Low cycle fatigue is a dominating damage mechanism for coke drums due to the batch cyclic operation and water quenching process. Resistance to low cycle fatigue is one of the major considerations when designing new drums. In a major coke drum replacement project, lab testing was employed to assist selection of weld profile, welding processes for clad restoration, and to generate the fatigue curves for the specific weld design. This paper summarizes part of the main testing results and findings from the lab testing program, using samples made with C-0.5Mo steel with Alloy 625 cladding. These testing results supported design selection for better fatigue resistance and are anticipated to increase service life of the new coke drums.
Delayed coking process is one of the major process technologies used for breaking down the heavy, long chain hydrocarbon molecules of the residual oil into shorter coker gas oil (and petroleum coke as by-product). Severe cyclic operating conditions are involved in these units. The main vessels, coke drums, are exposed to multiple damage mechanisms, as summarized in API 934G and J [1, 2]. The typical life limiting damage mechanism is low cycle thermal fatigue, caused by swinging between high temperature and low temperature operations, water quenching, formation of cold spots and hot spots during heating and cooling [3, 4, 5]. Industrial experience indicates a coke drum is typically being replaced after 20-25 years of service, with the trend of shortening coking cycles and increasing through put. Through wall cracks usually start to form in around ∼ 5000 coking cycles, and the total service life of a drum is ∼ 7000 cycles.
In a major project in an Upgrading Unit in Oilsands, eight coke drums are being planned to be replaced after decades of service. The design life of the coke drums is set as 10,000 cycles, approximately equivalent to ∼30 years. Such a design life is higher than the typical or "standard" coke drum performance in oil and gas industry, and therefore special material specification and drum fabrication/design are required. To support reaching this design goal, material selection of C-0.5Mo backing plate with Alloy 625 cladding was selected, rather than Cr-Mo steel with 410S stainless steel cladding which most of the coke drums are made of. This material selection is based on experience and information obtained from both field operation and lab testing program [6]. There are a few significant advantages by choosing this nickeled based alloy as the internal cladding material: 1) better fatigue resistance against thermal shocks caused by cold spots or hot spots, 2) lower thermal conductivity to diminish thermal shock to the backing plate, 3) higher strength against deformation/ bulging, and very importantly 4) elimination of the dissimilar metal restoration welds when using 410S cladding (where a Nickel based filler metal is used).
