Venting Methodologies for Fixed Roof Tanks
Normal inbreathing (bringing air or blanketing gas into the tank) and outbreathing (venting vapor or blanketing gas out of the tank) occur when product is pumped into or out of a tank, or when ambient temperature increases or decreases. Adding product to a tank compresses the vapor space and increases the vapor pressure. Similarly, increases in ambient temperature will heat the vapor and increase the pressure in the tank. If the pressure of the vapor in the tank increases too much, vapor will need to be vented to prevent damage. If vapor pressure decreases, due to product outflow or ambient temperature decrease, outside air or blanketing vapor will need to flow into the tank. This inflow of air will prevent the vapor pressure in the tank from creating a vacuum, which could damage the tank.
When a tank needs to vent vapor, or take in air, it can do so in several ways. The simplest of these is through an open vent at the top of the tank. This method does mean that the vapor space is in constant contact with the surrounding atmosphere, though, so it’s not recommended if the tank contains harmful vapors or if the product in the tank can be spoiled by contact with outside air.
Another way to vent, or take in vapor, is through a breather valve, also commonly referred to as a pressure-vacuum relief valve. A breather valve is a weight or spring-loaded valve that opens when pressure in the tank reaches a certain level. This limits the tank pressure to a safe level, while limiting the tank’s exposure to outside air. When the tank pressure drops below an acceptable level, this breather valve will open to allow outside air into the tank to prevent the pressure from decreasing further. For smaller required flow rates, certain types of hatches can also allow for selective breathing.
In the event of an emergency, the amount of vapor that needs to be vented from a tank can be much larger than the normal outbreathing amount. An example of an emergency would be a fire near the tank. A fire will quickly increase the temperature of the product in the tank and the pressure of the vapor space. To vent enough vapor to cover the emergency case, two methods can be used for fixed roof tanks. The first is an emergency vent. An emergency vent can be a weighted lid-style vent, which can be used to obtain low set points, or a hinged-style vent, which allows for excellent repeatability. Both types of emergency vents allow for the necessary venting in a worst-case scenario without damaging the tank or requiring replacement of parts. The second type emergency overpressure protection available for fixed roof tanks is a frangible roof. A frangible roof is a tank roof that’s designed to break off from the rest of the tank in an emergency condition. This protects the integrity of the tank, but it does mean that a new roof will need to be installed.