Pressure/Vacuum valve (PV valve) and its Working Principle!

Pressure-vacuum valves mostly refer as PV valve onboard the vessel. PV valves are used in tankers, the name itself says that it deals with pressure and vacuum. So let’s answer all the WH questions regarding this topic.

What is PV valve?

It can be simply be phrased as a device which is been used to maintain the pressure and vacuum in the cargo tank during loading and discharging or can also be defined as a device to prevent over and under pressurization of the cargo tank.

Why and Where PV valve is been used?

In both chemical and oil tankers, the cargo is in liquid form and we all know liquid cannot be compressed. While loading in tankers, the volume of cargo which is been loaded replaces the volume of air or inert gas which was already been in the tank. If this replaced volume of air or inert gas is not been released from the cargo tank, that will result in over-pressurization causing the hazard of damage to the tank.

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In order to avoid this, a PV valve is been fitted to each tank to release the pressure. In the same way, it is been used while discharging the volume of cargo discharged should be replaced with same volume of air or with inert gas. So the PV valves allow airflow into the tank whenever the tank gets under pressurize.

How does the PV valve work?

As we know PV relief valve is been used for venting of cargo tanks for tankers carrying different grades of cargo. Let us understand the mechanism of the PV valve. It consists of two valves namely pressure and vacuum for positive and negative pressure respectively.


It consists of one set weight of diaphragm for each valve which gets lifted up because of over pressurization and the air or inert gas is released, in the same way, because of under pressurization or vacuum the diaphragm is lifted causing flow of air into the tank. The weight of diaphragm varies as per the structural designs of the tank.

PV valves are set at a limit to activate. Generally, the limit is set approximately as:

  • Pressure: 2000 mmWG
  • Vacuum: -350 mmWG
  • Unit mmWG stands for Millimeters water gauge.
  • Following notes can be referred for more understanding

Check out this handout on the PV valve by IMO to get an in-depth idea.

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