I am always amazed at the lack of a basic understanding of atmospheric pressure and process pressures/vacuums. My Intermediate 3-Day PS course and my Advanced 5-Day PS course have a single slide, and every time, I find I have to spend an hour explaining to the class what PSIG is and how it’s different from PSIA or just “PSI” slang.
When we see an atmospheric storage tank, that vessel stands as it has the same pressure INSIDE as it has on the OUTSIDE. This pressure is NOT ZERO; it is 14.7 pounds per square inch. Just imagine that the tank has this visible grid drawn in square inches. Every one of those square inches has 14.7 pounds of force being applied to it. So imagine going to the gym and grabbing a 15-pound plate or dumbbell. That is the weight/force being applied to every square inch of the surface of the vessel by Mother Nature/our atmospheric pressure at sea level. (BTW – this same scenario should be used in teaching ZES in LOTO training so that people can visualize 0.5 psi on a 36″ manway is NOT ZES!).
Now, imagine what happens when I pump out the liquid in my atmospheric storage tank. As the liquid level falls, it creates head space, and that head space MUST be balanced with the rate of the level falling. If we drop the level too fast AND the atmospheric vent is TOO SMALL, we will create a “vacuum” inside the tank. Look at this as a pressure-differential, meaning the 14.7 psi pushing on the tank’s walls from the outside is now HIGHER than the pressure inside the tank, and that external 14.7 psi will begin to crush the tank. So we are not really “sucking the tank in”; instead, we are crushing it under the atmospheric pressure (i.e. 14.7 psi).
This is why all the atmospheric storage tanks have to have venting EQUAL to the inlet and outlet flows. If we introduce a liquid faster than the tank can vent the headspace as the liquid level rises (think of using a syringe when giving a shot to someone), we will OVERPRESSURE the atmospheric tank (which is ONLY allowed to see up to 0.5 psig). If the pull liquid out of the tank faster than the tank can pull in air to maintain the headspace at 14.7 psi, then we crush the tank with the 14.7 psi of atmosphere.
I NEVER include my N2 blanketing in my assurance that we will maintain this atmospheric balance when pulling out of the tank! If we have 6-inches of outlets, then we better have a 6″ atm vent. If it is PHYSICALLY IMPOSSIBLE to have all 6″ of outlets operational at the same time, then we can have a discussion of the sizing needs of the vent. But we ALWAYS start off with a 1:1 basis. The same goes for inlets; however, I have seen far more atmospheric tanks destroyed because of a vacuum inside the tank.
SAFTENG members can read more about this topic at the website and see my course contents.