Hazardous Area Operation
Hazardous Area
– General
A-6
Prima PRO & Sentinel PRO Mass Spectrometers User Guide
Thermo Fisher Scientific
When purge protected apparatus is initially switched on, it is
necessary to ensure that that there is no residual hazardous
atmosphere in the purged enclosure. For this reason the enclosure is
purged for a set time before the power is switched on. This time is
the time (determined during apparatus certification) to purge the
volume in the enclosure, including any partially trapped volumes.
Again, depending on area classification, it may be necessary to
automatically time this initial purge. Power is not allowed to be
switched on until this phase is complete.
Where the protected apparatus is an analyser, for example, it will be
necessary to introduce sample gases into the enclosure. Typically,
this gas is fully contained and would not enter the pressurised
volume of the enclosure. If the sample gas is hazardous in nature
then the possibility of leakage from this containment system into the
enclosure must be considered. The simple enclosure pressurisation
then becomes a continuous purge and the purge flow rate must be
such that the worst-case internal leak is diluted to a safe level.
A summary of the different purge types is given below.
Table A
–4.
Purge types
ATEX (Europe)/IECEx
North America
Area
Purge
Type
Purge
Characteristics
Area
Purge
Type
Purge
Characteristics
Zone 1,
2
Exp
Automatic power
interlock and
purge timing
Div.
1
X-
purge
Automatic power
interlock and purge
timing
Div.
2
Z-
purge
Manual power
interlock and purge
timing,
pressurisation
alarm
The point at which purge gas exits the enclosure must be protected
by a spark arrestor to prevent any sparks potentially generated by
apparatus in the enclosure from getting into the hazardous area.
Apparatus used to provide the required purge interlocking must
remain powered even when the purge gas supply is off. Hence, the
purge protection apparatus must use an alternative method of
protection.
Purge / pressurisation is the main method of protection used for the
Prima PRO Ex and Sentinel PRO Ex systems, and detailed operation
of these purge protection schemes is covered in later sections.
This method does not attempt to prevent sparks or to isolate
flammable gas from any potential source of ignition, and as such
Flameproof (Europe)
or Explosion Proof
(North America)
