The sound was exactly that of steam roaring from a locomotive's safety valve.
So you'll have to be my safety valve, confidant and confessor.
All keyed up for war and no place to 164 go, and this was a kind of safety valve.
They do no harm, and are a safety valve which should be understood.
Watt's brain, working at high pressure, needed a safety valve.
That one terrible, hysterical outburst was perhaps the safety valve.
There was but one safety valve, and that was on the other boiler.
That sounds like you were getting up a head of steam—the safety valve!
We had to have some safety valve, and if we had not been laughing we might have been crying.
It was a safety valve to her feelings, although she did not realize it.