T/P92 is being heralded as a superior and lower-cost alternative to T/P91 for new power plants with pressures above 3,600 psi and temperatures above 1,100F—such as the supercritical and ultra-supercritical units proposed to be built in the U.S. over the next few years. The switch from T/P91 to T/P92 would represent the next step in an evolution.
In the early 1990s, T/P91 began to replace T/P22 as the material of choice for high-energy piping in U.S. power plants. T/P91's higher allowable stress enabled engineers to design pipes with much thinner walls, saving plant owners a substantial amount of money in materials costs as well as reducing the bill for fabricating and installing the piping. And T/P91's higher creep-rupture strength allows piping made of it to survive higher temperatures and pressures than piping constructed of T/P22. T/P91 also has higher corrosion-oxidation resistance than T/P22.
The popularity of T/P91 grew so quickly that it became the preferred material for high-temperature steam systems (>1,000F) in a majority of U.S. combined-cycle power plants built in the 1990s. It is currently used as pipe material for main steam and hot reheat piping as well as for tube material in heat-recovery steam generators (HRSGs).
With the exception of the modulus of elasticity, T/P92's mechanical properties are superior to those of T/P91, especially at temperatures above 1,100F. Accordingly, the use of T/P92 instead of T/P91 in new coal plant design and construction should yield benefits similar to those produced by the switch from T/P22 to T/P91.
Coming to America
Following the successful use of T/P91 in the international power industry in the '90s, engineers began looking for a new material capable of handling the even more severe steam conditions (T>1,100F, P>3,630 psig) of supercritical and ultra-supercritical units in Japan and Europe. A major drawback of T/P91 is that its allowable stress falls precipitously at very high temperatures, to about 7,300 psi at temperatures above 1,100F. To withstand the extreme pressures of supercritical plants operating at such high temperatures, piping made of T/P91 would have to be at least several inches thick.
In Japan, the standard STBA29-STPA29 was developed for this purpose. In Europe, X10CrWMoVNb9-1 was developed as the European equivalent to STBA29-STPA29. In the U.S., the standard that T/P92 meets is Code Case 2179-3 of the American Society of Mechanical Engineers'(ASME's) Boiler and Pressure Vessel Code for Sections I and VIII, Division 1 construction. Whereas Japan and Europe have been using their equivalent of T/P92 since the 1990s, the U.S. is just now beginning to consider using T/P92 in steam systems of the next generation of coal-fired supercritical and ultra-supercritical power plants. T/P92 also is on the verge of replacing T/P91 in some HRSGs.
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