Sulfuric Acid and Stainless Steel Tubing/Swagelok

[picklegt]

Does anyone have experience using 316L stainless steel tubing joined with Swagelok fittings in 93% sulfuric acid service? This piping is part of a clean-in-place (CIP) process supporting a continuous microfiltration(CMF)unit and was specified/installed by the manufacturer of the CMF unit. The sulfuric acid will be pumped, via PVDF pneumatic diaphram pump, from a 55-gallon drum and transferred through approximately 40-ft of 0.75-inch OD by 0.120-inch wall thickness SS 316L tubing into the top of a 4,000-gallon CIP tank. Based on the pump size, velocity in the tubing will be less than 6 ft/s. Dosing time will be less than 2 minutes and it is expected that an acid cleaning will be required only once every thirty days. The entire system is indoors with climate control. I am very concerned about the safety of using Swagelok fittings in this application and would greatly appreciate any feedback or opinions. Thanks.

 

[cub3bead]

I would be more concerned about the materials selection than the Swagelock joining method. 93% H2SO4 is a strong oxidizer and will dissolve the passive coating from the surface of the tubing resulting in corrosion of the tubing. If you want alloy tubing, Carpenter 20 would be the better choice.

1" schedule 80, seamless carbon steel with welded joints is also suitable for this service if you can keep the acid concentration above 80%. I would be concerned about the tie-in at the injection point though if CS is used. Make sure to dehydrate the line following hydro before you introduce the acid.

You should ask how they intend to tighten the Swagelok fitting, 0.75" by 0.120" wall tubing is pretty stout stuff.

Also, remember that the flowing velocity follows a sine wave profile when diaphragm pumps are used with out pulsation dampners and backpressure valves.

Is the intent to leave the line packed with acid between cleanings?

 

[saxon]

pickelgt, I would be less concerned about the Swagelok fittings than I would be about the material of construction. You're pumping a high conc. of H2SO4, at ambient temp. I take it, you can expect a corrosion rate of greater than 0.05 in/year. In no time the acid will a have corroded your 316 SS lines & fittings to nothing. Better to take a look at a material like PVC or CPVC. That is, if the press. and temp. allow for it.

If you want metals, try Chlorimet 2 or 3, Hastelloy B-2, Hastelloy C-276, High Silicon Iron, Zirconium. But expect to pay through the nose for them.

Hope this helps.
saxon

 

[kenvlach]

Given the limited usage, you may be OK if you air purge and rinse with DI water several times after each use. I have used 316 SS hose barbs on 316 SS Wilden AODD pumps (& centrifugal pumps, too) with reinforced, PTFE-lined hoses to make transfers of 93% sulfuric acid over many years. After each use, pumped: air, water, air, water, air. No problems.

With the Swagelok® connections, there is a potential for crevice corrosion or stress corrosion cracking. The ferrules are a stronger but less corrosion-resistant SS grade than 316 (the seal design requires ferrules stronger than the tubing), and the tubing is near its yield stress where the ferrule presses in and seals. I once passivated a batch of Swagelok® SS fittings, and recall that it was necessary to use Type II passivation solution, rather than the Type VI (QQ-P-35C) customary for 304 and 316. Check the ferrules with a magnet and/or contact Swagelok:

http://www.swagelok.com/

cub3bead,
Check out the Unlined steel corrosion by sulfuric acid (pretty bad at 80%):

http://www.resistoflex.com/sulfuric_graphs.htm#7

I suggest you post this in the Corrosion engineering Forum forum338, with a link to this thread (thread164-92940).
Ken

 

[cub3bead]

Ken,

Thanks for the useful chart. I did not intend to recommend CS for use at less than 93% but in looking back I see how my response could be read that way. Thanks for catching the poor word selection on my part.

I still would like to see the size of the guy who manages to properly install a Swagelok fitting for 0.75" X .120" wall tubing.

There has to be a better solution to the problem for periodic CIP of the membranes than has been proposed.

PVC and CPVC acid lines tend to get brittle with time and I don't think they are worth the risk.

 

[WTP03]

I would be concerned with the connection point to the CIP. 93% and 316SS will not be a problem until it comes into contact moisture. Use of a back pressure valve at the CIP connection will keep positive pressure on the acid line through to the CIP connection not allowing moisture ingress and corrosion.

WTP03

 

[picklegt]

Thanks for all of your responses. You have all raised valid points for consideration and have confirmed my concerns that the existing SS316/Swagelok piping system is a potential hazard. When dealing with concentrated H2SO4, even small potential hazards pose to great of a risk to take.

I am leaning towards replacing the system with teflon lined steel piping. Another alternative I am considering is using threaded teflon piping and fittings. Obviously, the mechanical integrity is less than the lined steel but the piping run is only 40-ft and moderately well protected from being damaged by impact. Either way, I still have to deal with the connection to the pump (maybe teflon lined braided hose). Any thoughts on these ideas would be greatly appreciated. As cub3bead said, "There has to be a better solution...".
Thanks.

 

[cub3bead]

What amazes me about this situation is that the end user is having to re-engineer a vendor's design and will likely pay a nice extra for the pleasure of doing so. Too many vendors have adopted the attitude that if it lasts a year and a day then the solution is adequate. Maintenance budgets should not have to bear the burden of weak design practices.

picklegt you are to be congratulated on identifying a serious safety hazard and looking for alternative solutions.