safety valve 
noun
1A valve that opens automatically to relieve excessive pressure.He designed a safety valve to prevent the pressure of steam building up to dangerous levels....
- The harnessing of solar energy by photosynthesis depends on a safety valve that effectively eliminates hazardous excess energy and prevents oxidative damage to the plant cells.
- I wasn't satisfied with the strength of the cylinders in which the pressurised, volatile mixture was carried, and I wanted more tests carried out on the safety valve intended to prevent blowbacks.
1.1A means of giving harmless vent to feelings of tension or stress: laughter may be a safety valve that reduces tension...
- Showing images of mere reversal may in fact provide a safety valve for the social tensions that the women's movement has created by demanding a more dominant role for women.
- Certain elements within the ruling class regarded the party as a useful potential safety valve amid mounting social tensions.
- Releasing our emotions can act as a safety valve - relieving inner tensions, like steam out of a kettle.