sulfuric acid piping 1

[mooseye]

I know this has been gone over before.
I have recently had a piping failure with an acid system for ph level control. The piping is 316 stainless. The original piping appeared to be 1/2" socket weld but had a repair/replacement with threaded ball valves which were subsequently welded over the threads.
The ball valves had begun to leak at the stem from ball deterioration.
The line was repaired/replaced back to the last socket weld ninety using 304 pipe and fittings.
This portion of the line had a catastrophic failure within 48 hours. The 304 pipe was eaten through in 3 places and one 4" nipple had split.
This system uses a small diaphragm pump to move the 98% sulphuric acid about 40 feet to a 3" PVC riser
Prior to the repair, the line was flushed with city water for worker safety. I am curious as to whether the flush water may have accelerated the corrosion when the system was placed back in service. I know the 304 was probably a bad choice to start with. The original 316 does not seem to be affected.
Another question. What effect would possible internal sugaring of the pipe wall have on the corrosive action?

 

[vpl]

mooseye

I'm not an expert on 304 versus 316, but a couple items in your description gave me pause.

Your post says the 304 piping failed within 48 hours of being installed. You then ask if flushing the system with water before the repair could have accelerated the corrosion. I'm wondering how did the water get to the piping that wasn't there when the system was flushed? Even if there was water left in the original piping system, wouldn't it tend to mix and dilute the sulphuric acid?

Your post also asks about internal sugaring of the pipe wall. As this was new piping, what causes you to think there was internal sugaring? Was it there when you installed the pipe 48 hours earlier?

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[metengr]

What effect would possible internal sugaring of the pipe wall have on the corrosive action?

Sugaring is caused by oxidation of the stainless steel, which means there was no backing (inert) gas when the roots were deposited. Sugaring will result in reduced corrosion reisistance. In addition, this is a reducing acid that requires molybdenum containing stainless steel - 316 minimum, 20Cb-3 preferred.

 

[SeanB]

"Even if there was water left in the original piping system, wouldn't it tend to mix and dilute the sulphuric acid?"

In response to this - sulfuric acid becomes more corrosive and damaging to pipe as it is diluted, rather than when it is concentrated.

 

[vpl]

metengr and SeanB

Thanks for setting me straight.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[mooseye]

I guess I really knew the answers before I asked. Just looking to confirm my suspicions.
And, no, there was no purge gas used as it was a socket weld. In hindsight, It should have been purged with inert gas even so. I don't think there was actual sugaring present, just the possibility.
Thanks for the replys!

 

[stanier]

I would not use socket weld fittings in this service if water was being used for flushing. A socket weld is sealed only on the outside. Water can get into the crevice. When the sulphuric acid is introduced it becomes a dilute form in the crevice and corrosion will be very rapid.

Better to use butt weld fittings so there are no crevices. Avoid water if you can unless you can vacuum and hot gas dry the line.

 

[leakyseal]

No-one has touched explicitly on the material issue. My experience, and more importantly my handy dandy Outokumpu Corrosion Guidebook (which can be had for free and I highly recommend obtaining) says 304 is an absolute non-starter for sulfuric in the full concentration range. There are a couple of small bands of very low conc (0.25-1%), low temp and low conc (say 10-20%) where the service is "marginal" but the remainder is unequivocally "do not use".

Just my 2 cents, not adjusted for exchange.
Leaky

 

[pennpiper]

Get in touch with these people, they know a lot about piping material for sulfuric acid.

http://www.enviro-chem.com/MECS/layout/default.asp