Coke Drum Design and Fabrication Issues

Abstract

There are numerous design, materials and fabrication issues which significantly affect the cost, reliability and life of coke drums. Primarily in a pros and cons narrative, this paper discusses many of these critical decisions. It first outlines the potential damage mechanisms resulting from coke drum operation, which are primarily thermal fatigue and bulging and also embrittlement, sulfidation and erosion. Delayed Coking operation is described along with the ever present desire by owners to shorten cycle times to maximize unit throughput. Some examples of the decisions include the choices of alloys for base metals, cladding, and weld overlay, and the desire to maximize postweld heat treatment (PWHT) cycles while maintaining Minimum Design Metal Temperature (MDMT) toughness requirements to permit multiple future drum weld repairs. Design issues are reviewed such as uniform versus stepped thickness wall designs, and preferential placement of shell/cone plates to their optimum locations in relation to their individual thicknesses and yield strengths. Skirts also have options in attachment designs, thicknesses and the use of keyholes. The discussion of these and numerous other issues will hopefully assist the industry in the current development of a technical standard on coke drums being done by the American Petroleum Institute (API).

Microstructural aging and property degradation of service-exposed 1Cr–1/2Mo Coke Drum steel

“…From the instrumented indentation tests, fracture toughness was obtained for two locations (oxide at crack-tip and metal matrix away from the crack), shown in Table 3. Modulus values in this table were determined using the Oliver Pharr method [1]. Modulus near the crack (157 GPa) is very similar to the nominal modulus value for Fe 2 O 3 iron oxide scale, which has E = 151 GPa [3].…”

“…Coke drums are cylindrical-shaped pressure vessels used in the delayed coking process in oil and petroleum refineries. Coke drums are usually fabricated with carbon or low alloy steels, such as C-1/2Mo and 1Cr-1/2Mo, and typically cladded with Type 405 or Type 410S stainless steel on the inner surface [1]. Type 405 stainless steel clad was initially used in older coke drums, but changed to 410S by the end of 1970s.…”

“…Cracks initiated from the cladding on the inside of the drum shell and propagate towards the outside, or, if the bulging was outward, cracks would initiate from the outside shell and propagate towards inside of the drum. Cracking is normally not a serious safety issue, because cracks can be detected by steam leaks during the steam testing [1]. Repairs of cracking are necessary to prevent catastrophic failure.…”