In one of the Cooling Water Exchangers, the cooling water return line has been provided with a 3" connection to vent. In between there is small vertical vessel with a level gauge. The vent system with LG to indicate gas leakage to cooling water. The valve at the downstream of the vessel towards the vent is NC (normally closed).
I have no idea how the system will work. Please explain.
A copy of P&ID is attached.