Design of Large Diameter Steel Pipe Tees and Wyes

The Tarrant Regional Water District (TRWD)’s Kennedale Balancing Reservoir (KBR) bypass pipeline project included four 108-in. tees, two 30-degree 108-in. by 108-in. wyes, and several 108- by 30-in. tees, all of which were steel fittings. The wye fittings failed during hydrostatic pressure testing due to excessive localized stress, excessive strain, cement mortar lining, and concrete encasement failure. The tees showed no signs of failure. The fittings were originally designed with crotch plates per the Fourth Edition of the American Water Works Association (AWWA) Manual of Water Supply Practice M11 (M11) guidelines. However, an option to design the fittings with the American Society of Mechanical Engineering Boiler and Pressure Vessel Code (ASME-BPVC) section VIII, where suitable, was allowed in the specifications. The ASME-BPVC was allowed because it reduces the cost and the size of the fittings. The ASME-BPVC was selected for final design and fabrication of the fittings by the pipe manufacturer to reduce cost and because they believed the 30-degree wye with crotch plate was not constructible. After the initial hydrostatic test failure, finite element analysis (FEA) from two separate entities, and a FEA review from a third entity revealed that the ASME-BPVC design resulted in excessive localized stresses and strains for the wye fittings. After the failed field weld was modified, the wye fittings were able to hold pressure briefly during a “Proof of Design” test requirement allowed by the ASME-BPVC, but the wye fittings deformed, causing the ground to heave and the lining and encasement to crack. The original wye fittings were replaced with new fittings based upon the M11 design, and verified by FEA, which successfully passed the hydrostatic testing. The ASME-BPVC has been used successfully for design of steel water pipe tee fittings; however, this project revealed that the design procedure may not be suitable for all wye fittings, especially with large diameter openings and if cement mortar lining is used because it limits the allowable deformation. This paper compares the M11 and ASME-BPVC design procedures and proposes limitations on the application of the ASME-BPVC design procedure for large diameter steel wye fittings.