Thank you all for you help.
I know that boiler water needs to be treated before going to the boiler.
The reason I am asking this question is that I asked for a quote from a water treatment unit vendor. And I was told that since the boiler pressure is low (at atmosphere pressure) and there is no blowdown to remove the impurities left in the water after water treatment as it does in high pressure steam generator/drum, there will be scaling build up in the boiler tube. The boiler we will purchase is a gas fired steam boiler.
The water to be boiled off is produced water separated from process gas. It has high content of Na and Ca, Mg, K, Cl, HCO3. I meant to ask to what content the water won't cause much scaling in the boiler so we don't have to spent too much money of water softening or clean the tube too often.