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Heat loss from tank roof

Heat loss from tank roof

Heat loss from tank roof

(OP)
We have a client who stores coal tar liquids in atmospheric cone roof tanks. Storage temperature is around 175 deg F. While the tanks are mostly insulated, some have had insulation removed from the roof because of external (under insulation) corrosion. We want to estimate the heat loss from the tank roof, with and without insulation. Our problem is as follows. If the tank is half full on average with a liquid temperature of 175 deg F, what temperature would the vapour above the liquid be. This would be the temperature that drives the heat out through the roof. Would it be close to the liquid bulk temperature. Does it depend on the oil vapour pressure. Any ideas would be appreciated.

RE: Heat loss from tank roof

If well insulated, (and I know it's not), then temperature of the gas should be the same as the liquid once an equilibrium has been reached.  Since it's not well insulated, your heat loss will depend on the driving forces (delta T's), which will change with the atmospheric conditions.  If you assume a constant atmospheric temperature and humidity, take into account the heat transfer of the cover (i.e. material specific), you might be able to backsolve for the theoretical heat loss... Vapor pressure would tell you how much oil you have in the gas phase at a certain temperature.  So I suppose if you can get a sample of gas and analyze the amount of oil in that phase, you could backsolve for temperature, theoretically.  However, theory only goes as far as paper.  For the real world, I suggest you just get out there and measure it for a true value.  

RE: Heat loss from tank roof

(OP)
Thanks for the ideas.

RE: Heat loss from tank roof

Is heat conduction from the warm wall plates to the roof possible ? An affirmative answer would somehow affect heat losses from the roof by convection and radiation.

RE: Heat loss from tank roof

(OP)
Sounds reasonble. I will see what I can find. Thanks for the thought.

HAZOP at www.curryhydrocarbons.ca

RE: Heat loss from tank roof

You could reduce heat loss by installing a floating roof.  If interested, try sales@allentech.com

RE: Heat loss from tank roof

1) To tell if oil vaporization is a significant factor:  check for condensation on the underside of the roof.  If this is the case, then factor in the cooled droplets falling back into the tank, i.e., you have a heat pipe.  Find an area where dripping is uniform, then put out a large container for 24 hrs to determine the flux.

I actually think this is unlikely, you must have some thick stuff to keep it at 175 F.

2) aspearini1 is right, you need to measure some temperatures.  I suggest you get several data loggers & track the inside roof T, outside roof T and ambient T.  
On your way up the ladder, check some outside wall T’s, too. There may be a T gradient re 25362’s idea.
Probably, watch the Weather Channel & write down the wind speed hourly.
And, of course, any precipitation.

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