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API PUBL 937-A Document Information:
Title
Study to Establish Relations for the Relative Strength of API 650 Cone Roof Roof-to-Shell and Shell-to-Bottom Joints
American Petroleum Institute
Publication Date:
Aug 1, 2005
Scope:
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.
A study of roof-to-shell joint failure (Swenson, et al., 1996) showed
that for large tanks, the
roof-to-shell joint did indeed fail before tank uplift, but that for
smaller tanks uplift would
occur before roof-to-shell joint failure. Since uplift occurs for
small tanks, this increases the
possibility of shell-to-bottom joint failure.
The purpose of this study is to investigate the relative strengths of
the roof-to-shell and
shell-to-bottom joints, with the goal of providing suggestions for
frangible roof design criteria
applicable to smaller tanks.
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