Air Arms Why you shouldn’t overpressurize your S4xx/5xx

[Stubbers]

Actually, for recertification, they hydro test goes up to 1.6 x the working pressure. Going to 3X on every test shortens the life of the tank. Especially ones made from aluminum.

How to Check Whether Your Scuba Tank Meets the Safety Standards

A hydrostatic test is a safety check to ensure that your scuba tank will not fail while you are diving. Here's how it works!

dipndive.com

"Next, you will find the date of the original hydrostatic test as well as additional hydrostatic tests performed by an authorized inspection service. The aim of the hydrostatic test is to find out whether the tank is able to withstand the pressure. In particular, the testing measures the elasticity of the cylinder walls by filling it with water and then pressurizing the water up to 1.6 times the working pressure of the tank. If a cylinder fails the test, it is removed from service immediately."

Actually for scuba tanks between 150%-166% if you want to nitpick just to be right about something here, be absolute.

I quoted 3x because that is what Lloyd used on his test for an airgun tube, much like Marko did on his friends L2 to ensure it was built to his standards. That is considered destructive testing.

Add to that, what is 200 bar x 1.5, or worse 1.66? You correcting me here only drives home my entire point further, that OP's tube failed to meet such criteria. So thank you for that.

-Matt

 

[John "spysir" Blount]

Thanks. I thought I was keeping it well below any temperature that could affect the temper, but after having it deform like that I started to wonder.

I'm of the opinion that this particular aspect of the S4xx design was fine at the time, but is now a weak point if you abuse it. It seems Air Arms came to a similar conclusion as their new rifles are rated to a higher pressure. Back in the days of non-regulated guns, handpumps and aluminum scuba tanks charging over 200 bar just wasn't very likely to happen and would either make your gun shoot slow or lock up the valve. These were seriously nice shooting guns though and I'd be surprised if large numbers of them don't get retrofitted with regulators and once it's been regulated the gun won't shoot slow or lock up with a big charge of air.

Still thinking old style tube here (not the newer one rated higher) so do stay within rating.
An excellent well thought out and well (overly) tested in R&D design period. Guarantees a safe blow out if over pressurized. Works flawlessly every time. The higher pressure tube is entirely different and the only thing to use for higher pressures. People did have access ( just not as many) to 6,000psi tanks thus the fail prof design.

John

 

[subscriber]

Lloyd was testing a design to 3X. Design certification testing is not the same as proof testing a manufactured product; or life recertification testing.

Consider how low proof test pressures are for firearms: 125 to 130%. Very few can stand 2X rated peak pressure. Never mind 3X. It is usually the cartridge case that fails first. Why? To save weight; apparently.

 

[Stubbers]

Lloyd was testing a design to 3X. Design certification testing is not the same as proof testing a manufactured product; or life recertification testing.

Consider how low proof test pressures are for firearms: 125 to 130%. Very few can stand 2X rated peak pressure. Never mind 3X. It is usually the cartridge case that fails first. Why? To save weight; apparently.

ASME BPV Section VIII, Division 2, uses a much smaller safety factor on tensile strength than Division 1 does. The safety factor is 2.4 instead of 3.5 for non-bolting (see Mandatory Appendix 10 in SC II, Part D). Note G6 in Table 5A indicates an 85% multiplier like we see for Division 1 allowables, even though the Appendix itself does not spell it out. The yield allowable criteria is the same and the bolting allowables are still from Table 3 (for design by rule).

Division 2 only needs 2.4x FoS, better go call Lloyd and inform him.

-Matt

 

[Stubbers]

If it were my design, I want 3X over pressure based on the yield strength of the material.

AA made the calculation that they would lose more customers by making their air tanks heavy enough to stand 3X peak working pressure, than if the occasional tank blows an o-ring due to being mildly over pressurized. Their approach seems silly, but thinning the tube wall where the o-ring sits near the tube end acts as a fail-safe. So, not dangerous. But because damage due to overfilling is not covered under warranty, it seems like an expensive mistake that is a little too easy to make.

I'd rather rely on a burst disc failing than an entire air tube failing, or better yet have the entire assembly meet ASME standards to which I will never be able to reach with any compressor available on the market, like the majority of airgun manufacturers do. To each their own eh?

 

[Stubbers]

Designing a 'fail safe' well below 1.5x the stamped fill pressure does not protect the tube against falls, scratches, corrosion, temp swings, where the intent behind ASME standards do. Fail safes are NOT ASME standards and I will trust those engineers much more than any other, certainly from an airgun company doing their own thing. That's what burst discs are for, to be a middle ground between working pressure, and design pressure.

-Matt

 

[Stubbers]

There is nothing bad about using a greater factor of safety than a standard requires. Besides the fatigue properties of aluminum suggest a higher factor of safety would be smart, compared to that for steel, regardless of legal requirements.

Basing a the factor of safety on ultimate strength, rather than yield seems wrong to me. If you stay below yield strength strength there is no permanent deformation. For steel, there is very little fatigue at half yield stress. Aluminum shows significant fatigue life reduction, even if you cycle up to only half of yield strength.

If you get anywhere near ultimate strength you have already bulged your pressure vessel permanently. FoS based on ultimate strength is to cover a one-time high pressure event. Damage occurs, but the vessel does not rupture. For repeated filling and use cycles, an FoS based on yield strength makes more sense.

The Factor of Safety applies to while the assembly is in operation, not just the material walls UP to a point. They also take into consideration small differences in material tolerances, temperature variations the material is exposed to, corrosion...

Again most of our tubes apply to Division ll (2.4 FoS) not division l (3.5 FoS, not 3)

-Matt

 

[subscriber]

Designing a 'fail safe' well below 1.5x the stamped fill pressure does not protect the tube against falls, scratches, corrosion, temp swings, where the intent behind ASME standards do. Fail safes are NOT ASME standards and I will trust those engineers much more than any other, certainly from an airgun company doing their own thing. That's what burst discs are for, to be a middle ground between working pressure, and design pressure.

-Matt

It is not the whole AA air tube that has such thin walls. Only the section near the end where the O-ring sits.

I agree that burst discs are a more civilized solution.

 

[Stubbers]

It is not the whole AA air tube that has such thin walls. Only the section near the end where the O-ring sits.

I agree that burst discs are a more civilized solution.

You wouldn't catch me designing a tube in such manners privately nor commercially. If my yong heng compressor had such features I'd throw a fit, and intentionally let it fail to get a refund through warranty, opposed to the replaceable bursts discs it does use. It's not just more civilized, it's far more economical, and practical.

-Matt

 

[Stubbers]

 

[Sawney Bean]

I always had my doubts about the idea of proof testing a muzzle loader with 2Xs the usual charge or with 4Xs the powder. Even if it didn't blow up outright who's to say it wasn't damaged afterwards?

 

[Stubbers]

I always had my doubts about the idea of proof testing a muzzle loader with 2Xs the usual charge or with 4Xs the powder. Even if it didn't blow up outright who's to say it wasn't damaged afterwards?

This discussion about FoS (factor of safety) doesn't apply to a muzzle loader or the like as they aren't storing compressed air indefinitely. Likewise with fx's straw barrels. Although I would agree.

-Matt

 

[Sawney Bean]

This discussion about FoS (factor of safety) doesn't apply to a muzzle loader or the like as they aren't storing compressed air indefinitely. Likewise with fx's straw barrels. Although I would agree.

-Matt

That was meant as a reply to subscriber's post about proof testing smokeless powder rifles. I should have included the quote to avoid confusion.

 

[Stubbers]

That was meant as a reply to subscriber's post about proof testing smokeless powder rifles.

Likewise! He also compared pressure vessels earlier to gas tank capacity in automobiles, so ...but I didn't want to nitpick every little thing earlier but, eh why not now.

-Matt

 

[BSJ]

It is not the whole AA air tube that has such thin walls. Only the section near the end where the O-ring sits.

I agree that burst discs are a more civilized solution.

I haven't been inside my newer S5xx but I'm pretty sure the tubing itself is the same as the old ones. More or less 'standard' hydraulic tubing ground to a high polish externally.

The way it now handles over pressure is different. There is a relief hole that allows the air to escape after an oring fails in the now more complicated end caps. Replace the oring and you're back in business. Wiser I hope too.

 

[Stubbers]

ASME Division 1 uses the lower of 1/1.5 times the yield stress at temperature or 1/3.5 times the UTS (0-3000 psi)
ASME Division 2 uses the lower of 1/1.5 times the yield stress at temperature or 1/2.4 times the UTS ( (3000-10000 psi)

Aluminum included.

Barrels aren't held to the same ASME standards in their proof testing.

Per ASME:

Pressure vessels are enclosed containers that hold and store liquids, vapors, and gases at a pressure significantly higher or lower than the ambient pressure. Design, construction, repair, and testing of pressure vessels are governed by some regulations such as ASME BPVC and API 510.

More can be read here in regards to barrel proofing: To be clear this is UK specific.

Reasons for Gun Barrel Proof

and here https://www.legislation.gov.uk/ukpga/1978/9

 

[lloyd-ss]

................................................

Division 2 only needs 2.4x FoS, better go call Lloyd and inform him.

-Matt

This is Lloyd. I am always careful to design my tests and present information in an honest manner. It takes a lot of time. Because of the many variables that are always involved, and the improvised equipment that is used because of economics, I seldom draw definite conclusions. Readers can apply the knowledge they see as they wish. I have painstakingly prepared and offered that information in an honest manner in the interest of air gun safety.

This thread started out as a learning experience but seems to have been derailed.
That is unfortunate for the other members of AGN.

And finally, No offense meant, and none taken.
Lloyd

 

[Stubbers]

This is Lloyd. I am always careful to design my tests and present information in an honest manner. It takes a lot of time. Because of the many variables that are always involved, and the improvised equipment that is used because of economics, I seldom draw definite conclusions. Readers can apply the knowledge they see as they wish.

This thread started out as a learning experience but seems to have been derailed.
That is unfortunate for the other members of AGN.

And finally, No offense meant, and none taken.
Lloyd

I agree it's a shame the thread was derailed from its original sentiment, although I hope something of value could be taken here from everyone, including yourself.

Likewise, no offense taken, nor was any intended.

-Matt

 

[lloyd-ss]

..........

I agree it's a shame the thread was derailed from its original sentiment, although I hope something of value could be taken here from everyone, including yourself.

Likewise, no offense taken, nor was any intended.

-Matt

You are free to apply whatever standards you wish to tests that you design and perform. My tests are done only for personal edification, not for any type of certification, and are freely shared as such. No intention of adhering to specs of ANY particular jurisdiction are stated or implied, and none need to be, nor will they be.
Lloyd

 

[Stubbers]

You are free to apply whatever standards you wish to tests that you design and perform. My tests are done only for personal edification, not for any type of certification, and are freely shared as such. No intention of adhering to specs of ANY particular jurisdiction are stated or implied, and none need to be, nor will they be.
Lloyd

I think this discussion is best suited privately...as it only continues to derail the thread.

-Matt