How often should an interior fuel oil tank be cleaned out?

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Some background info:
I've got an interior 275 gallon heating oil tank of indeterminate age (though probably not more than 25 years old). The heating system gets the standard annual maintenance from my oil supplier (cleaning, filter, screens), including the addition of a pint or so of "acetene (A) tank treatment & fuel conditioner" / sudge dispersant & etc. Though I can't pick out a manufacturing date on the boiler, the manufacturer's service literature is present and has 1976 dates. The burner tested at 83% efficiency last summer, down from 85% two years prior. The burner's transfer coil and control unit have been replaced within the past two years, and the flow control valve shows evidence of some slight leakage (and I decided to defer the $450 repair). We use about 950 gallons of oil annually (in eastern PA).
My oil supplier offers a "tank cleaning" at $250. Being flush with cash at that time, I had them do this in 2000. Basically, they drained all of the remaining oil and did whatever other magic they do as part of that work. On at least one occassion since that time (although we're on automatic delivery) we actually ran out of oil, and so the tank was presumedly thoroughly drained.
Here's my question:
Is there enough value associated with cleaning out the tank as to make it worth doing periodically? And at what interval? Is the "tank treatment" fluid likely to be sufficient to maintain the interior of the tank in reasonably good condition?
Thanks in advance for your observations and advice.
Oscar.
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I have never heard of a "tank cleaning service" -- it sounds like a revenue generating idea created by the heating oil company.
Any tank that is 25 years old, cleaned or not, may be questioned by your insurance company with a demand to replace it with a new tank. Insurance companies don't want a huge payout for environmental cleanup if it should leak.

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Underground tanks have a life of 15 to 30 years. Above ground tanks considerably more. You can do ultrasonic tests that reveal rusting of the tank. I've never heard of one being replaced because the insurance company requested it, but I guess it can happen. I know of may tanks that are 50 years plus and still working.
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Here in Canada, it is common for Home Insurance companies to require tanks that are 25 years old or more to insist on them being replaced, because of this they actually caused a shortage in tank supplies as the replacements increased in number.
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I can understand that on tanks exposed to weather or ground moisture, but a basement tank? (Assuming a dry basement, of course.) As long as outside shows no signs of rust, I'd think a borescope inspection of interior every five years or so (I assume these things have a bunghole?) would be more than adequate. I know the pickup tube isn't on the bottom, but as long as no rust flakes are visible in the drained sludge, what are the odds of major interior rustout with no outside evidence?
(Having said that, any interior tank should of course have a spill berm around it, or be sitting in one of those giant kitty litter pans.)
aem sends...
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aemeijers wrote:

There can (and often is) moisture in a fuel oil tank. There are two outcomes. One is the tank rusts. The other is microbes that grow in the area between the oil and water and feast on the oil since oil is slightly hygroscopic causing "goo" which blocks equipment.

All (new/replacement) interior tanks have been double wall for some time.
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George wrote:

And all the water in the tank will collect at the bottom to cause rust there where it will do the most damage. Normally however a sudden failure is pretty rare and pinhole leaks occur well before any significant leak.

Perhaps in your particular area, however you can readily purchase and install a new 275 or 330 gal single wall steel tank in most areas.
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Pete C. wrote:

My buddy has a fuel oil company. He said the typical failure is that the rusted tank blows while being filled.

Might be old inventory or something and of course there is a lot you can do when no one is looking. According to my buddy there is some requirement for new/replacement to be doublewall.
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George wrote:

That is indeed the typical catastrophic failure, caused by not inspecting and noticing the pinhole leaks that were there for at least a year or more prior.

Again, probably a local requirement. MA perhaps?
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Pete C. wrote:

No, my understanding is that it isn't local. We are in PA but according to my friend it is a requirement in at least the NE and Mid-Atlantic states.
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George wrote:

Dunno, I know I saw regular single wall 275 and 330 tanks for sale in CT within the last couple years.
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George wrote: ...

http://www.epa.gov/OUST/faqs/heatoil.htm
Tanks used for the storage of heating oil for consumptive use on the premises where stored are excluded from federal UST regulations. However, state or territorial regulatory agencies may regulate these tanks. You can view a map showing which states regulate these tanks..."
http://www.epa.gov/OUST/graphics/heatoil1.gif
PA actually isn't on the map (but it notes it's not kept completely up to date)...
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George,
I don't doubt that there is some moisture in heating oil. Even if there isn't moisture will still get into the tank. In cars there are products known as Dry Gas that will solubilize and remove this water in the gas tank. Doesn't the heating industry have something like this?
Dave M.
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David Martel wrote:

Yes, and it is also available in jet fuel (kerosene) for the same reason.
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George wrote:

Additionally, there are water absorbent "socks" that you can put at the bottom of the tank (on a string for removal) that will absorb water and not the fuel oil. You replace them periodically and some are wringable and reusable.
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On Mon, 16 Jul 2007 12:16:06 GMT, "David Martel"

In the 60's one major oil company advertised that it included something like this in the gasoline. I think even at that time, all the major gas companies did so, and by not long after I think all the independants did too.
But Dry Gas is still useful when there is more than the normal amount of water in the gas tank.

I haven't heard about it.

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mm wrote: ...

...
Which is basically, just alcohol in which water is soluble. One advantage of ethanol...
--
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On Mon, 16 Jul 2007 12:16:06 GMT, "David Martel"

I haven't heard about it.
Ooops. I got confused by my own post. I havent heard about it being already included in the fuel oil when you buy it, but i'm not surprised the sell it separately.

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Edwin Pawlowski wrote:

I know a few friends whose insurer required tanks to be replaced only because of age. It also lessens further risk because all new interior tanks are double wall.
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I'd change insurers. They are welcome to come and inspect my tank, but I'll be damned if I'm going to replace a perfectly good tank just because it hit a certain age.
Our insurance company at work (as well as state law) requires we inspect our air tanks every two years. It takes about 5 minutes to do an ultrasound on one. Much cheaper that scrapping good material.
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