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Fitness for Service of DCU Coker Drums

Fitness for service evaluation of coke drums has been elusive since their introduction in the 1930's. Initially, the technology was not available; coke drums were constructed to the B&PV Code, and their deterioration was expected and experienced. By the late 1950's, the problems became sufficiently pressing that the first of several API surveys was undertaken to understand the mechanism and prescribe remedies.

The earliest attempt at remedy was to develop the UQF or Unit Quench Factor; it was apparent that the water quench phase of coke drum operations was the most damaging portion of a coke drum operating cycle as it imposed "thermal cyling" on the drum shell. This gave way to higher strength materials of construction, from SA 204 C steels to SA 387 22 and even Grade 21 (3Cr - 1Mo) but reliability did not improve. Other measures were proposed which were motivated by drum bulging. The BIF™ methodology was a trend that relied on comparitive bulge characterization. Recently, a technique designated as PSI™ / plastic strain index has been publicized. Both approaches are considered proprietary, insofar as the particular assessment techniques and detailed methodologies have not been disclosed. Hence, "black box" approaches have been presented as "state-of-art" in DCU coke drum fitness for service (FFS).

Why ASME FFS - 1 / API 579 - 1 Techniques Work

Proponents contend that the industry standard practice methodologies are not suited to coke drum FFS on account of the complex loading, in particular the thermo-mechanical cycling occassioned by water quenching. During water quenching, strains in excess of 3,500 microstrain have been measured.

Among the many reasons for not being able to process these measurements into meaningful results have been the failure to implement the ASME FFS -1 / API 579 - 1 methodologies correctly.   A significant conceptual hurdle is to recognize that there is a difference in the notion of cyclic design life versus cyclic service life. For engineers and analysts using the ASME VIII Codes, their use ingrains upon users a constraint that prevents correct and effective use of the FFS assessment methodologies, in this instance.

 

thermomechanical strain

This figure illustrates thermo-mechanical strain in a cold spot.


FFS for Coke Drums

ASME FFS - 1 / API 579 - 1 fitness for service assessment of coke drums can be effectively implemented when a number of particularizations and ancilliary techniques are brought to bear on the step by step guidance available in the FFS documents. A signficant paradigm shift occurs with this implementation; rather than a qualitative assessment that describes equipment condition as "failure", "likely bulging induced cracks", "scan in 6 months to year", a quantitative assessment can be determined. Hence, it can be established that the cyclic life at a specific shell location is "20,000 cycles at a strain exposure of 3,000 microstrain".

When bulging is present, the strain exposure will increase, either incrementally or severely depending on bulge characteristics. Fortunately, these can be ascertained with a high degree of confidence because of modern FEA methods. Rather that a strain exposure of 3,000 microstrain, a severe bulge can increase the strain to 10,000 microstrain and reduce service life to 1,000 cycles or some 33 months (2.8 years) of service life. This determination is much more informative than a recommendation to "scan in 6 months to 1 year".

Coke drums have been replaced in as little as 20 years of operation; others, have been replaced near 45 years of service, while very few coke drums are planned to be operated beyond 50 years. The difference among these decisions is the severity of operation and the availability of quantitative data on coke drum damage. The ASME FFS -1 / API 579 - 1 fitness for service methodologies allow accurate assessment of not only current damage, but also incremental deterioration from operational changes.

Another major consideration that has been overlooked by the "black box" approaches; thermomechanical damage to the coke drum shell is not dependent on bulges. While bulges accelerate damage locally, they are not necessary for thermo-mechanical deterioration in non-bulged or lightly-bulged regions. This is to be expected as cyclic life is finite as exhibited by the S - N fatigue curve. Many coke drums exhibit a lack of significant bulging; this does not mean they can expect an infinite life.

The design of coke drum skirts can be rationalized and designed more effectively using these same techniques. The details for executing the ASME FFS - 1 / API 579 - 1 methodologies to DCU coke drums is covered in detail in our course Delayed Coke Drum FFS & Damage Assessment to ASME FFS - 1 / API 579 - 1

John Aumuller, P. Eng., Ph. D.