118
The 118 model is derived from an old P2 general assembly drawing. It is the original model that demonstrated the software's ability to model a complete boiler. The model is not accurate as there is quite some detail missing in the GA. This led to a fair amount of guesswork in the model and some of the high levels of stress show that the guesswork could be improved upon. For example, the full length longitudinal stays are not in the optimum position and their flexibility compared to the barrel and wrapper means they do not support the top half of the backhead adequately.
The boiler has a copper fire box. A second variant with a steel fire box was produced to see if the use of copper helped stay stresses. Results for both copper ( B118 ) and steel ( B118SFB ) are shown in the stay plots below.
The picture below compares the 118 model with and without the throat/combustion stays. Their removal is detrimental to stress levels locally and at some distance from the throat/combustion intersection. It is not clear from the modelling work undertaken that their absence is a good thing.
Removal of the stays reduces the fire box tubeplate stresses but at the expense of very high stresses in the throatplate.
The stay stress plot shows the results of the original model and a variant ( B118NTS ) of it with the stays at the junction of the throat plate and combustion chamber removed to match the welded equivalent. One feature of note is that all the stay stresses are reasonable in the model with stays in this area but considerably higher if the stays are removed. Stay pitch is less than any of the subsequent models but it is not clear that this alone is responsible for generally lower stresses.
