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).

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The Welding Research Council (WRC) Bulletin 452 titled “Recommended Practices for Local Heating of Welds In Pressure Vessels”(1) was first published in June 2000. This document considers various issues associated with the local heating of welds in pressure vessels and addresses the application of controlled heat in the weld metal, heat affected zone (HAZ) and a limited volume of base metal around the weld. ASME Boiler and Pressure Vessel Code Section VIII Division 1, paragraphs UW-40, (a) 3 and 8 require that post weld heat treatment (PWHT) be carried out in a manner such that the thermal gradients are not harmful. WRC Bulletin 452 provides guidelines for local PWHT to satisfy this requirement. This paper provides examples of local PWHT (if not heated as a whole vessel in a furnace) based upon utilizing a soak band and calculated heated band and gradient band widths based upon WRC 452 recommendations. The paper addresses both circumferential and spot PWHTs and demonstrates that the guidelines provided in WRC 452 can also be used as a starting point for determining the band widths for spot PWHT.

Vanadium modified 2 1/4Cr-1Mo and 3Cr-1Mo alloys used for the fabrication of hydroprocessing reactors offer a number of important advantages over the corresponding conventional alloys. These include increased resistance to hydrogen attack, a lower susceptibility to temper embrittlement, increased resistance to weld overlay disbonding and higher strength resulting in thinner and lighter reactors. Since the first vanadium modified 3Cr-1Mo reactors first went into service in the early 1990’s, vanadium modified alloys have gained acceptance and today more than one hundred and forty vanadium modified reactors and pressure vessels have been placed in service and are operating in severe process environments. Despite the excellent benefits of these materials, they also exhibit less desirable characteristics such as reduced weldability, higher hardnesses in the base metal, weld metal and heat affected zones and the need for higher post weld heat treatment (PWHT) temperatures. Additionally, these materials have a reduced notch toughness at lower temperatures especially in the as welded condition and require intermediate stress relieving (ISR) in lieu of dehydrogenation treatment (DHT) in restrained and highly stressed joints such as nozzle to shell and head welds. These materials also require extra care and effort to be taken during fabrication. The paper presents a serious weld metal cracking problem that occurred with vanadium modified materials during the installation of a nozzle in a restrained and highly stressed weld when only DHT was performed instead of the more beneficial ISR. This fabrication problem is provided as a typical example of problems that can occur during fabrication with vanadium modified materials, and points out that additional care must be taken during fabrication when using these materials. The paper identifies the main causes for the cracking using information based upon mechanical, metallurgical and stress analyses and suggests steps that may be taken to circumvent similar reoccurrences.