There is a difference between the true discharge coefficient C-D for liquid flow through a safety valve and the more familiar ＇coefficient＇ K-D; this difference increases with the orifice-to-inlet diameter ratio. The use of C-D or K-D depends on whether the relief system design calculations are carried out in terms of static or total pressures. Confusion between C-D and K-D could result in undersizing of valves. These matters are discussed in this paper and the relationship between C-D and K-D is derived. Furthermore, a safety valve model is proposed in which the flow characteristics are described by a discharge coefficient C-Dn for the valve nozzle and a loss coefficient K-bn for the valve body. This two-parameter model permits a more detailed description of the flow than existing models. It is found that, for some valve types, it is important to take account of the area change between the inlet and the orifice. The implications of this for sizing and stability are discussed.