April 15, 2006

Why new U.S. supercritical units should consider T/P92 piping

—By P. Jason Dobson, PE, Cummins & Barnard Inc.

Will T/P92 replace T/P91

The answer to this question depends on your perspective, as the following three demonstrate.

Owner/engineer. From an owner/engineer's viewpoint, there are two factors that need to be considered when determining the type of piping to use on a project—especially the piping of high-energy systems such as main steam and hot reheat. The first is potential cost savings, and the second is delivery schedule (that is, which type of pipe can be delivered to the project site more quickly).

In terms of cost savings, the two key variables are pipe wall thickness and the required amount of pipe rack/support steel. Regarding the first, T/P92's higher allowable stress makes thinner pipes feasible, and that translates into lower materials costs. Because thinner pipes weigh less, using T/P92 in lieu of T/P91 also reduces the size (and costs) of required pipe hangers and support steel.

Although T/P92 costs more than T/P91, the former's superior mechanical and metallurgical properties lead to larger cost savings that far outweigh the capital cost differential.

The two (interrelated) factors that determine the relative availability of T/P92 and T/P91 are the number of steel mills producing the pipes and the size of the piping needed. For example, smaller-bore piping (<16 in.) is both more available and potentially has a shorter delivery time to the fabrication shop than larger piping. If seamless pipe is required for the project, its availability may be affected by the number of extruders each steel mill has as well as by the number of mills available with extruding capability. Due to the popularity of T/P91, delivery times for large-bore piping (>16 in.) are typically one year or longer.

Fabricator. From a fabricator's viewpoint, the two key factors in the choice of T/P92 or T/P91 have to do with welding and heat-treating considerations.

As mentioned earlier, because the welding processes used for T/P91 are directly applicable to the welding of T/P92, fabricators should not have to negotiate much of a learning curve. However, they may find it difficult to procure suitable welding material for the newer steel. In Europe, there's no shortage of companies producing consumables for most popular welding processes. But in the U.S., there currently is no classification in American Welding Society (AWS) codes for T/P92 base materials for stick-welding purposes.

The thinner pipe walls that T/P92 makes possible enable fabricators to reduce their preheat and PWHT costs as well as the costs of required weld filler materials. Thinner materials require shorter PWHT holding times, fewer weld passes, less filler material per weld, and less energy to perform heat treatment. Thinner pipes also minimize the potential for problems such as hydrogen-induced cracking in DMWs. Together, these factors can produce significant cost savings and justify the choice of T/P92.

Field erector. From a field erector's viewpoint, there is little significant difference between the erection of T/P91 and T/P92. Because of the similar metallurgy, the welding and heat-treatment processes will be virtually identical.

In terms of welding, any welder qualified to weld T/P91 should be able to qualify to weld T/P92 with little difficulty. As during fabrication, additional cost savings (involving welding preheat, PWHT, and filler metal) may be realizable during field erection of thinner-walled T/P92 pipe.

A bright future

As T/P92 begins to make inroads in the U.S. its long-term success will require the following:

• Continued research on creep-rupture strength and fatigue life for T/P92 materials.
• Continued monitoring of the failures of T/P91 to determine whether any failure mechanisms are applicable to similar T/P92 applications.
• Careful monitoring of the failures of T/P92-equivalent piping in European and Japanese power plants—especially those involving dissimilar-metal welds.
• Development of proper filler materials for welding T/P92, both to itself and to dissimilar metals.

—P. Jason Dobson, PE, is a mechanical engineer with the consultancy Cummins & Barnard Inc. (Ann Arbor, Mich.). He can be reached at 734-332-6409 or PDobson@cummins-barnard.com.