Vibration and chattering of conventional safety rel ief valve under built up back pressure

Safety relief valves are devices designed to open w hen the pressure in the process to be protected exceeds the design pressure. However, in industrial p actice, it often happens that the outlet of thes e valves are canalized through discharge lines which can be different from atmospheric, then there is a built up pressure generated by the flow in the pip ing which is superimposed to the back pressure in the discharge system. As a consequence, the initial s zing and selection of the safety relief valves, using results from tests conducted under conditions without back pressure are not necessarily valid. The manufacturers often use empirical rules of calc ul tions to assess the performance characteristics in this type of use. When a safety relief valve is used with substantial back pressure (up to 30% of the set pressure), it is common practice to use a balanced construction. The balancing effect is generally obtained by a balancing bellows. When the back pressure downstream the valve is con sidered low (up to 10% back pressure recommended value), a conventional relief valve (i. e. without balancing devices) can in theory be stil l used. However, even for lower build up back pressur e levels, fluttering and chattering of the relief valve disc may occur. This may not only lead to poo r perating conditions of the valve, but the damage can be dangerous even when this build up bac k pressure reaches a value far below 10%. As an alternative to complex physical modelling as fully coupled analysis of the fluid-structure interaction based on the three dimensional NaviersStokes computations, a composite coupled 1Dapproach is developed. Inlet and outlet conditions f the safety valves are modelled respectively by thermodynamic 1D-model and wave propagation model p r serving the physical phenomena. The safety valve is described by dynamic 1D-model where the hydrodynamics forces applied to the moving disk are given by empirical rules. These emp irical rules are predicted using CFD computations for different flow conditions. This methodology is validated by comparison with experimental data. Finally the composite coupled 1D-approach reproduce s th unstable behaviour of the safety valve and could allow drawing up more efficient usage rules.