This is my first time on this Xchange group, trying to understand the rationality behind NFPA 15 regarding Conveyor Belts.
They state at 220.127.116.11 "The water spray system shall be installed to automatically wet the TOP belt, its contents, and the BOTTOM RETURN belt", it looks to be a huge amount of water if you are dealing with a long and wide belt!!!
In case you go for Deluge System, there is no specific area limit neither time to reach de open nozzle, at least I was not able to find it under NFPA 15.
On the other hand, regarding other specific Data Sheets related with Conveyor Belts, there are no restrictions about the Double Coverage (top&bottom) for partially enclosed conveyor, do anybody can tell me about this significant differences. Think that the rate of fire spread would be less in these partially enclosed conveyor.