Materials considerations for equipment seeing increased cyclic duty

Steve Gressler followed Mastronarde at the podium and focused on the subject that the boiler designer had introduced a few minutes earlier: materials-life considerations for cycling plants. Gressler, a metallurgist, opened his presentation with these three thoughts regarding equipment lifetime management to assure personnel safety and optimal reliability:

• Materials and equipment have finite lives that vary with application and local conditions.
• More rigorous service duty—cycling, for example—accelerates damage rates.
• Fabrication defects and design shortcomings that were benign and tolerable under steady-state conditions increase uncertainty and risk under more demanding cyclic duty.

What owner/operators basically want to know, Gressler said, are the answers to these two questions: (1) Where are the highest risk locations on critical equipment? (2) When will failure occur?

He recommended an iterative process based on well-established phased methodology to get the answers. First identify weak links and their contributing factors. Then selectively progress through more quantitative analyses. Finally, integrate multiple disciplines—such as materials, NDE (nondestructive examination), analysis, monitoring, instrumentation, data management, and economics.

Critical to success, Gressler continued, are an early start (gather knowledge of current condition to serve as a baseline for comparison), consistency, and an increase in prediction accuracy when the expected time of failure nears. To accomplish the last goal, you must know what constitutes “failure” (crack, distortion, risk level, etc), what the active failure mechanisms are, and what the rate of damage is.

The materials expert told the group that damage can be caused by an independent mechanism, by several in unison, or by several having compounding effects, and then reviewed the failure mechanisms of greatest interest to personnel at generating plants:

• Fatigue is a progressive damage mechanism that develops over time because of repetitive and fluctuating thermal and/or mechanical loading. Extent of damage depends on the number of cycles, local stress, and temperature.
• Creep is a progressive damage mechanism that develops over time because of the sustained application of stress at high temperature (over 800F). Extent of damage depends on time, temperature, stress, and material. To illustrate: A 16% increase in stress (from 6 to 7 ksi) halves the expected lifetime; a 4% increase in metal temperature from 1000F to 1040F reduces material life by a factor of four.
• Creep-fatigue is of concern because the interaction of the two damage mechanisms can reduce material life to 20% of that predicted independently.
• Corrosion-fatigue, sometimes called corrosion-assisted fatigue, is characterized by crack initiation from fatigue and the acceleration of crack growth by corrosion and oxidation.
• Flow-accelerated corrosion, known simply as FAC, is the thinning of metal by dissolution of the protective oxide layer under certain chemical and flow conditions. It is found most often in boilers (cold-end heat transfer bundles), air-cooled condensers, and condensate systems.
• Over-stress conditions result from unintended movement or loading—for example, from water hammer, bending, etc.

To reduce the uncertainty in your predictions of remaining materials life, it’s important to know the current condition of your equipment. Accurate documentation is critical to this effort—specifically material specifications and fabrication and installation records.

Sounds simple, but many combined-cycle plants—particularly those built during the bubble years 1999-2004—are missing much of their important paperwork.

If that has occurred at your facility, it’s important to conduct the appropriate inspections and compile the needed information. You may find that the requisite materials identification tests reveal the materials specified in construction documents are not the ones installed and achieving the expected equipment lifetime may not be possible—even with flawless operation.

Example: Gressler and his colleagues at Structural Integrity Associates Inc found inappropriate weld material and heat treatment procedures while investigating the condition of high-energy piping systems at the New Harquahala Generating Co LLC, Tonopah, Ariz. Extensive work was required to correct deficiencies in the main and hot-reheat steam systems. The case history presented in the 2Q/2010 issue of the COMBINED CYCLE Journal, “P91 commands respect,” should be required reading for all plant supervisory personnel.

Once true baseline conditions have been established, it’s important to continuously monitor plant operations for such anomalies as temperature and pressure excursions, fast ramp, fast startup, turbine trips, etc. Only with this information is it possible to calculate remaining life accurately.

Recall from your gas-turbine experience how OEMs determine when engine inspections are necessary and when critical parts must be repaired/replaced. The same methodology must be applied to other equipment—piping, valves, high-temperature/high-pressure pumps, heat-recovery steam generators (HRSGs), etc.

To better understand the impacts of poor operating practices on equipment life, review the experience of Southern Company Generation with a new software tool designed to track the life remaining in critical HRSG parts. Access www.ccj-online.com/archives, click 3Q/2009, click “New software . . . .”on the cover.