U-Bolts Used as Pipe Restraints in CANDU Nuclear Power Plant

Abstract

In many existing cases U-bolts were used as pipe restraints. Restraining capacity and some physical properties were not known and/or properly understood while applied in the field. This created multiple problems. Many U-bolts were applied in seismically qualified systems. Because manufacturers do not provide any stiffness values for loaded U-bolts that act as pipe restraints, it was necessary to develop dependable calculation method and arrive at results that would be used in seismic stress analysis. Without such input values seismic analysis is inaccurate. In this case re-analysing piping systems and achieving reliable Code compliant results was a problem. Available capacity values provided by manufacturers created separate problem as they were found not dependable in everyday plant operation. Certain aspects of pipe restraining properties were not considered or given capacities were not trustworthy. Invariably it led to pipe support failures and urgent discovery work. In this case overloading U-bolts (with all attendant consequences) by using available capacity values was a problem. To remedy existing unsatisfactory situation and develop dependable information and reference source, analytical work was initiated to gain full understanding of the situation when U-bolts are used as active pipe restraint. First part, including assessment of the U-bolt stiffness is already finished, while capacity assessment is in progress.