safety valve vs relief valve

safety valve and relief valve both are nouns.

Word	Noun	Adjective	Verb	Adverb
safety valve	Yes	No	No	No
relief valve	Yes	No	No	No

As nouns, safety valve and relief valve are synonyms defined as:

safety valve and relief valve: a valve in a container in which pressure can build up (as a steam boiler); it opens automatically when the pressure reaches a dangerous level

Other synonyms of safety valve include escape, escape cock, escape valve.

safety valve (noun)	relief valve (noun)
a valve in a container in which pressure can build up (as a steam boiler); it opens automatically when the pressure reaches a dangerous level	a valve in a container in which pressure can build up (as a steam boiler); it opens automatically when the pressure reaches a dangerous level