Just because you don’t have the flame hitting your tubes you still have about 1700 F in the firebox. For what ever reason you start building coke at a spot, you will start heating this area to a point where you will or can start creep. This will start when you heat this area beyond its yield temperature. You should see hotspots in these areas. This does not happen in a short time. It takes heating in these areas some time to damage these tubes.