Welding or boss in stainless tank

[wookey]

I have just acquired a 180l main-pressure stainless tank with a big thermal coil in it from the top half of a dead 12kW heat pump (danfoss DHP http://heatpumpsireland.ie/wpcms/wp-con ... ual_en.pdf). Unfortunately the bodgit plumbers, who failed to fix it, drilled a hole in the tank 10cm up from the bottom, presumably to let enough water out to be able to move the thing. So now it's useless until I either weld it up or add a boss.

Anyone got advice on welding stainless? It's only an 8mm hole so shouldn't be hard to put a small plate over, or even just add a lot of rod into the hole. I want it to take mains pressure when I'm done and not corrode hopelessly (so I guess a bit of stainless plate is better than welding rod in the hole from that POV).

An alternative would be to add a boss so it has a drain point in the future. I know there are magic bosses you can add later from outside (are those only for atmospheric-pressure tanks?). It's quite thick-walled so cutting a bigger hole will be fun. Or I could try threading it and screwing a stainless bolt in. Then a bit of weld or epoxy to make sure.

[Moxi]

Hi Wookey,

Hope you are keeping well, whats your preferred welding type mig, tig, stick, gas ?

Welding would be a good first choice but you you can use some epoxy's that are specified for use with steel, or with the right filler rod you can braze stainless steel. Then there are the mechanical options you noted, can 8mm stainless pop rivets be purchased I wonder?

The epoxy will look messy but has the advantage of not affecting the metallurgy of the stainless steel.

Welding in general and the method applied specifically can change the metallurgical properties and change the passivity of the stainless allowing corrosion to occur I think this is referred to as sensitisation but basically you hit the recrystallization temperature allowing iron crystals to form above the passive oxide layers.

I depends on your ability, confidence and a little luck on the day as to what route you go I guess?

Moxi

[Joeboy]

I have just acquired a 180l main-pressure stainless tank with a big thermal coil in it from the top half of a dead 12kW heat pump (danfoss DHP http://heatpumpsireland.ie/wpcms/wp-con ... ual_en.pdf). Unfortunately the bodgit plumbers, who failed to fix it, drilled a hole in the tank 10cm up from the bottom, presumably to let enough water out to be able to move the thing. So now it's useless until I either weld it up or add a boss.

Anyone got advice on welding stainless? It's only an 8mm hole so shouldn't be hard to put a small plate over, or even just add a lot of rod into the hole. I want it to take mains pressure when I'm done and not corrode hopelessly (so I guess a bit of stainless plate is better than welding rod in the hole from that POV).

An alternative would be to add a boss so it has a drain point in the future. I know there are magic bosses you can add later from outside (are those only for atmospheric-pressure tanks?). It's quite thick-walled so cutting a bigger hole will be fun. Or I could try threading it and screwing a stainless bolt in. Then a bit of weld or epoxy to make sure.

Stainless is tricky to weld. I'd take it to a shop as its only a wee patch but critical. You need mig or tig gear.

[Joeboy]

You could try JB WELD epoxy metal and a s/s patch to up the surface area for contact. Make sure you scour both surfaces with sandpaper first if you go this route. Personally I'd pay to have it welded.

[Moxi]

Morning Joeboy,

Wookey, what about fitting a stainless rivnut and leaving the bolt in for the water tight seal - then in future you have a threaded, albeit small diameter "tapping" on the bottom of the tank ?

Moxi

[Joeboy]

Morning Moxi, I keep thinking 4 bar!!!(Maybe). I had a couple of threaded s/s bosses welded onto a 200 ltr milk churn i used as a brew kettle. The work was immaculate and even though I can stick, gas, mig and tig I could not have come close to the standard these lads did. Cost me a 6 pack!

[Gareth J]

A thin stainless tank that will be under pressure isn't the place to be learning anything.

For someone proficient, this will be a quick job with TIG. If you were a dab hand at silver soldering, that'd probably work but again, not a good place to learn.

Google/ask around for local companies working with stainless. Ones working with pipework, like dairy/brewery engineers better than architectural/fabricators.

I'd be nervous about anything screwed or glued in - especially if it's to live indoors - the risks are high! And a welded job should not cost much.

Is it worth pressure testing the coil first? Not much good if that one has split.

[Moxi]

Did you get the tank sorted Wookey ?

Moxi

[wookey]

Not yet. I bought myself a heat pump so that's much more interesting and has absorbed my plumbing time. I'll start another post about that.
And further machinations mean I've probably decided to buy a mixergy tank so this one is very likely redundant in fact.
I can MIG myself but my welder is bust so I'd get a friend to do it who knows what he is doing.