Trying to estimate the peak thermal out breathing flow rate for a 500 BBL produced water tank. API 2000 gives guidance of 300 SCFH (FP > 100F) & 500 SCFH (FP < 100F). Pretty sure the API 2000 was written assuming that these are petroleum products in the tank and as a result will produce vapors at a higher rate than water. The air above the water will certainly expand as an ideal gas with the addition of heat from the sun's radiation and the ambient temperature.

We have tried to model the tank as a building and apply ASHRAE recommendations which account for Lat Long, Sun Angles, reflectively of the tank, etc, We have gotten to the point that we are predicting a tank outer skin temperature, but then don't have a solid way to predict the temperature of the air above the water, in the tank. If we assume the air in the tank rises to the temperature of the skin, then we are predicting max flow rates of 310 cubic feet per hour, in November in the morning. We are using the ideal gas equation to say that V2=T2/T1 * V1

Each hour the skin temperature increase a certain number of degrees F. This is dT/dt. using the previously stated ideal gas T to V relationship, we can determine dV/dt, which is essentially SCFH.

There are also approaches for predicting thermal expansion and vapor loss in the EPA's AP 42, however this approach can only predict total losses (mass) over a month (30 thermal cycles). This doesn't help determine the max instantaneous flow that would need to be handled by a combustor or out of a thief hatch.

Don't want to buy more combustors than needed, but don't want to pop a thief hatch either.

-Dave

Process / Mechanical Engineer