Minimizing the Risk of Brittle Fracture in Existing Liquid Storage Tanks, 1998 Engineering Conference Proceedings

Liquid storage tanks which do not conform to the 1980 API Standard 650, 7* Edition or later[l], may not meet material selection requirements designed to minimize the risk of brittle fracture. Shell corrosion, excessive foundation settlement, improper repairs, etc., will increase stresses in the tank walls and may increase the probability of brittle fracture.

Unless it can be shown that a liquid storage tank can continue to operate safely in the winter months when the risk of brittle fracture is highest, the product level may have to be lowered to levels which make the plant operation uneconomical.

If it cannot be demonstrated that the tank design and operating parameters fall within a safe operating envelope, then the API Standard 653[2] “decision tree” states that:

“The owner/operator can also make a more rigorous analysis to determine the risk of failure due to brittle fracture by performing a j+acture mechanics analysis based upon established principles and practices. The procedures and acceptance criteria for conducting an alternative analysis are not included in this standard. ”

This article presents a brittle fracture assessment procedure and acceptance criteria for storage tanks, based on the principles of fracture mechanics.

A simple equation is developed which gives a safe fill height based on the tank diameter, the corroded shell plate thickness, product density and the lowest expected one day mean ambient temperature.

The equation has been tested against known brittle fracture failures of storage tanks and would have predicted all of them, except where the tank had very severe corrosion. To avoid this, tanks must be maintained properly and inspected regularly by knowledgeable individuals as required by API Standard 653 [2]. Otherwise, defects may develop which are sufficiently serious to cause a brittle fracture at liquid levels below those predicted in this article.