Service life temperature curves

If the objective in design or operation were optimizing catalyst utilization, then Figure 82 shows that the converter temperature-composition profile should follow curve (a), which corresponds to maximum reaction rate at all points. It is also obvious that in reality this ideal temperature - concentration profile cannot be achieved. For example, a synthesis gas with about 3 % ammonia concentration entering the converter cannot be heated to the ideal temperature by heat exchange because the very high temperature required does not exist in the converter system. To reach the ideal temperature, the first portion of the catalyst must initially operate adiabatically. Consideration of the service life of the catalyst requires that this maximum initial temperature not exceed that recommended by the manufacturer, usually 530 °C (cf. Section... 

Significantly, the two curves intersect at about a normalized catalyst mass of 1.6, where the service life is about 500 hours and the total production of hydrogen is 74 std m. For normalized catalyst less then 1.6, the same amount of catalyst produces a longer service life and greater total amount of hydrogen when run at 260°C versus 240°C wall temperature. When the normalized catalyst mass is greater than 1.6, it is clearly beneficial to decrease the wall temperature in order to significantly increase the service life. 

A plastic beam, 200 mm long and simply supported at each end, is subjected to a point load of 10 kg at its mid-span. If the width of the beam is 14 mm, calculate a suitable depth so that the central deflection does not exceed 5 mm in a service life of 20,000 h. The creep curves for the material at the service temperature of 20°C are shown in Figure 3.18a. The maximum permissible strain in this material is assumed to be 1%. 

Assuming that the logarithms of the service-life values t determined in tests at constant temperatures present a spread pattern around the mean value log according to a Gaussian error distribution curve, the failure probability of a component subjected during an operational period tg = 2to a temperature can be computed from the following equation ... 

The service life of a sealed nickel-cadmium battery, normalized to unit weight (kilograms) and size (liters), at various discharge rates and temperatures is summarized in Fig. 28.15. The curves are based on a capacity, at the C/5 discharge rate at 2(fC, of 30 Ah/kg and 85 Ah/L, reflecting the performance of standard-type sealed cylindrical batteries. Manufacturers should be contacted for performance characteristics of specific batteries. 

When displayed on the logarithmic time scale, the curves merge by shifting toward the x-axis. Here, the degree of shift represents the time compression, which can be described by the logarithm of the time shift factor log (a according to Eq. 1.36. This means that the service life curves are equidistant when the difference between the reciprocal, absolute temperatures is equal [104]. 

According to Figure 1.50, left, switchover points can be defined for the resulting service life curves at the individual temperatures. The aging mechanism and its... 

In practice, service life curves corresponding to two to four temperatures are determined. They provide the activation factors and slopes for the different aging mechanisms, as weii as the respective switchover points. They can be used to calculate service iife curves at operating temperatures. Figure 1.52. These curves provide the basis for correctiy determining reference time and activation factor [104]. 

ISO 9080 [114] describes a standard extrapolation method, abbreviated SEM, for predicting the service life creep behavior of pipes. The SEM method involves internal pressure tests at two or more temperatures, linear regression analysis with an estimation of regression quality, curve fitting and three different ways of expressing bend in hydrostatic pressure measurement. 

Figure 1.62 Service life characteristics as S-N curves under media influence and variable temperature fields for gear mating (PA 6)...

Appropriate use of the media iists [7] and of the recommendations for materiai reduction factors (see Section 1.6.3 ) can assure a iifetime of > 10 years for components under media influence. After successful proof of lifetime <= service life a binding agreement is reached for the supply of plastics or resin with a detailed media list, temperature data, creep curves, construction standards, etc., as well as the official disclosure of degradation data in company publications [649]. For example, for glass fiber reinforced sewer/pressurized sewage pipe, lifetimes between 10 years and 50 years can be selected. Figure 5.6. Specialty resin grades can be realized to adapt to most media. Media reduction factors Rj = 1.5 are recommended for service lives of 50 years. 

Determining the service life profile of an insulation system - in particular at various temperatures - is very time consuming, because at least 5 test specimens have to be used for each measurement point on the curve due to measurement value scatter. If the measurement points for one year can be connected by a straight line, this line can be extrapolated to longer time periods, e.g., to 10 years. 

Because creep strains are measured over much shorter durations than expected service lives, predictions are necessary to determine a long-term service life strain. This is accomplished by extending creep curves numerically or by time—temperature shifting. It is generally advised, and often regulated, to avoid extrapolation of creep curves more than a factor of 10. 

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