This Guide provides guidance on establishing operating periods of safety valves and gives further information, beyond the relatively simple statements contained in national codes, to provide a basis for the review of such operating periods. It covers a number of points which may be used as a basis for user company procedures. The main part offers general guidance, whilst the Appendix gives more detail in certain areas, not necessarily referred to in the main text, such as the effect of relief system design (Appendix A2), acceptance of new valves prior to use (Appendix A3), inspection, maintenance and testing (Appendix A4, A5 and A8), and advice on what should be done if a safety valve lifts (Appendix A9). This Guide applies to most pressurised fluid systems and safety valves, including thermal relief and pilot-operated valves, but with the exception of pressure-vacuum (tank breather) type valves.