theres about 4 liters per gallon, so measure your pipe, and adjust accordingly...
ie: i liter capacity in the tank would require .75 lbs of pure lye , if using draino, you would use 1.25 lbs of draino per liter of water (it's at 60% concentration of caustic soda) Draino comes in a 500 gr container (in Canada) this is just over 1 lb (454g)
I used a complete can for one liter, and it worked. I didn't leave it in long enough though.. (Had to go to work in the morning)
YES it's extreamly caustic - be careful.
Auto machinest will attest, that caustic soda is the chemical used in a "hot tank" or a "dip" , amongst many other uses a machinist would bathe a head block or other component in a bath of concentrated caustic soda, to clean it.