Several studies recognized that cracks in delayed coke drums resulted from low cycle fatigue induced by cyclic thermal stress [1], [2], [3]. According to a coke drum survey coordinated by API in 1996 [1], there are two different areas where cracks are produced. The first zone is located at the shell to skirt weld, and the second at the bulging areas found in the cylindrical section. In the second case, from 145 coke drums 57% reported that had shell bulging problems. Of the drums that bulged, 87% also showed cracks. In order to estimate the level of stress, finite element analysis of a bulged cylinder was performed. The study was carried out running a sequentially-coupled thermo-mechanical analysis. In the first step, a thermal analysis is used to estimate the variation of temperature with time. These temperatures serve as input to a mechanical stress analysis that was made in a second step. The assessment shows the influence of thermal gradients obtained for a period of 26 months for several bulging patterns that were identified from the analysis of 28 laser mappings of coke drums. The results indicated that the level of stress could reach the plastic deformation considering that the material has yield strength equal to the minimum specified in ASME Section II, part D. The zones where maximum axial stresses were found are consistent with areas where cracking has been reported in many coke drums.
Delayed coke drums are vertical thin-walled pressure vessels that operate under severe conditions by cyclic heating and quenching operations. The high stresses can lead to premature drum failure in the form of through wall cracking resulted from low cycle fatigue. One of the ways that has been used to reduce the growth of cracks localized near the shell to skirt weld is by the addition of different slots configurations in the skirt. In order to estimate fatigue life, a finite element analysis (FEA) was carried out using a decoupled thermo-mechanical assessment. In the first step, a thermal analysis was done to estimate the variation of temperature throughout the coking cycle. In the second step, this thermal response was combined with the mechanical loads to estimate the total stress and strain. The validity of the models has been demonstrated by several FEA results and by some field measured data. It is proposed a set of mathematical relations that allows predicting the behavior of fatigue lifetime as a function of slot size and hole radius.
Coke drums are thin walled pressure vessels that are subjected to severe thermal cyclic operation, which causes low cycle thermal fatigue. Because of that, they are considered as the vessels with the highest failure rate in a refinery according to API survey conducted in1996. In the last decade, a new technology in bottom blocking valve systems for coke drums has been introduced which induces a change in the traditional center feeding system to lateral feeding system; basically with the main goal to increase operators safety. Taking into account the mechanical integrity and remaining life of coke drums, the central feeding system has traditionally been considered as the best option, however; this hypothesis has not been fully demonstrated. Two central fed coke drums were heavily instrumented with strain gauges and thermocouples in bulged zones identified after performing a bulge severity analysis (BSA). Thermocouple arrays and several strain gauges were installed in eight specific locations of the drums. This instrumentation was installed three months before installing bottom blocking valves in the drums, and consequently, changing their feeding system to lateral. A statistical analysis was performed using 40 thermal cycles of the two coke drums with central feeding system and 120 thermal cycles of the same coke drums after changing to lateral feeding system. The usage factor was estimated for each cycle considering the axial stress amplitude and a fatigue strength reduction factor of 2 for the ASME S-N design curve Fig. KD-320.2. Finally, the remaining life was estimated for each instrumented zone taking into consideration that the coke drums would have the same cumulative damage in the future. The results show that average remaining life at instrumented zones (considering all locations) of one coke drum increased when the lateral feeding system was introduced; while the average remaining life at instrumented zones for the second coke drum remained practically unchanged after the lateral feeding system was put in to service.
Coke drums are thin-walled pressure vessels that experience low cycle fatigue due to thermal loadings. The delayed coking process is comprised by three major stages: heating, coking and cooling, which repeat at intervals between 20 and 48 hours. The cyclic changes of temperature increase the growth of bulges and cracks which with the passing of time, propagate and eventually cause failures due to the loss of containment. A better understanding of the phenomena of the thermal gradients and their influence on the generated stresses would reduce the effects of the damage mechanisms afflicting coke drums, for example; a continuous monitoring system could be implemented in order to control the cooling ramp to obtain a more homogeneous quenching around the cylinder of the coke drum and consequently increase its lifetime. It is been widely accepted that there is a relationship between high cooling rates in isolated zones and high axial stresses. However, this relationship has not been fully validated, since there are also been reported events of low cooling rates and high stresses. This study shows a predictable behavior (trend) that relates the spatial thermal gradients and the axial and circumferential stresses generated. A coke drum in an upgrader facility was instrumented with two arrays or grids, each of them having 24 thermocouples and 2 strain gauges in zones with distinct bulges. One arrangement was located at an inward bulge while the other was located at an outward bulge. Computational models were carried out to reproduce the behavior of the instrumented zones with their actual deformations obtained from laser scanning. Finite element models were developed using a sequentially coupled thermo-mechanical analysis to determine the transient temperature and stress distributions. The effect of the circumferential thermal gradients on the stress levels in the instrumented cylindrical sections were analyzed, considering two cases; the first of them a perfect cylinder (without deformation) and the second one considering the presence of bulges in the area of interest. The results indicate that there is a relationship between the circumferential thermal gradients [°C/m] or [°F/ft] and the axial stress levels, i.e., cold zones generate axial tensile stresses, and hot zones produce compressive axial stresses. This relationship is affected — exacerbated or counteracted — by the presence of the bulges. Additionally, the results obtained in this paper confirm those of previous investigations showing that outward bulges subject to pressure and thermal loading generate high stresses on its internal surface and low stresses on its external face whereas inward bulges produce the opposite effect.
Coke drums are subjected to batch cycles processing residue from refineries and upgrading the fuel streams back to the plant for further processing. As a result of operating conditions, these vessels are subjected to severe non-uniform thermal gradients that lead to localized hot and cold spots. The predominant failure mode in the coke drum shell is therefore the natural, but unwanted, progression of bulges and corrugations and eventual cracking. According to a coke drum survey coordinated by API in 1996 from 145 coke drums, 57% were found to have shell bulging problems. A common trend to increase profitability in coke drum units is reducing operational cycle length; which aggravates bulging and cracking mechanisms on these vessels. As part of the bulging monitoring process, laser mapping and bulge severity factor (BSF) analysis were conducted in a total of six coke drums. The vessel that exhibited the most significant bulges was subsequently instrumented with strain gauges and thermocouples in three specific regions. Finite element analyses (FEA) of the instrumented regions were performed using the laser mapping and 2-dimensional temperature gradients as inputs, and compared with the strain gauge measurements. The assessment shows the level of damage produced during operation, as well as the changes in damage from one cycle to the next. The usage factor can be used as a decision criterion by operation personnel for potential changes, as well as aiding in decision making on when to repair, replace or reinforce the sections of interest.
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