Technical data

1.3.3 Model DDX Type PL
The Reliable Type PL Double Interlock Preaction system utilizes an LPDI Double Interlock Actuator on
the outlet of the Model DDX Deluge Valve push-rod chamber. The lower chamber of the actuator is
held closed with pneumatic pressure, and provides a separation between the hydraulically pressurized
push-rod chamber and dry pilot line pneumatic pressure. The upper chamber of the actuator is held
closed with system pneumatic pressure, and provides a separation between the lower (dry pilot line)
pneumatic pressure and system pneumatic pressure. The Type PL system is described as a fully me-
chanical pneumatic/pneumatic double interlock preaction system. Two independent conditions must
coexist to open the Model DDX Deluge Valve and flow water into the system. These two conditions are
(a) loss of pneumatic pressure in the dry pilot line and (b) loss of pneumatic pressure in the system pip-
ing. Pilot line detectors are more sensitive than sprinklers on the system, and will activate and sound an
alarm prior to the sprinklers opening. The time period between the pilot detectors opening and open-
ing of the sprinklers allows the building owner a chance to respond and possibly avoid unnecessary
discharge of water into the water sensitive environment. In the event the alarm is not attended to, the
fire will continue to grow eventually fusing one or more sprinklers releasing pneumatic pressure from
the system. Loss of pneumatic pressure from the system operates the dry pilot actuator which vents to
a drain, relieving hydraulic pressure from the push-rod chamber and allowing the Model DDX Deluge
Valve to open and flow water into the protected area. Alarm devices are activated upon the flow of
water into the sprinkler system piping. Pneumatic pressure can be provided from a tank-mounted air
compressor, plant air system, nitrogen cylinders, or nitrogen generator, and must be maintained by a
Reliable Model A-2 Pressure Maintenance Device. Dry pilot line and system pneumatic pressures are
independently monitored by pressure switches* that will initiate a signal in the event of falling pressure
due to sprinkler damage, pipe damage, or failure of the compressed gas system. An independent low
pressure condition in either the dry pilot line or in the system will NOT result in activation main system
control. The singular condition can be corrected and the system returned to normal service.
*Note: For Type PL systems, System Sensor Model EPS 10-2 Pressure Switches are provided with the
trim.
Approvals and technical data for Model DDX Type PL Double Interlock Preaction systems can be found
on Reliable Automatic Sprinkler Company Bulletin 752.
16.
Figure 14