helium/LOX valve that is leaking - Anodized Aluminum housing is the problem?
helium/LOX valve that is leaking - Anodized Aluminum housing is the problem?
(OP)
We have a helium/LOX valve that is leaking even with heavy spring PTFE seals. The valve finish is hard anodized aluminum and even with a 4 Ra finish, leakage is higher than desirable.
Can we apply some type of second coating to reduce this leakage? Something to fill up the deep pores in the anodized aluminum?
Many thanks,
Can we apply some type of second coating to reduce this leakage? Something to fill up the deep pores in the anodized aluminum?
Many thanks,





RE: helium/LOX valve that is leaking - Anodized Aluminum housing is the problem?
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RE: helium/LOX valve that is leaking - Anodized Aluminum housing is the problem?
RE: helium/LOX valve that is leaking - Anodized Aluminum housing is the problem?
If LOX stands for liquid oxygen, aluminium could well be the problem, both for helium and oxygen at low temperatures. Even at low pressures my old (German in origin) user guide for different valve materials / parts Note: for solenoid valves) does not recommend anything else but mainly different high-grade stainless steels for housing and innerparts at 5 bar at minus 196 deg C.
PTFE (correct grade!) is the only accepted material in the same guide for dynamic sealings under the same conditions.
For helium aluminium is not recommended at high (plus 120 deg C)and low (minus 200 deg C). PTFE sealings are recommended for all practical temperatures. Most other alternatives for sealing materials will fall out under some of the conditions.
RE: helium/LOX valve that is leaking - Anodized Aluminum housing is the problem?
There are two [dynamic] seals and they have helical coil spring energizers. The jackets are made of modified PTFE. Modified PTFE is used because of its lower permeability compared to regular PTFE. At -320F, PTFE's modulus is 500Ksi+. The seals perform well at room temperature and then leaks when it goes down to -320F. Perhaps the PTFE jacket get too stiff and not able to flow into the crevices and imperfections of the hardware surface and allow helium to bypass the seal.
Hardware surface finish is 4 Ra, 30 Rz, and 10 Rp.
The hardware has gone through sulfuric anodization and is sealed per MIL-STD-8625.
Leak rate is too high with over 10 bubbles per minutes. Ideally it should be 1 bubble per minute or less.
RE: helium/LOX valve that is leaking - Anodized Aluminum housing is the problem?
2. What is the diameter of the PTFE seal?
3. What is the basis for 1 bubble of helium per one minute at 1000psi requirement?
4. Is there a similar application where such design is working with same requirement?
RE: helium/LOX valve that is leaking - Anodized Aluminum housing is the problem?
Uh oh. Yes, modified PTFE (I think you mean the DuPont Teflon NXT material?) has lower room temp. permeability, but I think it loses its "malleability" at a higher temperature than straight pfte will. Try switching to a straight ptfe spring-energized seal and see if the low temperature performance improves.
RE: helium/LOX valve that is leaking - Anodized Aluminum housing is the problem?
israelkk (Aerospace)
1. Did the valve tested in high temperatures (over 71C degrees) before tested in -320F?
No, it was tested at room temperature
2. What is the diameter of the PTFE seal?
Around 1 inch
3. What is the basis for 1 bubble of helium per one minute at 1000psi requirement?
Not sure, this is a customer requirement
4. Is there a similar application where such design is working with same requirement?
Yes, but the aluminum valve was untreated in previous cases. I have a nagging feeling it is the anodization that created a porous surface even with a 4 finish. I am investigating a sealant or somekind of coating to reduce the porosity.
RE: helium/LOX valve that is leaking - Anodized Aluminum housing is the problem?
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Uh oh. Yes, modified PTFE (I think you mean the DuPont Teflon NXT material?) has lower room temp. permeability, but I think it loses its "malleability" at a higher temperature than straight pfte will. Try switching to a straight ptfe spring-energized seal and see if the low temperature performance improves.
RE: helium/LOX valve that is leaking - Anodized Aluminum housing is the problem?