LDetek MultiDetek2 User Manual page 37

MULTIDETEK 2 User's manual
Handling highly flammable gas like silane or any other hazardous or explosive gases
requires a high level of safety and it is what LDetek offers with its built-in sample purging
and monitoring system inside the MultiDetek 2 compact GC. This system consists of 4
steps of safety:
Step 1: The sample gas flow path external to the purged box is fully welded offering no
possibility of leakage.
Step 2: A selection valve (V1) is mounted before the diaphragm valves used for filling the
sampling loops in the system. That selection valve (V1) is configured to allow the
hazardous gas going to the sampling loops only for a predetermined period of time just
before to start each analysis. The short period of time is configured in factory and is
dependant to the system configuration. The rest of the time, the selection valve (V1) is
switched on a purged gas which is normally the same gas type that the carrier gas of the
system for purging the sampling loops. In the eventuality of leakage on the sampling loops
or on the injection diaphragm valves, the reduced period of time introducing silane in the
injection valves combined with the low pressure operation and small volume will avoid
any potential hazardous situation.
Step 3: A sealed purge box containing the sample flow electronic pressure controller
(LDEPC2) and the diaphragm selection valve (V1) is mounted in the MultiDetek 2 chassis.
That box is normally purged with the same gas type that the carrier gas used for the system.
That environment requires low purge flow rate of about 10-30sccm through a fix orifice
depending of the system configuration. The box is fully ambient air free and the system is
ready to use after a short period of about 20-30 minutes depending of the system
configuration. This waiting period is only requires for initial start-up since once the
MultiDetek 2 has carrier flow going in it, the box is continuously on purge. Having a box
fully purged with UHP carrier gas eliminates the ignition risk in potential presence of silane
or any other explosive gases. A 1psig relief valve (RV) is mounted on the box to build up
a minimum sealing pressure and to maintain a constant purge with the carrier gas.
Step 4 (optional): The sealed purge box could be continuously monitored using a micro
PED (PLASMA2) to selectively measure trace N2 to ensure there is no air contamination
in the sealed box to avoid the ignition in potential presence of hazardous gas. If trace of air
is measured in the purged box by the micro PED (PLASMA2), then an alarm is activated
to shut off the flow of hazardous gas inside the MultiDetek 2. The shut off valve must be
mounted external to the MultiDetek 2 chassis. The feedback signal controlling the shut off
valve comes from the MultiDetek 2. It is requires to maintain the sample gas pressure
coming to the shut off valve below 10psig to minimize the risk of ignition. The flow type
selected by the selection valve (V1) is controlled with the electronic flow controller
(LDEPC2) to ensure a stable and constant flow rate whichever the gas type selected.
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