Deformation of welded-tube walls of once-through boilers is common. Although factors such as expansion restraints can cause permanent deformation, and flue gas pressure (in pressurised furnaces which is the case for subject boiler) can cause temporary deformation in operation, however one of the main reasons for deformation is temperature differences between adjacent tubes caused by unequal cooling medium and different heat input to tubes. Designers have taken several steps to avoid tube peak metal temperature and also temperature difference between adjacent tubes within permissible limits to prevent tube overheating and excessive tube panel deformation which can cause damage to fins, fin-to-tube weld joints and some times to tubes. Employing spirally wound smooth tubes and in recent years vertically-ribbed tubes for furnace of once-through boilers are major steps in this regard [1, 2]. It is also a known fact that in once-through boilers furnace cooling fluid may pulsate [3]. This phenomenon usually happens below certain loads called Benson load that is approximately 35% of MCR. Flow pulsation can cause tube overheating, thermal fatigue damage and temperature differences between adjacent tubes. Samples of fluctuation of middle wall temperature of heated sides of furnace floor tube caused by flow pulsation are shown in Figure 1. It is worth mentioning that in the boilers to them Figure 1 belonges extensive damage to furnace floor tubes were experienced and problem was finally solved by covering floor by refractory bricks.
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