American Petroleum Institute

Study to Establish Relations for the Relative Strength of  650 Cone Roof Roof-to-Shell and Shell-to-Bottom Joints
 N PUBL 937-A

Annotation

Introduction

This report documents an evaluation of the relative strengths of the roof-to-shell and shell-to-bottom joints in API 650 cone roof tanks. This information is supplied to the American Petroleum Institute as background material for development of design rules that govern frangible roof joints for API 650 tanks.

API 650 (American Petroleum Institute, 2001) provides design criteria for fluid storage tanks used to store flammable products. Due to filling and emptying of the tanks, the vapor above the product surface inside the tank may be within its flammability limits. Ignition of this vapor can cause sudden over-pressurization and can lead to the catastrophic loss of tank integrity. To prevent shell or bottom failure, the rules in API 650 are intended to ensure that the frangible roof-to-shell joint fails before failure occurs in the tank shell or the shell-to-bottom joint. Failure of the frangible roof-to-shell joint provides a large venting area and reduces the pressure in the tank.

Although the criteria in API 650 function well for large tanks, small tanks designed to the API 650 rules have not always functioned as intended. Morgenegg, 1978, provides a description of a 20 foot diameter by 20 foot tall tank in which the shell-to-bottom failed. Other such failures have been noted by API, providing the incentive for this study.

As presently written, the API 650 rules do not address the strength of the shell-to-bottom joint directly. Instead, the present rule is intended to ensure that the roof-to-shell joint fails at a pressure lower than that required to lift the weight of tank. It is assumed that with no uplift, the shell-to-bottom joint will not have significant additional loads and that failure of the shell-to-bottom will be avoided.