a device for controlling the flow of a liquid or gas, letting it move in one direction only 阀；阀门；活门；气门 
The plumber will fit some new safety valves. 水暖工将安装几个新的安全阀。 You need special tools to open the valve. 这种阀门要用专门的工具打开。
Oxford Collocations Dictionaryadjective- safety
- shut-off
- non-return
- …
- fit
- install
- open
- …
- (
anatomy 解剖学 a structure in the heart or in a vein that lets blood flow in one direction only （心脏或血管的）瓣（膜） Oxford Collocations Dictionaryadjective- safety
- shut-off
- non-return
- …
- fit
- install
- open
- …
- (
music 音乐 a device in some brass musical instruments for changing the note （铜管乐器的）阀键，活塞 
Word Originlate Middle English (denoting a leaf of a folding or double door): from Latin valva.