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Safe Fill Height for AST's

Safe Fill Height for AST's

Safe Fill Height for AST's

I need help in determining a safe fill height for my #2 fuel oil tanks.  They are horizontal, 20,000 - 22,000 gallon tanks built to UL-142 standards.  They have a shell thickness of 1/4", an outside diameter of 120", and an outside length of 408 1/2" to 449 5/16".  They are not heated and are vented to atmosphere.  The tanks are fitted with overfill protection valves, and monitored with a Pneumercator LDE740 level monitor.  I can not find a procedure for determining the safe fill height, and I don't want to rely upon the overfill valves or the level monitor.  The tank designer has passed away, and no one knows the location of his files.  My state pollution prevention authority says the height is up to me to determine, as they do not have any guideline for determining the height, just a requirement to post it.

RE: Safe Fill Height for AST's


1) Re-post on the Boiler & Pressure vessel Forum

  (reason: the pressure vessel guys usually know a lot about tanks and tank problems)

2) Consider posting on the www.ast-forum.com website

( there seems to be a lot of experienced help out there)


Has there been any recent tank inspections of either the tank wall thickness or pressure testing?

How old are the tanks ?

UL-142 allows both flat and formed tank heads....what kind exist on these tanks ?


RE: Safe Fill Height for AST's

If you haven't yet, you should first review the UL-142 standard.  I've only recently had to look at a UL-142 storage tank so I can't say I'm very familiar with all of the UL-142 requirements but I don't see much in the way of discussion about safe fill level.  There is a cross-reference in UL-142 for Overfill Protection that I believe references a Canadian standard ULC/ORD-C58.15 but I don't know what it says.

Other than that, you may want to review the American Petroleum Institute Recommended Practice 2350 (API RP-2350) which addresses Overfill Protection for Storage Tanks In Petroleum Facilities.  Though it may not be written specifically for your type installation, in lieu of any other governing requirements, I would think following API RP-2350 would be considered generally accepted good engineering practice.

To give you an idea what you might find in API RP-2350, the following is from the section Definition of Terms
1.3.1 2 safe fill level (tank rated capacity): The safe fill level up to which the tank is allowed to receive product delivery after the normal fill level is attained. The safe fill level is always below the overfill level. The safe fill level is established by determining the amount of time required to take the appropriate action necessary to completely shut down or divert product flow before the level of product in the tank reaches the overfill level. The safe fill level is established by operators for each specific tank and depends on the type of tank, its internal configuration and condition, and the operating practices.

Bottom line is, how fast do you fill the tank and how long does it take for you to stop filling.  You can then determine the freeboard required between overfill and safe fill level.

Of course there are other considerations that need to go along with the above such as how do you know when you have reached your safe fill level.

RE: Safe Fill Height for AST's

Thank you for the replies.  I will try posting this question in other forums before I fill my bookshelf with API's, although it may be worth it in the long run to procure some of the standards.  Cronin, the tanks are three years old, so no inspections have been performed.  They are horizontal cylinders.

RE: Safe Fill Height for AST's


You make the policy.......and from what I have seen, the number typically selected is 90%

Check this link:


About 3/4 down the page the 90% figure is stated

"There comes a time in the affairs of man when he must take the bull by the tail and face the situation." W.C. Fields  

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