Estimating the weight of a tank is important for seismic work, preventing tanks from overturning during high winds and storms, or for cost-estimating purposes. There are many challenges associated with frequently obtaining tank weights, but a tank weight estimation tool (TWET) eliminates the need for detailed specifications, designs and use of tank design software.
Phil Tran, Engineering Analyst PEMY Consulting LLC
In preliminary design stages of a seismic analysis, it is difficult to determine whether a tank will comply with API 650 Annex E. The components or details of the tank may not have been approved or designed. Changes in design will impact the weight of the tank, which can inhibit determining the seismic stability of tanks. A TWET allows a user to quickly determine the weight of a tank within a reasonable margin of error.
How it works
When developing the tank weight calculator, you should consider four main components: the weights of the bottom, shell, roof/framing and product. The weights of these components are estimated independently, and the weight of the tank is approximated as the sum of the four component weights.
To minimize the amount of data or inputs needed to calculate the tank weight, several assumptions are made about the design of the tank: steel is A36; the product of the tank assumed to have a specific gravity of 0.7; shell course heights of 8 feet and bottom thickness of 0.25 feet with no annular ring; and roof thickness of 0.1875 feet and slope of 0.75 per foot.
For the tank shell, the thickness of each shell is calculated based upon the 1-foot method. The design thickness, hydrostatic thickness and the minimum thicknesses are calculated as per API 650 Section 5.6, based on the tank diameter, height, material (A36) and specific gravity (0.7), with each of the thicknesses rounded up to the nearest 1/16th of an inch. These thicknesses are calculated for each course, where the height of each course is taken to be 8 feet.
The most difficult part of tank weight estimation comes from determining the weight of the tank’s roof and framing. Determine the type of roof that the tank will have, and then determine which weights to calculate.
Roof weight calculations can become inaccurate with the addition of columns, girders and rafters. Detailed design calculations should be performed for several tank designs, calculating the appropriate weight for columns, girders and rafters; each case varies in diameter, and the corresponding weights of the columns, girders and rafters are then computed at a given diameter. This data can be plotted and a trendline determined using least squares regression.
Geodesic domes and open-top tanks have negligible roof weights. When considering these tanks, the weight of the roof stiffener must be considered. Using a similar approach to the calculation of fixedconed roofs, several tanks were designed with varying diameters and appropriate stiffener sizes. These values were then plotted to determine the weight of the roof stiffener as a function of tank diameter.
Conclusion
The total weight of the tank is estimated by summing the different components: bottom, shell, roof/framing and product. Because of the assumptions, a user can provide only the tank diameter and height to calculate a quick and approximate value for the tank weight. Further, the weight calculation can then be used for seismic design considerations.
More sophisticated applications of this concept have been developed. These include allowing a user more control over variables associated with tank weight: specific gravity, bottom thickness, roof thickness, material type and so on. Additional variable knowledge provides greater accuracy in a tank weight estimation tool, but also increases the amount of inputs and data that is needed to operate it.
For more information on the full report as well as access to several helpful images and equations, visit www.pemyconsulting.com.
