Pressure Vacuum Valves
Small pressure rise can result in application of very huge forces when acted upon a large surface area. Since force is the pressure multiplied by surface area, pressure of 0.24 bar is sufficient to cause rupture and damages to the liquid cargo tanks of oil or chemical tanker ships. If the pressure inside the tank is not released effectively, this may result in buckling or bulging out of tanks at the top, bottom or sides. On the other hand if the pressure inside the tank falls, atmospheric pressure acts surround the tank and push it inward causing structural damages. Pressure Vacuum Valves are fitted to liquid cargo tanks of tanker ships to prevent abnormal rise and fall of the pressure in the tanks and to keep pressure at a pre-set value.
Figure above shows the working principle of a pressure vacuum valve. Bottom of the valve is connected to cargo tank and top is opened to atmosphere. There are two valves named Pressure valve and Vacuum valve. Pressure valve seats on the body while vacuum valve seats on the pressure valve itself. Both springs are in compressed state. Opening pressure of both valves are set by adjusting spring tension.
When pressure inside the tank exceeds set pressure, pressure valve lifts up along with vacuum valve and release excess pressure to the atmosphere. As pressure in the tank drops to normal pressure valve sits down. Same way when there is a vacuum inside the tank, atmospheric pressure pushes vacuum valve down, causing outside air rushes to the tank thereby filling the vacuum. A flame trap is fitted to the top of the valve to prevent passage of flame and sparks.
It should be noted that the primary use of pressure vacuum valves (pv valves) is accommodating minor changes in pressures. PV Valves must not be used for tank venting purposes. During cargo loading at high rates, gases in the the tank to be expelled out very fast which is attained by 'mast risers' or 'high velocity vents'. Mast riser and high velocity vents are specially designed for venting of flammable gases to a much higher heights so that vapours are not accumulated surrounding the ship. Tank domes, butter worths, manhole doors should never be used for tank venting as it causes accumulation of combustible vapours all around the deck.