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Water freezing in API650 tank

Water freezing in API650 tank

Water freezing in API650 tank

We recently built an outdoor, open top, non-insulated, non-heat traced API650 Stainless Steel tank. Shortly after completion, the owner asked the design firm if it's ok that 1ft water will freeze during winters since the tank's lowest drain nozzle is a foot off the bottom (no sump, no sloped bottom, no flush nozzle). Now the design firm is asking us that question, or what is a safe depth of water that can freeze at the bottom of a tank. I've scoured my resources and can't find anything that might answer this question, any help would be appreciated.

The tank is infrequently used, only when other tanks are having maintenance done. So, it sits -almost- empty for long periods of time.

RE: Water freezing in API650 tank

Clearly this is a question for the design firm, not for you as the constructor. You should not take on the responsibility (risk) associated with replying yes or no.

RE: Water freezing in API650 tank

Here is my two cents .....

Aside from "possible" tank damage to internal ladders and fragile level detection components, there seems to be no evidence of major structural damage to near-empty water tanks upon freezing.

Municipal water storage towers, that must remain near full adopt a strategy of electric heating and periodic recirculation. Remember that solid freezing for this type of tank requires an EXTENDED period of time. The tanks freeze, they do not freeze solid......


Your situation is more like freezing in a swimming pool than anything else ....

I do not think that you have any risk to the structure of the tank or the foundation .... but I cannot find any supporting evidence to give you.


Sr. Process Engineer

RE: Water freezing in API650 tank

For the most part, municipal water tanks are protected from freezing by the continuous circulation of fresh water into/through the tank. It is advantageous to avoid putting the tank on a dead-end line for this reason.

Fire-protection tanks more often are left without any water circulation, and NPFA 22 gives heating requirements (and exemptions from heating) for these tanks. Of course, these tanks may be unused for 10 years, but may be needed at any point in that time, so it is not permissible for them to freeze even if no damage occurs.

The information I've seen on damage caused by freezing was primarily damage due to an ice plug that moved down the tank when the level dropped, which can rip internal piping or ladders loose and tear holes in the shell while doing so. Another issue was that on an elevated tank, it is possible to get a large eccentric block of ice in there, and the tank is not designed for that eccentric loading. I've not heard of any failures due to ice overstressing the shell in hoop stress.

You might consider either putting a small connection down load in the shell, or mounting a suction line in an existing nozzle to allow it to be pumped drier when desired.

RE: Water freezing in API650 tank

Thanks to everyone for their responses. I figured I'd share what I found in AWWA D100 which is highly relevant to the situation.

AWWA D100-11 Welded Steel Tanks for Water Storage states the following:

This standard assumes that the purchaser (owner) provides sufficient water replacement and circulation to prevent freezing in the tank and riser pipe. Where low usage may result in the possibility of freezing, water may need to be wasted or heat provided to prevent freezing. The purchaser is referred to National Fire Protection Association (NFPA) document NFPA 22, Water Tanks for Private Fire Protection, for heater sizing. Purchasers are cautioned against allowing ice buildup for insulation, which may break loose and damage the tank.

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