Liquid Flow through Safety Valves: Diameter Ratio Effects on Discharge Coefficients, Sizing and Stability

There is a difference between the true discharge coefficient CD for liquid flow through a safety valve and the more familiar "coefficient" KD; this difference increases with the orifice-to-inlet diameter ratio. The use of CD or KD depends on whether the relief system design calculations are carried out in terms of static or total pressures. Confusion between CD and KD could result in undersizing of valves. These matters are discussed in this paper and the relationship between CD and KD is derived. Furthermore, a safety valve model is proposed in which the flow characteristics are described by a discharge coefficient CDn for the valve nozzle and a loss coefficient Kb 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 tipes, 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.

MORRIS Stanley David; 

1996-04-02

JRC12877