Conventional Spring-Operated Reliefs in Liquid Service

9-1 Conventional Spring-Operated Reliefs in Liquid Service 

Flow through spring-type reliefs is approximated as flow through an orifice. An equation representing this flow is derived from the mechanical energy balance (Equation 4-1). The result is similiar to Equation 4-6, except that the pressure is represented by a pressure difference across the spring relief  

AP is the pressure drop across the relief, and p is the liquid density. 

The volumetric flow Qv of liquid is the product of the velocity times the area, or uA. Substituting Equation 9-1 and solving for the vent area A of the relief, we obtain 

A working equation with fixed units is derived from Equation 9-2 by (1) replacing the density p with the specific gravity (p/pret) and (2) making the appropriate substitutions for the unit conversions. The result is 

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