A steam boiler is a pressure vessel with a difference. The vessel is subjected to heat stress of expansion and contraction, internal and external corrosion, as well as intense heating on some of its surfaces.
If we are not careful, steam pressure can rise up very high, and it can become a potential bomb!
Because of this, steam boilers have to be built to certain regulatory codes, with regards to materials used in their construction, their design, and their installation.
For operator of steam boiler, it is essential that boiler be operated safely. Although many safety devices are fitted in all boilers, they are not fail-proof. Humans are still needed to monitor condition of boiler all time latter is operated, even if it is just to acknowledge an alarm buzzer or flashing light in a control room.
One of most important ingredients of a boiler is water inside it. The use of untreated water will lead to scaling, corrosion or foaming. All of these have some detrimental effect on boiler or steam systems.
So, always treat water properly.
But however well you treat water, you will never know condition inside boiler until you actually see it.