Description Of Pressurization System - Emerson ROSEMOUNT MLT 2 Upgrade Manual

MLT 2 / BINOS
1.2

Description of Pressurization System

The intention of a pressurized analyzer is to
keep out of the instrument a surrounding po-
tentially explosive atmosphere by providing
an internal overpressure. The pressurization
system consists of a gas inlet valve, control unit
with micro processor and a bypass switch (Cat
2 only). The control unit monitors all relevant
parameters (pre-purge phase, purge medium
flow, internal overpressure, and more) and
ensures safe operation.
Category 2 analyzers, to be installed in Zone 1
are automatically pre-purged before the power
supply is activated. This ensures a potentially
explosive atmosphere is flushed out. The pre-
purge phase is controlled by the micro processor
unit, which also activates the power supply for
the analyzer when the pre-purge phase has
elapsed or disconnects the analyzer from power
supply in case of failure. It´s internal design has
redundant components to ensure safe operation
even in case of 1 failure.
The main difference of a Category 3 pressu-
rization unit for Zone 2 installation is, that this
unit does not control the analyer´s power sup-
ply (e.g. in case of a failure the analyzer is not
disconnected), pre-purging is only optional and
the internal controller design is not as redundant
(safe operation without failure). If an infallible
containment is available for the specific appli-
cation, it can be used for measuring flammable
gases. But, measuring flammable gases utilizing
The operator has to ensure that in case the analyzer is disconnected from
its power supply, applied external signals are disconnected, too, if they are
not intrinsically safe!
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1.2 Description of Pressurization System
EXPLOSION HAZARD
ATEX Instruction Manual
a differential pressure switch is not permitted: In
this case a Zone 1 instrument has to be used.
The different gaspath designs for measuring
flammable or non-flammable gases also influ-
ence the intruments operation parameters:
Measuring non-flammable gases:
The pressurization system provides an internal
overpressure of 14 mbar. The acceptable samp-
le gas pressure complies with the specifications
of an analyzer for non-hazardous (general pur-
pose) areas.
Measuring flammable gases utilizing a diffe-
rential pressure switch:
The internal overpressure is adjusted to
14 mbar. The opertor has to ensure the gas
pressure (sample gas, calibration gases) inside
the containment system remains below 12 mbar,
due to the fact that there is a differential pressu-
re switch installed in each gas path containing
flammable gas. Limiting the gas pressure ensu-
res the pressure inside the instrument is always
at minimum 50 Pa (0,5 mbar) above the con-
tainment pressure. This configuration ensures
that in case of a gas path leak flammable gas
cannot enter the analyzer enclosure.
In case the pressure difference is less than the
given values (due to a failure), the control unit
disconnects the analyzer power supply and
starts pre-purging.
Emerson Process Management GmbH & Co. OHG
ETC01035
02/2008