Fatigue of cantilevered pipe fittings subjected to vibration

ABSTRACT Vibration fatigue failures in piping systems often occur at cantilevered small‐bore fittings (SBF) such as pressure tappings and drain valves. Piping vibration has often been assessed by measuring vibration displacement or velocity. However, in the case of straight cantilevered fittings with a concentrated mass, a better method exists. This paper presents a simple robust method of calculating vibration induced stress for cantilevered fittings at both the fitting branch neck and the main pipe. The method is based on measurement of acceleration at the concentrated mass (e.g., valve) and simple mechanics calculations. Extensive laboratory and field verification of the technique is presented. The choices of a fatigue strength criterion, such as ASME BPVC VIII Div. 2, BS7608, or a fracture mechanics approach such as BSI PD6493, are discussed. Several case studies are presented as well as simple ‘good engineering practice’ rules‐of‐thumb for SBF design. These ‘rules‐of‐thumb’ are now incorporated in the piping specifications of the author's company.