Sets out the criteria for the design, construction and testing of safety valves, liquid relief valves, liquid level gauges, blowdown valves and other fittings for use on boilers and unfired pressure vessels and their associated piping. It does not cover the selection, operation or application of valves and fittings, as these are covered in the Standard applicable to the equipment to which the valves and fittings are to be connected. This Standard now aligns with BS 5154 in terms of valve body dimensions. This alignment will overcome the problem of differing valve body thicknesses evident in previous editions and will add to safety in the selection of correctly sized safety valves.