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Old 22-03-2019, 11:29   #61
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Re: Metal vs. Plastic Fuel Tanks

Originally Posted by Ike View Post
Interesting thread and lots of good info here. This is a subject that I spent many years on while working in the Coast Guard Office of Boating Safety. Yes I have seen a few plastic tanks fail, but it wasn't the tank. It was usually either a gasket that leaked, or someone (usually the boat owner) accidentally punched a hole in the tank with a screw or drill while installing something else.

Most of what I will say here only applies to gasoline tanks. Actually Cross linked Poly tanks were introduced in the 70's. My first encounter with them was while investigating boat fires in the late 70's. Not one of those were in any way due to a leaky plastic tank and quite a few involved metal tanks. In the 80's we started a research project to test the tanks for permeation and it's affects on the tanks. After soaking them in gasoline in a controlled environment, for five years, the differences in weight, permeation and fire resistance were zero. We also tested them for the affects of alcohol. Again, no change. What does all that mean?

First realize that plastic gasoline tanks have to meet the same safety standards as metal tanks. They have to resist a fire for 2 1/2 minutes without leaking. They are submitted to the fire and then pressure tested at a max of 3 psi, and cannot leak or they fail the test. The same test is true for metal tanks.

Also the USCG had a pretty lose standard (according to the EPA) on permeation, but our concern was not environmental. We didn't want boats blowing up because vapors permeated through the tank walls, so our standard was set to keep it below the Lower explosive limit. All the tanks we tested met this standard, but that is also why you could smell fuel vapors. Your nose is very sensitive to even tiny amounts of gasoline vapor.

In the late 80's we had UL do a long study of issues with aluminum gasoline tanks. I helped with some of the investigations of fires involving boats with aluminum tanks. A lot of information came out of that study.

Aluminum tanks on average last twenty years if properly installed and maintained. We had some fail in less than a year and some that were over 30 years old. But the average was 20.

The most common failure was pinhole leaks in the bottom. This was almost always due to moisture being trapped under the tank with no way to dry out. As an addendum to that we began to see a lot of aluminum tank failures due to phase separation of alcohol from the gasoline in boats that were stored with partially filled tanks. (one marina had over 40 boats with failed tanks in one winter)

In the early 2000's the EPA got into the act and started proposing changes to evaporative emissions and exhaust emissions. I was the liaison officer from the USCG to the EPA on that project for 3 years. The major thing that changed was the construction of PE tanks. The industry had to make them less permeable. They do this several ways but the easiest is a hard liner. The hose people used the same idea on fuel hoses. The added benefit is they permeate far less and don't stink (at least not to my nose)

But to answer the question in the quote, there are boats running around with plastic tanks that are 40 years old or more. I don't know if my boat came with a plastic tank but it got one somewhere in it's life and I have had it 11 years. The boat is going on 47 years old. The previous owner had it 10 years and he said the tank was already there. So that's 21 years or older.

As far as diesel goes I have seen every type of material used for tanks. When I visited Hatteras Yachts in 1985 all their diesel boats had Fiberglass tanks. I have seen aluminum, steel, stainless steel, monel and others. By the way, terneplate has been banned for gas tanks in boats for over 40 years and I have never seen one in a Diesel boat That doesn't mean there aren't some but they are few and far between because of serious corrosion issues.

The primary issue is installation, installation, installation. If the tank is installed right, so that it is kept dry on all sides (including the bottom and top), is inspected regularly and pressure tested annually it may outlast you. Check gaskets and fittings. Accident statistics show that most fuel leaks are at fittings. With diesel, scrubbing the fuel can help considerably and virtually eliminate things growing in the tank.

Great response!

As an API licensed inspector and 35-year refinery engineer, I've seen a lot of tanks in a lot of situations, including seaside and platforms. But boats have their own own twists.

On shore, most failures relate to either accumulated sludge (corrosion under sludge is worse) and ice breaking fittings. Fitting leaks are always a matter of maintenance.
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Old 22-03-2019, 15:41   #62
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Re: Metal vs. Plastic Fuel Tanks

At Eastman Kodak/Eastman Chemical almost all our tank (all were stainless steel, aluminum, or carbon steel) failures or near failures were caused by chloride induced stress corrosion cracking in the heat affected zones of welded stainless steel tanks, general corrosion, external mechanical damage, and over/under pressurization due to a blocked vent/relief.

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Old 22-03-2019, 15:54   #63
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Re: Metal vs. Plastic Fuel Tanks

^^ Bill: I guess if I think back...
* Several destroyed by bad vents (I actually watched a 30,0000 gallon tank sucking in--A workman had been directed to plug the vent because and over flow had come out there!). That was always one of my first inspection points. Surprisingly often vents were removed, blocked by birds or bugs, or modified.
* Explosion. One was was destroyed because the vent had been moved to near ground level and there was a spark nearby, and the other because the diesel delivered had some gasoline (about 5%) (static was the ignition source).
* In one case a copper float was resting on the bottom, which created a cell.

So lots of different reasons, many of which can happen on boats. I've inspected thousands of tanks.
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