jackssmirkingrevenge wrote:Isn't
this a pop-off valve?

***Edit ***

Sorry.. Dead form My bad. Replied to a random cannon pic.
Not quite, It is a pressure relief valve. A pop off valve has a second larger diameter so when the valve opens, it lifts the wide part of the piston so the valve stays open until the pressure drops considerably. This one with the seat at the exit will simply vent and reseat at 1 pressure, not two.
Pop off valves are designed to remain closed and when they open, they blow all the way open and stay that way due to the increased piston area exposed to the discharge stream until the pressure is low enough to allow it to re-close, where it suddnly snaps back fully closed. They are often used on steam boilers. Old steam train movies show this valve operating. It's the jet of steam from the middle of the top of the boiler that sometimes vents or not.
This page shows the anatomey of a pop off valve;
http://www.stanleymotorcarriage.com/Parts/PopValve.htm
I hope it shows the difference from a relief valve.