Hydrotest Air Compressor Tank

[Rosenbluth]

So I bought this old Rolair compressor at a good price...80 gal, 2 stage, 175psi.....turns out to be about 29 years old. Still working good. Pumps up and what not. No water when I drained the tank, but doesn't mean it never did have water in it. So I been reading about testing these old tanks and what not..usually to 1.5X the operating pressure. I found a fire supply/test company close by that can hydro test the tank for me for like $45..so I am going to do that. Sure I could do it myself, but they are close and cheap, so I'll just not mess with it and have them do it. So, if it passes the hydro test, do I assume that it is good to go. Even for being 29 years old, if it holds to 300psi for the test, I'd think it will be fine for 175psi sitting in my shop. Any thoughts? Thanks in advance.

 

[JerryB]

Should be ready to go. That is exactly the point of hydrotesting, and having it done by a company with the necessary equipment and experience really validates that testing. IMO, $45 is a great price!

Do make certain your pressure switches and relief valves are working.

 

[theoldwizard1]

I found a fire supply/test company close by that can hydro test the tank for me for like $45..so I am going to do that.

It is worth the $45 to have a certificate.

 

[Rosenbluth]

Thanks guys. Already planning to put on new gauge, two new relief valves and a new pressure switch. I'll probably even hit it with a DA and give it a fresh coat of paint to make er look nice again before I'm all done with it.

 

[tcianci]

Your post makes it sound like you could do this at home. A hydrostatic test fills the vessel with water and the vessel is in a containment area. There's more to a hydrostatic test than getting the certificate, it's the safety of the test process that you can't duplicate at home.

 

[matt_i]

The hydrotest is inherently safe, which is why its done. Think of filling the tank with water up to the very top point where it can be pressurized with something like the lever pumps from a grease gun. It takes minimal additional water to achieve hundreds of psi, and its valved off with a pressure gage to observe any drop. Im guessing its going to be tested to double the working pressure, so something in the range of 200-300psig.

The tank, if ASME certified, is designed with a low carbon steel and weld to leak before fracture. Essentially the highly ductile material stops crack growth which would be very fast if something like a brittle material were employed, and would be very dangerous if a compressible fluid like air were contained inside. In any case, if the tank were to suddenly leak, a little harmless water jet squirts out and is done. The volume of fluid released would be a couple of CCs. There is no accumulator or prime mover to maintain the pressure and the water is essentially incompressible as a hydraulic fluid.

 

[CNGsaves]

OP . . . sounds you found great local resource to do the hydro test, that's great!

Let's SEE that compressor !! What did you have to pay for it ??

 

[LS6 Tommy]

I'm no expert, but I'm not sure I'd use a DA to clean it up. You don't want to mess with any of the welds after the hydro test. I may be also be 100 % wrong.

$45.00 is a steal, BTW. Good job!

Tommy

 

[Rosenbluth]

Here is it. I got it for $450 and was 1hr drive away from me. I feel I did ok on the price, but wasn't by no means a steal. But these old ones, as long as they are taken care of, will run forever, IMO. The thing is a monster. Yes, it is a ASME tank, and I had even thought about not having it hydrotested because it is an ASME certified tank, and is built to not rupture, just leak if it did fail. but for the $45 to test, it's more of a piece of mind part for me to get it done.

I would not DA any joints or welds, just the smooth sides, maybe wire wheel across the weld joints or not even touch them at all. It doesn't need to be showcar finish, just a little nicer than it is now. Probable just spraya single stage enamel on it and call it good. I'm all about not weakening this tank, that's the main reason I'm not having it sandblasted to clean it up, I'm afraid it may weaken or fatigue the metal.

 

[G_P]

$45 is well worth it for the peace of mind. As for sanding it, unless you are going at it hard with coarse sandpaper, you wont hurt the welds.

 

[Rosenbluth]

I had talked with the local sandblaster, asking him what his experiences were with pressure tanks. He said he does them all the time, and the old units like mine, didn't have paint baked on or powder coated and didn't take a whole lot of pressure to get the paint off...and he never saw a problem with any he had done. But I will just feel a little better not getting the blast done. Even a light sandblast..I'll just do some sanding, get it clean as I can and give it a couple coats. Like I said, no show car or anything like that.

 

[rsanter]

Here is it. I got it for $450 and was 1hr drive away from me. I feel I did ok on the price, but wasn't by no means a steal. But these old ones, as long as they are taken care of, will run forever, IMO. The thing is a monster. Yes, it is a ASME tank, and I had even thought about not having it hydrotested because it is an ASME certified tank, and is built to not rupture, just leak if it did fail. but for the $45 to test, it's more of a piece of mind part for me to get it done.

I would not DA any joints or welds, just the smooth sides, maybe wire wheel across the weld joints or not even touch them at all. It doesn't need to be showcar finish, just a little nicer than it is now. Probable just spraya single stage enamel on it and call it good. I'm all about not weakening this tank, that's the main reason I'm not having it sandblasted to clean it up, I'm afraid it may weaken or fatigue the metal.

Scotchbrite and cleanser is all you need to clean and prep the tank.
Wire wheel any rust spots and then you are ready for paint.
Sanding and all that time consuming stuff I'd not needed

Bob

 

[sberry]

They blast tanks all the time but unless there was some super compelling reason to wouldn't bother. The test is a deal at that rate, I wouldn't have done it but you didn't get robbed.

 

[LifeLongWNYer]

The hydrotest process is not complicated, but as others have related not possible at home. They put YOUR tank inside a pressure vessel and seal both. The space between the two tanks is filled with water that leads to a calibrated tube. That is so if your tank starts to expand when they pressurize it, it takes more space in the vessel, and displaces some of water, outside your tank, but inside the pressure vessel, which the operator can observe by watching the tube. To run the test, they pressurize your tank, watching for a rise of water in the tube, which indicates that your tank is expanding. The pressure vessel containing your tank needs to be secure enough so that if your tank gives way, it is contained in the vessel.

Fire equipment dealers are easily able to test fire extinguishers, air pack bottles and welding gas bottles, but not many have a pressure vessel large enough to contain your tank. If they do, you lucked out!



JBP


.

 

[38Chevy454]

For the $45 fee, the hydro test is a good way to go. But you could also just do some ultrasonic wall thickness scan measurements to see if any thinning or significant loss of metal at the bottom of the tank. Kind of like sonic checking an engine block for cylinder wall thickness prior to boring. Your local machine shop could probably do it on the tank, it is the same principle.

 

[scissorman]

As far as I know compressor tanks are required to be SONIC tested every 5 years, (at least here in California) when in commercial use.

 

[sberry]

It cant hurt anything but compared to some of the **** I worked on in the early days lucky I didn't blow my head off more than once. This eclipsed electrical hazard maybe and as bad as rigging for sure.
Today I am aware and even though I can do this I really don't. I have and always rig up a relief and the ideal is to fill then bring it up with a regulated pump after the fact vs regulating the output where it could be so simple to accidently deadhead.
In the same vein I about electrocute myself a while back in some cord scheme I got backwards in the last minute. Its why they say not to do it,,, hahaha

 

[sberry]

So in the end if I get killed from an old tank my number will truly been up. On occasion faulty equipment is liable but for every 1 of those 10 is operator error magnified by the fact most of these operations are not something I would do specific daily etc.

 

[sberry]

Here is why I speculate as to the value. Its about like car inspections statistically and we see so much foolin with it, modifications, heat and beat and all to find out there wasn't much to do.
It would be different if it was full of rusty gunk extreme but my neighbor cut a hole in one to look in, welded it back up. It was small and he is a super good welder but he didn't gain squat and uses it about once a week in the end to blow off some chips.
Most of the dangers from equipment are from things I did to it, installed wrong, I wired a bypass etc, wrong circuit, added something or neglected factory safety, run it after a known problem existed.
The biggest culprits are small ones, they weld the wheels and brackets to the tanks with bird sheet. Most new ones are better as the welding became automated or farmed out to a Chinaman that gave a **** and was good at it. Nothing like a cheap 70's American made air bottle so prone to leak.

 

[Rosenbluth]

I agree with testing the tanks every 5 years. I don't know what the rule is here in Nebraska, but this is just in a shop behind my house, not a commercail business. Once I get it tested, cleaned, painted, and all the new pop off's, pressure switch and gauge installed, I plan to be pretty religious about draining it everyday I use it.

This unit has a centrifical unloader that is not working at the moment. Rolair sent me instructions to replace it with a pressure switch with an unloader built into it as the old centrifucal unloaders are not very reliable. So I also have that planned to get this thing back to good shape. Looking forward to having it done.

 

[LX-Markham]

Kind of off topic, but sort of related:
What is the risk of a tank that may be rusting internally due to water?
Wouldn't the rust just slowly eat away at the bottom of the tank and eventually cause a hole and air leak?

Or is there a danger of an explosive failure?



(regardless, $45 for a test seems like a steal!)

 

[Stuart in MN]

Wouldn't the rust just slowly eat away at the bottom of the tank and eventually cause a hole and air leak?

Or is there a danger of an explosive failure?

Either one could happen. I suspect a simple air leak is more likely, but there's no way to predict which one.

 

[crerus75]

Kind of off topic, but sort of related:
What is the risk of a tank that may be rusting internally due to water?
Wouldn't the rust just slowly eat away at the bottom of the tank and eventually cause a hole and air leak?

Or is there a danger of an explosive failure?

The end result can be a pretty spectacular explosive failure. The air is compressed in the tank, and if the tank ruptures (not pinholes, but actually ruptures), the sudden release of energy can do a lot of damage. There have been reports of entire buildings being blown down by exploding tanks, and plenty of people have been killed. A google search will bring up lots of pictures, some of them pretty grisly.

 

[dave89iroc]

^^ agreed, that most common failure is a pinhole leak, however, you cant rely on that always being the case

 

[LX-Markham]

never mind, search feature worked.
http://www.garagejournal.com/forum/showthread.php?t=27463&page=2
back to scheduled regular programming.

I was just curious because I have seen several tanks develop leaks (pinhole on the bottom), but never heard of one rupturing and exploding due to rust.

 

[Rosenbluth]

According to one of the engineers that I work with, an ASME certified pressure vessel is built with a certain steel (carbon content or something) that is supposed to not "blow" open upon rupture.It would simply split an opening to relieve pressure. Rusted thru tanks will simply create a pin hole and begin to leak, as opposed to exploding, and opening up like a pop can torn in half. There was someone posted on the Practical Machinist site that was challenging anyone to find an ASME tank that "exploded". I am no expert by no means, just saying what I was told and what I read. I know the ASME tanks are required to be a certain thickness to be certified. Non certified tanks are usually built overseas, and they use the calculation to figure how thin of metal they can use and it will still hold. Also, welds on ASME tanks are also certified and done by people that know how to weld pressure vessels. Same reason they say to never weld on your air tank, because it can weaken the steel in certain areas because of the heat and what not, and it will certainly void any ASME certification. Like I said, I am no expert and don't do ASME certifications, just stating what I was told.

 

[Mooky]

Pressure vessels like this are designed to "leak before break" per ASME Section VIII, as mentioned.

Testing frequency is specified by your state and insurance carrier, if the vessel is registered with the state (unlikely in a home shop).

The volume displacement test mentioned is used on small vessels to measure permanent deformation. A standard hydrostatic test at 1.5 X MAWP meets the code.

Replacing the relief valves is an excellent procedure. Testing is required by code, but these little ones are throw aways.

Mooky
NBIC / PA / API 510, 570 Inspector