Safe, Home Hydro Testing

[Rabbitstopper]

Evening all,
Always try to give back. The forum has been more than good to me.

Any way I was chatting to the fella who comes to take our stuff away for testing. He was a bit into his Airgun. And told me a safe way we can hydro test. The caveat on this is 3x redundancy. I.e I don’t go beyond 1500psi for sub 12 regulated .22 velocities. I can reach a test pressure for a vessel under these condition with a made in England 4.5k psi pump. I’ll let ChatGPT explain the rest. I’m to tired to write.

[Rabbitstopper]

Aaaa

[Rabbitstopper]

Aaaa

[Rabbitstopper]

I hope I’ve helped, all this could be common knowledge. Thought I’d put it out though. Sorry for chat gpt ing it. Tired to be fair.

[Rabbitstopper]

I’ve messed the order up a buy

[avator]

I am a bit confused by your intended goal here.
Is the purpose to hydro test your own vessels to insure your safety?
I believe you would need to be certified and insured to perform these tests for others.
Or am I on the wrong path with my understanding?

[subscriber]

Clive,

You probably won't get many replies to your post, because of a real potential for misunderstanding that could increase risk, rather than mitigating it.  The second reason why people may not want to talk is because the detail you posted about what happened to your fiance.  As horrible as that detail is, many are going to wonder why you would share such personal info with airgunners around the world.  If you are preoccupied with those events, then perhaps you should not be thinking of high pressure testing anything right now.

Context is everything.  There is a huge difference between periodically verifying the safety of a production pressure vessel, and verification of the design, materials and manufacturing processes performed pre-production.

Yes, it is common practice to have a factor of safety of 3 times over the yield stress, for any given pressure vessel design and material.  That is something that is confirmed during development, NOT during production, nor to verify the ongoing suitability of an air tube or tank during its life.

If you want to prove that an air tube or tank is still good, as is done at 5 year intervals for some air vessels, the test standard is 5/3 x the maximum working pressure indicated on the tank.  This is much less than 3 x the working pressure, because such a harsh test is likely to shorten the life of the vessel, rather than extend it.  So, to be meaningful you need to use the maximum working pressure indicated on your tank.

You are correct that stored energy is proportional to the pressure and volume of a compressed gas.  So, if you can ensure that a 100 ml tank under test actually contains 99.75 CC of water, then it is the much smaller air volume of the fill whip that has potential to do harm should it blow up or blow off.  By harm, I mean project something at a high enough velocity to injure you.

Despite a water filled tank possibly just splitting rather than fragmenting, that does not mean it is safe to be near, should it rupture.  The energy in the system would still be enough to generate a short duration high velocity water jet in your direction, if you are nearby, without an adequate shield.   

The connections, pipes and hoses between hydraulic cylinders and pumps fail from fatigue eventually, unless replaced via preventative maintenance.  A pin hole is such a system can generate a jet of fluid that can cut tissue or inject oil that can present as a pin hole in the skin, but cause major damage internally.  Certainly, even a brief jet of oil or water can blind a person if they close to the hole or split in the system. 

Think about the potential damage a 1200 PSI pressure washer can do when fitted with a small orifice nozzle, rather than a large fan nozzle.  Even the latter can hurt you:
https://washh.com/residential/pressure-washer-injury/#:~:text=Lacerations.,using%20a%20narrow%20nozzle%20tip.

Then there is an important difference between design and manufacturing verification pressure testing; and verifying the safety of an air cylinder to "re-life" it for another 5 years.

The US Department of Transport (DOT) testing requirements for pressure vessels does not apply to air or gas cylinders with an OD of less than 2".  Also, short of external damage or internal corrosion, steel air tanks that have not been pressure filled to over their yield strength have near infinite fatigue lives.  This does not apply to aluminum air tanks, and perhaps aluminum endcaps.   Aluminum has much less favorable fatigue properties, but the characteristics are well know to engineers that make light weight air tubes or tank.  Typically the ultimate burst strength for aluminum tanks is increased by using thicker walls, to offset some of the deficit in fatigue life.  There are prior discussions on this forum, and the conclusions were that it would take a decade of daily shooting and refilling to reach even aluminum air cylinders' fatigue life, providing the air used to fill the airgun was dry to prevent corrosion.   This, assuming that the user does not overfill beyond the rated pressure, to "get a few more shots per fill".

My point is that pressure testing is intended to reduce risk.  So, you need to be careful that you are not increasing the risk of failure, by testing in a manner that design verification units are tested.  You could be damaging your air tank or your own skin by inappropriate or unnecessary pressure testing, using metrics that are valid for development, but not for mid-life inspection. 

Certainly, inspecting the tank inside and out for distortion, damage or corrosion along its full length, and at the threads, at intervals, may be all that is required for many PCP airguns with typical air tubes just over an inch in diameter.  Air tanks over 2" diameter are required to be inspected and pressure tested to 5/3 times their rated working pressure.  Some of them cannot be recertified after the tank is 10 years old, regardless of how good the tank looks.  The reason that smaller diameter air tanks are exempt is probably that they would not contain enough energy to make a large enough bang to injure people beyond a certain distance.  Some airgun manufactures with removable tanks or cylinders will instruct you to replace them, regardless after a certain amount of time in service.  None of them advise their customers to do their own hydro testing.

Asking ChatGPT for advice is risky, if it or you do not understand the context of the search results.

Many PCP airgun air tubes are designed to fail safe, by allowing captive O-rings to extrude or steel parts to distort in a manner that the pressurized air is vented safely to ambient, rather than splitting the tube, or blowing the valve out like a projectile.  Although the latter can also happen, if the assembly is taken apart while still pressurized.

The videos below are of PCP air tanks being hydro tested to failure.  This is to prove that the materials, the design and manufacturing methods for that assembly have much greater strength than they would require in normal use.  And, to validate the factor of safety for the production units.  These tests or one just short of failure would weaken the parts, rendering them scrap, rather than proving they are safe to use.  Don't confuse the different types of pressure tests, if you want to keep your digits and sight.