what are the basic principles of pressure vacuum valves on working? thanks.?

Answer 1

on the principle of pressure. On the side that opens the pressure fluctuates. When there is a lack of pressure it sucks the valve open and when there is an influx of pressure it pushes the valve closed.

Answer 2

On a tank that doesn't normally operate with an internal pressure or under vacuum conditions, such as a water tank, all you require is a vent that is large enough to allow enough air to be discharged from the tank thru the vent while it is being filled so that an intermal pressure is not produced, and conversely, while water is being discharged from the tank, the vent must allow for sufficient air to enter the tank so that a vacuum isn't formed in the tank.
In tanks that operate under low internal pressures such as fuel tanks that are blanketed with pressurized nitrogen gas to keep air out of the tank, the vents must be set to open at a pressure setting equal to the internal design pressure of the tank, and the vent must be sized to pass the maximum amount of blanketing gas that could possibly enter the tank thru the blanketing gas system, without overpressuring it.
The same applies to tanks that are designed for external pressures. The vacuum valve must open at the design set pressure of the tank, and allow for sufficient flow into the tank so that the maximum design vacuum condition is not exceeded.
There are Codes and Standards for the design of all tanks, that define the venting and design requirements.