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View Full Version : Scary mishap in the shop today.



gundog
01-03-2007, 12:00 AM
I bought a used compressor today and it almost did me in. See the picture below.

http://img.photobucket.com/albums/v292/millnut/Myaircompressor.jpg

The pipe you see on the right side has 2 ball valves one on the bottom of the tee and one on the top side of the tee. The one on the bottom is for a water trap the top one is for the hose connector this is how I bought it. My friend and co-worker Jim helped me bring it home. After we get it moved into place I tell Jim check this out it has a valve on the bottom for a water trap and when I hit the valve the fitting broke off and the pipe swung around like a wind mill and put a huge hole in the wall. The tank had a full pressurized tank full of air in it probably close to 175 PSI. The fitting must have gotten cracked when we moved it probably when the guy moved it with his forklift. That thing hit the wall with such force I am afraid to think of what it would have done to me had it hit me or Jim. In fact I can't see how it missed me. I had not even wired it yet. There is a lesson to be learned here if you ever move a used compressor make sure and bleed the air from the tank before moving it. This could have been real bad.

Upon inspection of the broken fitting it was a close nipple between the ball valve and the tee, the close nipple was made out of very thin material. I have noticed at the big box stores which is where I am sure the nipple came from the close nipples are very thin until you get up to say about 4" long nipples they are very thin material the longer lengths seem to be cut from standard pipe but the short close sizes are very thin. I don't think a thicker nipple would have been this fragile. I stopped by my local big box store for a replacement and chose a longer & thicker nipple to replace it with.

I did get a very nice compressor for half the new cost and I am hoping it will last for a few years. I plan on installing a regulator on the outlet before my hose connection to drop down the pressure from 175 PSI to around 120 PSI. I really don't have anything that needs that much pressure. this compressor puts out 27.5 CFM@ 90 PSI that is going to be much better than the 10.3 my last one put out. I had to run a new service to feed it thought this motor draws 30 amps.
GD

EDMTech
01-03-2007, 12:06 AM
Glad you or your friend didn't get hurt. Compressed air is nothing to mess around with, it will haunt you, and quick!

When you say you didn't even have it wired yet, does that mean you MOVED it full of air???? :eek:

gundog
01-03-2007, 12:25 AM
When you say you didn't even have it wired yet, does that mean you MOVED it full of air????

Yes in hind sight a very dumb thing to do.

EDMTech
01-03-2007, 12:34 AM
Hey, no big deal, we all do dumb things! The important things are 1: nobody got hurt, and 2: you learned from it :)

CCWKen
01-03-2007, 01:08 AM
In addition to the regulator, you should be able to turn down the cutoff pressure going to the tank. No sense pumping it up to 175 if you're always/only using ~125. It could save you some money on electric too.

Evan
01-03-2007, 02:40 AM
Gundog,

I don't think you know how lucky you were. It's a very good thing the weather is reasonably warm around here right now. If you had been moving that charged tank just a couple of months ago it could have exploded due to brittle failure in the cold weather we had then. Some time ago I read a report of an industrial accident that was a result of exactly such an incident. An air temperature of zero to ten degrees F can reduce the impact strength of the air receiver by a factor of 2 or 3 times or more depending on the steel alloy. Even a minor impact can result in explosive rupture of the tank. In the report I read one person was killed.

Scatterplot
01-03-2007, 03:48 AM
Re what Evan said, I must admit (since nobody was harmed here) that I would really like to see what happens when one of these is detonated. I bet it would put some serious fear in us all!

John Stevenson
01-03-2007, 04:18 AM
Gundog,

I don't think you know how lucky you were. It's a very good thing the weather is reasonably warm around here right now. If you had been moving that charged tank just a couple of months ago it could have exploded due to brittle failure in the cold weather we had then. Some time ago I read a report of an industrial accident that was a result of exactly such an incident. An air temperature of zero to ten degrees F can reduce the impact strength of the air receiver by a factor of 2 or 3 times or more depending on the steel alloy. Even a minor impact can result in explosive rupture of the tank. In the report I read one person was killed.


Why don't truck air tanks explode in the winter then ?
Sure is a hell of a lot of them running around in all weathers and many are in accidents.

.

charlie coghill
01-03-2007, 08:23 AM
Gundog, when I am plumbing something like a compressor I use schedual 80 fittings. Over kill maybe, having it break not likley. I go to a pipe supply place and get mine and they are not too expensive.
Charlie

Evan
01-03-2007, 08:55 AM
Why don't truck air tanks explode in the winter then ?
Sure is a hell of a lot of them running around in all weathers and many are in accidents.
They are designed with that in mind. Stationary compressor tanks aren't.

Paul_NJ
01-03-2007, 08:56 AM
Gundog, when I am plumbing something like a compressor I use schedual 80 fittings. Over kill maybe, having it break not likley. I go to a pipe supply place and get mine and they are not too expensive.
Charlie

I couldn't agree more. As an engineer with experience in refinery process piping, I'd never use "Home Depot" quality pipe fittings in compressed gas service. If a water line ruptures, you get a mess, but not likely hurt. High pressure air is another matter. When any pipe is threaded, the cutting removes considerable steel, hence strength. A close nipple is particularly vulnerable, because there's little flex in between the threaded ends. Considering this and the vibration in a compressor, they're the weakest link. I doub't HD nipples are even Schedule 40 . . . more likely Schedule 10. Not much steel left. Running 175 psi makes it much worse. Listen to Charlie and buy process fittings for his service. Cheap insurance.

Paul_NJ
01-03-2007, 09:05 AM
P.S.

I'd adjust the pressure switch down to 120 psi . . . the outlet regulator, as you know, only drops the pressure of everything downstream. I wouldn't want a used compressor of unknown tank internal condition running next to me in my shop at 175 psi. Is it an ASME rated tank? At what pressure? Be careful!

ronm
01-03-2007, 09:58 AM
I've told this story before, but not here yet, I don't think. My Uncle had one of those little roll-around compressors, about 2 hp., maybe a 20 gal. tank. It was prob. about 20 yrs old. One summer night, somebody had forgotten to turn it off, & the pressure switch apparently stuck. The tank let go, & the whole thing went through the wall of the metal shop building. My cousin was sleeping on the back porch, & he thought WW3 had started...they found the tank, peeled open like a banana, out in the barnyard. The pump was about 50 ft. farther, & they never did find the motor...the hole in the shop wall was big enough for a man to crawl through. Moral of story? Drain the water...this tank was rusted out in the bottom...
Ron in CO...

wirewrkr
01-03-2007, 10:48 AM
Did anyone see the episode of Mythbusters that aired recently that they knocked the head of a compressed air tank? I had seen the aftermath of this before but never got to witness the destructive force of 2500 psi of force let loose. Really cool.
Robert

Wirecutter
01-03-2007, 10:56 AM
I saw the Mythbusters episode.

It took them a couple of tries to successfully shear the valve off the top of the tank. When they did, the tank sailed right through their cinder-block test wall without slowing down. It also went through a second cinder-block wall comprising the side of the building.

It's things like this that have kept me from getting an otherwise quite affordable OA torch rig for the home shop. Compressed air is one thing, but acetylene is what "Emil" would call "kicking it up a notch".

Use as directed.


-Mark

Dawai
01-03-2007, 11:13 AM
Yeah, pvc does not fare well because of the stretch and shrink of pressure.. Constant pressure equals no problems..

When I had the 4" water trap column set up for painting? People on here were having cows. I put it outside behind the shop. Much less dangerous than riding around in North Chattanooga late at night.

All air tanks have a destroy by date stamped on them, including the ones on trucks. This is engineered to cycles of pressure they might achieve under normal conditions. Heat, cold affect this directly.

Poor metal.. yeah, some home depot fittings I put in recently were crap. I went to the industrial supply, they throwed the same china crap on the counter. Threading pipe, sometimes it has inclusions that makes the threads tear out.. machining the crap, sometimes it has inclusions that tear holes as it is machining. What is it? I ain't got a clue since I am not a metalurgist.

ANYTHING under compressive or electrical load is dangerous, be it a flyback transformer in a tele *(killed more than one tech) or a pressurized tank and lines. I blow down new air lines when I install them.. I put a rag over the end since the shavings that "might be" in the pipe are like bullets. A small shaving will destroy a good air tool. OR a controller, Or your eye.

It is about to the point where I put on saftey glasses anytime I open the door of the shop. I can hardly see now and I'd like to keep the rest of my vision as long as the heart beats.

A.K. Boomer
01-03-2007, 11:41 AM
I saw the Mythbusters episode.

It took them a couple of tries to successfully shear the valve off the top of the tank. When they did, the tank sailed right through their cinder-block test wall without slowing down. It also went through a second cinder-block wall comprising the side of the building.

It's things like this that have kept me from getting an otherwise quite affordable OA torch rig for the home shop. Compressed air is one thing, but acetylene is what "Emil" would call "kicking it up a notch".

Use as directed.


-Mark


i didnt see it but i thought that was discussed on here before and the odds were stacked heavily in the favor of the tank -- like they greased the floor and stuff and gave it a massive head start, big deal, so the tank has some momentum, fact is is if you were on the blunt end of it and held it back it would never go anywhere, now being on the other side --- thats another story....


I just completed a study and for every person that dies in a compressed air accident there are 137 guys that die of stress related heart failure due to worrying about a compressed related air failure, use some caution by all means but its really very safe if you watch your p's and q's, i cringe when i see my bro's work trucks and whats in back of them sometimes -- the cheapest air tanks money can buy and there all beat to crap -- tanks, fittings, everything --- and usually filled to 25 psi over there ratings, and tossed around like rag dolls on the coldest temps, No -- im not condoning this behavior --- but its not like your going to walk by an air tank thats filled to its rated capacity - tic it with your wrist watch on a cold winters day and be missing your head a second later, it usually takes a real fuqe up to get one to pop, to me the biggest thing to worry about is old tanks, if there is some simple inexpensive way of accuratly testing metal thickness in the bottom part of a tank then put me down for that... otherwise worry about important things like trying not to get your shirt caught in the chuck of your lathe:p

Herm Williams
01-03-2007, 12:21 PM
A couple of observations, one run the tank at 110 or 115 it will use about half the power, two the tank can be checked for thin spots or excessive rust with an ultrasonic tester. The testing is required and done by California to get a permit to run a compressor in a shop. I relize that most people don't want big brother to look at their equipement but some things can ruin your whole day. IMHO compressed air is worse than welding gases.
my two cents worth
re

pkastagehand
01-03-2007, 12:51 PM
If you run the pressure lower (110-120 as suggested) I expect your compressor may live a little longer too. Less wear from run time as well as heat build up.

A.K. Boomer
01-03-2007, 01:49 PM
If you run the pressure lower (110-120 as suggested) I expect your compressor may live a little longer too. Less wear from run time as well as heat build up.


That can be another illusion in itself, its funny how most of us rely on those little tank guages like they were gods word yet they are a pneumatic/mechanical device complete with little half exposed gears and crap that are not infalible to incorrect readings and such, Im not saying your suggestion is a bad idea but just as important is to make sure the reading your getting is accurate and since most air comps are over kill for the amount of pressure we need they never really get compared to anything of the same PSI --- treat every tank with respect like it was a loaded gun and more importantly treat every gun as if it was a fully pulled back crossbow --- i think where many of us get into trouble is not being able to "see" the potentual energy, you wouldnt believe how many people need gun safety courses yet have already attended them, placing a loaded gun on a table and walking past the barrel --- think of a cocked crossbow, I think people automatically have more respect because they can "see" the potentual energy, some hand guns are hard to tell if thier "cocked" and ready to fire, and 15psi will kill you if you inject it into an open wound...

John Stevenson
01-03-2007, 02:04 PM
They are designed with that in mind. Stationary compressor tanks aren't.

Let's examine that.
On the right we have a truck air tank, made out of mild steel plate rolled into a cylinder and seam welded.
On both ends are two formed ends also welded. Various bosses are also welded on to take screwed fittings and usually a couple of feet also welded for mounting.

On the left we have a compressor air tank, made out of mild steel plate rolled into a cylinder and seam welded.
On both ends are two formed ends also welded. Various bosses are also welded on to take screwed fittings and usually a couple of feet also welded for mounting.

:confused::confused::confused::confused:

.

Evan
01-03-2007, 04:22 PM
Air brake tanks are specifically limited to a diameter of 10 inches or less and may not be operated above 150 psig. The diameter of a tank has a larger influence on its strength than any other factor, even differences in type of steel.

That is because the area upon which the pressure is acting goes up as the square of the diameter while the diameter (and amount of steel encompassing that diameter) increases only in linear proportion. The larger the tank the larger the hoop stress for a given pressure.

Hoop stress has two components: radial stress and tangential stress. Radial stress manifests as shear stress in the tank walls and tangential stress manifests as tensile stress.

The magnitude of the radial stress in the tank wall is proportional to the inverse square of the diameter while the tangential stress increases by the square of the diameter.

The low temperature ductile/brittle transition in body centered cubic crystalline metals such as steel alloys doesn't show an abrupt change from ductile to brittle fracture as the temperature is decreased. It is a continuum of change as the temperature decreases.

The change to brittle fracture from ductile fracture primarily affects the tensile strength of the material, not the shear strength. The hoop stress of a tank becomes predominantly tensile stress with increasing size.

Because of that the size of air brake tanks is limited to prevent brittle fracture in cold conditions.

Doubling the diameter of a tank over the maximum 10 inches of an air brake tank increases the tangential component of the hoop stress by a factor of about 4 times while reducing the radial stress proportionately. Your average stationary air compressor tank as shown above is far more sensitive to the effects of cold on the ductile/brittle transition than an air brake tank.

This graph shows the reduction of strength of a variety of low alloy structural steels with temperature.

http://vts.bc.ca/pics/steeltt.gif

rantbot
01-03-2007, 04:55 PM
The Charpy test is a measure of energy absorption before fracture. It relates to toughness, which isn't the same thing as strength. You're not talking about shooting bullets at these tanks, or whacking them with hammers, right?

Evan
01-03-2007, 05:49 PM
In general, the higher the yield strength at normal temperature, the greater the relative loss of impact resistance at reduced temperature. There is a direct relationship between tensile stress and the ability to absorb energy. The higher the tensile stress on a material the less energy it can absorb without failure. Note that the actual tensile strength of a flaw free specimen increases as the temperature decreases until the material reaches the glassy transistion temperature. However, increasing tensile stress reduces the amount of additional energy required to propagate a fracture.

When an air receiver is fully charged to maximum pressure and cooled so that it looses perhaps 75% of it's ability to absorb impact it may not take much of an impact to cause it to shatter. During WWII some liberty ships actually broke in half in cold seas because of brittle fracture.

I did word this paragraph somewhat poorly:

" The change to brittle fracture from ductile fracture primarily affects the tensile strength of the material, not the shear strength. The hoop stress of a tank becomes predominantly tensile stress with increasing size."

It should read:

" The change to brittle fracture from ductile fracture primarily affects the impact strength of the material, not the shear strength. The impact strength is dependent on the tensile stress. The hoop stress of a tank becomes predominantly tensile stress with increasing size."

nheng
01-03-2007, 06:21 PM
I had a Big Box (Home Depot) store experience with pipe fittings for natural gas recently. They actually have the audacity to import the Chinese crap fittings they do into this country and with all our rules and regulations, we can't seem to control it. Pipe tapers totally wrong or almost non-existent, porous fittings, poorly cut threads (IF they were even cut in some cases) and finally, employees who can't thread pipe suitable for an outdoor drain run.

Rant off ... this stuff cost me an extra day and a half of pain and I'll bet is costing some people their homes or even their lives !

John Stevenson
01-03-2007, 06:33 PM
Air brake tanks are specifically limited to a diameter of 10 inches or less and may not be operated above 150 psig. The diameter of a tank has a larger influence on its strength than any other factor, even differences in type of steel.


Somebody better tell Volvo and Scania then.
Daimler Chrysler, IVECO and MAN are even fitting aluminium air tanks.

.

Evan
01-03-2007, 07:55 PM
Aluminum isn't subject to ductile/brittle transition. It isn't a body centered cubic (BCC) crystal structure but rather a face centered cubic crystal (FCC). FCC metals generally do not undergo ductile/brittle transition. Aluminum in particular becomes stronger and more ductile as it becomes colder.

A.K. Boomer
01-03-2007, 08:13 PM
Air brake tanks are specifically limited to a diameter of 10 inches or less and may not be operated above 150 psig. The diameter of a tank has a larger influence on its strength than any other factor, even differences in type of steel.

That is because the area upon which the pressure is acting goes up as the square of the diameter while the diameter (and amount of steel encompassing that diameter) increases only in linear proportion. The larger the tank the larger the hoop stress for a given pressure.

Hoop stress has two components: radial stress and tangential stress. Radial stress manifests as shear stress in the tank walls and tangential stress manifests as tensile stress.

The magnitude of the radial stress in the tank wall is proportional to the inverse square of the diameter while the tangential stress increases by the square of the diameter.

The low temperature ductile/brittle transition in body centered cubic crystalline metals such as steel alloys doesn't show an abrupt change from ductile to brittle fracture as the temperature is decreased. It is a continuum of change as the temperature decreases.

The change to brittle fracture from ductile fracture primarily affects the tensile strength of the material, not the shear strength. The hoop stress of a tank becomes predominantly tensile stress with increasing size.

Because of that the size of air brake tanks is limited to prevent brittle fracture in cold conditions.

Doubling the diameter of a tank over the maximum 10 inches of an air brake tank increases the tangential component of the hoop stress by a factor of about 4 times while reducing the radial stress proportionately. Your average stationary air compressor tank as shown above is far more sensitive to the effects of cold on the ductile/brittle transition than an air brake tank.

This graph shows the reduction of strength of a variety of low alloy structural steels with temperature.

http://vts.bc.ca/pics/steeltt.gif



i just wanted to bring this whole thing up again, its classic Evan and its at its best, even a graph and everything --- you gotta luv that, the man is a machine --- kudos to you Ev, my hat is off (and thats mostly because i dont wear one but thats besides the point) Yup - been drinkingain:D

BadDog
01-03-2007, 08:17 PM
My Argon/CO2 tank is aluminum. It's about 8" in diameter, but I don't recall the full pressure...

gundog
01-04-2007, 12:10 AM
Upon further review and talking to my buddy Jim. The fitting did not break as previously described it was my impression of what had happened (it happened really fast). Jim says when I opened the dump valve it had a 90* fitting on the end and the pipe was about 18" from the wall. The force of the air pressure turned the whole pipe on the axis from the tee wind milling it in almost a complete circle then striking the wall. The force from the windmill affect swinging around hitting the wall broke the close nipple and then allowed all of the air pressure to escape. It all happened so fast when I looked up I saw the fitting broken in half I had bent down to operate the valve. Jim had said while we were moving the compressor he had noticed the tee fitting was loose and he had pushed the pipe back straight a couple of times. I had not known the fitting was loose.

I did make the repairs to the piping today and wired it up. It runs great I also removed that elbow on the bottom of the air trap so it blows straight down and I can just put a bucket under it to catch the small amount of water. The bottom of the tank has a nice drain with ball valve and it has been drained also. This compressor is a major step up from my old oil less.

For the condition of this compressor it is 2 years old and made in the US by Powerex. I did some research before buying this compressor Powerex is a very well respected compressor company dealing in many medical compressor applications. The compressor pump has casted into it made in the US the electric motor is also made in the US and the tank is ASME certified. I contacted Portland Compressor the local company that sold this unit new and was told this was a very high quality unit. The guy I bought it from had used it for 1 year spraying bed liners and then it has sat for 12 months in storage.

I will be lowering the pressure on the system to 120 PSI cut in & 140 Cut out or maybe a little lower I will try that to start with.

Thanks for everyones concern I mostly potsed all of this to maybe help the next guy in maybe not making my mistake.
Cheers Mike

A.K. Boomer
01-04-2007, 12:40 AM
Wow, Now there's is a lesson --- its not like something just blew off, in a way its even worse with the arm still attached it had the means to keep using the pressure to gather momentum --- how wierd if it never hit the wall but was able to unscrew itself maybe seven or eight revolutions and then come off at who knows what kind of imbalanced RPM's:eek:

jkilroy
01-04-2007, 06:33 AM
You have got to get rid of all of that ridgid pipe connected to the tank, very bad idea. Mount a quality ball valve right on the tank, then find a local hydraulic hose maker and have him fab up a hose that you can connect to the valve and run to the wall, or other suitable mounting sufrace, where you can mount you're regulator setup. With a compressor of this size get a hose with at least .5 ID and make sure this hose is long enough to have plenty of slack in it once everything is mounted up. But you don't want any loops hanging lower than the valve otherwise it will hold water.