MAX300-RTG: Installation Manual Hardware Installation
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safe source. The interior of the enclosure is maintained at a pressure above
atmospheric pressure. This overpressure prevents entry of the possibly hazardous
atmosphere that surrounds the unit and allows the use of electrical devices inside
the unit while in hazardous locations. If these conditions are met, the control device
will apply power within the instrument’s enclosure.
In the event that the control unit senses a failure, power to the enclosure is
automatically disconnected. The failure sensed may be due to interruption of air
flow, an air supply pressure decrease below minimum, or the enclosure
overpressure not being maintained.
The purge and pressurization system is controlled by a Control Unit (CU), a
pneumatic logic circuit, which is the heart of the system. Once the main power
and purge gas has been successfully installed on the mass spectrometer, the
purge and pressurization system can be started. When the purge gas supply is
first turned on, the purge cycle will begin. Although the unit is wired to the main
power source, no electrical power is established within the enclosure. For power
to be established within the enclosure, the control unit must determine if the purge
gas has adequate pressure and if the enclosure over pressure is sufficient. Also,
the purge gas will flow for a set time interval to ensure that any hazardous gases
within the enclosure are purged.
The purge system controls electrical power to devices inside the enclosure, and
provides two flow rates of protective gas. The initial flow, during which power has
not yet been connected, is to ensure that the enclosure is free of hazardous
materials that may have been introduced from the outside atmosphere prior to
startup. This initial flow rate is set at a minimum of 300 Nl/minute (10.6 SCFM) and
the system must sustain at least this rate for a minimum of 12 minutes. Upon
successful completion of the main purge cycle, the system is designed to turn on
electrical power, and maintain an internal pressure in the enclosure of at least 2.5
mbar (1.1” H2O) with a constant minimum flow rate of 80 Nl/minute (2.8 SCFM).
The purpose of this is to ensure that the pressure within the enclosure is always
higher than that of the surrounding atmosphere.
Because the instrument may contain parts that may be above the ignition
temperature of the surrounding atmosphere, i.e. T4 of 135° C, (T3 200°C option
available) the enclosure must not be opened, even when de-energized, until 10
minutes have elapsed. The delay ensures that the hot parts have cooled well below
the ignition temperature.
In Zone 1/Div 1 areas, any wiring into the enclosure, communications wiring for
example, must be disconnected in the event of purge failure or power loss, unless
it is intrinsically safe wiring. Fiber optic communications is recommended wherever
practical. As an alternative to fiber optics, a communications option is available.
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