Do pressure vessels become unsafe over time?

by Tom Gaylord
Writing as B.B. Pelletier

This report covers:

  • Do pressure vessels become unsafe over time?
  • Operating pressure
  • Water leading to rust
  • Oxygen!
  • Danger through work-hardening
  • Last word

Today we have a safety issue to examine. Here is the question I got last week that spawned this report.

“Wasn’t sure how to reach you so using this venue. Was wondering about safety issue on pcps with regard to repeated pressurization over the years.Most pcps are newer and certainly built with margin of safety, but is it possible as these age and perhaps are handed down that they (the pressure vessel) can become unsafe? Read somewhere repeated pressurization can lead to eventual metal fatigue (in relation to high pressure vessels of non air rifle type). Thinking of future owners down the road. Thanks for all you do for all us airgun fans, read blog everyday.”

This was such a good question that I felt it needed an entire report for an answer. I know that precharged pneumatics (PCP) are starting to catch on, and a lot of shooters are getting into them, bringing lots of fundamental questions. Let’s start with the one this anonymous reader has asked.

Do pressure vessels become unsafe over time?

The short answer is, yes, they always do. The longer the time, the greater the risk. But there are other factors that play into this. Let’s examine them.

Operating pressure

There are pneumatics that are hundreds of years old whose pressure vessels are still functional — although I would NEVER advise using them! The reason they have lasted so long is the low pressure at which they operate. Until the 20th century, 800 psi was considered an extremely high air pressure that most PCPs never achieved. Many of the older guns operate on just over 600 psi. I know that from their terminal velocities, as evidenced by splatology forensic examination, and also by the detailed testing Dennis Quackenbush and I did on the hand pumps of antiquity. I know that some of these old reports were boring to read, but on a day like today they are the only solid evidence I can point to, to say what is and what was with certainty.

Today’s PCPs operate at much higher pressures. A few operate on 4,500 psi, while the majority use 3,000 psi. And now a few use 2,000 psi. Manufacturers build or use pressure vessels that are rated for the pressures at which their guns operate, and they build in huge safety margins. But sometimes the boutique makers will skimp in this safety area.

You risk more when buying from a boutique maker — especially one with only a year of two of sales to his credit. Many of them are building airguns that are just as safe as the guns of the larger manufacturers, but I have met some who think nothing of gambling with your life. The most dangerous I ever saw was a Farco air shotgun that was running on 3000 psi air! I wouldn’t even stand close to that one!

Water leading to rust

Water in a pressure vessel is an overrated danger — though it is still a danger. Dennis Quackenbush and I intentionally introduced water into a rifle’s reservoir and then shot it several times to see what happens. We weighed the water going into the gun and again, after a number of shots.

Dennis poured 22.1 grains of water into the reservoir of his Light Sporter rifle and then assembled the reservoir and filled it to 3,000 psi with an Axsor hand pump. We dry-fired the rifle 16 times, which completely exhausted it of air. This was done with the muzzle held below the rest of the gun, which put the exhaust port higher than the rest of the reservoir. After shooting we disassembled the reservoir and found 10.4 grains of water remaining. So, about half the water was exhausted in 16 shots.

We then poured another 22.1 grains of water into the reservoir, assembled it and filled it to 3000 psi once more. This time when we dry-fired the rifle we held the muzzle straight up and once again shot until the reservoir was empty of air. In this test the exhaust valve was at the lowest point on the reservoir. After this test there were just 4.3 grains of water remaining. This simple test shows that PCPs tend to blow water out of their reservoirs as they shoot. But they don’t get completely dry.

I disassembled the air reservoir of a Career 707 Tanker Carbine once and did find a few water droplets inside. The owner had been filling the rifle from a hand pump pretty much exclusively. There was no rust in sight because the inside of the tank was oily with what I assume was silicone oil, but that’s not to say rust could not form this way. I am saying that there is some risk of rust inside an air reservoir from water, but the risk is lower than is commonly discussed on the chat forums.

The fear that water vapor will rust out a pressure tube is not as great as many shooters fear. However — there is some risk.

Oxygen!

The greatest risk I know of to PCP operations is the use of the wrong gas — with oxygen heading the list. The use of oxygen in a PCP is nearly a guarantee of disaster. Medical workers see oxygen tanks and reason that air is mostly made of oxygen. Why not use it? They fail to acknowledge the risk that a pure oxygen environment creates, even though their training teaches them otherwise.

oxygen-fire
These aluminum parts were incinerated by an oxygen fire. It self-ignited while the shooter was shooting the airgun.

Danger through work-hardening

This is the question the reader really asked. I addressed it partly in the beginning of the report, but now I will address it fully. As a pressure vessel is filled and then discharged, the material of the vessel expands and contracts. That causes the material to work-harden over time. Eventually the vessel will become so brittle that it can fail under pressure. This is why air tanks of a certain size (2.0 inches in diameter and greater in the U.S.) are required to be pressure-tested periodically. They are tested to see if they are still flexible — i.e. not work-hardened too far.

But what about airguns? Most airgun reservoirs are smaller than the 2-inch minimum size required for pressure testing, so they don’t get tested. Some companies, such as Feinwerkbau, say their removable air cylinders must be replaced after a certain time, like 20 years. The date of the initial pressure test is stamped into the body of the tank, and that starts the clock.

But the majority of PCPs do not follow any testing regimen. And that is what the reader is asking about. Is there a point at which those guns will become unsafe? Without a doubt there is such a point. It will be different for each gun and will depend on the construction of the gun, as well as the use it has been put to. Failure is inevitable over time, but the exact amount of time is impossible to determine.

Last word

Okay, now you know the story. But let’s be safe about this. NEVER test a pressure vessel with air! That creates a bomb that can injure or kill if it ruptures. Pressure testing is always done with non-compressible water, so if the vessel ruptures, all it does is spray water.

51 thoughts on “Do pressure vessels become unsafe over time?

  1. B.B.
    Really good report, concise and to a needed informational point. Especially concerning the use and misuse of pure oxygen. I won’t divulge too much truly embarrassing information but the ignorant and foolish (that would be like me) can be amazed and astonished at what can happen to a lit cigarette should said item intersect with a goodly draw and a (previously) innocuous flow of pure O2.
    The worst part of that fiasco was my hot-date that evening and attending with a blister on the tip of my nose.
    Despite my hopes, everyone will notice something like that and pretty much everybody in the room will want an explainatiom of just how that came to be.
    If they can keep a straight face.


  2. Those Farcos are usually meant to use bulk fill CO2 which should probably only require a pressure vessel rating of 1,200 psi max. But even then, I have never seen any engineering calculations for most any Philippine airguns whether they use CO2 or HPA. Nobody ever mentions any technical specifications in terms of the strength of the materials used. Usually it is just some sort of seamless brass or steel tubing that looks to be thick enough for the fabricator. Bulk fill CO2 tanks there however seem to be produced scientifically to test rate as much as 3,000 psi and they are equipped with overpressure rupture disks. The only Philippine airgun manufacturer I would really trust in terms of pressure vessel integrity would be Armscorp (formerly Squires Bingham) They are the same guys that make the Rock Island, Charles Daly, High Standard and Armscorp 1911s being exported worldwide. They have decades of engineering to back them up.

    The Philippine cottage industry airgun makers however, do not even mention if they test their pressure vessels at all. And they make all sorts of airguns, Mostly CO2 fueled, but ranging in variety from . 22 caliber single shot pistols and rifles, to 16 ga. air shotguns, to .30 cal. steel ball bearing smooth-bore muskets, all the way to steel ball bearing, full-auto, smooth-bore air machineguns. And now they are venturing into making PCPs filled by hand pump and scuba tank set-ups using 3,000psi. Some of the custom shops charge up to as much as $500 per air rifle or depending on the features. About 10 years ago I ordered one from a well known shop. The only discussion of the specs was about whether it was fueled by CO2 or HPA, whether the stock was rigid or folding and whether I wanted a rifled brass barrel or an imported Lothar Walter tube. No mention as to the materials specifications. There didn’t seem to be any any change in spec in terms of the pressure vessel material whether you chose CO2 or HPA. The choices only affected the valving mechanism used. They have been making airguns in the Philippines for at least 50 years now. And yet I know of no documented castastrophic failures of Philippine airguns inspite of the seeming absence of scientific design engineering, materials testing and pressure vessel testing. You would think enough of them would be grenading all over the place and be such a menace to end up being mentioned in the evening news.

    We don’t read any warnings from PCP makers about the life of their pressure vessels. Even in liability conscious America. You would think the product liability lawyers would be licking their chops at the prospect of preparing class action suits against Daisy, Crosman, and the other well known airgun manufacturers in the States. Maybe there is something we don’t know about airgun strength over time. Maybe the danger has been exaggerated or is non-existent for small pressure vessels?


    • Lioniii,

      About two years ago there was a wake up call with a picture of a man whose face was ripped apart when the tank nestled against his face blew up. Investigation of the origin of the picture revealed that it was of an Air Force knock-off with a tank built to handle CO2 filled with HPA. Soon after the picture was circulated there was a lot of talk on the message boards regarding safety resulting in makers giving reassurances of the safety of the materials being used. Some do a form of hydrotesting as in filling the tank with water then inflating the remaining space to more than normal then measuring for any abnormalities or cracks. The use of T6 aluminum has also increased with some testing if the material they received is actually T6 rather than T5. As you have mentioned some still use their time tested Mk 1 eyeball and use thick enough materials.

      Most however have gone to using ready made commercially available tanks usually Catalina bottles or Hatsan tubes. There are some experimenting with “titanium” tubes from China although I suspect they are depending on the figures listed on the material data sheet rather than testing it themselves. Prices have increased though from your last visit with makers charging up to $1000 in making a unit. Due to taxes and tariffs an imported unit in comparison such as the FX Wildcat which is marketed over there for about $1,400 costs after taxes and tariffs nearly $3,000. So a $1000 dollar locally manufactured unit that has proven itself in competitions becomes a reasonable price.

      Siraniko


      • Good to know they are importing known reliable brands of pressure vessels for use in Philippine PCPs. However, over there, you have no guarantee what you are getting is not a fake. There was no way for me to tell that it was really a Lothar Walther barrel they installed on my custom CO2 air rifle. For all you know it was an Armscor barrel they installed. I would steer away from those Chinese “titanium” tubes. Unless these materials are subjected to scientific testing by the airgun manufacturer through a reliable lab, then it’s like Forrest Gump’s box of chocolates. You never know what you’re gonna get!



      • I read that 1,200 psi figure somewhere. But you are probably correct….that figure is probably too low. Actually, designing pressure vessels doesn’t simply involve specifying a particular psi number for the gas being used. It also really involves an understanding of Boyles Law, the strength of the material being used, the operating temperatures expected, design safety factors…..etc….etc. Because the hotter a gas is heated the higher the pressure will develop for a given volume contained. Most CO2 guns however, will probably only experience a maximum of 120 degrees F in the field or in a vehicle’s trunk.


  3. Lioniii,

    Thank you for that look into Philippine air guns.

    For all,… I had a tuff time finding any place that did testing when I looked. I did find this, which may have been provided from another reader. It is a start at any rate.

    https://portal.phmsa.dot.gov/rinlocator

    I have no idea of the cost. This would be for the air tanks that you fill with, (rather than the gun reservoir), like my Guppy that goes to 4,500 psi.

    Good Day all,….. Chris


  4. Since we are on PCP safety, I saw a YouTube video of a popular on line air gun tester awhile back. When “tuning” the gun, while apart, while (charged), the 1″ valve let loose and shot a 1″ hole (clear through) his upper thigh. The screws that held the valve in apparently sheared.

    I have done this with the M-rod when various modifications. I will not do it again after seeing that. I suppose that is why they say never to work on a PCP while charged, though many do.

    I do not have the time to re-find the video right now. Perhaps someone can link it. Worth the watch. On the video he removes the bandage and you can clearly see a big hole in his leg. Be safe Ya All!


  5. I feel certain that catastrophic failures are possible from metal fatigue over a period of time, but a more likely failure will be that of a seal. This is from what would be normal usage by someone who has knowledge of what they are doing.

    Then you have those with no experience with HPA who will experiment. Someone discovers dieseling in their sproinger, so they intentionally introduce a light oil into the compression chamber to make the pellet go faster. Fortunately the usual result from such is the seals are blown out.

    Someone takes their new PCP rated for 2000 PSI and figures that if they run the pressure up higher it will have more power, so they fill it to 4500 PSI.

    Most problems occur from ignorance or stupidity.


    • Ignorance is fine. it leads to someone doing such a thing, finding out it does not work that way and learning not to do it again.

      Stupidity however is dangerous. The stupid will get away with that 4500psi trip in their 2000psi gun once and even though they learned it was wrong and did not work, they will keep doing it over and over adding more and more striker force to get that valve to open. When they do they will announce to the world that they have a 4500psi airgun now and gained much power while saving many dollars by not buying a gun meant for that. Thus encouraging others to follow in this path.


  6. B.B.,
    “Medical workers see oxygen tanks and reason that air is mostly made of oxygen. Why not use it? They fail to acknowledge the risk that a pure oxygen environment creates.”
    By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere.
    Atmosphere of Earth – Wikipedia
    Pete


  7. Modern airliners pressurize and decompress every time they take off and land. The bodies are designed to expand and then “relax” back to their original configuration upon landing. Over years of travel, I would imagine this cycle must have been repeated thousands of times in the life of an airplane. Rigorous design parameters and government regulations and inspections make modern air travel very safe from a mechanical standpoint. So compression and decompression are normal modern happenings, but I doubt other unregulated industries are as rigorous as the airline industry where the passengers safety is a daily concern. As BB mentioned, most main stream manufacturers probably do some sort of basic pressure testing as they want to avoid lawsuits from injuries. Businesses want to stay in business, sell more products to happy customers, and not enrich lawyers. A happy customer is a repeat customer.


    • Airplanes use Aluminum, and they work harden it over their service lives.

      PCPs largely use steel and normally there should be enough steel to resist the level of expansion that leads to work hardening on human observable timescales.



        • The technique, then, is making humans’ subjective observations of time out of the equation. With commercial aircraft scrupulous records are kept regarding flown hours, flown distances, flown hours and distances between inspections, maintenance performed, parts replaced, even perhaps numbers of take-offs and landings. These records follow the craft for its entire service life.

          Of course air rifles, with the exception of the Diana Model 30 gallery gun, do not have counters. But I would hope manufacturers would over-build all of the parts of a PCP that might become dangerous over time. Last week I watched a documentary on the DC-3, the quasi-official greatest airplane of all time, and so many of them flew safely for many decades because of Donald Douglas’ decision to apply a principle of overkill in the build. No plane before had such a passenger capability, so out of caution he decided to make it as stout as it could be and still fly properly. Pilots who flew them back from France after dropping paratroopers at night before D-Day commented that the planes (modestly modified DC-3s) might have been anti-aircraft-gunned into Swiss cheese, but they flew as usual all the way back to England.

          Famously, a DC-3 with total engine failure was abandoned in flight over the middle of the U.S., its crew and passengers bailing with parachutes. The plane was found perhaps a 100 miles farther on, in near perfect shape, in a farmer’s field. It had glided to a safe landing unmanned. Now THAT is a great machine!

          I would hope that PCP makers overbuild the s**t out of every air gun to make it safe for many generations. When in doubt, overbuild for strength and then overbuild some more.

          Michael



        • I am confused as to why you posted this.

          I already said airplanes are made of aluminum and as such have serious metal fatigue and work hardening concerns.

          This is why air tubes are usually not made of aluminum or titanium. Some are and they must be even more carefully designed, and frankly I would not buy one. The risk is too high for me. I prefer nice heavy steel that handles this kind of stress over time better.


    • You have made some excellent points here, B-I-L; I work at an air base, and I work on metrics for the aircraft. The life of an aircraft is measured in pressurization cycles, the number of takeoffs and landings. Safety is a MASSIVE concern here, and it is just as well in the civilian industry. While it’s a bit dated, the information is still relevant, and you can see a good article about the life cycle of aircraft here: http://www.airspacemag.com/need-to-know/what-determines-an-airplanes-lifespan-29533465/
      Thank you.


      • Also of note is that airplanes undergo 100% cycling with every flight.

        PCPs are rarely cycled 100%, we normally leave them charged. Which again reduces fatigue, and we use materials that are not as susceptible as aluminum.


      • Thedavemyster,

        Please forgive me for my above redundant post, written and posted with my having not yet read your similar, earlier, post. I choose to blame light-headedness from coming in after a shift of snow shoveling.

        Michael


    • True enough, but not as true as a happy customer be ng one that lives through the flight experience.
      I’ve a relative who’s a certified aircraft mechanic who let me amuse myself one day by peeking into the manual…no very small thing in itself. I’ll not bore you with what exactly it takes to construct and repair a sealed pressure vessel, suitable for large numbers of people. On such vessel at at a time, much less on a mass production basis, but it must be done JUST SO.
      We take all this stuff for granted, what with pooting off for the weekend to Maui and all, cheerfully flirting with an environment much closer to space than the green hills of earth. It’s amazing we get away with it at all.
      Research:
      –B-29/B-36 mass-produced pressurized aircraft subtitled, “How to get to the front of the aircraft from the rear, in flight, without wearing a space-suit.”
      –The Comet airliner disasters, Proving, yet again, what can go wrong, will go wrong, and just ’cause you never thought of the cause doesn’t make it less real. Hint: There’s more than one way to generate metal-fatigue.
      Overall, humans are remarkabley non-pressure sensitive…until we’re not, And just because you work on a submarine, or just a silly SCUBA type diver in no way makes you immune to radical pressure changes, It’s complicated, whether more or less.
      (There are cases of sport-divers happily on their way home after higher altitude dives, having meticulously followed their decompression tables, suddenly developing “The Bends” in flight.)


  8. B.B.

    If PCP air tanks have to be labelled with expiration and replacement dates, what are airgunners to do if their rifle doesn’t have an easily removable tank? Perhaps it’s time for PCP manufacturers to considering changing their rifle designs over to removable tank modules instead of permanently mounted tanks.



  9. If I remember correctly, an AL80 scuba tank has an estimated lifespan of 100,000 fills. It is good as long as it can pass it hydro test (every 5 years). The local shop has old steel tanks that date from the late 50s that are still in service.

    Jim


  10. FYI- The HW100 air reservoir is made from stainless steel and its cylinder wall is thick, that is why it is heavier than other maker’s air reservoirs. You want an air cylinder that last, this one is it.



  11. B.B. I think You really meant Gambling with your life instead of gamboling. As far as Chris’ remark, he is correct about the valve shooting through the man’s leg. Evidently he didn’t pin it correctly with enough screws or the correct ones. He had added a couple of after-market extensions on the air tube which some felt caused the catastrophe. The main lesson which should be learned is do not take apart a gun that is not depressurized.



  12. Since we are talking about pcp’s. Figured I would mention my Benjamin Wildfire shipped today. Should have it before the weekend.

    Can’t wait to get it. It’s going to be a long, long work week is all I can say.


    • Gunfun1′

      Got mine Friday, but one of them was a leaker on the second fill. 🙁 Had it laying in my lap while I filled the mag and it just suddenly went “whooosh”. Just got off phone with Pyramyd (had been a few days and no word on my “return authorization” so I called them.) Was told they are short staffed and behind right now, but took care of it over the phone) Having some issues with a wrist injury right now so I can’t really operate my hand pump without aggravating it, so real shooting will have to come later anyway.


      • Halfstep
        I remember you saying that yours were shipped. Was wondering if you had got them and shot them yet.

        And I wondered about something like that happening that you described happening to yours. I know on some pcp guns that the first fill you need to cock the bolt so the striker/hammer is not resting on the valve stem. Sometimes the top hat won’t seat and you can’t pump the gun up. And I have had them dump like what yours did also. Sometimes it helps to cock and fire the gun several times. Even when it dumps all the air. Particles of stuff gets inbetween the top hat and seat. And in the case of the Wildfire just pull the trigger a bunch of times without the clip and magazine in the gun. But face the gun with the magazine opening down and hold it away from you. It will discharge some air with out them in place.

        If you still have yours and not boxed up you might want to try that before you send it back. Of course if your up to it with your wrist. Or maybe have someone else pump it.

        If you try let me know if it helps.


        • Gunfun1,

          The first fill of the gun went fine. I got 4 mags on that fill ( I’m not setting really strict criteria at this time,135 fps range from 2000psi to 1000psi) The first mag on the next fill went fine as well, but as I was refilling the magazine all the air blew out of the gun. I did try dry firing both on a completely empty reservoir and also with a small amount of stored air. ( I could barely fill faster than it leaked out) No joy either way so I sent it back. By the way, both my guns fill from 1000 to 2000 on 50 pumps of my G6 pump. using my loose standards that’s just 1 pump per shot, approximately. (shooting RWS Basics) My wrist is getting better but I still won’t be foolin’ with my guns for awhile,as I’m leaving on a week long crappie fishing trip to LaGrange , Georgia on West Point Lake on Saturday and want my wrist at 100% for that.


          • Halfstep
            Love crappie fishing. And bluegill too. And eating em too. 🙂 Sounds like fun to me. We got a pond a couple hundred yards from the house that has nice bluegill and crappie in it. And some big bass at that. And there’s a lake a little farther into the woods that I haven’t fished yet. But going this spring. But hope your wrist gets better for your trip and your shooting.

            And hope I get a good Wildfire. Hope what happened with yours doesn’t continue to happen with others. Guess we will start hearing soon enough one way or another though.

            But keep me posted on yours and how it goes when you get back at it. And have fun fishing.


            • Will do GF1. I like any fishing as long as I can do it under a bobber with a cold one in my off hand, as it were.Got a great blue gill and redear( shell cracker) fishin’ spot about 70 miles from here in the Hoosier National Forest and a friend with a huge lake house at Barren River Lake here in KY that has super crappie but later in the spring. We like to get an early start in Georgia.


              • Halfstep
                Same here with my off hand. Got to keep that cold one ready.

                And yep that’s what we have to is the big red ear. I just call them bluegill. Some people don’t know what I mean when I say read ear. But yep nothing like some good battered and fried pan fish.

                And we have some real nice big lakes in mid and southern Illinois. Use to go around to them all the time when I was younger fishing. Haven’t done that in a long time. I need to try to do that some this year.


  13. B.B.

    “Safety First”!

    I live in a “harsh marine environment”. Everything rusts here. Rubber oxidises. Plastic become brittle.
    I would be worried about what seals the air tank failing. I would worry less about the tank itself. Are carbon tubes on air guns susseptible to moisture damage from the inside? Would titanium tanks last longer?
    Thanks,
    -Y



      • The problem with carbon fiber tanks is that they probably should be viewed as disposable with a definite short expiry date. Back in the 1980s. one of my scuba buddies announced that he had come across a treasure trove of US Air Force surplus fiberglass tanks that were so much lighter than the aluminum and steel scuba tanks we had. Those corrosion-proof plastic lightweight tanks that were used aboard aircraft, could hold up to 4’500 psi, so imagine how much more air we would have for our dives if we could adapt our scuba regulators to them. A professional diver friend however, brought us back down to earth. He explained that there was a reason why the Air Force discarded those tanks. They were expired in terms of strength. They should be destroyed, and would have been if the Philippine scavengers had not sneaked them out of Clark Air Base to sell them on the black market. He explained that the fiberglass tanks could not be reliably hydrotested. That was because with each fill/empty cycle, an amount of those glass fibers would develop microsopic cracks and breaks. Add to that the degradation of the plastic resin matrix over time, and you have a limited lifetime for the safe use of the tank. Carbon Fiber pressure vessels probably undergo the same kind of physics, even if they are stronger.


      • B.B.,
        there are cylinders that have plastic liners too.

        As for the “clean air”, it depends on where you get your air. I inspect compressed gas cylinders and I can tell you that not all compressor owners are maintaining their filters and moisture separators, and those with hand pumps don’t have any at all.

        Water in a pressure vessel is a danger! albeit not an immediate one. What happens is that water in a cylinder helps to accelerate the oxidation (aka rust or corrosion) of the cylinder walls due to an increased number of oxygen molecules in the smaller space of a pressurized compressed air cylinder. It still has the same 21% of O2 but more of it in a smaller space, for example an 80 cuft cylinder has an 8×10 foot room with a 10 foot high ceiling at normal pressure stuffed into an 8 inch diameter x 22 inch high cylinder at 3000psi.

        Any cylinder should not be stored a full pressure for long periods of time (6 months or more), cylinder manufacturers recommend storage at low pressure (20-100psi)


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