helium/LOX valve that is leaking - Anodized Aluminum housing is the problem? 2

Is it a seat leak and/or elsewhere? What kind of a valve is it (type, size, class, end type, body and trim material)? What are the operating conditions? What design/testing standards are applicable?

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What is the leak rate, and what size is the seal? Realize that ptfe is only a diffusion limiter for gaseous helium.

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.

Application data: 1000 psi, -320F, linear travel with .030" stroke length. Yes, LOX is liquid oxygen. Valve Size is about 1 inch.

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.

1. Did the valve tested in high temperatures (over 71C degrees) before tested in -320F?

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?

"The jackets are made of modified PTFE. Modified PTFE is used because of its lower permeability compared to regular PTFE."

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.

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.

Yes, Teflon NXT. From our experience it out performs regular PTFE because of its lower porosity and more resistance to wear/shedding in dynamic applications. I will discuss your suggestion with other engineers.

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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.

I have calculated the equivalent orifice that under 1000 psi at 140 Rankine (-320 F) will give a leak of 1 bubble per minute. Assuming bubble diameter to be 5 mm at room temperature and pressure, it gives a mass flow rate of 0.000011 gram/min. The equivalent orifice diameter to get such a flow rate is approximately 0.000014 mm (0.00000906 inch). This is very small orifice. Any defect around the 1" diameter seal with similar section area will contribute to 1 bubble leak of helium per minute. If my calculations are correct this makes the task almost impossible. Usually, for such cases a welded flexible metal bellow is used as a seal instead of rubbing PTFE/Rubber or any other type of seal.