Advice sought

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First of all let me introduce myself. I am Canadian 61 years old and own a construction business, I am a pipefitter by trade and have worked in New Zealand and Australia in the trade.We are building a 50 foot 304 stainless clarifier. The tank is is 50 feet in diameter and 16 feet high. 1/4." 304I will have 4 only 16 foot verts to do to xray quality in the field and about 250' of flat welds on the tank. All to be wire feed.There are structural and mechanical components as well but I am looking for advice on welding the shell.The job will be taking place near the water and will kick off in Sept. So I can expect some good and bad weather.I will have to use diesel powered welding machines and a suit case type wire feed. Unless someone has another suggestion but I do not think I will have power to run a rectifier.I have used Lincoln suitcases on carbon steel work in the past with mixed results, perhaps they are better now.So what do you think hard wire or flux core. What type of gas. Any advice you can give me would be most appreciated.ThanksKenRead more: http://welderfab.co.uk/index.php?top...#ixzz2bQrdMukK
Reply:Will all work be done in the field? Or some in shop and then pieced together in field?
Reply:Originally Posted by the vapricatorWill all work be done in the field? Or some in shop and then pieced together in field?
Reply:Ken,My advice to you is to plan to tent each vertical weld made on the exterior of the tank.You have few options for welding the tank.  Since you mentioned X-ray quality I'm going to assume that you need 100% penetration on the joints with typical reinforcement inside and out.  First question is will your customer allow a backing strip on the inside of the tank? Or do you need an open-root butt joint?A backing strip makes these really easy to do with a 308L gas shielded flux cored wire.  I'd use 0.045" diameter with a backer and about a 30° Bevel, 3/16 to 1/4" root width.  These wires typically run on 75/25 mixed Ar/CO2 gas.  They are easy to weld vertical up and give excellent penetration and X-ray quality welds; so long as the wind doesn't become a problem.  The backing strip will solve any sugaring issues.  The only worry is that the single sided weld might peak, so you'll need to dial in the fitup so that the finished welds come out with the proper radius to match the tank.If you have to do an open root butt joint, then I'd opt for 308L solid wire and buying or renting a welder that has a modified short circuit process; Miller's RMD is great or Lincoln STT if you have a preference.  The root can go in vertical down with a 30° bevel 1/8" root opening and little or no land.  Ideally you'll use a Helium Tri-mix for the root pass; although you can do this with 98/2 Ar/CO2 if you have to.  You'll likely need to shield the back side of the weld to prevent sugaring.  Miller claims that pipe welds on 304L can be made with the RMD process without backpurge; but I think this large plate weld outdoors will demand some kind of shield on the root side with 100 Argon back purge.  If the customer will let you back purge with Nitrogen, then this could save you some money.  The fill and cap can be welded pulsed MIG vertical up with the same solid wire, or you can switch to the 308L FCAW wire as you prefer.You could also weld these from both sides with the joint beveled but slammed shut.  But you likely will need to do some grinding on the back of the first pass to ensure the joint is clean.  The advantage of doing this is you can use just the FCAW wire and a simple CV power supply to make the welds. Also, welding from both sides should serve to cancel out some or all of the welding distortion; so the fitup for a good finished tank shape should be easier.Regardless of how you do this, I think you're only looking at 2 or 3 beads for each weld joint.That's my suggestions...Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:a back purge wouldn't be hard to maintain.  use cardboard boxes taped together and taped to the shell to maintain the purge.  simple, easy and cost effective without having to fill the entire structure with purge gas
Reply:Thanks for the great advice, I appreciate all the time you have spent to help me. We intend to make our first pass from the inside and we intend to put angles on the outside which will have holes drilled in them, these we can put studs in and use as pusher/pullers, also a place to drive a wedge for alignment.  I will scaffold both the outside and inside and hoard it in as required.  I was hoping to find an articulaing manlift (JLG/Genie) that I could go up and over with and put a shelter on that, but I have not yet found one that will allow the platform to rotate 180 degrees.Thank-youKTK Originally Posted by A_DAB_will_doKen,  My advice to you is to plan to tent each vertical weld made on the exterior of the tank.You have few options for welding the tank.  Since you mentioned X-ray quality I'm going to assume that you need 100% penetration on the joints with typical reinforcement inside and out.  First question is will your customer allow a backing strip on the inside of the tank? Or do you need an open-root butt joint?A backing strip makes these really easy to do with a 308L gas shielded flux cored wire.  I'd use 0.045" diameter with a backer and about a 30° Bevel, 3/16 to 1/4" root width.  These wires typically run on 75/25 mixed Ar/CO2 gas.  They are easy to weld vertical up and give excellent penetration and X-ray quality welds; so long as the wind doesn't become a problem.  The backing strip will solve any sugaring issues.  The only worry is that the single sided weld might peak, so you'll need to dial in the fitup so that the finished welds come out with the proper radius to match the tank.If you have to do an open root butt joint, then I'd opt for 308L solid wire and buying or renting a welder that has a modified short circuit process; Miller's RMD is great or Lincoln STT if you have a preference.  The root can go in vertical down with a 30° bevel 1/8" root opening and little or no land.  Ideally you'll use a Helium Tri-mix for the root pass; although you can do this with 98/2 Ar/CO2 if you have to.  You'll likely need to shield the back side of the weld to prevent sugaring.  Miller claims that pipe welds on 304L can be made with the RMD process without backpurge; but I think this large plate weld outdoors will demand some kind of shield on the root side with 100 Argon back purge.  If the customer will let you back purge with Nitrogen, then this could save you some money.  The fill and cap can be welded pulsed MIG vertical up with the same solid wire, or you can switch to the 308L FCAW wire as you prefer.You could also weld these from both sides with the joint beveled but slammed shut.  But you likely will need to do some grinding on the back of the first pass to ensure the joint is clean.  The advantage of doing this is you can use just the FCAW wire and a simple CV power supply to make the welds. Also, welding from both sides should serve to cancel out some or all of the welding distortion; so the fitup for a good finished tank shape should be easier.Regardless of how you do this, I think you're only looking at 2 or 3 beads for each weld joint.That's my suggestions...