Critical Uncertainties and Probabilities: Heat Impact on Exhaust System Components—A Case Study

This paper addresses an issue with gas turbine manifolds. The manifolds were developing cracks once the exhaust gas output properties were changed. Exhaust gas velocities and temperatures have increased as the gas turbine efficiency increased putting thermal strain into the system. One of the other effects of the increase in gas turbine flow was the introduction of flow induced vibrations into the system. The source of the vibration will not be addressed in this paper and will be investigated in the future. Data for this case study was gathered at various gas turbine sites throughout the United States, either first hand or by representation. Site photos were taken, field reports were written, and a field fix was developed by a gas turbine system component Original Equipment Manufacturer (OEM). This fix has been working and the OEM had asked for analytical data to determine its reliability. Principle used to determine Critical Uncertainties and Probabilities (CUP) on the system were engineering Computation Tools, Algorithms, Methodologies, and Engineering Interpretation (CAME). The principles of CUP and CAME are being further developed my Mr. Ciras and Prof. Mustapha S. Fofana at Worcester Polytechnic Institute in Worcester, MA. The principles of six sigma, axiomatic design, and lean manufacturing were used as techniques to evaluate and valuate the most suitable solution to this problem. For this case study, the entire system around the fault area was looked at and analyzed. The system is composed of the turbine inlet, the gas turbine and its mounting system, the exhaust manifold, the exhaust diffuser, and the Heat Recovery Steam Generators (HRSG) along with interacting components within the system. CUP was use on each system component to determine with certainty what was causing the failures to prove the reliability of the field fix established by the OEM. The solution to this issue was established by suggesting proper manufacturing techniques; such as welding, material selection, allowance for thermal growth, proper installation of the external installation, proper fastening methods for the external installation and suggestions for improvements in the other system components. The proof was completed and the reliability of the fix relies heavily on the interaction of the system components in the area of the manifold.