Steel tanks and rust

[reefugee]

Hello,

I have four HP steel 100 tanks. I have had these tanks for about 4-5 years now. In those four years, I have had several of my tanks tumble due to rust. This latest round of vis, three of the four tanks needed tumbling. Today, I got a call from my dive shop telling me that one of my tank has pit rust so bad that they need to have their hydro guy take a look at it. I asked the guy why my tank needs so much tumbling? He asked me if I boat dive. Yes. Every year, I go on a 4 day liveaboard. Then the tank sits for the next 4-5 months with very little use. The service guy told me that dive boats are notorious for filling tank with moist air, and that's probably what is cause the rust. Anyone heard of this? I am wondering if I should have my tank hydro'ed after the live aboard (in August) instead of the normal time of Dec/Jan/Feb?

Minh

 

[Jim Baldwin]

I have four HP steel tanks, 100 and 120's that I have had for over 10 years. Have had to have the two 120's tumbled a few years back. The 100's have some slight discoloration and I will probably have them tumbled at the end of the year.

I would have the tanks visually inspected immediately after your liveaboard trip. Letting the tanks sit up for several months with moisture in them as you have found out is bad business.

A fill system on a boat gets a big workout and if the crew doesn't keep the water drains clear and filters changed you will definitely get moisture.

 

[spectrum]

The wet boat air is a well worn path. In a boat setting there is also an elevated chance of external water getting pushed in, much like wet dive shop filling.

Scheduling your VIP, or taking a peek yourself if so inclined would make a lot of sense.

Pete

 

[duckbill]

There's no excuse for that, boat or not. Notify your dive boat operator and let them know. Do you do local boat dives, or in some third-world country? If it was that much trouble and I just HAD to dive from his boats, I'd seriously consider renting cylinders for those four or five days.

 

[oxyhacker]

Do you mean visualled or hydroed? Hydros are done every 5 years, so changing the date won't help water in the tank much, not at least, 4 years out of five. Scheduling yearly visuals for just after your live aboard trips might help, if only by giving the tanks a bit of a chance to dry out. But you'd probably be better off to learn how to pull the valves, so you could inspect the insides yourself, and give the tanks a proper drying out.

I got to add, I have never understood why people take water in the air on diveboats so casually. The water that makes it through a compressor is almost always (Rixes aside) in the form of an emulsion, that is to say, mixed with oil, so if water is getting in your tanks the odds are oil is too. And you really don't want oil in your tanks, because it is harmful to breathe. If water in quantities sufficient to rust a tank as badly as you describe is getting past the compressor's separators, then it will quickly saturate the media stack, and the compressor will begin outputting virtually unfiltered air. There's no secret to clean dry air - just keep the separators drained (or add autodrains if one cannot) and change the media cartridges a little before they need it rather than a little after. And for especially moist conditions, add another media stack.