Assessment of TRAC-BF1 for Waterhammer Calculations with Entrapped Air

The TRAC-BF1 computer code is used to analyze the fluid pressure response for a waterhammer event in a water-filled pipe with entrapped air. TRACʹs capabilities are assessed by comparison against a methodof-characteristics (MOC) solution of pressure-wave propagation in a gas/liquid interface system. A vertically oriented pipe with air initially occupying up to 10% of the pipe volume is considered. A step increase in pressure is imposed at the inlet, and the fluid pressure response in the pipe is calculated. TRAC correctly predicts that the peak pressure with entrapped air is substantially higher than it would be in a purely liquid system. For an initial air volume equal to 10% of the pipe volume, the peak pressure calculated by TRAC compares within 1% of the MOC result. For smaller initial air volumes, TRAC underpredicts the peak pressure disturbance by up to 14% compared to the MOC. The TRAC solution exhibits a degree of long-term artificial damping, but in all cases it captures the basic features of the pressure response for a waterhammer event in a system with entrapped air.