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Re: Purge

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Posted by Rod Nissen on July 10, 2002 at 06:51:57:

In Reply to: Re: Purge posted by JC on July 06, 2002 at 05:13:41:

When a boiler is first commissioned the volume of the furnace, convection passes, flue ducting up to the base of the stack is calculated. This is the volume to be purged. Theoretically if we assume "plug flow"(ie what you put in one end, has to come out the other)this volume divided by the blower air rate will give the time required for one (1)air change. In the real engineering world we always use safety factors and the relevant authorities require additional purge time. Here in Australia we are required to provide 5 air changes (sometimes they will allow less if you demonstrate efficient plug flow), but I think in the US you only need 4. Once this minimum required time is calculated the automatic burner programmer is required to have at least this purge time. In doing this the infallable human element is removed. So there is no reason to bypass this system and put it back in the hands of an operator. So why do you require to do this?

HOWEVER, if you have a situation with say an oil burner where a number of consecutive flame failures have occurred, then raw fuel is present in the furnace. In this case it is foolhardy to further continue resetting the control. The furnace should be manually cleaned out and the reason for the missfires fixed. Unfortunately this is often not done, and people do get away with it, or sometimes NOT and there is a BANG.

As engineers at sea we were taught to only reset a control 3 times max before checking and cleaning out a furnace. I leave that decision up to you!!
Having said all that I would still be interested in knowing your reason for a manual purge?

Regards
Rod
E-mail: nissenr@iprimus.com.au
Web: www.canded.com.au

: : : What method is used to purge low pressure boilers?

: Yes manualy purging the furnace. I know the boiler will pre-purge and post-purge through the fireye control automatically, but how would the operator manualy purge the boiler?

:
: : What do you mean by purge? Are you talking about purging the furnace and convection passes prior to fuel ignition? Or removing air prior to steam generation?

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: When a boiler is first commissioned the volume of the furnace, convection passes, flue ducting up to the base of the stack is calculated. This is the volume to be purged. Theoretically if we assume "plug flow"(ie what you put in one end, has to come out the other)this volume divided by the blower air rate will give the time required for one (1)air change. In the real engineering world we always use safety factors and the relevant authorities require additional purge time. Here in Australia we are required to provide 5 air changes (sometimes they will allow less if you demonstrate efficient plug flow), but I think in the US you only need 4. Once this minimum required time is calculated the automatic burner programmer is required to have at least this purge time. In doing this the infallable human element is removed. So there is no reason to bypass this system and put it back in the hands of an operator. So why do you require to do this? : HOWEVER, if you have a situation with say an oil burner where a number of consecutive flame failures have occurred, then raw fuel is present in the furnace. In this case it is foolhardy to further continue resetting the control. The furnace should be manually cleaned out and the reason for the missfires fixed. Unfortunately this is often not done, and people do get away with it, or sometimes NOT and there is a BANG. : As engineers at sea we were taught to only reset a control 3 times max before checking and cleaning out a furnace. I leave that decision up to you!! : Having said all that I would still be interested in knowing your reason for a manual purge? : Regards : Rod : E-mail: nissenr@iprimus.com.au : Web: www.canded.com.au : : : : : What method is used to purge low pressure boilers? : : Yes manualy purging the furnace. I know the boiler will pre-purge and post-purge through the fireye control automatically, but how would the operator manualy purge the boiler? : : : : : What do you mean by purge? Are you talking about purging the furnace and convection passes prior to fuel ignition? Or removing air prior to steam generation?

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