Gas Waterhammer

Gas process plant includes pressure relief valves, or pressure safety valves, which enable vessels containing high pressure gas to be emptied quickly into a relief main which takes the gas to a flair stack. The pressure safety valve opens suddenly and sets up acoustic waves, in the manner of a waterhammer, that propagate backwards and forwards in pipework until a steady flow is obtained. Such acoustic waves may set up large forces in pipework that may cause damage. Consequently, an assessment process is required that includes the forces generated by the gas and the resultant stresses in the pipework. One particular problem, investigated here, is the initial conditions for the acoustic waves as the pressure safety valve lifts. The main difficulty with an assessment is the near impossibility of modelling the pipe dynamics adequately. This difficulty is due to the lack of information available concerning pipework support systems and the need to model pipework vibration to high frequencies. A conservative method for assessment based on looking at extremes of support conditions is proposed.