21 JANUARY 2003
TEST PROCEDURE FOR SETTING THE PRESSURE DIFFERENTIAL CONTROL (CDHS-3)
A3.1. In General. The CDHS-3 control operates from a DP produced by the orifice plate on the outlet
of the main valve. The orifice plate bore size is on the flange of the orifice plate. Once you identify the
orifice plate bore size, you can compute its DP at a given flow rate (Figure A3.1). By knowing this DP,
testing becomes a matter of producing the DP across the diaphragm of the CDHS-3 control and
adjusting the control until it trips. For this method you add shutoff valves in the CDHS-3 sensing lines to
shut off the fuel supply, then add external pressure equal to that produced by the differential across the
orifice plate and make the adjustment. Figure A3.2 shows the location of' the shutoff valves and the
equipment needed for the adjustment.
Figure A3.1. Flow Chart.