A couple of years ago, in a PHA, I came across a scenario where a “capture device” had quite “the pull” on several small tanks that were NOT rated for a vacuum. As usual, my inquiry was met with the old “this is the way all the other plants were built and they have not experienced any issues“. But I learned about the power of even a small vacuum the hard way in 1996, and now I am very keen on identify process opportunities for these vacuum scenarios. Plus, I am always surprised at how many people literally believe the vacuum “sucks the container in“, when in fact, it’s the atmospheric pressure that crushes the container from the outside. So lets us a vac of 0.6 psig (or 16.63 inches of water vac) as the “small vac” scenario…
To visualize the force, consider a standard vertical storage tank that is 12 feet in diameter and 15 feet high:
- Total Surface Area (Side Walls): Approximately 97,784 square inches.
- Total Inward Force at 0.6 psig: 97,784 X 0.6 = 58,670 lbs
That is over 29 tons of force pushing inward on the tank walls. Most atmospheric tanks (built to standards like API 650) are designed to withstand the outward pressure of the liquid, but they have very little structural resistance to inward “buckling” forces.
A blower generating 0.6 psig vacuum can easily trigger a collapse in these common industrial situations:
- Pumping Out Without Venting: If a high-capacity pump or blower is removing liquid or vapor faster than the vacuum relief valve (VRV) or vent can replace it with air.
- Steam Cleaning: If a tank is steamed out and then blocked in, the steam condenses into a tiny volume of water, creating an internal vacuum.
- Failed Conservation Vents: If a “mushroom” vent or PVRV (Pressure Vacuum Relief Valve) is frozen shut, painted over, or clogged by bird nests or product buildup.
- Rapid Temperature Drops: A sudden heavy rainstorm on a hot afternoon can cool the internal vapor space of a large tank fast enough to create a vacuum that exceeds the vent’s capacity.
Why 0.6 psig is the “Danger Zone”
Many thin-walled atmospheric tanks are only rated for a vacuum of 1 to 2 ounces per square inch (about 0.06 to 0.12 psig). A blower capable of 0.6 psig is roughly 5 to 10 times stronger than the structural limit of a standard non-rated tank.
Safety Note: If you are using a blower for an exhaust or ventilation task on a tank not rated for vacuum, ensure the inflow air capacity (the “make-up air”) is significantly higher than the blower’s pull to prevent any negative pressure buildup.