For chemical vessels operating above creep temperature, the criteria for failure are different from those for general pressure vessels, usually considering two forms of failure:

The first type is due to excessive deformation caused by creep, resulting in component failure. For this, the allowable deformation amount should be set within the design life range;

The second type is due to the material reaching creep fracture life and cracking under constant stress. Therefore, the corresponding durability strength value should be found based on the design life of the container, and the working stress should be limited to be less than the durability strength.

（1） Conventional Design of High Temperature Pressure Vessels

The conventional design approach is to limit the maximum principal stress of important parts. In China's regulations, methods for setting allowable stress at high temperatures are specified for the two types of failure modes mentioned above.

(1) The deformation criterion is based on the limit that the strain does not exceed 1% (0.01) after 100000 hours. The strain rate found from the creep data is:

The stress at time is called the creep strength of the material, with the code s10-7. The temperature of creep test data should be the same as the working temperature of the container.

At this point, the allowable stress is:In the formula, the safety factor nn=1.0.

(2) The fracture criterion is based on a durability strength of 100000 hours, ensuring that the material does not fracture after working for 105 hours. If the endurance strength of LO5h is designated as s105, the allowable stress is:

In the formula, the safety factor nD is taken as 1.5 to 1.6. The allowable stress for actual design is taken as the smaller of the two.

It can be discussed that for pressure vessels and pipelines, a slight deformation does not pose much obstacle; Moreover, the deformation limit of l% seems to be too strict.

Therefore, in recent years, British regulations have abolished the design based on deformation criteria. In some countries, the regulations have not been abolished, simply due to the retention of traditional practices.

It is widely recognized around the world that the real danger for high-temperature pressure vessels is creep fracture.

On the other hand, a design lifespan of 105 hours may not be sufficient for some important devices. Modern chemical, petroleum, power and other equipment tend to be larger and larger in size, with high equipment costs and long service life requirements.

Many countries' boiler industries have set the design life as 20 years or 2X105h, and nuclear power plants tend to be higher. The problem brought by this trend is the lack of creep data, as conducting creep and endurance tests is expensive and time-consuming, and the reliability of data extrapolation methods still requires a certain amount of real-time data to verify.

The accumulation of creep data requires extensive international cooperation, which has been ongoing for several years.

（2） Thoughts on Analysis and Design of High Temperature Pressure Vessels

The American standard ASME VIII-1 is a conventional design, which includes the design of high-temperature pressure vessels. The specific treatment is to use the creep limit or endurance strength as the reference when the working temperature exceeds the creep temperature under the allowable stress of the given material.

ASME VIII-2 is an analytical design that specifies in its scope of application that it does not involve pressure vessels with creep, and is limited to below creep temperature when given allowable stress.

ASME III is the same as ASME VIII-2, but in practical applications, there is a problem that some nuclear reactors operate above creep temperature. For this reason, ASME compiled a "Code Case N-47" in the late 1970s as the basis for the analysis and design of high-temperature pressure vessels, and expected it to be included in the main text of ASME III after several years of use.

N-47 is also applicable to non nuclear pressure vessels. Strictly speaking, modern large-scale hydrogenation reactors in the petrochemical and coal chemical industries should also be considered as major equipment designed according to analysis,

Analyzing and designing high-temperature pressure vessels involves many theoretical and practical experience issues, as well as more comprehensive material performance testing problems. Here are only some important considerations mentioned:

(1) The creep calculation method must take into account that the container components are always in a state of multi-directional stress, and the basic data for creep is obtained under uniaxial tension [ec=f (T, s, t)].

——How to choose the appropriate equivalent rheological stress to make it possible to utilize basic data is the first problem to be solved in stress analysis.

——Secondly, the multi-directional stress in actual components often varies along the thickness or is unevenly distributed. Over time, the creep strain at each point is different, leading to "creep stress redistribution".

——In short, it is not only necessary to study the initial stress distribution, but also to analyze the stress history.

(2) The estimation of creep fracture life under multi-directional stress, how to utilize uniaxial tensile stress life data, is an unsolved problem in scientific research, and designers must choose a reasonable assumption. Furthermore, as mentioned in the previous paragraph, the influence of stress history on lifespan should be considered, that is, an appropriate cumulative lifespan law should be adopted.

(3) The interaction between creep and fatigue must be considered when designing high-temperature pressure vessels for fatigue. This effect varies for different materials and is related to factors such as stress level, stress amplitude, and strain rate. At this point, sufficient experimental data is needed, and consideration should be given to how the unidirectional tensile and compressive specimens are related to the multi-directional stress states of the actual structure.

(4) Relaxation analysis should be conducted on the bolted connections of high-temperature containers. At the initial installation, the stress and strain are constant, and this strain is elastic strain. During work, the total strain remains constant. Due to creep, creep strain gradually replaces elastic strain, resulting in a decrease in stress in the bolt. Measures should be taken to keep the seal tight or estimate the time interval for regular tightening of bolts.

The above briefly mentioned some projects that should be considered, and truly conducting in-depth thematic learning and research is required for analysis and design.