High Velocity Vent on Tanker Ships
Huge amount of vapours are escaped from tanks while cargo operations, especially during cargo loading. These vapours may be toxic, corrosive and flammable in the case of chemical tankers. Opening tank covers while cargo operations cause these vapours to accumulate on deck where persons work. This can be disastrous in both safety and hazard point of view. Any sparks or naked flame on deck cause an explosion in case of flammable vapours. Similarly toxic vapours cause health hazards to the working personnel. So these vapours must be effectively cleared out from the ship's vicinity. High velocity vents are fitted to cargo tanks for this purpose.
Refer to the figure shown above. This is a commonly used high velocity vent. The major parts consist of a moving orifice fitted to a bellow to which a counter weight is fitted and a fixed cone where the orifice seats. The moving orifice sits down on the fixed cone because of the weight of the counter weight. Bellow act as a spring for movement of the orifice. When pressure increases inside the tank, it acts on the bottom area of the orifice, causing orifice to lift up against the counter weight, and forms a narrow opening between cone and orifice. Since the gap is very narrow pressure of the gas is converted to high velocity, hence gas is released at higher velocity of 30 meters per second which is sufficient to clear the gases out from ship's surroundings. If the pressure of the gas inside the tank is still increasing, moving orifice lifts up more and forms a larger gap between cone and orifice. The shape of the cone ensures, area of opening increases exponentially with increase in lift.
The higher velocities of the gas at the orifice causes creation of a partial vacuum, which draws fresh air to the gas and dilutes it. A flame screen is fitted around the cone as shown to prevent flame travel. This screen is to be periodically inspected and cleaned if found any deposits, since they can choke or reduce the capacity of the vent. A cover is fitted which can closed when vent is not in use.
Simple High Velocity Vent
Figure above shows a much simpler design of high velocity vent. Here two flaps closed with weights allow passage of tank vapours when pressure increases inside the tank. Gap between both flaps creates same effect of nozzle as explained before. Opening of the orifice is proportional to the pressure rise in the tanks.