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Plant and Instrument Air

By: Mihail Editoiu, Process Engineer

Main issues:

Instrument air is a critical service and operation reliability of its system components is extremely important.

The dew point of instrument air at maximum operating pressure shall be lower with 10°C that the lowest expected ambient temperature for the air system, free of toxic, flammable vapor, oil and liquids and the solid particle shall be lower than 3.0 microns with solid content minor than 0.1 gr/m3.

Plant air usually is not dried but in cold climates it becomes important and economical to remove water also from plant air – an economic analysis is necessary.

Spare air compressor shall be provided, remember that the instrument air is critical. I would recommend oil free type and considering the capacity the type could be:

– Reciprocating: 50-1000 Nm3/h

– Rotary = 900-2500 Nm3/h

– Centrifugal >2300 Nm3/h

The discharge pressure is in the range of 8.5-8.8 barg – seems that is the most economical value but needs to be checked with the vendor to be sure that margin is provided to allow for the correct operation of the antisurge system.

The inlet of the compressor shall be in a non-hazardous area, at least 5-6 m above ground with wire mesh, pay attention to provide winterization where is necessary – check lowest ambient temperature and also on suction a 3.0 microns dust filter shall be foreseen.

Air buffer shall be sized for 20-30 minutes as hold-up based on plant air shutdown

p = 7.2 barg and instr. air min p = 4.2 barg – the mentioned values shall be defined in Basis of Design. Also 150 microns of water droplets need to be considered for sizing. Usually depending on air plant capacity 1, 2 or 3 buffers are necessary.

Dryers with silica gel up to -65°C pressure dew point, activated alumina shall provide -75°C pressure dew point and mol sieve can remove moisture below 0.1 ppmv.

For instrument air demand please see the movie and regarding plant air please consider continuous consumers and the maximum peak demand of simultaneous intermittent user:

– utility station: 500 Nm3/h corresponding to 3 hoses

– air driven pump shall be added

– regeneration facilities used for catalyst regeneration: usually one at a time but check for simultaneity.

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