Jul-2006

Coke drum design

Issues to consider for extending the turnaround schedule of a delayed coking unit. Theory behind coke drum failure is discussed, with detailed solutions centred on drum design and support structures

Coby W Stewart, Aaron M Stryk and Lee Presley, CB&I

Article Summary

The global supply of crude is getting heavier. In response, refiners are making their refineries flexible enough to handle the changing feedstock. These crudes are cheaper than sweet crudes due to their limited processing capacity. One of the major additions and targets for upgrades are delayed coking units, which process the bottom-of-barrel material into valuable, lighter products such as gasoline and distillates. Adding a delayed coker unit to a refinery flow scheme provides a two-fold financial incentive: cheaper feedstock and a higher volume of light products.

In several parts of the world, North America being more severe, refining capacity is struggling to keep up with the demand for products. Refiners are making strong efforts to improve the reliability and availability of process units, with the ultimate goal of decreasing the frequency of turnarounds and making process units safer.

In an environment where it is very profitable to keep refinery availability as high as possible, efforts are directed toward process units and equipment that bear the most wear and tear. Also, it is financially preferable to run heavier crudes, especially when a refiner has an existing delayed coking unit. But nothing is easy: delayed coking units require more maintenance than normal process units in a refinery, and increasing the availability of this unit has a major impact on the long-term profitability of a refiner.

Delayed coking process
Delayed coking is a process in which a heavy residual feedstock is superheated and then introduced into an insulated, vertically oriented cylindrical pressure vessel, commonly known as a coke drum. Vapours are then removed to be further refined into various petroleum byproducts, leaving behind a high-density hydrocarbon residue referred to as petroleum coke. Petroleum coke is a solid coal-like substance that has been commercially manufactured for nearly 80 years and is used primarily as a fuel or for the production of anodes for the aluminum industry, electrodes for the steel industry, graphite or similar carbon-based products. This residue is then water quenched not only to allow for its removal once the vessel has been depressurised, but also to cool it to the point where it will not self-ignite when exposed to air. As part of the delayed coking process, coke drums undergo severe thermal and pressure cycling, as the vessel is filled with hot product and subsequently water quenched after coke formation. 

Due to the severe heating and quenching rates of this process, the useful life of coke drums is much shorter than that of other pressure vessels operating in non-cyclic conditions. This is because the severe operational thermal cycling causes the plate and the weld to be stressed with each cycle and, due to their differential strengths, the drum may bulge and eventually crack in the vicinity of the circumferential weld seams. This leads to coke drums experiencing significant downtime throughout their useful life to make needed repairs or partial shell replacements. It is a problem that has plagued refineries for decades.

Depending on the operating parameters, type of coke produced, feedstock and other variables, the cycle time of a given unit can be from as little as 14 hours to more than 24 hours. The industry recognises that the shorter and more severe the cycle, the sooner and more pronounced the bulging and cracking will appear. To illustrate this, Figure 1 is a photo that was taken from a coke drum shell replacement project, illustrating the severity of the distortion.

In the last decade, the demand for delayed coking capacity has been steadily increasing. Experts attribute this trend to refineries having to process more lower-quality crudes than in the past, as previously discussed. Thus, few refinery owners have the option of increasing cycle time, as they must balance drum distortion with the desire to meet rising throughput requirements. Instead, refiners must often resort to shortening the coking cycle, adding more units or doing both.

Bulging and cracking phenomenon
Weil and Rapasky1 (1958) identified radial bulging as a “re-occurring difficulty” that existed in essentially all operating coke drums of the time. Through extensive research carried out on a total of 16 coke drums erected between 1938 and 1958, they identified a radial growth varying from almost negligible to as much as 0.3in per year. The rate of bulging was found to be directly attributable to the “quenching portion” of the operating cycle. They also recognised that the girth seams, due to the higher yield strength of the weld metal, tended to augment the bulging, causing a “constrained balloon shape”, as illustrated in Figure 2.

As a result of the restraint caused by the weld seams, the base material tends to become thin and ultimately fails via through-wall cracking. The bulging is most severe in the lower cylindrical portion of the vessel, usually 40–50ft above the cone section. This section of the vessel experiences the highest quench rates during the quench cycle and typically has four to five circumferential weld seams, depending on the width of plate used for each shell course.

Through their studies, Weil and Rapasky observed that high quenching rates produced thermal gradients in excess of 10°F per inch, while lower quenching rates produced smaller gradients, thus less bulging. To measure this effect, they developed a “unit quench factor” (UQF, Table 1), which is the ratio of water-quenching time in minutes to the coke yield per drum in tons. Utilising the resulting data, they theorised that when the UQF is greater than 0.5, the bulging is minimal, and when the UQF is greater than 0.8, the bulging is all but non-existent. The UQF is directly proportional to the rate of water injection during the quench cycle — the slower and more controlled the quench is, the greater the UQF, which translates into less bulging. However, as previously mentioned, fewer owners have the option of longer cycle times. To the contrary, the trend is for even shorter cycles, the result of which is a higher UQF that will ultimately reduce the overall lifespan of the vessel.

In recent years, there have been a number of studies undertaken that substantiate the conclusions of Weil and Rapasky: 

- Penso et al5 suggest that low cycle thermal fatigue is the most common failure mechanism of a coke drum. The authors state that thermal shock is the main mechanism for the initiation of these cracks. They further propose that additional analysis of the cooling cycle is warranted, since the cooling cycle affects the severity of thermal shock