All vessels has tanks for fuel, water, ballast and cargo. These tanks need to breathe, much like ourselves. 

Each vessel needs several tank vent check valves or so-called G-valves. Ship designer Marin Teknikk have developed ship designs since 1981. The company has achieved solid reputation for its many innovations, but also for the important stability- and weight calculations:

When we make our stability calculations, we presume that the air vent check valves are weather tight. That means that they should close and protect the ship against water coming in as a reliable guard, every day for the lifetime of the vessel.
Marin Teknikk

When calculating stability, the valves are assumed to be working reliably at all times – but what happens if they don’t? If a ship is listing heavily, with valves in the wave zone or even underwater? If the valves fail, for instance by the float being stuck, damaged or simply gone, the valves will take in water. Worst case, the pumps could be out of order due to electrical failure or blackout, or the machinist could have evacuated. This situation may rapidly become very serious for the ship.

The valves reliability is of the highest importance for safety at sea. If the valve is slightly too narrow for the airflow stream, suction blocking
will occur. With a blocked valve, suction forces will create tension in the tank which might cause a breakdown in the tank’s bulkhead structure.
 The opposite may also occur: high pressure drop due to overfilling, forcing the tank to break down outwardly. Correct sizing, or even better an oversized valve, could prevent tank breakdown. 

In case of an unannounced inspection from the port authorities, the valves could be one of the most critical points that may be checked. If they are not accepted, your ship can be detained until it is repaired. The resulting off-hire time may be days or even weeks, depending on where you are in the world and how quickly the problem can be solved.