Seeking an Engineer's Opinion on FWB PCP Air Cylinder Life Span

[Reddleader68]

I just purchased an FWB 500 PCP target rifle and after reading the instructions, the air cylinder is supposed to be disposed of after 10 years from the date stamped on the cylinder with no mention of cycle count on refills.

My question is aimed toward anyone one who has a engineering back ground, specifically in material fatigue with regards to high pressure air cylinders....if I charge the air cylinder once a year and shot it until it's empty that same day and repeat this once a year for 10 years (recharging the cylinder only 10 times in 10 years), is the cylinder going to deteriorate the same compared to refilling the cylinder every day for 10 years potentially refilling it thousands of times over the course of a 10 year period? 

My guess is that it's just some bogus liability issue and the metal fatigue on the unit would not be an issue for a casual shooter well after 10 years has passed. What's your technical take on this?

[anuthabubba]

Not an engineer and don't play one on TV either ...

My guess is that it's just some bogus liability issue and the metal fatigue on the unit would not be an issue for a casual shooter well after 10 years has passed. What's your technical take on this?

but your guess would be my guess.

[straightshooter]

I would start with contacting FWB, they may be able to supply you with their testing results and specs etc.?

info@feinwerkbau.de

E   

[anuthabubba]

Bet you could find out something over here.

http://www.targettalk.org/

[Wayne52]

That just makes me wonder why guns like the Mrod and Prod wouldn't have similar warnings about their air cyclinder incorporated into the gun.  How much metal fatigue or just replace seals every so often or what?  They don't specify things like that in their description's.  They give you their normal guarantee but nothing of what beyond would be needed.  It's better to always keep air in them I know.

[JMJ in NC]

It's a new air rifle, right? To estimate life span, engineers will assume a reasonable duty cycle for the product (towards the high end of use, though), look at the stresses induced, apply a safety factor, and then calculate the life span of the critical components. Bear in mind that they will balance this with the liability of having a product fail. Being this is an older German company, they will be conservative.

If it was me, I'd assume the air rifle would be used to practice with 3-4 times per week, with a match or team practice every weekend. This is based on it being a match-grade air rifle that would be used in junior to intermediate level competition.

If you're anywhere close to this duty cycle, 10 years is a pretty long time. If you're not shooting it this much, I wouldn't worry about it.

JMJ
BTW, I'm a mechanical engineer.

[Reddleader68]

Thank's for the comments everyone. Looks like everyone is pretty much thinking the same thing I am.

[Reddleader68]

By the way, Jim, it is a new rifle and the cycle count will not come close to someone practicing for competitions. I like it cause it's accurate and you can attach all kinds of cools widgets to it.

[AmBraCol]

It has nothing to do with actual fatigue and everything to do with competition rules.  European rules do not allow you to compete with an air cylinder that is over 10 years old, even the old CO2 cylinders (that are under a lot less pressure).  It's a "one size covers everyone's rear end" rule.  If you're running 100 pellets a day through it, you'll be filling it very frequently and theoretically it MIGHT develop stress points that would cause it to explode without warning.  There's no way to know if someone's cylinder was filled and emptied daily or if it's merely a 10 year  old New Old Stock cylinder that's under it's very first load, so they just banned all cylinders over 10 years old.

For personal use, of course, you do what you want to do and what makes you feel safe.  I just rebuilt a Modell 2 FWB pistol for a friend.  It was his mom's and he has no qualms about using it with CO2, even though the cylinders are way older than the European limit.

[Rob M]

it depends on the metal used in the tube construction. and even then there are extremes in aluminum

[Yogi]

I would imagine that the cylinder's in a FWB would be more precision manufactured than MRod cylinders.  Perhaps thinner material?

-Y

[AmBraCol]

Guys, the issue isn't the material or the quality, it is the "safety consciousness" of the sport shooting community in Europe.  They set an arbitrary 10 year age for all pressure vessels used in sport shooting events in Europe. That's what it's all about.

[HectorMedina]

Michael;

Fatigue is a complex mechanism that debilitates materials through a process where the material becomes "crystallized" and fails.

If you take a piece of sheet metal and bend it forwards and backwards a number of time along the same seam, it will break along that seam. This is something you can try yourself. The material gets bent, which means the elastic limit has been exceeded and when that happens the internal structure of the material changes. Once it breaks, if you look with a loupe at the edge, you will see like the material was made up of "granules", these granules have much less resistance than the homogeneous material that the original sheet was rolled from. Same material, different internal structure.

Now, in pressure vessels the situation is not the same as in the bent sheet of metal. The material will expand and contract as many times as the pressure is increased and decreased, and the "duty cycle" will have several components to it:
-Length of the excursion.- it is not the same if you completely deflate the tank than if you reach the point where the regulator drops out and top it off. The longer the excursion the faster fatigue will set in.
-Relation between maximum pressure in the tank (and the stress level produced) and the maximum elastic limit of the material at hand. The closer you are to the elastic limit the faster the fatigue will set in.
-Time spent not only at each point in the excursion, but the time it takes to execute the excursion (superfast fills/degas's are not advisable, in other words).

It is not true that tanks yield and explode without warning. The truth is that the signs are there but we cannot see them. What we CAN do is measure them.

If your cylinder is new, take good measurements of the diameter of the cylinder at several points along the length of the cylinder when empty. Measurements need to go down to the 1/10,000" so always do it at the same temperature and always at the same point.
Once you have your battery of data, fill the cylinder to recommended pressure and take the measurements again.
Make notes.

You need to keep an accurate log of the cylinder's measurements for the first week of actual work (about 15 duty cycles). If you take proper measurements you will see that new cyilnders "take a set" but once the dimensions have been "set" by pressure/duty-cycles, they stay constant.
The EXCEPTION is some cylinders that get tested at 150% the nominal working pressure at the plant. These take a "set" during that test (usually 5 cycles).

And then just re-measure after every 300 cycles.

When the cylinder fails to return to the ORIGINAL "SET" DEFLATED dimensions by more than 0.2% (for ALUMINUM tanks), you can consider the tank gone. If that never comes, then you should be good assuming no overpressure or abuse of the cylinder has happened.

The 10 years is not only a sporting limitation, it is in the LAW of several countries that exempt certain sizes of cylinders from visual and hydro inspections every certain number or years (you are spared the interim expenses of the tests, but you have to dispose of the tank after 10 years).

Yes, I am an engineer, and so I have to add a disclaimer: "since I have no control over the accuracy of the measurements taken by the reader, the procedure laid down here has proven safe and reliable IN MY CYLINDERS and GUNS, no further claim is made, or implied, here".

HTH







Héctor Medina

[amb5500c]

Good info here. Being a fireman, I am somewhat familiar with the testing of SCBA tanks. But I have never heard it explained in such "user friendly" detail. Thanks, Mr. Hector.
Richard

[AmBraCol]

Thanks for clarifying that, Hector.

[Reddleader68]

Hector, thank you for the informative response. I will take a measurement and start a log book and start to monitor the diameter of the tank.

[HectorMedina]

Glad to be of service!

Keep well and shoot straight!






Héctor Medina