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Hazardous Area Classification Of Combustion Systems


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#1 ianmcq28

ianmcq28

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Posted 04 October 2013 - 06:56 AM

Hello All,

 

I would like to raise a discussion point about hazardous area classification when applied to combustion equipment.

 

Generally speaking codes such as IP15, NFPA and UK HSE guidance etc imply that HAC cannot sensibly be applied close to combustion equipment since such equipment is inevitably hot and other measures to minimize leaks etc should be adopted. 

 

It is also generally recommended that such equipment is located away from the main processing plant in a 'safe area'.  This is something that I have commonly seen at petrochemical sites; the boiler plant and fired heaters are not zoned and are located outside of the boundaires of the main plant HAC zones.

 

I am working on the DEED for a replacement gas disposal plant.  The plant consists of a down-fired incinerator and quench tube directly mouted onto a scrubber sump tank where the exhaust gases pass through before entering the scrubber, which is also directly mounted on the sump tank.  There are also other scrubbers used for scrubbing toxics gases and emergency releases and the extraction header is common to all scrubbers.  So the incinerator is very much integrated with the process plant.

 

A HAC report was completed as part of the FEED and concluded that the scrubber and ductwork internals should be allocated with internal zones due to the risk of unburnt flammable gases, the incinerator internals were not allocated a zone obviously, but the natural gas supply train to the incinerator was allocated an external zone 2 to account for flange leaks etc.  The calculated zone extent from the closest flange to the incinerator is not sufficient enough to encroach upon the incinerator itself.

 

Some of the various guidance and internet sources suggest that HAC is not appropriate to electrical equipment close to combustion equipment because history has proved that a lot of combustion equipment fire/explosions have been due to major uncontrolled gas leaks encroaching upon or entering the hot equipment and the risk/probability of this occuring makes HAC/ATEX not sensible.  However HAC is not intended to address 'catestrophic' leaks and only considers zones based on leakage from set hole sizes.

 

I have determined that a 0.5 mete radius should be applied to all flanges and fittings.  The 0.5 metres radius zone 2 does not encroach upon the incinerator (which is well insulated) itself and is thus perhaps not considered to be 'close' enough to disregard as 'HAC cannot be sensibly applied'.  I have therefore deemed it a sensible approach to classify all electrical equipment on the gas train to be suitable for zone 2 to protect against the risk of small leaks.

 

However the burner supplier we are dealing with is arguing that they have never been asked to provide a gas train suitable for a HAC zone!  If we want to go down that route then they will have to re-validate the failure rates of the components included within the Burner Management System to validate the SIL capability to EIC61808 etc.

 

It seems that they adopt an IGEM standard (IGEM UP16) to get around this.  I have reviewed this standard and generally speaking it has been written with the intention to ensure the correct installation and maintenance of gas pipelines (including those supplying burners) to enable the allocation of a zone 2 of negligible extent (Zone 2 NE) to be applied (ATEX rated equipment is not required).  However the jury is very much out with applying a Zone 2 NE since a judgement is required to determine whether the available ventilation is adequate enough and that the installation is not confined or congested.

 

Has anyone else had to consider a similar situation where combustion equipment cannot be located remotely? 

 

Am I really asking for something unusual by asking the gas train to be suitable for zone 2?  IP15 does imply that the risk of electrical equipment should be considered even if there is a risk of fire due to hot surfaces.

 

How should HAC be addressed for combustion equipment and connecting pipework / gas train etc or should ATEX rated equipment be applied to combustion system gas-trains regardless?

 

It is worth mentioning that the previous plant was not zoned and the reason we are replacing it is because the incinerator burnt down and melted most of the GRP/Polypropylene scrubbers adjacent to it.  The client and insurance company require a 'like for like' replacement only, so unfortunately I don't have the luxury to re-design the plant properly.

 

Any feedback would be appreciated please.

 

Best Regards,

 

Ian


Edited by ianmcq28, 04 October 2013 - 07:05 AM.





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