ABB EasyLine Series Instructions For Installation Start-Up And Operation page 130

130 EasyLine EL3000 Series Commissioning Instructions
Failure of instrument air supply
Shutting-off gas supply if instrument air supply fails
It must be ensured that the gas supply to the analyzer module is shut off
if the instrument air supply fails.
As a rule, this is ensured by installing a pneumatic shut-off valve in the
gas supply line (recommendation (see page 29)); this valve must be
controlled by the instrument air supply in such a way that if it fails (and as
a result continuous case purging fails (see page 34)), the gas supply is
automatically shut off.
If such a pneumatic shut-off valve is not installed, the following
precautions and measures must be taken:

The Overall Status or the "Failure" status of the gas analyzer must be
monitored.

If the status appears, the cause must be verified in the gas analyzer
on site:

If the gas analyzer is not in operation (e.g. as a result of a supply
voltage failure), the supply gases must be shut off (see section
"Gas Analyzer Shutdown").

If the gas analyzer is in operation, an adequate instrument air
supply must be verified. If this is the case, the status messages
must be verified. If this is not the case, proceed as follows:
1. Shut off the combustion gas supply.
2. Reestablish instrument air supply.
3. Purge gas analyzer for 20 minutes.
4. Turn on combustion gas supply.
5. Gas analyzer starts automatically.
Note for measuring flammable gases
When measuring flammable gases, it must be made sure that in case of
a failure of the instrument air supply or of the analyzer module itself the
sample gas supply to the analyzer module is shut off and the sample gas
path is purged with nitrogen.