We found that the standard weld end geometry described in the various ASME codes do not adequately account for startup / shutdown thermal stresses on piping components that are used in cycling service; the fatigue damage mechanisms are not addressed in those situations. The problem has been espescially problematic when welding thin wall P91 piping ( operating at 1000-1120F) to a much thicker walled F22 valve, as with HP steam turbine stop valves. In particular the 30 degree permittted slope specified for the change in OD local to the weld line is inappropriate for such cycling service, and the use of a transition piece with a slope of the change in OD not greater than 18 degrees near the weld line generally may provide longer fatigue life. It was demonstrated on fast startup combined cycle power plants that the actual life of this critical weld was less than 18 months before failure if the default value of 30 degrees is used, as the change in wall thickness local to the weld line was in excess of 2:1. The creep fatigue life in such cases is additionally worsened due to the "metallurgical notch" effect of the "soft zone" of the P91 HAZ. If in doubt, conducting a transient 3d finite element model of the valve and piping under the max expected startup /shutdown rate can provide a ( scarey) estimate of the thermal stresses that are apparently not considered in the default code ( eg Progress Energy Richmond/Rowan/Effingham duplicate units had identical weld line cracking).
A related problem can be found when using flanges , as the standard B16.5 flanges ( originally designed circa 1930) are extremely heavy and were designed well before modern finite element methods were available to optimize the flange. It was found that when such flanges are replaced with "compact flanges" designed as per ASME sect VIII and using seal rings in lieu of gaskets are generally less than 50% of the B16.5 weight and are more amenable to the fast thermal transients that occur on cycling plants. Such flanges should also ensure the weld line is removed from the flange face as far as practical to ensure the weld line is not impacted by the thermal stresses generated at the flange face. The most extreme example of such use is on the retrofit upgrade of the Alstom GT24/gt26 family of combined cycle plants, wherein the critical disimilar metal weld at the HP once thru cooler (P91 to 316H DMW, 600 C/ 1112 F design temp) has been successfully replaced with compact flanges with seal rings and belleville washers.
The bottom line is that the legacy code rules work fine for base load plants and for low temperature service, but these old designs generally have no consideration for limiting fatigue damage that becomes the primary failure mechanism for fast startup cycling plants.
"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick
