Fitness for Service of DCU Coker Drums
Fitness for service (FFS) evaluation of coke drums has been elusive
since their introduction in the 1930's. Initially, the FFS 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 cycling"
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 comparative 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. Another recent approach is to develop bulge transition characterizations and correlate these
to inspection frequencies.
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 μstrain
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.
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, for example, that the cyclic
life at a specific shell location is "20,000 cycles at a strain exposure of 3,000 μstrain".
When bulging is present, the strain exposure will increase, either incrementally or proportionately
depending on bulge and transition 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 μstrain, a severe bulge can increase the strain to 10,000 μstrain 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
John Aumuller, P. Eng., Ph. D.
| |
|