welding an extra point into an air tank

[brendan h]

plan is too cut off the 2 parts that join the tanks together and weld caps, in my case i will be probably be using some 3-5mm steel plate. then after thats welded on i will be welding a nut on for fittings.

tank will need to hold 135 PSI. my only concern is one of the welds braking and shooting me in the head

i have a tig welder cabable of 200amps. stick welder capable of 160amps and a mig capable of 125amps. i would say i am best with the tig


EDIT- probably should have said the tanks will be going into my car and run of a 12V compressor for when im our 4wding.

 

[Justme]

Good luck, nice knowing you.

Job for a coded welder I think.


Justme

 

[Wozzaaah]

"Fly me to the moon........."

Seriously though Brendan, if you really have faith in your welding then you might be ok but please be careful!

 

[English Steve]

Unfired pressure vessel - welder should be coded to ASME VIII or BS5500, or whatever the standards are in your country.

 

[brendan h]

if i do deside to give it ago. what would be the safest way to pressure test it? i was thinking about pumping it up under water or under a heavy blanket

 

[malcolm]

It's not really possible to test these effectively on a DIY basis - failures will be fatigue due to pressure cycles and rust around the weld which will both be influenced by how you weld it and what materials you use. I don't know what they are. Failure isn't welded bit falling off so much as tank taking off.

Manufacturers will weld the things up properly (certified welders with their procedures) and test to much higher than the pressure you can get out of a compressor.

Personally I'd buy a compressor that doesn't work on eBay and use the tank from that. But what pressures are you planning to work to? Can imagine a 12V car compressor isn't going to manage much more than 60psi.

 

[English Steve]

Unfired pressure vessels are tested to 1.5 x working pressure. As I mentioned in a previous thread, get yourself a lorry brake air tank - available in all sizes and properly tested etc.

 

[brendan h]

the 12v compresser will pump up to 150psi. and the pressure i will be using in the tank is 135PSI max.

i bough the compresser brand new for a fairly resonable price purly for use in my car. the tank will only be holding pressure when i am filling up my tires not while driving.

also a nother option.....
maybe silver soldering it?
also if i where to just keep it in one peice would welding a braket on the tank be harmfull? and same will cutting brackets off the tank be harmfull?

 

[peterd51]

Hi,

sounds dodgy whatever you do. I'm usually willing to try anything but I steer clear of welding pressure tanks, etc.

The problem is you will never know just 'when' it's going to fail. It could pass your test now but in a years time go with a bang.

Isn't that why gas cylinders, etc, are tested at regular intervals?

Regards
Peter

 

[snowcat]

I can only speak from experience with Steam vessels 2X working pressure hydraulic test on build 1 1/2 times annual, hydraulic being the safest test method, procedure to be done to an approved WPS, inspection would be required at pre determined stages of the job, usually prep/root then final fills and caps before signing and stamping, this may be slightly out of date now, as it all depends on the barltr of the boiler to determine the exact inspection, compressed air I think is simlar...

BTW no silver solder would not make a jot of difference, the only way round it is to build the whole lot of copper, this is deemed a safe practice, as a lapped silver solder joint, cannot unstitch and completely fail..

 

[Master-of-none]

So why not just use the compressor directly to pump the tyres, is it a race (to heaven)?

Seriously NO don't do it.

Pressure Vessels are designed using rather complicated equations noting the deflection of ends, branches, thickness and shapes as well as weld shape & no. of passes. Any alterations should be done with the same care!
In the UK the proposal would probably be illegal for a 'work' situation, why should we be happy to have less safe leisure activities?

 

[Luke]

135psi really doesn't make a mess, in this sort of situation.

Someone used his rollcage tube as an air reservoir. he made a test piece, a bit of tube welded up at each end. he pressurised it to over 200 psi and then went about puncturing/rupturing it in as many ways as he could.

Each time, it just hissed, and that was that.

I'm not saying you'll get away with a shoddy job. Of course it needs to be a decent job, decent metal, properly welded etc, but as you're not using it commercially it doesn't need to be tested/inspected etc.

By no means am i advocating bodgery, i just don't think visions of exploding tanks and flying lumps of steel are realistic at all.

 

[English Steve]

135psi = 135 pounds per square inch. There are a lot of square inches in the surface area of that air receiver and a lot of volume of compressed air, and so a lot of stored energy if it goes pop - certainly considerably more than you would find in a roll cage.
Look at the size of the welds holding the dished ends on and you will get an idea of how much force the designer expected the receiver to cope with. The weld sizes are all part of the required design calculations for pressure vessels, as are the filler rod materials etc etc.
It is not worth the risk to yourself or anyone else

 

[Luke]

yes, but he's not talking about modding the end caps or the cross section, just capping off the joining pipes.

Some dimensions from the o/p would be good - it's difficult to judge scale.

 

[English Steve]

The stored energy within the receiver is the same throughout. Look also at the welds on the joining pipes - they are substantial for a reason.

 

[Luke]

yes the stored energy is the same, but (given a constant pressure) the force on any surface is proportional to it's area. so a small cap will take less force than the large end cap.

Hell, if the pipe's an inch or two dia, thread it and plug it - that's what's done on many production air tanks.

 

[mishka]

Other than the advice about welding, since this is going in your car and it is a pressure vessel you should check with your motor insurance company. They may take a very dim view of this - not just a pressure vessel, but a modified one at that. As it is they use anything they can to get out of settling claims. So while you may or may not kill someone, if there's an incident you may find yourself without support from your insurance company.

 

[English Steve]

If an end cap blows off the pipe, the entire contents of the receiver will try to come out through the hole and the tank will try to take off in the opposite direction.
I doubt if the pipe wall will be thick enough to accept a pipe sealing thread. Either way, ASME VIII and BS5500 are there for a reason ie to stop people getting hurt.
I spent 5 years designing pressure vessels for the brewing and food industry. Most of these were only spec'd at 5bar ie quite a bit lower than the O/P intends to operate, but it still made me twitch at first pressure test because I have seen what can happen when they fail.

This is my last post on this thread as I have to go to a meeting in a bit.
No offence meant to anyone but I stand by what I said ie don't do it.

 

[Satsuma]

I agree with pretty much what has already been said, this is something to not be taken lightly.

I go diving with 300 bar (4400 PSI) cylinders strapped 6 inches away from my skull, even though I handle them with care and I know that they are pressure tested to 465 Bar (every 2 1/2 years) I still see them as a danger to life and limb.

In reflection pumping something up to 150 psi doesn't seem that big a deal, but as said already the forces involved are still enough to more than ruin your day.

If you still insist on trying it, I believe the safest way to pressure test it is to fill the cylinder with water and pump it up to test pressure. Because water does not compress, if the cylinder fails then the pressure drops rapidly and you should only get wet! HP cylinders are tested this way.
Good luck!

P.S. YouTube Pressure Vessels Fab (Dig the music)

 

[Hitch]

Funny isnt it..... all that pressure in the tank....then you plug a bit of rubber hose into the end of it held onto a fitting with a jubilee clip.... What happens if a hose comes off the fitting, you get a big hiss untill you fire the connector out of the quick release.