When a high-pressure boiler plant only has one boiler, which statement is true?

In a high-pressure boiler plant with only one boiler, an automatic non-return valve is not necessary because the primary function of this type of valve is to prevent backflow in systems that have multiple paths or parallel configurations. When there is only one boiler in operation, the risk of backflow is significantly reduced since there are no alternative routes for the flow of steam or water.

This simplicity in the system design allows for effective operation without the need for complex mechanisms like non-return valves, which can add to maintenance and operational costs. Therefore, not having an automatic non-return valve in such a configuration is valid, making the statement true. In contrast, the other options pertain to scenarios that either imply complexity where it is unnecessary (like requiring both valves to be manually operated) or suggest unnecessary redundancy (such as requiring safety valves when they are standard in all boiler settings regardless of number).