Every radiant tube is designed against a rupture life, not a wall-loss allowance. Creep is what spends it, quietly, whether or not the thickness gauge notices. Why it matters Every fired heater radiant tube is sized against two numbers at once: an internal-pressure hoop-stress requirement, and a creep-rupture life at its design metal temperature, per API Standard 530. The pressure calculation gets checked at every turnaround with a thickness gauge. The creep calculation gets