Refractory-Lined Pipe Failure Analysis

In lines for high-Temperature fluid solids (e.g., in fluid catalytic cracking units), it is sometimes necessary to calculate the metal temperature with an internal, insulating refractory lining. In this case, the metal temperature may be significantly less than the fluid temperature. The refractory lining resists erosion while at the same time permitting use of carbon steel with a fluid that would otherwise be prohibitively hot for its use. Inspection of these lines are difficult due to lack of access to internal part of the line or if needed, the pipe has to be out of service which is not possible all the time. In this article RCA of failed refractory-lined pipe is briefly introduced.

In SRU (Sulfur Recovery Unit) of a gas refinery, there was a failed 20" pipe of A335-P11 material with 2 bar internal pressure of acid gas. Cracks were in both longitudinal and circumferential direction of pipe. Thermography was used to determine the temperature gradient of pipe in different locations after insulation was removed, so damages of refractory and metal thinning of pipe will appear in lighter points. Temperatures are in Fahrenheit and the probable damage mechanism are described below.

Sometimes, more that just one failure mechanism is active.

• As pipe is working in more than 350 degrees centigrade, and material is Cr-Mo, temper embrittlement is one of the probable damage mechanisms.
• Circumferential cracking in elbow-to-pipe weld, could be the result of SCC, due to lack of PWHT of elbow-to-pipe weld.
• If we take a look at the support under elbow, we see that temperature gradient is substantially high as the color is blue. It seems that thermal fatigue is also active here which is the 3rd damage mechanism. It has to be noted that insulation on support would reduce the effect of thermal fatigue in such a situations.
• And last, but not the least, lack of proper refractory inside the pipe, could make acid gas touch the metal and corrosion would happen especially when condensation of moisture exists. Signs of wall thinning was observed after removal of pipe.

Sometimes more than just one failure mechanism is active and the RCA could be more through if fractography (e.g. metallography shows type of cracking that could be intergranular or transgranular ) of the pipe and weld material would have been done.