Designing storage tanks
Software based on the latest international codes enables straightforward design of storage tanks in a range of load conditions
SCOTT MAYEUX and JANA MILLE
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The design and maintenance of atmospheric and low pressure vessels for oil storage is becoming ever more vital as crude oil storage capacity utilisation rises and oil storage capacity grows globally. The US Energy Information Administration shows crude oil storage capacity utilisation rising steadily.
Standards for tank design
From the perspective of the casual observer, these storage tanks just sit there doing their job day in, day out. And then in a spark of enlightenment, questions start to pop into the minds of the curious:
• How thick do the walls and floors of these structures have to be?
• Why are some tanks bolted down to the ground and others are not?
• What internal support structures are needed if the tank has a fixed roof?
• What happens to a tank during a hurricane or earthquake?
The answers to these questions are readily available. They ultimately lie in the pages of the following codes and standards:
• American Petroleum Institute (API) 650
• BS EN 14015:2004
• API 620.
Tank design codes reflect the culmination of decades of work by many dedicated individuals. Using these standards helps to ensure that tanks will be able to stand the rigours of the elements and conditions to which they are subjected.
The API 650 code is entitled Welded Steel Tanks for Oil Storage. At the time of this, the latest edition is the 12th, addendum 2, January 2016. This code can be used for designs where the internal pressure is less than or equal to 2.5 psig. These tanks have historically been used to house petroleum for use by chemical plants and power production facilities, as well as basic and strategic reserves.
A group of dedicated individuals meet on a regular basis to maintain and update the 650 code. These sessions typically involve lengthy discussions of various agenda items that are important to the refinement and development of the code. As one would expect, all aspects of the code, both analytical and non-â€¨analytical, are addressed in the meetings.
But getting back to one of the previous questions, how do we decide how thick the wall of the tank should be? The answer can be found in section 5.6, Shell Design. In this section, there are two methods for consideration:
• 5.6.3 Calculation of Thickness by the 1-Foot Method
• 5.6.4 Calculation of Thickness by the Variable Point Method.
The 1-foot method computes the required plate thickness at a distance of one foot above the bottom of each shell course and is applicable to tanks 200ft (61m) and less in diameter. The basic equation in US customary units looks something like this:
The variable point method is an alternative to the 1-foot method and can be used for tanks in excess of 200ft in diameter. The variable point equation in US units is as follows:
H is the design fluid height in feet.
D is the nominal tank diameter in feet.
G is the specific gravity of the contents.
S is the tank wall material allowable tensile stress for the operating or test condition.
CA is the corrosion allowance, if any.
API 650 storage tanks are often designed to work at temperatures of up to 500ºF (260ºC). For these higher temperature designs, the allowable stress of the material decreases. As a result, the required wall thickness increases in a linear fashion when using the 1-foot method and in a slightly non-linear fashion when using the variable point method.
In addition to causing hoop stress and longitudinal stress in the tank wall, the slight internal pressure causes a tensile force (pressure × area) to be produced. This force pulls upward on the tank wall. This positive upward force is countered by the weight of the tank and roof (if not column-supported). If the net force is upward in any case or condition, the tank must be held down by anchor bolts.
The basic internal pressure case is just one example. There are several other uplift formulas in Tables 5.21a (metric) and 5.21b (imperial), which must also be considered.
The net uplift due to design â€¨pressure formula from Table 5.21b in API 650 12th Edition, Addendum 2 is as follows:
Pi is the design internal pressure in inches of water.
D is the nominal diameter in feet.
W1 is the static weight of the tank components in pounds force, which resist the pressure/force uplift effect.
BS EN 14015:2004
BS EN 14015:2004 is the European design and analysis code for tanks. Its descriptive title is Specification for the design and manufacture of site built, vertical, cylindrical, flat bottomed, above ground, welded, steel tanks for the storage of liquids at ambient temperature and above.
This EN code shares some similarities with the API 650 code. Like API 650, EN 14015 computes the shell course required thickness via a fairly straightforward equation as shown here:
c is the corrosion allowance in millimetres.
D is the tank diameter in metres.
e is the required thickness in millimetres.
Hc is the distance from the bottom of the shell course under consideration as defined in 9.2.1.
P is the design pressure at the top of the tank in mbar.
S is the allowable stress for the appropriate condition in N/mm2.
W is the density of the liquid under consideration in kg/l.
It is interesting to note that the maximum design pressure for 14015 designs is 500 mbar or about 7.25 psig. This is well beyond the maximum of 2.5 psig allowed by API 650.
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