Information Gap in Butt-Welded Pipe Fittings As Considered for Design

Abstract ASME B16.9 is the standard for butt-weld fittings which are used extensively in all kinds of pressure piping systems. ASME B16.9 governs aspects of fitting geometry such as surface contours, end preparation and provides requirement with respect to ASME B36.10 or ASME B36.19 nominal diameter and nominal wall thicknesses at ends to be as specified by purchaser. It also specifies the positions of welding ends with respect to the centerline of fittings or the overall dimension of the fittings enabling standardization which ensures their fitment in a piping system. Fittings are designed in accordance with section 2 which has no restrictions on the upper limit for fitting thickness away from the ends, straight end lengths, or crotch radius etc. These dimensions can be critically important to two aspects of pressure piping system design; process design which calculates the pressure drop through these fittings and strength design which calculates the stresses in the piping system. A common example of piping systems where pressure drop is critical are the inlet to pressure safety valves for protecting pressure vessels within the limits of API 520. An example of piping systems where strength design is critical are high temperature piping systems undergoing frequent cycling where stress intensification factors (SIFs) of the fittings are crucial in calculating accurate primary and secondary stresses. From a manufacturing perspective, ASME B16.9 and ASTM standards permit various manufacturing methods. For example, two different manufacturers may have a cold forming or hot forming method for the same fitting; the starting raw material thickness for the fitting may vary. These differences in manufacturing may affect critical fitting dimensions such as crotch thickness and crotch radius of a tee which in turn affect the SIF. This information gap between fitting manufacturers and piping designers results in differences between engineering calculations and actual performance of the installed system. This paper will be useful in educating piping system designers about these gaps and how to address them in design calculations.