bladder tank be made into an air tank?

[Scotty370]

The 'plus's' with Hydro testing, is that there is no "Line-Pack' as they say in the industry. A vessel filled with water will just 'spurt' for a second, and relieve pressure, because the liquid is non-compressable. With an air test, the vessel is charged with compressed air, or Nitrogen and must 'blow-down' in failure, something you don't want to be near!! I've got a "Captive-Air" tank (With bladder) That I know has failed, because at our cabin, the lights flicker ever ten seconds when using water. So the bladder isn't doing it's job!! I've either got a leaking bladder, or tank that isn't holding the 'blanket' air! In either case, it's going! ~Scotty

 

[SensibleNick]

I have 2 industrial 100 litre bladder tanks at home. These are designed to absorb the water-hammer effect from hundreds of yards of 2" dairy pipes.

One I use as a remote reservoir in the workshop that I have in the house (the compressor is in the garage and that's 30 meters away from where I use the air the most), and the other I use as a vacuum accumulator.

Both are rated to 10bar, but both are hydro-burst tested to 20bar. My compressor's output is limited electrically to 13 bar and mechanically to 14 bar, and a safety valve will trip at 15bar.

While a simple "water storage tank" should never be used as a pressure vessel, Bladder-accumulators are well suited to the job so long as common sense is followed and everything is well tested with a good safety margin.

The major concern that has popped up so far in response the OP has been a busting tank.
One example was given - and attributed to a faulty safety valve - which is only ever a second line of defence against overpressure. In that case I'd call the competence of the owner/builder into question.

J_J once more raises the fair concern of test pressures. From what I've seen in his responses on this forum, J_J's a level-headed and very helpful bloke. He rightly says "some have been hurt": While you can never say that the people who've succeeded have done so because they did everything right, It's fair to say that those who've failed have done so because they did something very wrong.

This is why we recommend hydro-testing the tank: ensuring there is NO air in it at all, and using water as the device to build pressure to a measured maximum level in order to ensure the tank's ability to withstand a far greater pressure than you'd ever use it for.

A really simple way of hydro testing at home is to install a grease-nipple on the tank, and use a hand-pumped grease-gun to pump water/oil to pressurize it.....

It's all perfectly safe so long as you've tested the tank at a pressure that markedly exceeds the pressure you wish to use it at.


Why the **** should you trust me?
I'm a newbie when it comes to tractors. I'll happily ask questions, and tentatively offer solutions when it comes to tractors and implements, but I have no experience to back up my suggestions: just engineering know-how and ruthless mechanical logic.
This area however is one where I have a bulk of expertise and experience. I'm a Mechanical Engineer working as Machine Designer in one of the world's largest food/liquid packaging companies. I specialise in mechanical motions, the design, manufacture and assembly of them, and all the stress/fatigue problems that come with systems that repeat their functions 5 times a second. If you need an automated 20 ton lump of stainless steel and aluminium to get foodstuffs packaged at high speed in an FDA approved way, I'm your guy. Aside from this, I am the pneumatics specialist for our large R&D department - this leads to collaborative projects with some of the best pneumatic companies in the world.

Take or leave whatever advice you wish, but if you're going to do this, I insist that you Hydro-test your tank to at least 150% of the maximum pressure you intend to use it at. Not only is it easy to do, but it'll set your mind and rest and give you well-placed confidence.

Failure to heed that one bit of advice could well put you in running for a "Darwin Award".
I'd also suggest you take photos to put other people's minds at ease.

 

[rambler]

I see it says operating pressure of 100 lbs.But assume that it could stretch up to about 120 possibly without failure.

I'd be worried about that 'stretch' part. That last 20# of pressure has _way_ more energy than the 1st 20# of pressure. You are adding a _lot_ of stress.

I get the point of hydro testing, but - if something does go wrong, ain't no one going to care about your test, they will care about what the label says, 100#

--->Paul

 

[SensibleNick]

That last 20# of pressure has _way_ more energy than the 1st 20# of pressure. You are adding a _lot_ of stress.

Sorry to contradict, but that is incorrect. The last 20 stores exactly the same amount of energy as the first 20.

The "Stored Energy" of a compressed gas is a direct function of the pressure at which it is stored. Each pound of pressure you put in stores the same amount of energy.
The Energy stored in a vessel of air at 120psi is 1.2 time the energy stored at 100psi.
Also, the stress (or in this case, Strain) in the walls of the cylinder is a linear function of the pressure inside. (So the last 20lb of pressure adds the same amount of stress as the first 20lb)

Put Simply.... Double the pressure = Double the energy = Double the stresses.

 

[Train]

I watched a show on TV where they took an air tank and dropped it from about two stories up several times. They dropped it straight on the pavement, and also onto a spike that pierced the metal. Not once did the thing take of like a rocket or explode into a shrapnel.

I also worked in a cabinet shop that had virtually no maintainence going on. Was standing about four feet away from a compressor charged to 125 psi when it failed from rusting through. All it did was start leaking air like it had a bad fitting.

I'm not saying "dam the torpedos" but I know there are a **** of a lot of horror stories out there that fall under the " my uncle had a friend that knew a guy" catagory. As far as air vessels failing I'd think that a fitting blowing off would carry far more oppertunity for injury than a tank failure. And to add to Nick's comment about the builder being responsible, I'm in the process of cleaning up a farm estate. I found a homemade compressor made out of a paint sprayer compressor and two freon tanks. Now I've seen the fellow use it and he had it for many years. When I tried it to see if it still worked I noticed there was a guage but no pressure switch. :shocked: I sure that's the kind of thing Nick's refering to.

 

[Egon]

Put Simply.... Double the pressure = Double the energy = Double the stresses.

On the typical tank those stress are different for the hoop and the end are they not?

 

[SensibleNick]

On the typical tank those stress are different for the hoop and the end are they not?

You're just trying to get a rise out of me aren't you?

Yes, you're absolutely right. It's hard to design a tank that doesn't have that paradox. However, the fact remains that the increase of shear-stress between the two components will be absolutely linear when plotted against the internal pressure. i.e. the statement I made still stands.

If you were to plot the maximum Von-Mises stress Vs Internal pressure you'd see a nearly straight line until you exceeded the material's yeild strength.

This is one of the reasons that a grease gun is a nice tool for hydro-testing a tank. You'll see the same increase in pressure for each pump of the grease gun.. if the pressure stops rising as you pump you either have a failure, or you're achieving plastic deformation of the tank.... which is obviously a long way from groovy

 

[SensibleNick]

I'm not saying "dam the torpedos" but I know there are a **** of a lot of horror stories out there that fall under the " my uncle had a friend that knew a guy" catagory. As far as air vessels failing I'd think that a fitting blowing off would carry far more oppertunity for injury than a tank failure. And to add to Nick's comment about the builder being responsible, I'm in the process of cleaning up a farm estate. I found a homemade compressor made out of a paint sprayer compressor and two freon tanks. Now I've seen the fellow use it and he had it for many years. When I tried it to see if it still worked I noticed there was a guage but no pressure switch. :shocked: I sure that's the kind of thing Nick's refering to.

Scary scary compressor. That's exactly the kind of stuff I'm talking about.

There is one scenario of tank failure that could cause a tank to take off like a rocket: A tank with a decent amount of water in it could do that. Just like the water-rockets we used to make as kids. the kind of ignorance that builds an unsafe compressor might well come hand in hand with a lack of draining ability/routine.

Somewhere I have photos of a tank that DID go skywards.
A 30 meter high and 6 meter Diameter water storage tank at a fruit-juice factory in Spain. Some poor bugger was welding on that, unaware that a decent amount of methane was in the water that they'd pumped from their well. A good sized pocket of combustable gas was filling the top of the tank. A very slow burn occurred, pressurizing this water tank that had a few hundred tons of water in it.
The guys on the ground saw that base of the tank begin to bulge as the pressure grew, and suddenly the bottom blew out. It turned into a giant water rocket, and the welder on the top sadly fell from a massive height and was killed on impact.
The tank landed 300yds down the road through the roof of another factory.

I'll see if I can dig the photos out... it was rather interesting.

 

[SensibleNick]

Can't believe I managed to find them...

Here you go. I'm sure I don't need to point out that this tank was for storage only... and never meant to be a pressure vessel.

Captions and album description will tell you more.... and if you dig around a bit you'll find other pictures of my life

Picasa webbalbum - Nick - Nightmare in ...