API Standard 620 covers the design and construction of large, welded, field-erected low-pressure carbon steel aboveground storage tanks (including flat-bottom tanks) with a single vertical axis of revolution.

The rules presented in this standard cannot cover all details of design and construction because of the variety of tank sizes and shapes that may be constructed. Where complete rules for a specific design are not given, the intent is for the manufacturer-subject to the approval of the purchaser's authorized representative-to provide design and construction details that are as safe as those which would otherwise be provided by this standard.

The tanks described in this standard are designed for metal temperatures not greater than 250 F and with pressures in their gas or vapor spaces not more than 15 pounds per square inch gauge.

The basic rules in this standard provide for installation in areas where the lowest recorded one-day mean atmospheric temperature is -50 F. Appendix R covers low-pressure storage tanks for refrigerated products at temperatures from +40 F to -60 F. Appendix Q covers low-pressure storage tanks for liquefied hydrocarbon gases at temperatures not lower than -270 F.

The rules in this standard are applicable to tanks that are intended to (a) hold or store liquids with gases or vapors above their surface or (b) hold or store gases or vapors alone. These rules do not apply to lift-type gas holders.

Although the rules in this standard do not cover horizontal tanks, they are not intended to preclude the application of appropriate portions to the design and construction of horizontal tanks designed in accordance with good engineering practice. The details for horizontal tanks not covered by these rules shall be equally as safe as the design and construction details provided for the tank shapes that are expressly covered in this standard.