...and to comment further on the actual pressures you mention:
16 bar operating pressure is the value you are allowed to operate with a EN PN16 pressure class valve. This pressure class is normally tested with a body pressure of 24 bars, valve open, and with 1,1 x 16 = 17,6 bar for seat tightness.
BUT: with a DESEIGN pressure of 25 bar it is indicated as Jim says, that 'nothing happens' at 25 bar, and you have to use the pressure class above, EN PN 25. EN PN25 is tested for body at 37,5 bar and seat thightness of 27,5 bar, allowing a normal occuring pressure up to 25 bar.
Compare with your car-tires. You do not have to drive 210km/h all the time even if your tires are designed for it, but you can do so , and it will be at a risk to do so with tires designed for lower speeds. It is illegal to mount tires with lower design speed on a car than what the car is built/designed for.
Many engineers believe that the test pressure can be used as the maximum normal pressure, but this is a safety margin.
The EN norm has a varied description and definition of the different pressures occuring and a detailed explanation of what is allowed on operating pressures, pressure peaks etc.
