To let off steam is to release built-up energy or anger harmlessly. Let off steam at the gym.
The picture is a boiler, which would burst without a valve. Children need somewhere to let off steam far more than adults admit.
1phraselet off steamTo release built-up energy or anger harmlessly.
“He would run for an hour to let off steam after a bad shift.”
let off steam is a phrase.
blow off steam Relief valves released the excess pressure in a steam engine to prevent the boiler from exploding.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms