⁴⁷Such applications are typically found when the purpose of the control valve is to regulate process fluid pressure. Consider, for example, a control valve regulating upstream gas pressure in a vessel by venting gas from that vessel to atmosphere. In such an application, the valve’s upstream pressure (P₁) will be nearly constant due to the control loop’s action, and the valve’s downstream pressure (P₂) will be constant due to it being atmospheric pressure.