All tube support castings are subject to warping and breakage. The frequency of failure depends upon the firebox temperature and the design. Figure 400-4 shows a typical tube support design. A good wall or roof hanger design should:
• Provide for free expansion of the tube supports. If it is connected to the structural steel at more than one point, one support foot must have a slotted connection to permit movement.
• Provide restraint so that tubes cannot slip off the support. Tubes bow toward the fire because they are hotter on that side, and have in some cases slid off and dropped free of the supports.
• Be arranged so that individual radiant section castings can be removed for repairs and be replaced without removing tubes. This is important. Convection bank castings should be designed in sections to minimize the number of tubes that have to be removed for casting replacement.
• Be ribbed and have sufficient cross-section to resist the side-wise stresses induced by thermal expansion of the tubes.
• Be thick enough to obtain good sound castings, no thinner than 5/8 inch.
• Have sufficient resistance to impact so that they will not fracture when tubes shift suddenly during heating or cooling.
• Have good quality castings to reduce warping and breakage.
• Allow removal of the tubes without requiring removal of the supports.