Final answer:
The Type III hydrant system valve that opens to relieve thermal expansion is the pressure relief valve, which automatically opens when the pressure exceeds a certain limit.
Step-by-step explanation:
The Type III hydrant system valve that will open to relieve thermal expansion is the pressure relief valve. This valve is designed to automatically open when the pressure within the hydrant system exceeds a certain limit, allowing excess fluid to be released and relieving the thermal expansion.
When the temperature of the hydrant system increases, the fluid inside expands, which can lead to an increase in pressure. This can be dangerous if the pressure exceeds the system's capacity. The pressure relief valve acts as a safety measure to prevent any damage or explosion caused by excessive pressure.
Once the pressure relief valve opens, the excess fluid is released, and the pressure within the system is reduced. This helps to maintain the stability and integrity of the hydrant system, preventing any potential hazards.