e. Purge System. The purge system is used to exhaust potentially dangerous contaminated air inside the
laboratory to the outside. The system consists of the following components:
(1) Power control relay located in panelboard assembly number 1.
(2) Time delay relay located in explosion proof distribution box A3.
(3) Blower control relays located in explosion proof distribution box.
(4) Two purge intake doors and two purge exhaust doors located on the outside wall of the laboratory.
(5) Four purge door limit switches.
(6) Two intake dampers and two exhaust dampers located behind the external purge doors.
(7) Three ventilation dampers mounted in the ECU plenum located in the mechanical room.
(8) Seven purge damper motors.
(9) One purge alarm buzzer located at the power entrance panel.
A purge cycle can be initiated in two ways, one of which is by the application of external power after a power
shutdown, the other is by an alarm generated by the gas detector system. When external power is first applied, the
power required for a purge cycle is picked off ahead of the main circuit breaker. Main circuit breaker is held open by
power control relay. This prevents power from being applied to any equipment or lighting in the laboratory until the purge
cycle is completed. Relay routes the power that was picked off ahead of the main circuit breaker to start the 5-minute
time delay relay and to energize the four blower control relays which provide power to the ECU blowers. Power is also
routed to the purge door damper motors and to the ventilation damper motors so that the purge door dampers are
opened and the ventilation dampers are closed. The ECU blowers suck outside air into the ECU ducting where it is
dumped into the laboratory creating a pressure which forces any gases or contaminated air to the outside via the purge
exhaust doors. After 5 minutes have elapsed, time delay relay de-energizes. This causes power control relay to change
state which de-energizes relays. Power is removed from the ECU blowers, the dampers are driven to their normal
positions and the main circuit breaker may now be closed to supply normal power to the laboratory. If at the end of the
purge cycle the gas detector alarm system is in an alarm condition, the time delay relay is activated causing the cycle to
be repeated. This will continue until the alarm clears. When external power is applied and a purge cycle initiated, one of
the external purge doors is closed, the purge alarm buzzer will be activated by the purge door limit switch and will remain
activated until the door is opened.
Air System (Figure 1-7). The air system consists of the air compressor, air tank, pressure switch, moisture trap,
and associated piping and valves. The air compressor is located in the mechanical room and operates on 208V, 60Hz,
3-phase power. It can supply air at 120 psi to the system. In automatic operation, the compressor is controlled by a
pressure switch which shuts it off when the pressure has reached 120 psi and turns it back on when the pressure in the
system drops to 60 psi. The air tank relief valve is set to operate at 150 psi. There are air service valves located within
the main laboratory work area and are color coded orange.