads to me. Also, I've seen them a lot in pressure vessels and such. Although I have no real experience.

Aluminum (650^oC)?

 

[Majorana]

Maybe a flange with a soft metal gasket, like Indium?
IDK, maybe it would melt (157^oC). Maybe you would want it to melt, like solder?
The simple mechanics of squeezing a seal between two flat plates seems more reliable than screw threads to me. Also, I've seen them a lot in pressure vessels and such. Although I have no real experience.

Aluminum (650^oC)?

Dave, your idea of using a metal gasket is really smart! Indium (if pure) is not suitable due to its melting point, too low given that helium in the vessel may exceed 350°C and the vessel is partly copper and partly stainless steel. But another soft metal, or alloy (to raise the melting point), will be just fine. Like an alloy of lead (melting point 327°C) with something else. And I have just thought to replace the conventional safety valve on the vessel with a rupture disk (burst diaphragm), that is hermetic by definition when intact, while a spring-loaded safety valve, with internal gasket, is not.

 

[.Scott]

Another strategy for dealing with leaks is to recover the leaking fluid and pump it back into the system.

For your Helium system, I suggest these layers: the hot Helium core, regular piping, thermal insulating material, a vacuum, exterior piping and fixtures. I would have the recovery system operate at room temperature and pump any Helium captured from the "vacuum" back into the hot system.
The shafts on the valves would protrude through the out layer to a handle - and that valve shaft would include a temperature-insolating segment.

 

[jrmichler]

Any valves should be bellows sealed to eliminate leaks past the valve stem. The image below is of a Swagelok bellows sealed flow regulating valve:

The above valve is not the right valve for a shutoff application. It is the first diagram that I found in a quick search that clearly shows how bellows sealed valves work. Search terms bellows sealed valve will find numerous manufacturers of these valves. One such manufacturer states on their web site: "Every bellow assembly is helium leak-tested to assure zero leakage."

 

[Majorana]

Another strategy for dealing with leaks is to recover the leaking fluid and pump it back into the system.

For your Helium system, I suggest these layers: the hot Helium core, regular piping, thermal insulating material, a vacuum, exterior piping and fixtures. I would have the recovery system operate at room temperature and pump any Helium captured from the "vacuum" back into the hot system.
The shafts on the valves would protrude through the out layer to a handle - and that valve shaft would include a temperature-insolating segment.

We - the developers - have ruled out from the beginning any helium recovery system, because it would be far too impractical and actually far more complicate and expensive than the "hot system" itself! If appropriately enginereed, the pressure vessel can be made as hermetic as a welded closed circuit, like a domestic refrigerator that can operate literally for decades after manufacturing, without servicing the refrigerant fluid. As there will be some allowable tolerance in the operating pressure, we can even fill the pressure case to a pressure slightly higher than nominal, to allow for a slight decrease (if any!) along the years.

A much more delicate problem will be the choice of the lubricant used for the few moving parts inside the vessel. The total quantity will be really small (no more than a teaspoon or two) so it can be, in principle, even very expensive, but it has to be stable under those high temperature conditions, so stable that even 20+ years later if must retain its lubricating qualities. I'm wondering if some exotic lubricant used in space applications (where you cannot reach the craft and pour a good can of oil ) could do the job, or maybe some more terrestrial stuff is adequate enough...

 

[Majorana]

Any valves should be bellows sealed to eliminate leaks past the valve stem. The image below is of a Swagelok bellows sealed flow regulating valve:
View attachment 369221
The above valve is not the right valve for a shutoff application. It is the first diagram that I found in a quick search that clearly shows how bellows sealed valves work. Search terms bellows sealed valve will find numerous manufacturers of these valves. One such manufacturer states on their web site: "Every bellow assembly is helium leak-tested to assure zero leakage."

Thank you for the hint! ) Swagelok valves look really very well engineered, very clever design! However, keep present that in my application a safety valve, theoretically, isn't even necessary at all. The pressure case is hermetic, filled with helium, and no combustion or uncontrolled addition of any gas can take place. Helium pressure is only a function of temperature, and temperatures higher than the normal operation values (already very hot per se) are possible only in case of a catastrophic event, like a fire in the room. So the safety vent is something that would operate only if a larger scale disaster occurs. In that case, there would be much serious problem to care about... the purpose of a safety valve would be only to avoid adding disaster (pressure vessel explosion) to disaster. A safety valve will NEVER open under less-than-disaster conditions. For that reason, and also because of the absolute hermeticity guaranteed by a burst disk, it's better to have a device that opens and stays open, rather than a valve that could give off - unnoticed - a small quantity of gas, that one would never know unless installing a detection system at the valve outlet. A rupture disk is much more economical, reliable, hermetic, and its operation cannot go unnoticed.