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).
ASME Code Case 2235 and the new adoption of this Code Case into ASME Code Section VIII, Div. 2 has acceptance criteria which were based on predicted flaws and fracture mechanics modeled after the nuclear industry. These criteria were developed in response to a query about how to apply the acceptance criteria of ASME Article 4, Appendix 12 when using non-amplitude based ultrasonic methods such as phased array and time-of-flight diffraction. Since the traditional acceptance criteria were based on amplitude, they could not be applied for these alternative methods. The Code committees responded with a flaw size acceptance criteria based on the fracture mechanics properties of the materials and service conditions found in the nuclear industry. This was a major improvement and added a technical basis for the criteria lacking in past standards, however, there are no limitations or qualifications in CC 2235 on its applicability to other materials or service conditions. This is a concern for some oil refining and other plant services, especially those leading to embrittlement, various types of hydrogen-induced damage or high cycle fatigue cracking. The CC 2235 criteria were also not adequate for the recent major industry problem with reheat cracking in 2 1/4 Cr-1 Mo-V reactor fabrication. This paper summarizes the basis for CC 2235, describes the concerns with applying the acceptance criteria without consideration of material or service conditions, and suggests how to approach the issue from a better informed perspective and in some cases, establish stricter maximum flaw sizes.
As an industry consensus, API 934-A is an excellent recommended practice on the materials and fabrication requirements for Cr-Mo reactors. However, it is cautious and somewhat vague on the topic of Intermediate Stress Relief (ISR) versus Dehydrogenation Heat Treatment (DHT) for the different types of welds — which reflects the industry’s varying practices. For the advanced steels, API 934-A states that DHT should only be used with Purchaser approval, and that it should not be used on restrained welds such as nozzle welds. As a result, it is common for a DHT to be permitted on longitudinal and circumferential seams to achieve the cost and schedule savings, and ISR is used for nozzle welds. There are risks to the fabricator however, as the welds remain extremely brittle after DHT (the toughness is restored after postweld heat treatment {PWHT}, and at intermediate levels after ISR), and welding defects that are acceptable per ASME Code criterias can lead to brittle fractures during subsequent fabrication steps. The costs of the repairs and delays can then be very high, especially if the cracking is not detected until after PWHT. This paper shows the risks of acceptable defects causing brittle fractures by fracture mechanics calculations, and presents some case histories of cracking. The relative costs of ISR versus DHT, versus repairs before and after PWHT are also reviewed.
UNS N07718 (commonly known as Grade 718) is an age-hardenable nickel-chromium alloy which primarily has applications in aircraft components and engine parts, cryogenic tankages, and for downhole and wellhead components in oil and gas. It also has been used primarily as bolting for ASME Section VIII vessels, exchangers and other high strength applications. ASME SB-637, UNS N07718(1) has the highest design allowable stresses of all the bolting materials in ASME Section II, Part D(2), Table 3. There is a second industry standard which covers UNS N07718 components, namely API 6ACRA(3), which was developed for Oil and Gas equipment. Since this standard is for very different applications compared to the ASME applications, the recommended heat treatments and required mechanical properties of the two standards vary considerably. Bolting to either standard can be specified to meet “NACE hardness limits” for wet sour services, but for the ASME materials, this requires strict controls of the various heat treatment steps as the “box” for meeting all the required properties is tight. The NACE standards and requirements for wet sour service are discussed in the paper, along with the specified heat treatments and mechanical properties required by the two industry standards. The paper will review the effect of the heat treatments on the expected microstructures and properties. By using testing results of N07718 materials specified to both the materials standards, the heat treatment and properties were optimized for a specific application that was specified to meet NACE hardness limits as well as ASME Code strength requirements. The paper also makes a recommendation to ASME and ASTM committees to consider incorporating an additional grade of N07718 bolting with lower yield and tensile strength requirements and subsequently slightly lower allowable stresses which will provide lower hardness levels, improved toughness and improved ductility, and make it easier to source a viable N07718 bolting that is endorsed by the Code for wet sour services, and improved for low temperature, cryogenic and other services.
Within Sulfur Recovery Units (SRU), many equipment and piping items are built with internal refractory linings, but between the various process areas, the refractory is being used for completely different functions. Understanding the “purpose” of the refractory is important for both the initial selection of the optimum type of refractory, and also for repair decisions whenever refractory damage is found during turnarounds. This paper will describe each area where refractory is used, the operating and turnaround conditions, and the four unique purposes of the refractory to provide a mechanistic understanding of the function of refractory. It will also discuss the relationship between the purpose and repair philosophies for use as a general guideline.
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