What is a Boiler Furnace drain.why is a boiler furnace drain used?.why is it adviced to wash the furnace side?. won't it affect the refractory strength?

Boiler furnace drain is provided at the bottom furnace side to remove the soot ,ash which is accumulated in refractory side.Normally in older ships ,soot and ash were led to bilges.But in few ships they were collected into a seperate tank located below boiler platform. Don't mistake Boiler furnace drain with soot collecting tank.Chemicals used for this is some ash modifier.

A scotch boiler.