二、High velocity relief valve with gas freeing cover
The pressure valve will automatically open when the pressure in the tank reaches to the set pressure of the high-velocityelief valve and the gas in the tank will be released with the speed of 30m/s once assemble the high-velocity relief valve on the ventilation system of the cargo oil tank A manual ventilation system gas-freeing cover is necessary while do other works of the tank such as cleaning，removing the gas because such works need ventilate and the pressure cannot work while the pressure is low or without pressure (remark ：the pressure valve is not manual and it will work with the velocity of smaller than 30m/s) .
According to the regulations of MSC/Circ 677 the gas-freeing cover can prevent the flame into the tank while the gas-freeing cover meets to the demands of the pressure/vacuum valve.
Shell，upper cover and flame screen (flame retardant bed) constitute gas-freeing cover. It can directly assemble on the vacuum valve of the high-velocity relief valve.
assemble the gas-freeing cover on the vacuum valve
If the ship Design ＆ Research Institutes and ship-owners want to as semble both gas-freeing cover and some other accessories on the high-velocity relief valve，the gas-freeing cover can fix on the branch of the high-velocity valve or branch of vent pipe as the gas-freeing cover is a separated one.
assemble gas-freeing cover on the branch of the high-velocity relief valve