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Elevating the Pressure and Temperature PUBLIC ACCESS

Mechanical Engineering 136(11), 38-39 (Nov 01, 2014) (2 pages) Paper No: ME-14-NOV4; doi: 10.1115/1.2014-Nov-4

This article discusses the modernization of the rules of Section I of the ASME Boiler and Pressure Vessel Code to better accommodate the challenges of increasing temperature. At very high pressures and high temperatures, the current Section I rules require components to be comparatively thick, but making things thicker is not always better. In thick components, temperature gradients and consequent differential thermal expansion produce large secondary stresses. When pressure and temperature drive a component’s thickness to be very large compared to the size of the component, it can compromise that component’s ability to endure thermal transients that occur in service. One of the biggest challenges in addressing elevated temperature service is understanding creep and fatigue interaction and developing appropriate design rules to manage that. Another challenge is that corrosion mechanisms change with increasing temperature. The push to higher temperatures will spawn development of new materials to meet all the design goals. The BPVI standards committee on Power Boilers will also need to evaluate whether some of the construction details traditionally used will be appropriate at higher temperatures.

Codes and standards have a twofold purpose. The paramount reason for standards is to assure that equipment is safe–safe for people who work around it and safe for the general public. The second main reason is to promote commerce by achieving uniformity, interchangeability, or at least compatibility. Within that backdrop, standards should also strive to assure that equipment can be used for economic benefit.

In the process of give and take that produces standards, each stakeholder not only presents his concerns, but also shares his expertise. Products that meet the Code are not only safe as long as they are built, maintained, and operated according to appropriate standards, but they are also expected to provide a reasonably long life over which they can be reliably operated for economic benefit.

As an employee of an electrical generation company, I am interested in technologies that will make units more efficient. By burning less fuel, my company saves money, and it is easier for us to comply with new emission regulations and remain competitive. Ultra-supercritical steam generators are a current trend toward greater efficiency, promising to push to ever higher temperatures and pressures than today’s supercritical units.

Supercritical steam generators aren’t boilers in the strict sense. Water heated above critical pressure of 3,208 psi (221 bar) does not boil, per se. Since the pressure in the furnace walls is above the vapor dome of the water-steam system, there is no phase change from water to steam. Unlike in a subcritical drum-type boiler, there is no point in the cycle where evaporation occurs at constant temperature; rather each unit of thermal input raises the fluid temperature. Final temperatures of the fluid to the turbine rarely exceed 1,100 °F on existing units.

For tomorrow’s units, the industry will push for higher temperatures, since thermal efficiency is limited by the peak temperature of the cycle. Higher efficiency means less fuel for the work output, saving money and reducing emissions, especially of carbon dioxide.

Nuclear Beginnings 1955

The Boiler and Pressure Vessel Committee appoints a special committee on nuclear power, the predecessor to the Boiler and Pressure Vessel Code Section III Committee.

David L. Berger: New Challenges involving creep, fatigue, and corrosion.

Grahic Jump LocationDavid L. Berger: New Challenges involving creep, fatigue, and corrosion.

I Chair the BPVI standards committee on Power Boilers (Section I of the ASME Boiler and Pressure Vessel Code). In that capacity, I charged a task group to investigate modernization of Section I’s rules to better accommodate the challenges of increasing temperature.

There can be no question about the safety of ultra-supercritical steam generators under the ASME Boiler and Pressure Vessel Code. Safety is never a negotiable goal.

At very high pressures and high temperatures, current Section I rules require components to be comparatively thick, but making things thicker is not always better. In thick components, temperature gradients and consequent differential thermal expansion produce large secondary stresses. When pressure and temperature drive a component’s thickness to be very large compared to the size of the component, it can compromise that component’s ability to endure thermal transients that occur in service.

Repeated cycles of heating and cooling can thermally fatigue components. And those cycles are certainly a reasonable expectation because tube leaks, air heater washes, and other events require a plant to shut down for repair or replacement of parts. Many of the Section I devices built for domestic power generation over the last two decades are heat recovery steam generators (HRSGs), essentially waste heat boilers that use the heat from the exhaust gas of a combustion turbine in combined cycle plants. Such units are characterized by fast startup and shutdown, plus a large turndown to follow demand, consequently their components endure a lot of thermal fluctuation.

One of the biggest challenges in addressing elevated temperature service will be understanding creep and fatigue interaction and developing appropriate design rules to manage that. Another challenge is that corrosion mechanisms change with increasing temperature. The push to higher temperatures will spawn development of new materials to meet all the design goals. The committee will also need to evaluate whether some of the construction details traditionally used will be appropriate at higher temperatures.

As a person who has many friends who work in operating power plants on a daily basis, I am very concerned with “keeping the steam in the pipe.” ASME and other standards are one source of help to that end.

Participating in Codes and Standards development helps me in many ways. It helps me understand how to apply the rules properly. I learn the rationale for new rules. I interject my thoughts as a user and have a voice to shape the rules with a focus on what happens to the equipment after it is built

Above all, my colleagues and I better protect our people working in the plants.

Copyright © 2014 by ASME
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