Similar to establishing the minimum allowable thickness for tubes, the metal temperature of the fittings must be established so that the appropriate allowable design stress of the material can be used. Generally, if the fitting is outside the firebox the fitting temperature is considered to be the same as the temperature of the fluid flowing through it plus 55°F (30°C). The metal temperature of a fitting inside the firebox is considered to be the same as that of the corresponding tubes. The allowable working stress value for fittings is determined in the same way as it is for tubes. Minimum allowable thickness can be determined from applying calculations from the appropriate ASME piping codes. Because of stresses that may be set up by closing and holding members and by thermal expansion, the calculated allowable thickness may be too small to be practical. As with tubes, it may be advisable to add some thickness, based on judgment and experience, when setting the minimum thickness at which a heater fitting should be replaced.
When plugs are used in a heater fitting like plug-type or mule-ear fittings or when a sectional L is used in a sectional fitting, the width of the seating surface in the fitting must be sufficient to prevent leakage. A width large enough to prevent leakage generally provides adequate strength against blowout, but a lesser width should never be used. The proper seating width required to prevent leakage is difficult to calculate and is often determined by experience. When there is no previous experience to be used as a guide, one way to determine these limits is to wait until evidence of slight leakage is found and then set a limit at a point that is a little greater than that at which the slight leakage was evident.
Because of the great number of variables affecting the limiting thickness and the variety of types, sizes, shapes, operating methods, and constructions of boilers, it is not possible in this recommended practice to present a set of pre-calculated minimum or retiring thickness. However, it may be quite feasible to prepare one for the boilers in a given refinery. Formulas for the thickness of drums, headers, and tubes are given in the ASME Boiler and Pressure Vessel Code, Sections I and IV. ASME B31.1 also provides calculations for wall thickness of power boiler piping. These formulas can be used as guides when repairs and replacements are needed.