All energy plant gear has a restricted lifetime, however not all parts age on the identical fee. Some gear might outlast the helpful lifetime of a plant, whereas different equipment could also be changed greater than as soon as by means of the years. Boiler parts supply a living proof. Typically, tubes uncovered to greater temperatures and extra excessive stress degrade sooner. Understanding the mechanisms and figuring out the place to search for early warning indicators might enable repairs to be made previous to failure.
If water chemistry management has been good, economizers normally last more than radiant superheaters (SH) or reheaters (RH). The degradation course of is brought on by publicity to temperatures excessive sufficient for the supplies of building to function within the creep vary, for thermal fatigue to be a big degradation mechanism, and, after all, for microstructural adjustments to develop.
For the needs of this text, no wastage or wall thinning shall be thought-about, that’s, ignore hearth gas ash corrosion, fly ash or sootblower erosion, and water/steam oxidation and corrosion. Wall thickness surveys will usually discover these varieties of issues and tube replacements might be made as wanted.
Concurrent with microstructural adjustments are decreases in hardness, energy, and ductility. Those adjustments embrace spheroidization of carbides in chromium-molybdenum (Cr-Mo) steels, graphitization in carbon (C) and C-Mo ferritic steels, and sigma section formation and sensitization in austenitic stainless steels.
For high-temperature SH and RH outlet headers, creep and thermal fatigue work together in a singular style, usually referred to as creep fatigue, on the stub tube to header welds towards the ends of the headers. For headers with steam temperatures beneath the creep vary, “easy” thermal fatigue might develop at these places. Differential enlargement between the warmer header and cooler waterwalls results in a deflection of the “stub” tubes between the waterwalls and header. The “bending” or deflection is best on the ends of the header, assuming the enlargement is symmetrical concerning the midpoint in size.
Creep Fatigue and Thermal Fatigue
The type of injury, creep fatigue or thermal fatigue, will depend on the temperature of the person stub tube. Microstructural evaluation of the cracks is normally needed to determine the trigger. Not all tubes function on the common steam temperature within the header. For particular person tubes in an intermediate header, a steam temperature of 850F could also be excessive sufficient for SA-213 T2 stub tubes to be within the creep vary regardless that at 850F T2 isn’t normally anticipated to fail by creep at code-allowable stress ranges.
Estimates of the stresses imposed by differential enlargement on the stub tube welds at or close to the header could also be calculated from easy beam principle. Assume the load on a versatile stub tube is some extent on the waterwall/roof penetration, as proven in Figure 1.
|1. Stub tube instance. Stresses imposed by differential enlargement could also be calculated from easy beam principle. Courtesy: David N. French Metallurgists|
The deflection is given by:
the place δ is the deflection brought on by the differential enlargement (in.), l is the size of the stub tube between the header and waterwall (in.), E is Young’s Modulus (about 22 x 106 psi at 1,000F), I is the Moment of Inertia (infour) and for pipes and tubes is given by π / 64 x (exterior diameterfour – inside diameterfour), and P is the load essential to trigger the deflection (lb in easy bending).
The bending stress on the floor is given by:
the place S is the utmost stress within the outer fiber (psi), M is the bending second (in-lb) and is the same as P x l (load x size), c is the gap from the impartial axis to the floor (in.), and I is the second of inertia (infour).
What is to be calculated is the stress, S, from the deflection on the cracked tubes close to the ends of the header. The two equations might be solved for P as…