Ok, So I work as a Marine Engineer but I do not have an engineering degree so I am having a hard time wrapping my head around this problem.
So currently on my ship we had a minor spill overboard from our black water collection tank (lower left) where air was drawn in from our grey water (lower right) tank. The air that was drawn in agitated whatever liquid was in the black water tank, causing it to foam and go overboard.
The grey water tank is in a machinery space that is very well sealed in case of a fire, lets make the assumption that it is a sealed box. The water from our sinks, showers and drains gravity feed into this tank. When the tank reaches a certain level the pressure head from the sensing tube will cause the valve activator to send a signal to our vacuum discharge valve to open. This will allow our vacuum collection system to draw water from the grey water tank. The vacuum discharge valve will close when the pressure head in the sensing tube goes down to a certain level.
A vacuum in the system is provided by a pump recirculating the liquid in the BW Tank through an educator.
The Problem / Proposed Cause
The proposed cause of air being drawn into the Vacuum Collection System was due to an exhaust fan in the space being set to high while the intake fan remained at low. This caused a vacuum in the machinery space which apparently caused premature pressurisation of the sensing tube tube to act as if the pressure head was great enough to cause the Valve Activator to open the Vacuum Discharge Valve eventually allowing in air.
What I cannot wrap my head around.
I can't seem to understand how a vacuum in the space would cause over pressurization of the sensing tube? I would think a positive pressure in the machinery space would cause that.
Is there some sort of law in physics that can explain this for me? Or does the proposed explanation have flaws?