API PUBL 937-1996

$40.30

Evaluation of Design Criteria for Storage Tanks with Frangible Roof Joints

Published By	Publication Date	Number of Pages
API	1996	80

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Category: API

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Description

API 650 "(API, 1993) provides design criteria for fluid storage tanks with frangible roof joints. The rules are intended for fluid storage tanks used to store flammable liquids which could be subjected to sudden over-pressurization. Over-pressurization can occur due to the ignition of flammable vapors and can exceed the capability of the pressure relief vents specified in storage tank design.

Sudden over-pressurization can lead to the catastrophic loss of tank integrity. One undesirable mode of failure is the loss of the shell-to-bottom joint, which results in loss of containment and spillage of the contents. One such example is shown in Figure 1.1 for a tank not designed to API 650 rules (Gugan, 1978). A similar failure occurred at the Monsanto Company(Morgenegg, 1978), when lightning ignited the vapor above the liquid surface of a 20 foot diameter by 20foot tall tank, resulting in a tear of the shell-to-bottom joint. To prevent such a failure, the rules in API 650 are intended to ensure 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.

This report describes research that evaluated the ability of the present design criteria to ensure the desired frangible joint behavior. Particular questions include:

Evaluation of the area inequality as a method to predict the buckling response of the compression ring.

Effect of roof slope, tank diameter, and weld size on the frangible joint.

Effect of the relative strength of the roof-to-shell joint compared to the shell-to-bottom joint.

All objectives of the research have been completed. The research was performed in three phases.

Phase I (Swenson, et al., 1992) of this project included:

1.Initial calculations of pressure loads due to deflagration

2.Linear structural analyses of storage tanks.

3.Static testing of joints and scale models.

Phase II (Swenson, et al., 1993) tasks included:

1.Completing development of the capability to predict pressure loads due to deflagration. These loads were used as input to the structural analyses of tanks and in the design of scale model tests.

2.Performing simplified analysis of tanks to evaluate yielding of the compression ring and the appropriate w_hand W_cvalues for calculating compression area.

3.Dynamically testing scale models (the major focus of the Phase II effort). Two model tanks(7.5 feet diameter and 8 feet high) were tested under deflagration loads using a combustible gas mixture ignited in the tanks. This provided data on pressure loads and the response of the tanks.

Phase III of the project synthesized what had been learned in the previous phases into a computer program for the design and analysis of storage tanks. Tasks included:

Developing a PC based computer program, API-Tank, to design and analyze storage tanks with frangible roof joints. API-Tank includes design, analysis, and post-processing modules. The user can input tank parameters and API-Tank will develop a design following API 650 guidelines. The user can then analyze the stresses and displacements in the tank at pressures corresponding to selected tank failure modes. The pressures at each failure mode can be used to help evaluate safety of the tank due to overload pressures.

2.Using API-Tank, develop failure pressure charts for a range of tank sizes.

This report is a description of the entire research project. Additional information is provided on a video of the testing that has been submitted to the PVRC and API.

Additional information