When is it necessary to turn the hydrant valve?

Turning the hydrant valve is necessary when pressure readings are needed because it allows for the measurement of the water pressure available at that particular hydrant. Opening the valve enables water flow, which can then be measured with a gauge or other equipment to determine the pressure in the system. This is crucial during operations to ensure that sufficient pressure is available for firefighting tasks or for understanding the capabilities of the water supply system.

In the context of the other choices, closing the hydrant does indeed require turning the valve, but it is not the primary reason for doing so in scenarios related to pressure readings. Inspecting the hydrant may involve visual checks and assessments, but typically does not necessitate turning the valve unless flow conditions assessment is part of the inspection. Testing flow rate directly involves measuring how much water can be flowed through the hydrant, which would also require the valve to be turned, yet the act of turning the valve itself is more directly linked to pressure measurements than to merely testing flow rates.