Sizing emergency venting for atmospheric flammable liquid storage tanks is a safety-critical requirement, yet we often find it undersized. The core objective is to provide sufficient relief capacity so that, in the event of an external “exposure fire,” the tank does not rupture due to rapid internal pressure buildup from vaporizing liquid. Unlike normal venting (which handles thermal expansion or breathing during filling/emptying), emergency venting is sized for fire exposure. We must ensure that the combined capacity of our normal vents and our emergency vents prevents the tank pressure from exceeding the tank’s Maximum Allowable Working Pressure (MAWP). The industry standard for this calculation is NFPA 30: Flammable and Combustible Liquids Code.
Step 1: Determine the Wetted Area (A)
This is the surface area of the tank that is in contact with the liquid and is subject to heating from an external fire.
- Vertical Tanks: Usually the first 30 feet of the shell height above grade.
- Horizontal Tanks: Typically 75% of the total exposed surface area.
- Rectangular Tanks: The total surface area of the bottom and four sides (excluding the top).
Step 2: Calculate Required Venting Capacity (CFH)
Using the wetted area (A), we calculate the required venting capacity in Cubic Feet of free air per Hour (CFH). The formula is generally derived from heat input equations found in NFPA 30:
CFH = 1,107 X A0.82
(Note: This is a simplified version; specific heat absorption factors (F) and latent heat constants (L) apply depending on the specific liquid stored.)
Step 3: Account for Piping Losses
The capacity ratings provided by manufacturers for emergency vents are usually based on a “free discharge” condition (i.e., the vent mounted directly on the tank). If you have piping between the tank and the vent (e.g., to route vapors away from a building), the flow capacity will drop significantly. You must:
- Calculate the equivalent length of the pipe, including all elbows, tees, and fittings.
- Adjust the required CFH to account for the pressure drop along that pipe length, ensuring the pressure at the tank top does not exceed the MAWP.
Don’t Forget Normal Venting! Our total relief capacity is the sum of your normal and emergency vents. We can count the normal vent capacity toward your total requirement, provided it is properly sized.
Tank Construction: If the tank is designed to “weak-seam” (where the roof-to-shell joint is intended to fail before the shell), the tank’s construction is the emergency relief.
Always document the calculation, including the assumptions made about wetted area and piping equivalent lengths.