The Cla-Val Model CDB-7 Differential Relief Control is normally closed and responds to the differential pressure changes across the diaphragm of the control. An increase in the differential pressure tends to open the control and a decrease in the differential pressure tends to close the control. This causes the main valve cover pressure to vary and the main valve modulates (open and closes) maintaining a relatively constant differential pressure between the main valve inlet and outlet. When differential pressure is lower than the set point of control, the control closes. This pressurizes the cover of the main valve and the main valve closes. The CDB-7 Differential Control is adjusted by turning the adjusting screw clockwise to increase the setting.