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SurfLung
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DIY Hydro Proof Test?

Wed Jan 02, 2019 11:06 am

DIY Hydro "Proof" Test
- So, we have a scuba tank that that hydro shops have refused to test. Its an odd vintage tank with a valve thread they haven't seen before so they just say "No, we won't do that."
- We're primarily interested in just checking it for safety so we can fill it partially and dive it once in awhile. So, I'm wondering if it's feasible to do a "Proof" test. No measurements, just make sure it isn't going to blow up at working pressure and then only fill it to 50%.
- Here's what we're thinking: We can fill the tank with water and put the Scuba valve back on. Then connect it to a scuba tank that has the test pressure of air... 2250 psi. Connect it like a tank balancing whip. Submerge the test tank in the lake completely and then open the two valves to balance the two tanks at 2250.
1. Would this apply 2250 psi of hydrostatic pressure in the test tank?
2. Is it Safe? In the unlikely event that the test tank exploded, would it be contained in the lake water due to the uncompressibility of water?

P.S. The "Test" tank in question looks like brand new inside and out... No corrosion. The only thing keeping it from a normal Hydro test is the odd tank thread.
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antique diver
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Re: DIY Hydro Proof Test?

Wed Jan 02, 2019 12:07 pm

That should put 2250 in the tank. I think that in case of failure it would possibly just split open instead of exploding.... I hope. I tested my shop made test chamber the same way, but only to 150 psi.

I'm curious to know more about the markings and threads (pitch, diameter etc.) How about photos of both?
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SurfLung
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Re: DIY Hydro Proof Test?

Wed Jan 02, 2019 2:16 pm

- Thanks for your reply. I actually posted about this tank before in this forum.
viewtopic.php?f=41&t=9381

- It's owned by my buddy Rich (50,001 Questions) and I think he has maybe tried to be too "up front" with the hydrotestors he has approached about it. The first one was a traveling Hydrotest man... Does fire departments mostly. He didn't have an adaptor to fit. The second was our Hydrotestor from Brainerd. He saw the bushing and flat out refused to do anything with bushings.
- We have found a new Hydro testor we're going to try next week or so. This one does all kinds of tanks... industrial, welding, medical, scuba, etc. So we're thinking he probably has the adaptor. But what we're going to do is just bring the tank without the bushing and if he's got the right adaptor, maybe he'll just do the test with no issues.

- The tank is over at Rich's house right now but next time I get access to it, I'll get a pitch gauge and measure the threads.
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luis
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Re: DIY Hydro Proof Test?

Wed Jan 02, 2019 3:28 pm

My first question is are the tanks DOT or ICC stamped?
What are the marking on the neck?


If the cylinder is a DOT (or ICC) approved cylinder, it is more likely that they just don’t have the correct adapter to test that cylinder. It cost money to buy adapters for all kinds of cylinder threads.

My LDS (that went out of business) had their own hydro machine and they tested a lot of foreign liferaft cylinders so they had all kinds of cylinder adapter, both imperial and metric, also male and some female threads.

Yes, it is totally legal for them to test non-DOT cylinders and stamp the test date. They just could not stamp their DOT license number (RIM number?).

This same dive shop didn’t happen to have the adapter for Sportways cylinders. At first they didn’t think it was economical to buy it just for one set of Sportways cylinders they ran into. Later they found out that it was the same threads as SCBA, so they bought the adapter. This ties to that other thread.


About doing your own hydro test.

First question #2: putting the cylinder in water is a very bad idea. As you mention, water is basically incompressible and it will transmit all the energy of an explosion. It will not absorb the energy.

It is not like the crumple zone on a vehicle. It is more like the old heavy vehicles that used to survive a crash, but the passengers inside received the full impact killing them.

It is also the same way a modern torpedo will sink a ship or submarine. A modern torpedo will not even try to hit a ship. It is a lot more effective for it to explode below or near the ship. The incompressible water will transmit all the energy of the explosion and tends to break the ship in half. A torpedo hitting a ship will only blow a section which the ship may survive.

This is also the same reason why filling tank in water is reasonable for cooling purpose, but it gave a false sense of security when dive shops used to think that the water would help absorb the energy of an explosion. A very bad assumption.


About the test itself.
A hydro test is performed at 5/3 of its working pressure. For example a cylinder stamped at 2250 psi is tested at 3750 psi. This test is not just an arbitrary over pressure test to see if the cylinder survives and doesn’t rupture or explodes.

The 5/3 is not an arbitrary number, it is the inverse of 60%. The designed working stress of most DOT pressure vessels is 60% of the yield strength. That is where the test pressure is derived.

The primary purpose of the hydro test is to take the material on the cylinder to the lower limit of the yield strength and take measurements of permanent expansion to confirm that this is still the actual yield strength limit.

The permanent expansion is what we call plastic deformation as opposed to elastic deformation. The elastic deformation is the pressurized expansion that returns back when the pressure is removed. Just like a rubber band returns when tension is removed.

The deformation on a cylinder (both the elastic and any permanent deformation) is so miniscule that in order to be able to measure it has to be done by measuring total water displacement in a burette. It is a way of amplifying a very small measurement so that it can be read using conventional means.

There are several reasons why it is important to determine the yield strength.

A steel (or aluminum) cylinder is supposed to be a very elastic material, not a brittle material. A brittle material will fracture into many pieces like a fragmentation grenade. A ductile or elastic cylinder will stretch until the weakest spot will rip, but it will only rip on that weakest area.

The material on steel cylinders is heat treated to intention keep its ductility, but if the cylinder has been exposed to a fire or other high heat, it could change its material condition. It can also be worked hardened at high stress (fatigue life), but this is much less likely to happen to a cylinder, but not impossible.


One other point. De-rating the working pressure of a cylinder will increase the safety margin between the pressurized stress the material exposed to and the strength of the material. This sounds like a great safety factor, but… and this is an unlikely scenario but far from being impossible. If the material has become brittle (specially steel in cold temperatures) and it get hit or dropped it could cause a chain reaction. It is not very likely with the DOT- 3AA alloy of steels, but it did happen to (man rated) decompression chambers back in the 70’s and to some liberty ships back in WW-II.


BTW, this is an abbreviated, very short answer to try to give a little understanding of the intention of a hydro test. The purpose of a hydro test is often misunderstood even by the hydro test technician. It is not up to me to to give advice on what you are going to do. It is up to you. But if you are going to take a risk, IMHO it is probably good to make it an informed risk.

If you know the history of the cylinder (it has never been exposed to high heat, etc.) would play a big part on making a decision of risk versus benefits. What else do you know about the cylinder?


I hope this helps.
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Re: DIY Hydro Proof Test?

Wed Jan 02, 2019 3:39 pm

The adapter has to be removed to perform the test.

The hydro tester should have known that, but it is a good idea for you to go ahead and remove it.

If it is your typical converted Scuba cylinder, the adapter is a standard tapered thread. Any self respecting hydro facility should halve a full range of tapered cylinder threads. Many industrial cylinders use tapered threads.



Good luck
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Re: DIY Hydro Proof Test?

Wed Jan 02, 2019 3:57 pm

I should also add:

During a hydro test it is very important to minimize the amount of stored energy. This is done by totally minimizing any compressible air in the pressurized system.

The cylinder is filled with water and all the lines and connection are also bled to remove all the air. The only air in the system is in the pressure gauge. It is a very small volume. To increase the pressure a pump is used. The pump is driven by low pressure air, but it is not storing any significant energy in the pump itself.


Filling a cylinder with water is a good start, but how do you control all the stored energy that it being applied from the other cylinder?
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Re: DIY Hydro Proof Test?

Wed Jan 02, 2019 4:15 pm

I felt like I should tack on one additional comment about the fiber-wrapped aluminum cylinder. It is suggested by some sources that they not be submerged for filling to prevent water from entering any damaged surface area. The concern being that it could ending up getting between the fibers and the aluminum inner shell where unseen but damaging oxidation may occur.

That seems to rule out diving with them, but I do know some individuals that use them for sump diving in caves where item weight is an issue. Packing heavy cylinders far into a complex dry cave to the water interface can really be a bitch. Of course someone still has to get all that extra lead weight in as well, but it can be spread out over several people's packs to be more manageable.

One caving person ( I started to refer to him as an Idiot, but thought better of it because I'm such a nice guy and he actually is too) that I know over-fills his out of date 25 year old wrapped 4500psi cylinders to 5000 psi for his sump diving adventures. I refused to be a party to those outings due to the terrible banging around the already stressed cylinders get, and also declined to include him in my cave projects. Anyone that reckless doesn't get invited into a cave with me.
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Re: DIY Hydro Proof Test?

Wed Jan 02, 2019 11:27 pm

Like Luis said, you have no way to test the expansion. So if by some slim chance the tank is bad, you wont know, and you're now one fill closer to it failing. But if you do decide to fill it with water, be sure to have some way to dry it. Would be a shame to rust spot a tank that was spotless to begin with. If it was me, I would find out the thread pitch, find a hydro facility in a bigger city that will do it, and send it in through a local welding supply shop that uses that hydro center.

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Re: DIY Hydro Proof Test?

Thu Jan 03, 2019 1:45 am

Eben, my only suggestion is that you get one of those smartphone apps that automatically dials the paramedics when a sudden concussive force hits it. 8)

M
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Re: DIY Hydro Proof Test?

Thu Jan 03, 2019 2:38 pm

Thanks fellows... That's a lot of great information. I agree its not a good idea to try this but it has turned into a fascinating topic to discuss. I thought I was asking a fairly simple question but it looks like its a lot more complicated than I thought. So, I'm still unclear on a few things.

First, the in-compressible nature of water. I was under the impression that the reason they fill a test tank with water instead of air is because if the tank fails physically (cracks open), the "explosion" will only amount to the expansion of the water inside, which is minimal due to the incompressible water... So the test tank might just crack open and stop. Is this a correct assumption?

Second, if placing the water filled test tank in open water would enhance the explosion because of the in-compressibility of water, what if you just did it in open air?

Third, If an air tank and high pressure hose were used to raise the internal water pressure in the test tank, why would they have any contribution to an explosive failure of the test tank? It seems to me that the test tank would crack open and the air tank and hose would simply free flow until you turn the valve off.
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Re: DIY Hydro Proof Test?

Thu Jan 03, 2019 3:39 pm

Youtube is a wonderful source if you can weed out the garbage.
https://youtu.be/AB9QvkvQuvM

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Re: DIY Hydro Proof Test?

Thu Jan 03, 2019 3:55 pm

tripplec wrote:
Thu Jan 03, 2019 3:39 pm
Youtube is a wonderful source if you can weed out the garbage.
https://youtu.be/AB9QvkvQuvM

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Excellent! A simple video shows us all what would happen! Thanks Tripplec!

- Although its far less catastrophic than if the tank was filled with air... I STILL don't want to be near it when that happens! :)
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Re: DIY Hydro Proof Test?

Thu Jan 03, 2019 9:15 pm

SurfLung wrote:
Thu Jan 03, 2019 2:38 pm
Thanks fellows... That's a lot of great information. I agree its not a good idea to try this but it has turned into a fascinating topic to discuss. I thought I was asking a fairly simple question but it looks like its a lot more complicated than I thought. So, I'm still unclear on a few things.

First, the in-compressible nature of water. I was under the impression that the reason they fill a test tank with water instead of air is because if the tank fails physically (cracks open), the "explosion" will only amount to the expansion of the water inside, which is minimal due to the incompressible water... So the test tank might just crack open and stop. Is this a correct assumption?

Second, if placing the water filled test tank in open water would enhance the explosion because of the in-compressibility of water, what if you just did it in open air?

Third, If an air tank and high pressure hose were used to raise the internal water pressure in the test tank, why would they have any contribution to an explosive failure of the test tank? It seems to me that the test tank would crack open and the air tank and hose would simply free flow until you turn the valve off.

First question is mostly correct.

But here are a few clarifications:

The amount of energy stored in the compressibility of the water can be considered truly negligible. The compression and therefore re-expansion of the water at these pressures can be basically ignored.

The elasticity of the steel cylinder in itself does store some elastic energy due to the expansion. The same applies to hoses, but their diameters is very small therefore the stored elastic energy is small.

The air in any gauges (or any trapped air that was not totally bled) will store some energy.

The video shows the inertia generated by the stored energy (in the elastic steel and other elastic materials).


Second question.

As you can see on the video that is the preferred media if you are going to intentionally take a cylinder to a rupture condition.

The only reason the hydro test is done inside a water jacket is to use the water on the outside of the jacket to measure the volumetric expansion. This method is used to amplify a very small cylinder expansion into a burette that can be read with reasonable accuracy.

This is the most economical and accurate way of amplifying a deflection that would be very hard to measure directly. But the total volume expansion of the cylinder can be measured because the displaced water is concentrated into a relatively small burette.

BTW, the water jacket (container) around the cylinder has a large blow disc that is always installed pointing away from any personnel in the area. The blow out disc is about 7 or 8 inches in diameter. I have done many hydro tests, but I have never actually being present during a rupture. I have seen the results shortly after and it is just a mess, but no harm occurred (other than a new disc has to be installed). Well I don’t know about the clothing of the operator at the time.


Question three.


If you can close the valve in milliseconds you may be OK, but even then, probably not.

Take a look at the motion on the cylinder in the video. You heard a water pump and there was minimal amount of stored energy in the entire system.

The energy stored in the attached cylinder will released and accelerate all the water like liquid projectiles until the valve is closed. That water will hit like a pressure washer cleaner but a lot more volume at once. It could cut right through flesh like a hot butter knife on butter.

At the same time (for every action there is an equal and opposite reaction) the cylinder will be propelled in the opposite direction.

A human would have a hard time closing that valve fast enough to minimize the reactions. You could try barely opening it and maybe even putting a restrictor orifice. But there is too much risk of adding too much compressible (energy storing) air into the system.

I think you mentioned about having a high pressure "bicycle style" pump. That could be safer (due to lower air volume) if you have a hose long enough to get far away. But make sure to fill the hose with water and maybe even add a restrictor orifice in a joint near the cylinder.

Restrictor orifices are commonly used to slow the flow of compressed oxygen when transfilling, and most modern Scuba HP gauge hoses have them on the fitting at the regulator end (look t the end of the 7/16" fitting).


Added:
I should add that because of the sudden nature of the rupture. The abrupt energy release creates a shock or energy pulse. Even a relatively small amount of energy when released suddenly in a pulse can cause some damage due to the sudden acceleration of any mass involved (the water or the cylinder).



I have been thinking about making a clear pressure chamber to test gauges and dive computer. I am thinking about using a clear water filter container. They are rated to city water pressure . To avoid the danger of elastic energy (from compressed air) I would have to evacuate all the air out of it and I would use a bicycle pump to pressurize it. I have thought about connecting it using an HP hose with its built in restrictor orifice.
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