Frequently Asked Questions


General Tank Questions

Unfortunately, not. To give you an idea of just how careful you must be an electrical engineer could be talking about a capacitor which is a tank to store electrical charges, and another engineer may be talking about fuel tanks in an aircraft, while yet another maybe talking about petroleum frac tanks. In the oil business the word “tank” is just a general term and as you go through these FAQs the precise meaning and how to ensure that your meaning of “tank” is precisely communicated will become evident.

No. In fact lower cost options such as UL or STI or other tank standards are possible tank standards that can be applied. Tank life and corrosion rates as well as how hazardous the substance is if it leaks should be the governing considerations. Sometimes local or state regulations govern which liquids must be stored in which types of tanks and standars. For flat bottom tanks over 50 or so feet in diameter the only realistic standard is indeed API 650, although you could use standards from other countries but contractors would not be as familiar with the construction practices – so API 650 would be the primary choice for a large chemical or petroleum tank. If the pressure exceeds 2.5 psig then you would probably use an API 620 tank. Of course, if the stored liquid is cryogenic then an API 620 tank would be required.

The most economic choice is typically carbon steel. Various alloys are used that provided enhanced properties such as improved toughness when the design metal temperature is low or higher strength alloys when the tank diameter and thus wall thickness large which helps to keep the thickness and costs within reason. In some cases stainless steel tanks are used for chemical storage or boiler feed water. Aluminum tanks are sometimes used for storage of urea ammonium nitrate or other chemicals. These materials are all covered in API 650. However, plastic and fiberglass tanks are commonly used as well. API uses API 12P for fiberglass tanks. Plastic tanks are not covered by API standards.

In general both tanks have the floating roof to reduce emissions. However, wind does drive the emission rate so covering tanks (i.e. the IFR) has less emission when compared to and EFR.

Part of the answer is historical. If a tank was originally built as an EFR, then it is a simple matter to cover it which converts it to an IFR. However, the conversion costs money and doesn’t typically payout in terms of reduced emissions. There have been cases such as in southern California where the regional air quality districts required conversion of all EFRs to IFRs. While this does cut emissions somewhat the benefit-cost ratio may not be nearly as good as reductions in other ways. So the answer is complex and tank specific involving specific circumstances as to if and when tanks should be covered. An IFR also has some disadvantages compared to the EFR so there is not a clearcut answer to “should all EFRs be converted to IFRs” not to mention the cost-benefit considerations such as safety, operability, severity of fires should they occur and so on.

I won’t be precise about this because of the many details. But API 650 tanks are for the most part operating at atmospheric pressure with no more than a few inches of water column pressure. But API 650 has provisions for higher pressures up to 2.5 psig when the tank is design with the rules of Annex F. For temperature the lowest temperature is XX? up to 200F. Beyond 200F you and invoke Annex M to go from 200 to 500F which is the upper limit. ADD criteria for API 620.

A cone roof tank is a steel roof, usually made of steel 3/16 in thick. The cone allows rainwater to drain off the roof. A fixed roof tank is a generic term meaning that the tank has a roof – but does not specify what type. A fixed roof tank could be a cone roof tank or it could be a dome roof tank. Domes come in 2 flavors – the steel dome or the aluminum dome. The steel dome is not used for low pressure storage nor for larger (60 ft dia) tanks so it is not nearly as common as aluminum domes. Aluminium domes dominate the tank covers for larger tanks when cone roofs are not used which are the most common. Aluminum dome roofs have some significant advantage. One is that they can free span any tank diameter without a center column or bays of columns to hold up the roof as required for cone roofs. However, they are taller than cone roof tanks. If a reputable manufacturer is not chosen they can leak at the many panel seams that they have. Dome are often used to convert EFRs to IFRs. New FRs or IFRs are often fitted with aluminum domes. The deciding factor is usually costs on whether a cone or a dome is used. Many engineers forget to make a fair comparison which is the cost of a cone roof with external coatings versus the cost of a dome with no coating. Since coating costs are significant the best choice really depends on the cost of aluminum to steel. But an engineer should consider other trade offs such as the free span of the aluminum dome, the potential for rainwater leaks, and the difference in emissions that results from the floating roof wells that are needed for cone roof tanks with supporting columns.


Definitions and Specialized Tank Terms

(See sketch) The plates that are directly under the shell plates are called “sketch plates”. The plates that fill in the rest of the bottom are called “bottom plates”. Annular plates for a ring whose inside edge may be circular or polygonal. Annular plates are required where a higher quality joint is wanted or required than possible with sketch plates. Because annular plates are butt welded and form a flat surface for the first shell course, the quality of the welds is considered superior to other kinds of bottoms. Also, annular plates can be thicker than the typical ¼ inch tank bottom. There are many benefits to annular plates including

A shell is the cylinder of the tank. Because steel sheet is usually ordered in widths of 8 feet, a typical tank which has 6 courses is 48 feet tall. A course is one circular band of the storage tank so Course 1 starts at the foundation and Course 2 is the next up and so on,. Since the hydrostatic stresses decrease as we go up due to the reduced depth of fluid being stored the thickness of each shell course can often be reduced as you go upwards from Course 1 through Course 6.

Here is where it is important to ensure or ask for clarification. At the broadest level there are fixed roofs which sit atop the cylindrical shell of the tank. Then there are floating roofs which float on top of the liquid level and hence these roofs are always moving up or down with the liquid level. We recommend always prefacing the word roof with the words fixed or floating to ensure clarity here. Now if we are talking about fixed roofs then there are several types that can broadly be classified into cone roofs or dome roofs. Cone roof tanks usually have a very shallow cone angle of 1:16 slope which is the minimum permitted by API 650. But fixed roofs can be fitted with cones up to about 10 degrees from horizontal if necessary. The other dome roof category has 2 types of constructions. By far the most common is the aluminum geodesic dome. This roof is meant as a competitive alternative for the fixed steel cone roof. They are typically operated at atmospheric pressure and have eves that are open to the atmosphere. The big advantage of an aluminum geodesic dome is that they do not require any column supports across the span of the tank diameter no matter how large the tank is. Steel domes are usually used for smaller tank that must carry significant internal pressure.

These refer to the slope of the flat bottom tank which is not really flat. See General Tank Questions for more details.

A portion of the tank shell and bottom can actually lift up above the ground or foundation level. This might be caused by excessive internal pressure. Other causes are the forces introduced during strong ground shaking caused by an earthquake.