August 2007

Maximizing liquid sales from tank batteries

A quick review of how proper stock tank setup, care and maintenance can save you money.

Vol. 228 No. 8  


Maximizing liquid sales from tank batteries

A quick review of how proper stock tank setup, care and maintenance can save you money.

Teddy M. Pledger, AGAPE Inc.

Many, if not most, of the practical aspects of stock tank economics are known to operators. But a brief review is never a bad thing, and if just one aspect of the construction, layout, equipping and coating of an oil/condensate storage tank battery is not carried out with planning and care, it could significantly affect the amount of liquid sold from the facility.


When the tank battery site is laid out, make sure to align the tanks in an east-west direction, with the tank’s hatch located on the north side of the tank (south side in the Southern Hemisphere). Temperature is one of the main correction factors applied to the gross sales volume. The north side of the tank is the coolest side, since the sun doesn’t shine much on the north side of the tank, and even when it does it’s only in the summer near sunrise and sunset, depending on latitude. On a 40°API-gravity oil, east-west orientation will increase sales about 0.5% over the life of the facility.

The storage tank’s location must be perfectly level for the tanks to be absolutely vertical. This is essential, since the tables that are used to determine sales volume assume that the surface area of the liquid in the tank is a circle. If the tank is tilted, the surface area of the liquid becomes an oval.

When the sales volume is determined, it is based on the area of the circle measured between two heights (opening gauge - closing gauge). However, an oval has more area than the circle. So, if the volume actually removed from the tank is an oval area measured between those same two heights, then the more the tank is tilted, the greater the volume “given away.”


A “down-comer” on the inlet of the tank should be installed such that the oil/condensate is released into the tank near the bottom. By entering the tank through the down-comer, flash calculations (40°API-gravity oil at 120°F, entering the tank at atmospheric pressure and 100°F) show that sales will increase by about 5% compared with that same liquid entering the tank at the top and spraying into the tank.

The tank hatch must be able to hold a few ounces of pressure. The gasket on the hatch must be in good condition. Install a vent valve on the gas outlet from the tank. When the tank hatch and vent valve are working correctly, there should be a sound of rushing wind when the tank hatch is opened. If this sound not heard, something has malfunctioned and repairs should be made immediately.


Where possible, use a paint that has a low Solar Heat Absorption Factor (SHAF), such as real aluminum paint, which has an SHAF of 31. Oil-based paints are among the poorest options, since they all have about the same SHAF, regardless of color.

According the book, Heat Transfer (Benjamin Gebhart), the SHAF of oil-based paints varies between 0.92 and 0.98 (the higher the number, the greater the heat absorbed). Whatever paint is used, the heat absorbed from the sun can be minimized by blending into the paint additives that increase the paint’s insulating properties, such as micro-balloons (see

One final note: It should be understood that the higher the API gravity of the liquid, the more critical all of the above points are to the volume of oil/condensate actually sold. WO



Teddy M. Pledger is president of AGAPE Inc., an Oklahoma oil and gas producer and consulting firm. He graduated from Louisiana State University in 1960 with a degree in petroleum engineering and has worked for service companies and major oil producers. He spent much of his career teaching oil and gas related seminars around the world. He can be reached at:


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