Basic Experimental Study on Improvement of Seismic Capability of Grouped Piping System

Abstract

Improvement of seismic capability is one of the most important aspects in the design of nuclear power plants. Amongst various major components, primary heat transport system piping is required to have extremely high seismic capability, because their damage during an earthquake would have a great influence on the safety of nuclear power plants.

Aiming at searching the basic measure to improve seismic capability of piping system, a forced vibration test was conducted on a simplified small model representing a grouped piping system using a small shaking table, where the emphasis was laid on the improvement of damping effect. For a grouped piping system, spacers and restraints are usually employed between adjacent pipes in order to avoid contact which might occur due to thermal and seismic movement. However, they are deemed not to make a significant contribution to the damping effect on a grouped piping system. This is because, in many cases, the vibration characteristics of each pipe constituting a grouped piping system are similar, and as a consequence each pipe vibrates in phase and the relative displacement between the pipe is small under earthquake excitation condition.

In light of these facts, EPDC (Electric Power Development Co., Ltd.) and MISM (Muto Institute of Structural Mechanics, Inc.) performed a basic experimental study on seismically improved piping system models such as seismic support type and steel mesh type models. Improvement by seismic support was based on an idea that, the vibratory energy of a grouped piping system is designed to be mainly absorbed by the seismic support when subjected to relative displacement between building and piping system. On the other hand, steel mesh type was a new idea employing a mesh tie for pipes, thus in- tending to restrain their out of phase vibration. Additionally two other models were tested, spacer type and restraint type models. This paper deals with an outline of testing and test results. The damping effect of each model was evaluated in comparison with that of a conventional piping system in order to demonstrate validity of proposed improvement schemes.