The upstream oil and gas industry relies heavily on storage tanks built to the API 12F specification. Shop-built, small, transportable storage tanks are a cost-effective solution that can help reduce overall installation costs compared to the cost of a field-constructed tank
Now in its 12th edition, the “API 12F Specification for Shop Welded Tanks for Production of Liquids” traces its history back to 1954, when API 12F was known as the “Specification for Small Welded Production Tanks.” The 12th edition was published in 2009.
Standards are constantly evolving to meet the requirements of those who use them. For instance, the original standard was for tanks built up to and including 400 barrels. Today, oil and gas production operators demand tanks with greater capacity and higher working pressures, while also maintaining a cost-effective solution for a market where commodity pricing is controlled by outside forces.
Many factors influenced the revised specification, such as:
• Larger-capacity tanks to reduce the number of tanks on location. Fewer tanks can potentially reduce the overall project cost, including such things as the number of over-pressure protection devices, instrumentation, piping and freight cost.
• Increasing the working pressure of the tanks to better contain and control emissions from the tanks.
• Changes for improved safety, such as the inclusion of a fall arrest attachment point and emergency venting.
The 13th edition of API specification 12F will soon be published, and many welcomed changes will be evident in the new edition. Previous versions of the standard had design pressures ranging from 8-16 ounces per square inch (oz./in2) and a vacuum of 0.5 oz./in2. The design condition in the new edition is 16 oz./in2 of pressure and 1. 5 oz./in2 of vacuum for all tank sizes. New tank sizes have been added, most notably a 1,000-barrel tank and a 16-foot, 6-inch-diameter tank design.
A lot of design and engineering effort has gone into the 13th edition, but the end result will be well worth the wait. The increased working pressure for tanks greater than 12 feet in diameter posed some design challenges, especially for the square corners of the cleanout door design. Specification 12F tanks are generally not anchored, and the increased pressure of the larger-diameter tanks created an uplift concern. The relative failure ratio of the top shell course to the roof joint, as compared to the bottom shell course to the floor joint, was evaluated. Engineering results showed that by increasing the thickness of the bottom shell course from 3/16 inch to 1/4 inch and making a slight modification to the tank shell cutout for the cleanout door (which eliminates the square corners), these concerns could be alleviated. The minimum 1/4-inch bottom floor was maintained, except for the 16-foot, 6-inch tank, which must use a 3/8-inch floor. A big effort went into the design of the venting for all the tanks. While the piping external to the tank is outside the scope of the 12F specification, the standard takes into account the type of equipment most commonly used upstream of the tank and considers such factors as liquid flow into the tank, gas flow into the tank — including an upstream valve hanging open — and liquid pump-out based on common oilfield pump-out rates.
Users of the standard will see a change to the vent opening size. An increase in the vent opening size helps operators to meet higher emission standards by reducing tank back pressure, which can help to keep the venting devices closed. Due to the wide band of operation for the various types of pressure-relieving devices commonly found on upstream production tanks, it was determined that a tank normal operating pressure of 10 oz./in2 would enable a tank with a suitably designed vent header system, and properly maintained pressure-vacuum devices on the tank, to operate in the closed vent condition.
The tables and figures in specification 12F were also updated and designed to provide a better user experience. Lastly, clarity was added to ANNEX B for the requirement that ladders, platforms and stairs must meet local, state or federal requirements such as 29CFR1910.
For more information, contact John Slupsky at john.slupsky@dvn.com.
