If a boiler pressure is greater than or equal to 400 psig, what is required?

When the pressure of a boiler reaches 400 psig or higher, it is crucial to ensure safety and accuracy in monitoring the water level. The requirement for two direct-reading mechanical gauge glasses is implemented to provide redundancy and to minimize the risk of failure.

Having two gauge glasses allows for cross-verification of the water level reading. If one gauge were to fail or give an incorrect reading due to factors such as blockage or damage, the second gauge can serve as a backup to confirm the actual water level. This is particularly important in high-pressure steam systems, where improper water levels can lead to catastrophic failures, including boiler explosions.

In summary, the requirement for two direct-reading mechanical gauge glasses at pressures of 400 psig or higher is a safety measure designed to ensure accurate monitoring and prevent potential hazards associated with boiler operations. This is why option B is the correct answer.