When looking through the cause and effect charts for a new steam boiler installation, I was surprised to find that there was no "post purge" time after an emergency shut down or abnormal trip of the boiler.
For a normal shut down of the oil or the gas burners, the combustion air fans would run on for some time . The idea of this is to clear any unburnt fumes or fuel out of the furnance and uptake. But the C&E chart shows that after a trip, the fan shuts down immeadiately.
Does any one have any idea of the logic behind this?

Hello Vegman. There will always be a pre-purge. Presumably, if it is an emergency then a complete stop of everything is required. In my experience manually flashing is the dangerous bit. When the bridge and chief are screaming at you to get the boiler back on and your manual pre-purges are getting shorter and shorter until you get that 'light' blow-back at the furnace front like a whooof. You are then are reminded to do a full pre-purge!

Fond memories of singed and non existant eyebrows from flashback days whilst peering through that glass peephole with a manual torch flaming away and thrust into the furnace with cotton waste on the torch.
All the torches were welded onto the ends with the firemans initials and who betide anyone who used someone else's torch.
Fans were steam powered with solid whitemetal bearings and screaming all the time, the only sensor here was the steam valve admitting steam from the bulkhead stop valve.
Zero or max was the setting.
When firemen retired or went to another ship the initialled torch went with them.
Knew a bloke who came ashore after many years and took a pub, his torch was on the wall in the bar for 30 years plus.
And memories of after a shutdown of tools in icebuckets and into that corrugated furnace, up the end to the tube faces in a scotch boiler locating leaking tubes.
Scary stuff for a boy indeed but very fond memories of manual setups back then.