Double Interlock Preaction systems...
To model a Double Interlock preaction scenario in FDT when the air pressure is comparatively low (15 psi) and the water pressure is high (165 psi), you have to increase the dry pipe valve's differential to a number that will hold the clapper shut until the time delay on the DPV accelerator has expired. Although this is not the case in reality, this is the method we use because the valves used in FDT are based on the Dry Valve concept, not the Deluge Valve concept of clapper operation. Using the example values above, a water pressure of 165 psi divided by an air pressure of 15 psi yields a minimum differential of 11+. Consider using a differential of 20 (this would hold the clapper until air pressure dropped to 8.25). You could as easily use a differential of 100 (holds to 1.65 psi). Either way, after the delay imposed in the valve's accelerator settings (normally 4 seconds for the DPV-5 Electric-Electric) the clapper pops open and the rest of the analysis is performed. Note that the increase of the differential serves to hold the clapper shut without changing the air pressure conditions in the piping. This is important for a proper analysis. Should you raise the air pressure, the water would have a greater air pressure to overcome at trip. Using the increased differential, the air pressure is properly modeled from the baseline of the opening of the head(s).
