An Investigation into the History and Use of Welded Lap Joints for Steel Water Pipe

Abstract

It is common practice to weld joints in water pipe. The practice is more recent than welded joints in pressure vessels. Therefore, it is inevitable that welding standards for pipe welds would have been influenced by the older pressure vessel codes. There are significant differences between high-pressure tanks and pipes for transmission and distribution of water. Successful lap welded pipelines and test programs on welded lap joints provide information on the performance and performance limits of welded lap joints in pipes. Single welded lap joints are adequate for most welded steel water pipe. Double welded lap joints are roughly ten percent stronger (than single). Single welded lap joints can resist longitudinal stresses when valves are closed, i.e., zero flow, (highest pressure and worst case pressure analysis). Longitudinal stress caused by change in temperature must be included if the pipe length is fixed. Design is based on safety factors, which should be ratio of longitudinal weld strength and maximum longitudinal stress. Longitudinal weld strength should be determined by full pipe sections – not coupons cut from a joint. Maximum longitudinal stress is generally less than the hoop stress. Hoop stress is the primary design consideration.