A.
Components
(Figure 1)
1.
Pressure regulating valves maintain 80 psig to the airlock system from
control air supply pressure. Pressure
from the valve is piped to the master trip valve and passed to the individual
trip valves when the master valve is open.
2.
Control panel panel contains two 0-200 psig range pressure gauges and a
reset valve with a push button. A pressure switch set at 75 psig is
connected to an indicating light panel and
a buzzer. The light panel contains a
transformer, a white light bulb to indicate the airlock panel is energized, and a red light bulb to warn of
impeding loss of control air pressure.
A rotary switch enables the operator to silence (cutout) the buzzer
signal once the alarm has been sounded.
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Figure
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3.
Master trip valve is a switching valve manufactured by Fisher Control
Co. The unit contains a diaphragm,
three 1/2 inch ports, and a steel spring set a t 65 psig. There is only one master trip valve in the
system. The valve is connected to all
the individual trip valves at each final control element. The valve is held in the open position by
control air pressure and is calibrated to close when the air pressure drops to
65 psig.
4.
Individual trip valves are
Fisher 3-way switching valves (individual trip valve) installed at each
final control element located in the main machinery space. The valves are identical in construction to
the master trip valve. The spring
however, is set for 40 psig. These
valves are held open by 80 psig pressure provided from the airlock regulator
through the master airlock and are calibrated to close when the airlock system
pressure drops to 40 psig. Individual
airlocks are installed on the fuel oil control valves, forced draft blower
steam admission valves, main feed pumps (if automatic controls are installed) ,
and feedwater control valves.
5.
Three-way vent valves (one for each individual trip valve) are installed
between the individual trip valve and the actuators of the final control
devices. The valve allows the operator
to release or vent the control air trapped between the individual trip valve
and the final control element when shifting the final control element to local
manual control. Venting the signal
allows for full range of motion from minimum to maximum position of the final
control element.
6.
Reset buttons are used to reset the air lock system. When the valve is depressed, control air
rushes up under the diaphragm in the master trip valve, thus opening it and allows
air to pass to all individual trip valves in the system, opening them.
B. System
operation
1.
As air pressure drops from 125 psig:
a.
115 psig high start/stop compressor starts
b.
110 psig low start/stop compressor starts
c.
85 psig priority valve closes ensuring all air is sent to vital air
systems
d.
80 psig alarm sounds in the engine operating station (EOOW)
e.
75 psig alarm sounds in the fireroom
f.
65 psig the master trip valve closes aligning air from all the
individual trip valves to the atmosphere.
When the airlock system pressure drops to 40 psig all the individual
valves will close, thus locking in the last signal to the final control
element.