Ken:
Let me recap what you’ve stated so I can refer to each point:
1) You’re storing heavy fuel oil in an API 650 tank, near the Equator latitude;
2) You will use a steam boiler, presumably to heat the tank through the use of internal coil(s);
3) You are capable of calculating tank heat losses under your conditions;
4) You speculate that tank insulation would not be justified on your tank;
5) You think that radiation energy from exposure to the sun’s rays would be sufficient to discard the need for insulation.
My comments are as follows:
1) I’ve recently done a crude oil project that called for heated oil storage right at the Equator. I didn’t insulate, nor did I paint the storage tanks;
2) You say you will use a boiler. This means you are committed to heating the fuel oil with external means other than radiation. So, why worry about any radiation positive heat input? All you have to consider is that should local sun radiation be of assistance in heating up the tank contents (which I doubt), you merely have the boiler make up the remaining incremental amount that you require. The boiler size is not going to be any different than it would be without the radiation addition, in my opinion.
3) If your location undergoes rainfall – as mine did – you will be wasting your time in trying to maintain the tank(s) surface paint quality and color. The tank exterior will rust and assume its own natural color. This effect is inevitable if you don’t protect the outer surface with a primer and proper insulation.
4) You must take into practical consideration (you say you want a practical solution) the fact that the tank would undergo cooling at night, on rainy days, and on windy days. The boiler (or any other auxiliary heating device) is required for continuous and reliable temperature maintenance.
5) You fail to tell us whether the tank contents are essentially “static” or if you continuously pumping in and out. From a practical point of view, you have to fill, or “top-off”, your tank(s) – regardless of what your storage scope of work is. And when you add fuel oil to the tank(s), what is the feed temperature? In other words, it’s obvious you’re not storing fuel oil for the sake of posterity but rather for the sake of consuming it or pumping it somewhere else. When you pump out fuel oil, you must replace it and when you do that you’ve inherited another heat-up rate load if the fresh, make-up fuel is not at least at 120 oF – and I have to assume it isn’t, since you fail to state so.
Basically, and on a practical note, I found it was not economically feasible to insulate large API 650 oil tanks for hot crude oil in my remote location. Rain, alone, made the idea of insulation even more costly due to the need to maintain it in reasonable working condition. I applied a corrosion allowance to the tanks’ plate steel. I believe that any heat absorbed through radiation by an exposed tank is subject to subsequent loss as soon as night fall approaches. If your location is within the Amazon Basin, this effect is compounded by rainfall; if you find yourself in some desert, then the cold, desert nighttime will take its toll. In my opinion, if all you’re talking about is 120 oF, you’re not going to suffer that much natural convection heat loss during the daytime hours near the equator. And you can’t rely on the weather remaining constant and your side all the time. Size the required boiler appropriately.
