Premature weld cracking had occurred in exhaust ductwork. The cracking occurred in corner weld joints and in stitch welds used to secure the shell to stiffening angles approximately 3 weeks after it was placed in service. OCE evaluated the quality of construction/ welds and the design of the exhaust ductwork. Finite element analysis (FEA) and engineering calculations were employed to evaluate the thermal stresses.
The exhaust ductwork is used to convey high-temperature exhaust from large diesel engines through the roof to the atmosphere during engine testing and qualification. The diesel exhaust temperature is approximately 1100°F when it enters the ductwork. The hot exhaust enters the ductwork vertically. It then immediately turns 90 degrees at the lower elbow and travels horizontally for approximately 10 feet before passing through a second elbow (upper elbow) which directs the exhaust upward. The exhaust then travels upward through two silencers before exiting to the atmosphere. The region of the stack exhibiting cracking in the welds is from the exhaust entrance through the two elbows and up to the first expansion joint.
In order to quantify the thermal stresses in the exhaust ductwork a finite element model was built. A partial model was built to represent the section of the exhaust stack where the temperatures were recorded.
It was determined that the cause of failure was the change of ductwork material from carbon steel – which has a thermal expansion coefficient that is approximately 30 percent less than 316L stainless steel. The thermal expansion coefficient is directly proportional to the thermal stress. This means that, given the same thermal conditions, the thermal stresses in the stainless steel ductwork would be 30 percent higher than the thermal stresses in identical carbon steel ductwork. Also, carbon steel has a thermal conductivity which is approximately 2.5 times greater than that for stainless steel at temperatures up to approximately 600 FO. Consequently, the temperature gradients through the stainless steel material would be greater than the thermal gradients through the carbon steel material under the same thermal conditions. This also results in higher thermal stresses in the stainless steel material.