Safety Systems - Purging; Purging Overview - Siemens Maxum II Manual

Safety Systems - Purging

2.1

Purging Overview

For safety purposes, Maxum enclosures that are installed in hazardous locations where
flammable vapors or gases may exist must be purged. Purging refers to the process of
supplying the interior of the enclosure with protective gas in order to increase the pressure
within the enclosure above that of the ambient outside air. This increased pressure creates
flow out of the enclosure and prevents flammable vapors or gases from entering the enclosure.
It also removes any flammable vapors or gases that may exist within the enclosure.
The primary electronic devices of the Maxum II analyzer are housed inside the electronics
enclosure (see Figures 1-1 and 1-2). All electronics in the Maxum II are designed to meet North
American Division 2 and European Zone 2 safety requirements. This means that these
electronics will not cause combustion of flammable vapors or gases under normal conditions.
However, safety codes also require that equipment be designed to accommodate abnormal
conditions as well as normal conditions. The concern is that electronics inside an enclosure
might fail, causing a hot spot, a spark, or some other possible source of ignition. To mitigate
this risk, when operating in locations where flammable vapors or gases are present, the
enclosure is normally closed and supplied with a flow of clean instrument air. The air ensures
that the interior of the enclosure is maintained at a positive pressure relative to the outside and
that air always flows out of, rather than into, the electronics enclosure. This "purging" makes
sure any flammable vapors or gases located outside the enclosure cannot migrate to the
interior of the enclosure where they might contact a possible source of ignition.
The type of purging within the Maxum II depends on location-specific requirements. The
certification of the analyzer (and hence the applicable purging method) can be identified from
the labeling on the analyzer. See Section 1.7 (
document for more information. There are two purging methods available in the Maxum II.
1. Purging without Automatic Purge Unit: This is the standard purging method for the Maxum.
In this purging method, constant purging pressure is applied. Circuitry on the Power Entry
Control Module (PECM or PECM-DC, depending on configuration) of the Maxum II is used
to verify positive purging pressure. This circuitry compares interior pressure to exterior
pressure to determine whether the proper pressure difference exists. An alarm is generated
when purge pressure is lost.
2. Purging with Automatic Purge Unit control: This optional method of purging utilizes a device,
called an Automatic Purge Unit (APU), to control the purging system. The APU disconnects
electrical power to the Maxum if purge is lost.
Explosion Protection Safety Standards
Manual, August 2017, A5E02220442001 Rev 8
Applicable Standards and Regulations ) of this
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