In gravity systems that are open to the atmosphere, which boiler trim item is considered optional?

In gravity systems that are open to the atmosphere, the safety relief valve is considered an optional boiler trim item because these systems operate under lower pressure conditions due to being vented to the atmosphere. The primary purpose of a safety relief valve is to protect against overpressure situations that can lead to catastrophic failures, which is less of a concern in atmospheric systems where pressure is not incrementally increased as it is in closed systems.

In contrast, expansion tanks, pressure gauges, and flow meters serve important functions that enhance the operation and monitoring of the heating system. Expansion tanks help manage thermal expansion of the water, pressure gauges allow for close monitoring of system pressure, and flow meters help in assessing the distribution of water throughout the system. Each of these components contributes to the efficiency and safety of the boiler's operation, making them essential in both gravity and pressurized systems.