Written by Samir Baydoun | Power Plants

In high-temperature turbine exhaust environments, material selection is often the difference between equipment that lasts decades and equipment that fails within years. Thermal fatigue, creep rupture, and structural degradation occur when materials are used that are not specifically engineered for elevated temperatures and thermal fatigue. Field failures and plant safety concerns continue to underscore the importance of selecting the appropriate alloys for silencer components, particularly baffles.

Recognizing the Risk of Material Failure

Cracking, component collapse, and safety hazards are common in poorly specified silencers. Within two to three years, the silencer begins to crack, break apart, and fall apart. This has led to safety risks in some cases, some of which cause safety issues and eject parts to the outside.

These failures are often traced back to improper material selection in the baffle construction. We receive many calls from power plants saying, “I have parts of the silencer ejected. What can we do?”

Understanding Creep and Fatigue Behavior

The driving mechanisms behind these failures include creep rupture and thermal fatigue. We base the material selection on its ability to withstand the creep rupture at elevated temperatures. Creep is governed by three primary factors: load, temperature, and time. For example, carbon steel becomes susceptible to creep rupture at temperatures above 750 degrees Fahrenheit, while for stainless steels 304 and 316, the threshold is 1050 degrees Fahrenheit.

Failure can develop subtly but persistently. If you load it at over 1050 degrees Fahrenheit for a short time, it is within the proper strength range, but after a prolonged period (over 10,000 hours), the creep effect will begin to produce high strain and distress.

Creep rupture not only compounds the issue, but we also analyze thermal fatigue during startup and shutdown, ensuring that material stress under the design loads remains within the allowable stress listed in ASME II Part D at the design temperature.

Choosing the Right Material for the Job

Three primary materials are commonly used in silencer design, each with different thermal and mechanical properties:

  1. 40930 (Ferritic Stainless Steel): A common choice for moderate design temperatures, such as 850°F. However, with elevated temperatures (such as 1250°F), the allowable stress decreases to 1.28 ksi after 100,000 hours of exposure to this temperature.
  2. 11 Cr-Cb (40955): Also ferritic stainless steel and used in car mufflers before being adapted for power plants. While slightly stronger than 40930, it is not the proper material for elevated-temperature applications, such as those requiring temperatures of 1250°F. Additionally, it is not listed in the ASME code, and it is prone to cracking in cold weather due to a ductile-to-brittle transition temperature that is near room temperature.
  3. 304 Stainless Steel (Austenitic) is the premium option, especially for high temperatures such as 1250°F. It is an ASME code material with allowable stress 4.7 ksi after 100,000 hours of operation. This allowable stress is approximately three times better than that of the 40930 and the 11 Cr-Cb. Its higher chrome content and austenitic structure offer superior long-term performance under thermal cycling.

For applications with temperatures exceeding 1290°F, additional factors such as intergranular corrosion and sigma phase embrittlement must also be addressed. We select material with titanium or niobium as a stabilizing element to prevent intergranular corrosion.

Avoiding Repeat Failures

One case involved a failed silencer using 11 Cr-Cb material: The baffle installed in that project broke on the ground. Although cost-effective, materials like these can fail under repeated thermal cycling. Investing in higher-grade materials can significantly improve long-term reliability and reduce safety and maintenance risks. It’s expensive, but sometimes it is worth the additional cost.

 

Power plant operators can avoid premature silencer failure and enhance safety by selecting materials engineered for high temperatures, stress, and cycling. Properly analyzing creep resistance, fatigue life, and corrosion behavior ensures that the right material is chosen, ensuring silencers perform reliably for years without cracking, falling apart, or posing a hazard to plant operations.