Are socket welded connections acceptable for raw water?

[Design-fab]

Hi folks, the situation is 2-1/2" 316 schedule 10 pipe connecting to "commodity" type CF8M 3 piece ball valves. The proposal is to GTAW with back purge. A couple of general questions if anyone has experience with this:

1. Is pickling required?

2. If yes I'm not sure that the pickling solution would properly wet out the back of the weld given the geometry of the sockets?

Any insight on this would be greatly appreciated.

Regards, John.

Edit: We would consider using a CF3M valve and 316L pipe if that solved the problem.

 

[1503-44]

Why not butted fittings?

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[TugboatEng]

1503-44, the labor and QC costs for butted fittings are substantially higher than socket weld.

Design-fab, no socket weld fittings are not acceptable for wet services treated or not. Stainless steel fittings are subject to crevice corrosion in the socket. Even if the system has corrosion inhibitors, the additives cannot protect in crevices.

 

[Design-fab]

That makes sense, thank you.

 

[EdStainless]

Does this raw water have any Cl in addition to all of the microbes?
If so then the extra money for but welds is little to pay against the assured crevice corrosion failure of the socket weld fitting.
No, you don't need to pickle as long as you can weld with nothing worse that a little bit of straw coloration.
And blue or dull heat tint areas and you need to address it.

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P.E. Metallurgy, consulting work welcomed

 

[Design-fab]

Hi Ed, no Cl. Appreciate the tips on HAZ coloration.

 

[EdStainless]

None as in <10ppm? I suggest that you double check that.
For heat tint you can buy AWS D18.2 which is an 11"x17" color chart of SS welds made with different oxygen levels.
We usually required #3 or less.
There is another chart also in the ASME BPE, however its range is all #3 and below so it is only really useful for high purity systems.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Design-fab]

Less than 10ppm Cl for sure. The water is roof collected rainwater (untreated) stored in 316 tanks. The site is around 300 miles inland.

I should mention that we're not commited to using socket weld. It was more of an oddball idea that I was assessing to see if it had any merit. Sounds like the answer is definitely not...

 

[1503-44]

Wait a minute.
It's rainwater. 0 pressure, nonflamable, not too dangerous, no plant shutdowns necessary to repair/replace, cheap, probably go a long time without any problem. Not near the ocean, right?
If the price is right. Go for it.



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Why aren't you using PE or similar?

Stl Stl for rain water collection!!

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Another great question.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[EdStainless]

For that matter if they want SS tanks they should be 2101 or another lean duplex alloy.
They will use 45% less metal in them thanks to the higher strength.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Design-fab]

Hi LittleInch, we do use PE for the most part. There is a short section we're going metal due to risk of damage. Copper was ruled out as the water is mildly acidic and tends to leach copper, and DICL is only locally available 4" and larger.

The line of thought was that socket weld valves are the same price as threaded, we have GTAW capability in house, welding would allow the use of schedule 10 pipe, not to mention eliminating a whole bunch of threaded connections.

However after reading all the comments I'm leaning more towards sticking with traditional threaded 316 fittings and schedule 40 pipe. The water is pretty harmless but it will be a major disruption to repair / replace later on.

John.

 

[r6155]

Stainless steel (good material) with socket welding (bad design) is not consistent and is not good engineering practice.
I suggest butt welds.

Regards

 

[EdStainless]

I would take a harder look at the welding and using sch10.
It just makes a lot more sense.
Rainwater is always acidic because of the dissolved CO2.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[1503-44]

Full circle.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Design-fab]

Full circle indeed. Butt weld valves with sch 10 pipe seems to make the most sense doesn't it...

 

[1503-44]

Well, I'm OK with that.
SS is costly, so why mess up that anti-corrosion choice by using sockets? I don't think it makes sense to be considering SS for corrosion resistance, then fit it all up with sockets. If you want a cheap system, be consistant and use cheap pipe with cheap fittings and cheap welding and rebuild it every 10yrs.
Using cheap construction methods with expensive materials gets you the worst result of all... expensive junk.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

Also PE is a pretty robust material and forgiving of a lot of things unless you mean heat.
Or sleeve it inside a thin steel pipe?

No galvanised available if you really want metal.

Still shaking my head at Stl Stl for rainwater...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Some things are not meant to be understood by us mere mortals.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."