18/02/2024
🔺 Fired Heater
Fired heaters use controlled combustion of fuel to heat a process fluid. The heat of combustion is transferred to the process fluid in the radiant section.
While any type of process involving combustion carries hazards and safety concerns, the hazards are even greater when a combustible process fluid is heated inside tubes. Hence, fires and explosions are significant risks of fired heater operation.
🔷 Overheating: Tube failure occurs when the tube metal overheats due to loss of fluid flow, or hard internal/external scaling or fouling. Coke formation can occur when there is uneven flow in multipass heaters, improper two-phase flow regime operation at a localized spot in the tube during turndown, or flame impingement for an extended period. Any localized over heating dramatically increases the temperature of the tube, causing the temperature of the metal to exceed its design temperature. Prolonged operation at such high temperatures or frequent and rapid temperature changes can cause thermal fatigue of the tube, affecting its mechanical strength and integrity.
🔷 Corrosion: Unusual operating conditions (napthenic acid, corrosive sulfur, caustic carryover, etc.) can accelerate internal corrosion rates within the tubes, which reduces effective tube thickness. Corrosive waste fuel streams from burner leaks or waste fuel in the firebox can cause external corrosion when they come in contact with process tubes. External corrosion can occur in the convection section on the outside of tubes or downstream in the air preheater when the temperature is lower than the dewpoint of the corrosive fluegases.
Erosion: High fluid velocity or impingement of solid particles on tubes can erode the tube metal. Erosion effects are more prominent in v***r applications (bubbles in a liquid) or where restrictions in fluid flow exist (elbow or return bend). Erosion may also remove any protective passive layer of the tube, which can accelerate corrosion.
🔷 Restricted Thermal Expansion: Fired heaters are generally designed with adequate clearance between tubes and the refractory wall to accommodate longitudinal thermal expansion of the tubes, based on tube material and its design tube metal temperature. If the tube expansion is restricted, the tube can expand unevenly, causing it to sag, bow, etc. In extreme conditions, restricted expansion can cause tubes to crack and fail. If the tube material is changed from its original design, the new material’s thermal expansion should be carefully considered. For example, stainless steel tubes have a much higher coefficient of thermal expansion than carbon steel.
Source:https://lnkd.in/dTfCQZ3Y
