A probabilistic method of determining the remaining life of reformer furnace catalyst tubes is presented. Method of analysis, required data input, and model outputs are described. The methodology employed will allow calculation of the cumulative probability of failure, time-to-crack initiation, and time-to-failure in terms of furnace operating hours. On the basis of program results, recommendations can be provided for the optimum time of tube replacement and non-destructive examination. Tube replacement strategy can be determined on a row-by-row basis. This model is highly effective when used to manage operating conditions for the life optimization of new tubes.
For a given tube material, design, and quality, the life of steam/methane and hydrogen reformer furnace catalyst tubes is determined by the manner in which they are operated. Relatively minor changes in the manner of operation can increase tube life by 30 percent or more. Manner of operation includes steady-state tube temperatures, pressures, gas flow rates, and the frequency and severity of startups/shutdowns and process upsets.
The service life of reformer catalyst tubes will depend on the tube?s response to the combined action of steady-state creep and thermal stress resulting from transient conditions. Generally speaking, the relaxation of thermal stress accounts for the majority of the damage. Tube damage due to thermal stress takes place in reformer furnaces much more readily than in typical fired heaters due to 1) greater wall thickness, 2) lower ductility of the tube material, 3) lower thermal conductivity, and 4) greater frequency and severity of process transients. The relatively thick-wall design of centrifugal cast stainless steel tubes results in significant thermal stresses. These stresses, which are circumferentially oriented, can be greater than the pressure-induced hoop stress by an order of magnitude. Thermal stress will diminish (relax) with time at subsequent steady- state conditions. Given the relatively low ductility of the heat-resistant stainless steels, however, creep life consumption during stress relaxation must be considered. The ductility of the centrifugal cast tubes will decrease with time as secondary carbide precipitation and grain boundary carbide growth take place.
Rupture of the tubes will typically occur from creep-thermal strain interaction resulting from exposure to operating temperatures of 1500 to 1800F and thermal stresses during transient conditions. If thermal transients are not taken into account, estimates of remaining life may be grossly non-conservative (by a factor of 10 or more).
ERA Technology has developed proprietary software that integrates creep and thermal strain damage, and takes into account the variability in operating parameters and material properties by a probabilistic analysis. On the basis of known values for operating temperature and pressure and material properties, histograms are developed and converted into frequency curves. The frequency curves are input into a Monte Carlo simulation that calculates the cumulative probability of failure given the full range of operating conditions. Approximately 10,000 iterations are required for each set of histograms. In addition to probability of failure the method will determine
. time to crack initiation
. time to failure
. crack growth rates
as shown in Figure 1.