Sure you can. You might want to consider a couple of things though. With 18lbs of grain after you pull your bag, the grain will have absorbed almost 2 gallons of your starting liquid. I would do a 2nd pot sparge of your grains and top off your brewkettle with this liquid to around 9 gallons to start your your boil. Be watchfull of boilovers! After your 1 hour boil you will still need to add 2-3 gallons of water to your fermenter(s) to make your final volume. ( I preboil 3-4 gallons of water the night before brewday in a large pot on the kitchen stove, cover it, and let it cool overnight for this purpose) Good luck!