How to Prepare a Large Propane Tank for Safe Oxyfuel Cutting?

This probably won't satisfy you but here goes. I've seen a lot of welders vent a truck exhaust into 55 gal. drums before cutting them. I think no matter what you do, it will take someone with brass balls to strike that first spark. A response usually has a snow ball effect. Maybe this will help start the discussion. (We called this reconnaissance by fire)

James

I had just read somewhere that a WWII fighter aircraft used engine exhaust to pressurize and clear oxygen from their fuel tanks prior to entering combat. I think it was Macchi.

Gasoline leaves a varnish in fuel tanks that will vaporize into a very explosive gas when the tanks are welded or brazed. Tanks that have been dry for months can explode. Blowing air through the tank when welding on it can prevent an explosive build-up of fumes.

Pressurized vessels have strict code issues regarding modification, repairs etc. In the USA most such work has to be done by ASME certified welders following prescribed procedures and inspections. Canada is probably more strict.

You would think that the local propane distributor could direct one to someone who decomissions propane tanks.

I've got a call in to a pipe-fitter buddy on this but considering the specific gravity of propane is 1.5, I would open a low-point drain and supply 5 psi of compressed air to the top. Rent a confined space sniffer and test the air coming out of the drain. When the propane concentration drops below the lower explosion limit, you're safe.

As an aside, I assume you're having a certified welder do the modifications on a pressure vessel?

More to the point, I hope you have a professional engineer DESIGN the modifications you propose to the tank, as they will have to be approved by the relevant pressure equipment safety body (ABSA here in Alberta), before the tank can be used in pressure service, and they will want to see the stress calculations.

If you are cutting a large hole in the tank to install the manway, you also need to consider how this will weaken the pressure vessel; additional reinforcement around the opening may be required, hence the need for the stress calculations.

Good luck & be careful!

Maybe run a fan into it for a day or so...

I agree with the post that says test it...

I'd also run some sort of remote hot wire* into it so it could be tested at distance before risking 'striking that first flame/arc'..at least that way it will be an impressive spectacle rather than a fatality if something goes wrong with the purging.

*say a car headlamb bulb with glass carefully broken, placed inside the tank then connected to a battery from a safe distance?

(I liked the exhaust idea)

Del

Regarding the engineering behind the installation of the manway and nozzle: You betcha!!! I have professional engineering help with doing the design as well as spec'ing the welding. As an aside, I found out that here in BC you don't need to have a P.Eng. behind your name to be able to design a pressure vessel alteration--as long as you walk thru the prescribed calc procedure.

I didn't mention this because I was focussing on the specifics of making the vessel safe for working on, which is pretty much a topic on its own.

Jon.

Heard back from my pipefitter buddy...he says there is a formula for pounds of dry ice based on tank volume, but did not have the actual calculation available. Sorry it's not a hard answer, but maybe knowing the basic procedure will help you wrangle the critical piece of information out of someone.

If it were me...and I have seen the aftermath of several oil field errors, so I am pretty paranoid.

First, I'd vent diesel exhaust through the vessel for a hour. That should reduce the O2 content to less than 5%. Then either vent CO2 or nitrogen from a welding supplier into the vessel or add 200 lbs of dry ice. 1 lb of dry ice sublimates into 8.3 ft^3 of CO2. A 12K gallon tank holds about 1600 ft^3.

The Lower Explosive Limit for propane is 2.1%. The above procedure should reduce the propane content below 2.1 and the oxygen content well below the level at which it will maintain a fire. Again, erring far onto the side of caution, I would then run a sniffer as suggested earlier on the lowest vent point as well. If that doesn't do it, I'd wait for spring.

Or..I guess you could rig up a radio control servo and a bic lighter and check the mixture at a distance. Be sure to set up at least 3 cameras to document that one, though.

I am glad that somebody finally mentioned L.E.L. for propane, although the suggestion of running exhaust gas into the cylinder is a little dodgy on the account of carbon monoxide and Nox emissions.

All gas purging at our plant is done with nitrogen gas and a combustable gas sniffer on the outlet of the pipe/tank. Anything other than 0% is unacceptable for welding and cutting in the area. Once this has been acheived periodic testing should be carried out to check and prevent further build up in the work area.

Call a local tank repair facility and let them do ALL the work. My pal repairs gasoline tankers welding on them everyday. Let professionals do this and certify it. You owe it to your family.

Your arms and legs do not know if you have money. Only your brain knows. Spend the money and condition yourself to remember that only your brain knows you wrote the check, not your arms, legs, hands . . . whatever.

All those body parts will still function regardless of the check amount for the full job.

I cannot guarantee the same thing if you write 30% of the full amount and do some of the work yourself.

Hardly anyone will do this job for you, too many people have blown themselves up....

The official process is to either Steam the tank by passing high pressure steam into the tank while venting, FOR AT LEAST 4 FOUR HOURS!

Some states forbid any welding or cutting on ex-hydrocarbon tanks, so you had better check your local safety rules. It will be the same as for car or truck fuel tanks.

or fill the tank with carbon tetrachloride while cutting it. As this requires a ozone depleting substance and you need 12000 gallons, you really only have the firsst option...

Note that Propane is a hydrocarbon and will have embrittled the steel, particularly on the inside of the tank- making it liable to crack under pressure, consequently you should consider the need to anneal it by some means before using as a presssure vessel.

I believe that in shipyards, this is usually done by pressurizing the vessel with compressed nitrogen gas until practically no trace of propane fumes remains.

The best approach to purging as you say would be filling the tank with water, however with the difficulties that you have indicated I would use an inert gas to displace all traces of propane. Have the tank outlets opened to allow for natural ventilation for a while. Measure the LEL inside the tank and start cutting.

With regard to using a cutting torch, have you given conderation to cutting the tank using alternative methods? API653 provides some guides on Tank maintenace. I would be using pnuematic cutters to cut out the holes and continously apply water on the on the steel surface while cutting. This is the same principle to cooling machine tools on a lathe machine. This cutting method removes one part of the fire triangle, "ignition source".

No, No, No!!!!!!! Don't be tempted by the idea of saving money in doing this project. Nothing is worth the loss of life. Not only are you trying to get around the ASME code, just the modification of a tank that has held combustibles is a FOOLISH act. If you want an air receiver of this size, buy one. There is no economy in wasting human lives. Trust me, what you might save in initial cost may result in disaster.

DON'T DO IT!!!!!!

Try Ajaks Services (ajaks.com) Googgle Oil refinery cleaning or decoking.

Generally we steam out vessels and test with a combustible gas meter. If this held commercial propane there should be very little oil inside the tank, so I don't think I'd worry to much about contaminated water.

The steel embrittlement guy is crazy; the stress calculation guy is dead one; ignore the "run diesel exhaust through it" advice. Generally speaking any opening should be less than 1/2 the diameter of the vessel and reinforced with the same area of metal removed.

This is just a shot in the dark and will cost you a phone call.

I would call Beall Corporation in Billings, MT. (406)252-2999. You may be familiar with them as they make lots of semi-trailers and tanks for petroleum products and they repair them. It's been 30 years since I had anything to do with them but they weren't too bad to talk with. I remember carbon tetrachloride being mentioned at various times and I think that's how they cleaned the tanks before working on them.

Just tell them your situation and see what they will tell you.

Good luck,

Randy

To add to the pessimism of your project, it is probably not a deep dark secret that you have discovered, but more likely something that is not done. Pressure vessels are replaced, not repaired.

You have a very big LPG tank, I think you should contract with a firm for repairing the tank. That company have responsibility to purge your tank with processes that meet fully your area laws.

I don't know how many propane remained in your tank? Can you use it to heat your house by burning it before repairing the tank.

Good sucess

Hi

Just purge with oxygen free nitrigen, then leave the nitrogen flowing as you weld, gives a great oxide free joint.

As propane is heavier than OFN purge into the top and vent at the botton, ensure no drains hollows etc nearby where propane can collect.

As previous note I have also welded fuel tanks containing petrol by filling all free air space with vehicle exhaust gasses, how brave do you feel

I repair allot of gas tanks at my shop and I too recently came across an underground propane tank that I intend to use for myself. I also am the R-stamp ASME administrator at a large paper mill on my day job. I have to wonder if you have the original paper work for the vessel? This would be the first step to doing alteration to the tank by the ASME code here in America. if you don't have the original paper work such as U-1 forms then ASME and the inspector will not go along with any work. The next step would be to verify thicknesses and do new calculations for the alteration you are planning. At this point I would have to contact my authorized inspector and review what I am planning. After the inspector blesses my initial calculations then I can start to do the fabrication work. After finishing the fabrication work I would then perform a hydro-static test at app. 1-1/2 time of design pressure for the alteration. AT this time the inspector would sign off on my ASME paper work and I could start using the tank. Doing these alterations correctly by the ASME code is hardly ever a money saver on smaller tanks.

The first thing I am doing to my propane tank is soaking it with water for about one month and I occasionally turn over the contents with new water. Although I am not trying to convert my tank to anything more than an atmospheric storage tank I will still have some nozzles and supports to install by welding on the tank. The next thing I will do is to drain the water and purge the tank with argon or another inert gas for around an hour so that I have all the propane fumes displaced. I leave a vent on the top of the tank and try to purge towards the bottom because the inert gas will be heavier. After I am convinced that the tank is purged then I will start the work. Please keep in mind that you are taking a chance anytime you cut on a tank that has had anything flammable in it and do so at your own risk. From a company liability and safety standpoint you would be probably be better of just throwing the tank away and buying an air receiver.

Pipwelder

My dad and I learned the almost hard way that water is not a great purging agent for gasoline. We were trying to patch a fuel tank leak on a 63 Chevy gas tank. Filled and emptied it twice with water. When we approached the leak with an open flame, we heard a small"WHOMPF" and the tank jumped about a foot off the ground. After we collected our wits, we took the tank to a local repair shop and watched from a distance as a "Old-timer" sued a large soldering iron to seal a screw into the opening.

Moral of the story: Be very careful. Steel will tend hold hydrocarbon traces after purging. Let someone else handle the work who understands and has years of experience.

Propane is heavier than air, and will vent better out the bottom of the tank. When using an inert gas to displace oxygen, be aware of asphyxiation issues for people in the area. Know the ASME requirements or get someone that does.

How do you sue a soldering iron? Some lawyers will take any case.

Hey, this reminds me of a science experiment from school.

get a metal container with a leaver removable lid (for those who know the Milo containers) 1 hole in the lid, and 1-2 in the base.

Put flammable gas into container (Natural gas, I think it was) and light the hole at the top, the flame burns like a candle, close the blinds and turn off the lights.

Watch quietly as the flame drops down to the surface of the can then as you nicely put it above "WHOMPF", the flame sucks into the can, then blowing off the top with a nice flame effect around the top and the air holes at the base, also scareing the crap out of most of the students (with the teacher suppressing laughter at the back of the room)

Ahhh, from the above, your supposed to purge the water from the tank, *thats* where I was going wrong, I was leaving the 2nd charge of water in the tank as I welded it up...

Sounds like the Norwegian goose guns we built in college before beer cans had the "pop" tops.

Thanks, all, for your replies. Not long after I posted this question we found a consulting engineer in Northern BC who does this stuff every day of the week for the oilpatch. He will assist us in our little project. Here's what I found:

Bottom line is testing for LEL (lower explosion limit) at the discharge vent while putting in steam, inert gas, etc, at another vent. No room for wondering...vent until LEL is virtually zero. There is some sort of table available for the number of cylinders of inert gas one needs to use for a given tank size. We need about a dozen or so large bottles of Nitrogen for our particular tank. There is a venting regimen, too, as some of your suggested--In at the top, out at the bottom, or vice versa--I didn't catch that detail. Since we need to perform a caustic wash to get rid of the "propane smell" (methyl mercaptans) we will probably steam the tank to get the LEL to zero, even though the weather might be an issue. I'll be standing on that tank beside the welder when it is cut open because I won't ask anyone to do something I wouldn't do. So I will have to be 100% comfortable about the science behind the purging.

For those that were concerned about the engineering of the tank modifications:

Code issues are not a problem. I am not and never suggested in any of my posts, that I was trying to circument ASME code requirements. I have already posted previously that I have a P. Eng. backing up the design of the vessel modifications. The work will be done by a fab shop certified for vessel alteration, and the actual work will be done by a pressure welder with a current certification. There will likely be a third party expert on site when the welding commences, at the recommendation of the local Provincial Boiler Inspector. Testing will be done on the finished welding, as per Code requirements. I'm not sure exactly what the test regimen will be, but my engineer suggested that 100% ultrasonic is all that is required--no Xray necessary.

To answer other concerns:

The tank will be internally checked for stress corrosion cracking from hydrogen embrittlement. It has already been ultrasonic thickness surveyed and we have the paperwork "pedigree" and drawing that was submitted by the original manufacturer years ago.

This is truly turning out to be a job where there is 10 man-days of paperwork for 1 man-day of actual fieldwork! The actual tasks involved to purge the tank, do the welding and NDT testing, and then do the final Hydro test are not large or costly. The next tank repair or alteration will take a fraction of the time this one has taken!

Thanks again, All. I hope I can offer some help to someone in the future when one of you post a question! (But not about purging tanks!!!)

Jon.

Thanks for the update! We all really appreciate it. Good luck with your project!

Sawmilleng, Thanks for the update, I think allot of the posters (including myself) where pushing the safety and ASME issues just in case. The truth is that we do allot of alterations to pressure vessels and tanks in the paper mill and a good many of them had contained or contain flammable contents. In the past we had a barometric condenser blow off and kill 2 men because of a pocket that no one new about between 2 of its water seals. A complete purging and many atmospheric sniffs were performed but the water seals had trapped turpentine and when the welder started to cut the bolts from it the vessel blew up.The purge and common since is always the trick with the safety and having the atmosphere sniffed to be sure is the only way to know if it is safe. There can be allot of money saved from using a good pressure vessel for things other than it was designed for as long as one follows the rules. I my opinion of what I have read of your post you know what you are doing with the codes and such and I wish you good luck and savings in this project.

pipewelder

Good luck and sucess.

You should use a bottle of voska while performing your project

Voska? Isn't that some kind of a Bulgarian automobile?

I'm really impressed. A real sincere question by a pro who needed some help finding sources for his project. Not only did he ask for help, he continued his own efforts and was successful. Here's a 1st class person who wants a project done right. I'd work under him as I can see he's concerned about people and project.

There were some good sincere efforts by CR4 members and guests to help. That shows this forum is an asset for those needing some help with their projects. Good job!

I'd only have one more question for sawmilleng because a lot of us have an interest in how the project turned out: could you let us know in a post how things turned out and your soon to be expert opinions on the whole thing?

Thanks

Randy

Very good answers- I will print out- I myself when welding petrol tanks wash them out with water- then refill with water to just under where the hole needs to be welded, then crouch behind a barrier while flashing a lit oxy-acet torch at the hole- I have never had a pop even- that said, I much prefer to use 50/50 solder with a patch- but really the best answer is a r/p good tank- if you can find one( in the case of an old car, where water lies at the bottom of the tank from condensation, & rusts thru the steel).

back in the early 80's when I was just starting out with my first small welding shop and money was tight I figured out 2 easy ways to make some grocery money. One was to rebuild small allum. trolling motor propellers for the retirees around town. it seemed they were constantly running up on a stump at the local lake. The other was to repair Ford 8n and 9n tractor gas tanks (they would nearly all leak on the bracket to tank spot welds from the vibration) Anyways once the word got out it was common to have 2 or so farmer/retirees at my shop in the morning. I did the repair for 35.00 back then and was very proud to get that. I found out the hard way about filling the tank with water when I tilted one and trapped just a small amount of air right where the repair was. All was ok till I really got it hot and had just started to add brazing rod when BAM! water went every where and it blew the tank several feet over as well as made a very nice hole in the side of it. After this incident i used the exhaust from my forklift and I never had it happen again. Back in those days I was just learning about GTAW welding and purging and really couldn't afford to use a purge type gas on the repairs at 35.00 a piece. I do know that carbon monoxide will purge a tank but I do not recommend it for safety reasons. I always wondered if to rich mixture of gas and air could cause extra gas fumes to be introduced through the exhaust?

pipewelder

In the 70's I was making two gas tanks out of a saddle tank for a truck. I had washed it out with water and soap which seemed to cut some of the residuals out of the tank. I had it cut about 2/3s in half when my wife called me in for lunch. After about an hour I went back out, fired up the torch and "WHOMPF". Thats exactly what it sounds like. I didn't have to finish cutting it in half either but I did have to straighten some edges. It's also the fastest my wife has ever come out to see what I've done.

A few months later, I put some blow off tops on some 20 and 30 thousand gallon gasoline tanks. It was under the supervision of the fire marshall and fire department. The tanks were filled with water and all the stuff they had at that time to keep them from blowing. I got them opened up with big holes without any problems. After that everyone thought every thing was under control and went about their business. I then discovered that there lots of "WHOMPF" left in those big holes. I learned the fine art of firing up the torch and waving it behind my back to let it get the "WHOMPF" out of its' system (same with the arc). The went on many times during the next few days.

Now that I'm kind of retired and reading these posts, it brings back some old memories and makes me thankful I've survived some of those educating experiences. Also makes me glad I eventually went into the computer business and just occassionally do a little welding on some personal fun projects.

Without those "educational experiences" we have all survived, we wouldn't have any expert knowledge to share with others would we?

Just to think.

Those things that educated you in the past are such highly dangerous that your not taught of the dangers, but do get shown the propper way to evac the tank, but the lingering thought will remain ..... what if .....

I was told in the past that for gasoline you should rinse the tank with alcohol, which disolves gas residues. Throw away the alcohol and rinse with water. Alcohol is soluble in water, and voila!

I did not try it and therefore I cannot tell you if it is safe.

Hi,

Just a final note to all those who had some interest in this challenge. The job has been done and placed into service sucessfully, with no problems.

The purging took a bit of time. We first pushed 10 large bottles of nitrogen through the tank, entering from the bottom and exiting the top (Or vice versa--I've already forgotten!)

The exit gas was then checked by a gas sniffer for lower explosion limit. It was still no good.

We then changed to low pressure compressed air and continued purging with a low but steady flow of compressed air over a weekend. The change to compressed air was to help get rid of the propane "Smell" which are the mercaptans which attach to the steel inside the tank. Oxygen helps oxidize this odor which we needed gone because the end use was to be a compressed air tank, with the air exhausting within a plant.

At the end of the weekend, we tested the gas issuing out of the tank. Clean. nothing. Nada. The gas meter didn't even blink.

A few days later, the shop was ready to cut into the tank. They emptied another dozen nitrogen bottles into the tank to be 100% sure that the tank was good to go, gave it another sniff, and fired up the torch. And they didn't tell me when they were going to do it, so I'm embarassed to say that I wasn't standing on the tank beside the guy doing the cutting like I wanted to be. The cutting proceeded with no issues.

As an aside, the reason we put more nitrogen in the tank just before cutting is because certain hydrocarbons in a tank will "bounce back" after purging. This happens with things like diesel and the like if there is a bit of liquid or sludge left in the tank which will outgas if left for any length of time after the purge.

After that, the job got real boring--The poor guy with the welding stinger spent nearly a week welding in the 24" manhole access port and an 8" airline nozzle. There were 3/8, 1/2", 5/8" and 3/4" fillet welds, all of pressure weld quality, with repads and all of the required pressure vessel stuff.

We found we didn't have to do an internal wash of the tank to get rid of the propane smell. The compressed air purging and the time the tank was sitting with the openings cut made the smell disappear completely on its own.

Once the tank was completed, we made new support saddles for it, cleaned it up and gave it a nice coat of white paint. Then onto a truck and off to our plant site.

The hydro pressure test was interesting, too. This was done at site, with the tank on its foundation, since 12,000 gallons of water weights a fair bit! We filled the tank with water right to the top of the new inlet nozzles we had just welded in. Blind flanges were bolted onto these nozzles and a guage port was empolyed to let nitrogen in to bring the tank up to 210 psi, which was our 150% test pressure.

Now, how much nitrogen would we use to get this tank up to 210 psi with the whole tank already full of water? I expected a few squirts of nitrogen would cause the guage to rise very quickly. NOT! We dumped a whole bottle into the tank and the guage wasn't much over 100 psi. It finally got to 210 after the second bottle was nearly empty!

All I can attribute this to is the flex in the tank. It must have stretched just a little, but that little stretch in such a big tank accomodated two bottles of nitrogen.

The boiler inspector signed the job off and we all left happy.

Would I do a tank conversion again? Well, this job cost us pretty much 100% of what a new tank would have cost. But at least half of that cost was learning curve. I think I would try it again, now that the mystery of how to do the job safely has been eliminated. It was a great learning experience!

Jon.

we read about you welding your tank and we are getting ready to weld on a 1000 gal propane tank. we have an oxygen sensor that tells us 25% down to 0%. what is a safe limit to weld on a tank with inert gas in it. how would you advise us to do this.

I can only comment on what we did, since I'm not an expert in doing this!! But our consultant and our plant safety guy recommended that we go down to ZERO on the sniffer. It was really quite easy to get to this level--the trick was using a low flow of compressed air over a weekend to vent the tank, get rid of the mercaptans, and then do the fill with nitrogen.

I can't remember what kind of sniffer we used--I'm checking with our safety guy to see if we were detecting oxygen or hydrocarbons. I'll add to this posting when I get the answer.

Jon.

Just an update to my comments above--Our safety guy used an LEL sniffer that alarmed at 10% of the LEL level. He ignored the oxygen level, which was also available on the instrument he had because it is only really used for checking when doing confined space entry, where you need to ensure there is a minimum oxygen level to keep from a man from dying inside the tank.

I noticed a comment above in this thread where someone who worked in a plant used the same standard as we did--0% of the LEL level was the only level acceptable. Those guys would have known the accepted safe way, given that they probably do this kind of work every day.

Maybe you can rent an instrument to read LEL if you don't have one.

Jon.