If I've read the question correctly, you should need a feedrate of 1800 lbm/hr to make up the outgoing 1800 lbm/hr of steam. Now, this says nothing about the amount of startup water required to fill the system and related piping, but to provide 1800 lbm/hr in a correctly designed system you'd need 1800 lbmH20/hr to replenish the outgoing steam.
Assuming feed water is at 1 atm and about 70F, it should be about 216 gallons an hour. Of course, if you have any losses in the piping systems you could raise your requirements, and if the feedwater is at wild pressures or temperatures you could see some different water densities.
Take this info with a grain of salt. As I said, I don't know your entire system and am just going on the fact that you need to replenish 1800 lbm of steam/hr.