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  Assessing Legacy Equipment for Brittle Fracture

Brittle Fracture
ASME VIII Division 1 Code for the construction of pressure vessels has been in publication since 1915 and has served the industry well since that initial 50 page document. Code developments in recent years have seen progress in reducing the design margin (previously labelled a "safety factor") from 5:1 to 4:1 to, currently, 3.5:1. In the recent past, the treatment of cold temperature exposure of pressure vessels has also seen changes. Primarily, this has seen the evolution of brittle fracture design.

Design for Brittle Fracture
Previously, cold temperature design was not required when that cold temperature was due to seasonal variation in ambient temperature. This provision allowed certain pressure vessel equipment to be designed without regard to cold temperature exposure during winter operation. An example is the construction of refinery propane storage drums, Figure 1; these drums were constructed of plain carbon steel such as ASTM A 515 70. In winter, location exposure temperatures can dip to - 45 oF or lower (think Canada, northern US states, Alaska).

The design standard of today requires consideration of the material and its MDMT i.e., minimum design metal temperature. Applying the rules of today to construction of past years is not typically done, i.e., so-called "grandfathering". However, when flaws occur in these vessels, most notably, general metal loss and cracking, an engineering review will reveal that the design metal temperature of these vessels is well above their critical exposure temperature as determined from Figure UCS-66 in ASME VIII D1 shown in Figure 2.

Figure 1    Horizontal Propane Drums in a Refinery

Propane storage drums can be of large diameter and length but can, nonetheless, be of thin wall construction e.g., on the order of 1 inch [25.4 mm] thick due to their relatively low operating pressure varying from 35 psig to 250 psig pressure over the course of a season.

Although the vessel may have been operating for multiple decades, the development of service induced flaws places the integrity and reliability of the vessel at risk. Given the large stored energy and explosivity of the contents, the risk level is considered extremely high across safety, environmental and economic categories.

The appearance of crack-like indications in shell welds gives rise to great concerns that brittle fracture or non-ductile fracture may be likely to occur.

Figure 2    Impact Test Exemption / Minimum Allowable Temperature

As seen in Figure 2, for a 1 inch [25.4 mm] thickness component, the minimum allowable temperature / minimum design temperature is 68 oF [20 oC] from Curve A i.e., for SA / A 515 70 material.

API 579 presents a number of evaluation options using Part 3 Assessment of Existing Equipment for Brittle Fracture or Part 9 Assessment of Crack-Like Flaws.

Part 3 provides a screening level effort and consists of a number of equipment specific options; a Level 3 analysis may make use of Part 9 or consideration of the loadings which could vary between dynamic loading of the impact test and the quasi-static loading of actual operation.

If crack-like flaws are present, these are addressed by Part 9.

Part 9 also presents a number of differing levels of effort; Level 1, Level 2, Level 3 of which Level 3 uses advanced stress analysis techniques.

While some operators have replaced equipment on the basis of a Part 3 FFS effort, a careful and thorough evaluation has demonstrated acceptability of the legacy equipment nothwithstanding apparent non-compliance with the modern rules of construction.

A very informative and essential effort is to benchmark existing equipment in both as-constructed and damaged states to current acceptance criteria.

The application of either Part 3 or Part 9 should be completed by or, at the very least, assisted by personnel well-informed and well-skilled in the application of the underlying engineering concepts. The use of and reliance upon software tools should be discouraged as these tools reflect the skills and intent of the software coders rather than, as often the case, those of the user.

The use of software tools in these evaluations must be rigorously demonstrated by validation and verification. The pressure to provide "quick & dirty" evaluations must be resisted given the risk level for this particular type of equipment.

If you are interested in these real examples of applying API 579 / ASME FFS -1 to the evaluation of brittle fracture and crack-like flaws, contact us for more details.

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