Large water storage tank Immersion heater for freeze protection

[AlaskanME]

Hello,

I wanted to get your thoughts on a task I am dealing with and make sure as an entry engineer I am asking the right questions before I continue to proceed.

Background:
We have an API 650 50,000 bbl water storage tank (110' x 30') built in 1967 that is used for firewater and all other facility water needs. The tank currently is freeze protected in the winter using a 40# internal steam coil using 400# letdown steam. When it was originally built the tank was also outfitted with a forced draft natural gas heater for freeze protection but I have no documentation on the heater or if it was ever used ( I know it hasn't been used in the last 20 years). The tank currently does not have insulation. For economic and operation concerns the plant wants to shut down its steam generation, so I am investigating multiple options to maintain the freeze protection @ around 40-42F.

Options:
Electric immersion heater

- insulate tank to reduce heat loss (this will create inspection issues)

installing electric heater would require demoing old forced draft heater and internal piping​

Forced natural gas heat

​

-would need to find new heater and inspect current heater internal piping
Let tank freeze during the winter
- ensure no instrumentation, internals, or vents will be affected
- NFPA 22 requires ambient temperature control
recirculating type heater

- allows for online repairs and maint.​

- can be natural gas or electric​

Add agitator/mixer to tank to prevent freezing

- Not sure how to quantify that this would be an option​

These are the options I am considering right now. Doing some heat loss calcs will be my next step to see if insulating is the way to go and to size the heater.

Let me know if you have any thoughts.
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[JStephen]

Alaskan, it sounds like you're approaching the problem in a reasonable fashion, so I don't have much to add to it.
There is some limited information on required heat input in NFPA-22.

Regular municipal tanks are generally prevented from freezing by the water flowing through- the water coming into the tank is nearly always somewhat above freezing, and if heat loss relative to this flow isn't too bad, no additional measures are required in those cases. Many firewater tanks have zero flow until needed, so it's a different situation there. If your tank has any significant flow through it full-time, you might want to incorporate that into your analysis. Or if there is significant flow that bypasses the tank, but can be routed through the tank, that would be worth checking into.

The electric immersion heaters are generally the simplest and easiest if you look at installed cost ONLY, on the tank part ONLY. But I would assume are most expensive to operate. In some marginal climates where heating may be required only a few days a year if at all, or if no other power source is available, they make more sense. Don't forget to figure the cost of bringing that amount of additional power to the tank site.

The agitator/ mixer option would make sense if you had problems with ice in some isolated areas of the tank, but the overall tank was in no danger of freezing. Otherwise, it might just make the problem worse.

 

[AlaskanME]

Thanks for the input JStephen.