A pressure relief valve that is actuated at a set temperture. ie the device remains closed until it reaches 150F at wich point pressure is released.
There are a lot of them out there, for instance those provided by pump manufacturers (Cat, Hypro) for thermal relief of positive displacement pumps (eg pressure washers).  Knowing more about your requirements (case pressure, max. differential across valve, flow rate, response time required) would aid in others helping you search.
Most hot water storage tanks have such a valve. These are very common so check with your local plumbing material supplier.

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