The selection of length and diameter is one part but the availability of a suitable precast vault is probably the most important. Pouring concrete for a special size vault is expensive. Also, is it to be an indoor storage area or outside, covered or not and are sprinklers required? Careful attention must be paid to the tank saddles and their placement due to the associated plumbing for the installation. The calculation of the diameter and length is fairly straight forward as the diameter will be set by the available diameter of the dished ends. The supplier will provide the volume of the dishes and the length can be calculated knowing those two volumes and the diameter.
Suggest to use follow procedures to size the vessel:
1. Divided the required surge volume by 0.8 to 0.9 to get the design volume of the vessel.
2. Pick a vessel length (for example 5 or 10 ft). Calculated the required vessel diameter, rounded to the next 6 or 12 inch. There will be multiple vessel sizes can be used for this application. List a table with vessel length at 5 ft increment.
3. Calculated the vessel weight for each vessel in the table, and use the vessel with the smallest vessel weight for final design. Usually, the slender the vessel, the less is its weight. Keep the final design vessel L/D ratio at 3-5.
Note: The required liquid surge volume = design liquid volume flow rate x (desired liquid residence time + 3 min residence time for HHLL + 3 min residence time for LLLL) [3 min is the allowable time for operator to response. The last two items is optional, up to designer to choose.]
It has been my experience that the diameter will be set in two ways: First, the upper limit in the diameter that the tank fabricator can handle and, secondly, by the diameter of the available dished ends the tank fabricator buys from his supplier. The dished end is formed in a press and so the upper limit is set by the available equipment at that supplier. It is not unusual to seek proposals from a number of tank fabricators and to also request they shop for particular size and shape of dished ends.