Corroded Stainless Steel (303) Nuts in Tank

Simple periscope?

Endoscope?

Change the nuts for something a bit more exotic at the next Planned Maintenance - how about 316, Monel or Hastelloy? What about an engineered plastic?

What would be the consequences of a significant number of nut failures: will the pipes float? What's in the pipes and at what temperature?

Install a redundant tank?

Install a replacement tank in different materials?

Is there an acidity problem with the materials in the tank? If it is waste and is being discharged to sewer then some additional pH correction may be required to stay within local consent limits, and may be advantageous to the longevity of the nuts?

Try those.

PWSlack seems to have covered everything. I just can't understand why you'd go for regular inspection of the nuts instead of replacing them with better ones. If you have the money for an endoscope, that would be appropriate but it seems to be overkill.

If your waste water is naturally acidic, you might try neutralizing it before it goes into the tank but, again, that's additional long term expense.

There's one other point to consider. If the material that your nuts are in contact with is of a different metal, corrosion will occur (something to do with electrolysis, I think). I'm not an expert on this but I've read about them and seen them happen. I'm wondering it that might have something to do with your problem.

I agree that PWSlack has covered all the basic and not so basic suggestions. The nuts are corroding but what about the pipe hangers? What material are they made from. Waste water from a fresh water system.....is this a municipal water filtration plant? Is this the backwash water? What chemicals are being added for pre-treatment and flocculation? What are the tanks for and what are they made of?

It is possible that simple cadmium plated nuts would work very well. If this is a public water supply system then the lab could provide an analysis of the water to tell you what the waste water is composed of to give you a basis to determine what needs to be protected against. Electrolysis could be part of the problem but I would think the pipe hangers would be affected, also.

Hi Mr. Robbins. This is an easy one, just change out the nuts and bolts with cobalt stainless steel, otherwise known as acid resistant stainless. This material is used a lot out in the north sea on oil rigs, and it works. Spencer.

I agree, going into the tank and replacing the nuts would be great and 316 stainless steel is what the nuts will be replaced with if the tank is entered. The problem is that manned entry of the tank is EXTREMELY expensive. I'm looking for a way to inspect the nuts without manned entry of the tank.

I take it that your question is more about how to inspect rather than what to replace failed components with.

Obviously you have done some inspection to have determined that some components are corroding, if the method of inspection you have used is sufficient to determine failure and it did not require entry into the confined space, simply extend it (camera on a stick).

If your further inspections reveal the need for replacement (hence, confined space entry) then you should replace all questionable components with more suitable materials. In the event that your further inspections show that the situation is still acceptable, a simple sacrificial anode(s) placed in the tank may help reduce the rate of corrosion.

In the past we underwent the expense of manned entry on some of these tanks (there are many). I'm looking for a way to cut costs on the inspections of future tanks by eliminating the manned entry and using a camera to inspect the tank internals. The issue is: can I adequately inspect the fasteners with a camera? The corrosion on these fasteners in previously inspected tanks had to be scrapped/cleaned off in order to determine the extent of the corrosion. If the tanks weren't so large I could probably insert something (wire brush on a stick perhaps) through the handholes and scrape at least a portion of the corrosion off and be able to judge the extent of material lost, but the tanks are very large and trying to scape corrosion off a nut using a 20 ft stick with a wire brush on the end would probably be difficult at best.

I'm not sure how you perform an adequate tank inspection, nuts or other components without manned entry. What is it about the tank atmosphere / exposure potential that is driving confined entry costs? Was your previous confined entry event subcontracted? I know this does not help with your stated request but on the surface it doesn't seem confined entry and nut replacement is such an onerous task. Sorry, just curious.

The tanks are radioactively contaminated, which makes the expense of manned entry large.

Joseph..................... I think I can now grasp your situation. Thanks for humoring me.

Whoa........well that, to borrow a phrase from The Wizard of Oz, is a horse of a different color! I appreciate the dilema much better now. You don't want to go in until you have to but you can't tell if you have to unless you go in.

Just on the surface it seems that it might be the best thing to go ahead and gear up for the tank entry and nut replacement but I'm sure that is much easier said than done. If the nuts are corroding significantly then the real problem is replacing them whenever you do go in because they could be extremely difficult to remove. What about the studs/bolts the nuts are on? Are they of the same material so they will need replacement as well?

The tanks are radioactively contaminated

Now there's a bit of information we should have had at the onset.

Get an endoscope. A camera at the end of a stick will be pretty hard to maneuver due to the camera's weight especially if the stick is long. Plus there's the problem of manipulating the camera's controls. That stick can get pretty large and heavy.

The endoscope is light and it's tip can be manipulated so getting it to point in a certain direction won't be difficult. Furthermore, I believe there are endoscopes that can be used underwater so it should do nicely for your application. The camera stays where you are and not underwater so that complication is eliminated.

One other thing. I remember reading something that radioactivity accelerates corrosion in some metals. If that is the case, I don't think you can escape the need to replace those nuts. If you can get nuts that last longer, or forever, your inspection expenses will go down since you won't have to do it frequently.

'Sounds like a dangerous job. You've earned my respect, sir. Good luck.

While I'm at it, does anyone know of a camera that would work well for this application? Ideally the camera would:

1. Have good enough resolution to detect corrosion pitting on the inside surfaces of the tank

2. Be waterproof so the tank wouldn't have to be pumped down before the inspection

3. Be articulated so it could be steered around obstructions and fit into tight spots

If you live near a port or major waterway,there should be a local diving contractor that does ship inspections or repairs. Often these contractors have remotely operated camera systems, they would certainly have adequate underwater video.

I did some consulting for a dive company in Southern Ontario a few years ago, they did not have remotely operated equipment but their competitors did.

JosephRobbin,

It occurs to me that even with an effective observation system you will inevitably encounter a confined entry situation. Even in your difficult situation it seems that your gonna have to bite the proverbial bullet. How many corroded nuts equal entry?

stainless anything..., will rust or corrode when oxygen is lost in liquid.