API RP 579-1/ASME FFS-1: Fitness-For-Service

Introduction

Fitness-For-Service procedures for evaluating fitness for service (FFS, Fitness For Services) of pressure equipment that contain defects or damage have been developed since the late 1960s and many procedures are now available for engineers choose. These evaluations, according to API 579-1/ASME FFS-1, are defined as: the quantitative engineering precisions that are executed to demonstrate the structural integrity of a component or asset in service; which may be faulty or damaged, or may be operating under a specific condition that could cause a failure.

An evaluation under the appropriate guidelines can extend the life of equipment that shows signs of deterioration; which are compiled and documented in API RP 579-1 recommended practice, for example. It should be mentioned that it is not the only one, however; ASME also has the FSS-1A .
The completeness of this practice takes into account the type of damage for the evaluation; consequently, the optimization of the analyzes that are carried out allow a diagnosis to be made with enough precision of the equipment or system under study. If the approval of the equipment for its commissioning under the conditions of the case is positive, it translates into savings for the company.

Refineries deteriorate as time goes by, presenting several problems, reducing their production capacity. For the industry, this is a concern of the first order, replacement costs, the execution of processes and activities safely, everything is compromised with the service time of tanks, containers and pipes together with the wide diversity of mechanisms of failures that compromise the structural integrity of the facilities. The recommended practices for evaluating fitness for service published by the American Petroleum Institute (API) in API 579¹ and the guide for the evaluation of structural metal defects published by British Standards².

The access to an immense power of calculation and management of enormous volumes of data, the incorporation of new tools of all kinds to obtain said data (laser profilometers, for example) allows once again to carry out work activities that until not long ago were quite difficult.

Fitness-For-Service Standard

It is important to highlight the following message addressed to technicians and engineers who are starting out in this world of inspection. “The precision of an evaluation under the mentioned practices and others like it is subject (and in a fairly solid way) to the accuracy of the measurements of the defects that are made in the field.” These measurements will support a series of computer procedures framed within API-RP 579-1 , to determine the possibility that the equipment fails in a certain time, or can operate in safe conditions. However, when making field measurements; this activity is still far from being an exact science, even though the NDT tools are very sophisticated.

Extensive research has been carried out to determine the most accurate methods, and large databases have been compiled with the results of a very wide range of studies. Various analysis methods have also been used on these databases with the same purpose of verifying the adequacy of different methods to use the most convenient for a particular measurement.

Another fundamental fact must also be established: most evaluations are carried out using manufacturing standards as criteria, which are to some extent conservative. API RP-579 uses factors of the mechanical behavior of materials that allow predicting with a high degree of reliability the margin of safety when making the decision to put a piece of equipment or system back into operation.

Conclusion

One advantage of the API RP-579 damage assessment is that it categorizes damage assessments into three different levels. The decision to stick to your application provides a technical basis for monitoring service conditions, at least until the next scheduled shutdown or until the next inspection, under well-defined conditions, such as the presence of hydrogen, corrosion rate, etc One of its inherent limitations is the need for precise measurements of defects, which is not easy to achieve, and is usually expensive.

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References

1. API Recommended Practice 579, Fitness for Service, API Publishing Services, First Edition, January 2000.
2. British Standard 7910, Guide on methods for assessing the acceptability of failures in metallic structures, 1999 incorporating amendment No. 1.