For a high pressure boiler above 1,400 psi (9.7 MPa), the boiler drum level Control and Protection systems serve two distinct, but complementary functions. The Control system needs to control drum level. Protection is done for safety. If the drum level reaches a critical level, the Protection system shuts down the boiler to avoid damage.
One must meet ASME Boiler Code requirements, get the job done correctly, and minimize the number of instruments required. By ASME Boiler Code, one must have at least one Boiler Water Gauge Glass. How many Control instruments are needed? If a single boiler drum water level instrument is used and this instrument suffers a failure, there is a problem. Even if two instruments are used and one fails, one does not know which to trust. Logic dictates that one requires a minimum of three Control instruments. Before discussing Protection, it is good to discuss the characteristics of the instruments available: