To let off steam

After the industrial revolution, boilers were commonly used in steam-driven machines. Once heated, the machine would be packed with steam, which kept it pressurised enough to operate. But if the pressure becomes too high, the machine may explode and result in a tragic disaster. To keep this catastrophe from happening, a blow off valve is added to relieve the pressure in the boiler. Nowadays, ‘To let off steam’ becomes figurative imagery which means to relax as if letting off some steam from our body.

Example: He doesn’t hear advice from anyone. He needs to let off steam.