11CrCb vs. 409SS or 304SS.
Seal Welding vs. Skip Welding
The eventual FATIGUE cracking all Gas Turbine Silencer Panels experience is amplified by thermal movement. So, doesn't it stand to reason that if the panel surface remains flat and the thermal movement is consistent along the frame/skin connection, the fatigue cracks will take longer to form?
We are convinced by traditional engineering practices and real life detailed inspections, that solid welded perforated sheets out live all other designs period.
Question: How does Schock design silencers to extend life and combat failure modes?
Answer: Schock designs silencer panels with the aid of an FEA Analysis, as well as applying industry experience. There are no one-size fits all. We analyze our silencer panel frames and designs by using your plant GT start up and shut down information. We are interested in how your GT starts up and shuts down (time vs. temperature curves).
It is through these conditions from ambient temperature to run temperature, and conversely that all silencer frame materials are going through the thermal growth cycles. These cycles are where fatigue cracks will begin forming over time.
The design truly boils down to designing so the cross-section frame & outer Perforated materials act and thermally move as one unit. In other words, we do not want the weld filler (or lack of in skip welds) to move any differently than the base materials. Failure to follow this rule causes thermal cracks to form pre-maturely.
These thermal movement variants are enhanced by improper material type and thickness selections. Therefore, the FEA analysis predicts the amount of lifetime thermal cycles the design can support BEFORE cracks begin. As we run these different designs, Schock Engineering adjusts the parameters of the model and re-runs until we get the maximum desired Cycles based on the site GT data.
The physical movement failures are caused by the Exhaust Panels having too much thermal growth room during startup/shut down conditions. All silencers require some sort of panel guide to keep them in position, and some sort of support to carry the load to the stack base. Our proven Stack Support brackets and guides are over-designed to thermally move, carry minimum heat to the casing plate, and keep the silencer panels in position for decades. Additionally, Schock ties silencer panels together in groups of 2-3 to further limit the movement from high velocities during running conditions.
Question: Is protecting or “tucking” the leading edge of the silencer panel perforated material necessary?
Answer: NO. Some designers claim by “tucking” the leading edge under the frame materials, it is protected from the high gas velocities. Firstly, the Schock leading edge is seal welded, it is not exposed to “flow” under the sheet. Secondly, the ideal gas velocity through the silencer sections is 180/ft. per Second or approximately 122 Mile per hour wind. It is not possible for this wind speed to bend or alter 11 gauge (0.125” thick materials). Schock uses a multi ton break press to form 11-gauge materials.
Question: Is 11CrCb material a better fit for F class Exhaust Gas temperatures.
Answer: YES. 11CrCb (details below) has a much longer creep life that 409SS, but not quite as good as 304SS. HOWEVER, 304SS thermally grows almost twice as much as 11CrCb and 409SS so this drastically different thermal growth rate for 304SS induces additional stresses into the materials and weld through cycling. 11CrCb was specifically developed for the Gas Turbine and Automobile Exhaust Market.
Question: Is continuous welding of the perforated seams better than skip welding.
Answer: YES. Beyond the reasons mentioned above, by nature of application, the skip or stich welding is performed manually by a human hand. Because of the application method, we have seen little consistency in weld bead shape, penetration, angle, etc. The Schock application method utilizes a bug-0 track system that is robotically applied while a qualified welder oversees during the operation.