The definition of emergency venting (per UL 142) is, “Venting sufficient to relieve excessive internal pressure in storage tanks caused by fire exposure.” If a fire occurs, a correctly sized and properly functioning emergency vent will allow excess pressure to relieve through the vent opening allowing the tank to maintain its structural integrity.
The tank fabricator creates the vent opening in the shell of the tank in accordance with tables in UL 142 and NFPA 30. The opening size is calculated based on the wetted area of the tank, also known as the exposed surface or shell area of the tank. UL and NFPA have established the specifications for the tank fabricators to follow. Unless the opening type is specified by the tank owner, the fabricator chooses to install the appropriate size male threaded riser connection, female threaded coupling, or a flanged connection.
With a known opening size and connection type, the next steps in the selection process are:
- Choose a vent with a UL 2583 Listing
- Select the appropriate opening pressure setting—eight ounce per square inch or sixteen ounces per square inch.
- Determine the preferred vent style—lightweight aluminum spring-assisted style or weighted style with cast iron covers.
- Determine if screens are desired to serve as a theft deterrent.
- Verify if California CARB, EVR approval is a requirement.
- Confirm if the fuel is compatible with the materials of construction used in the manufacture of the vent. Morrison vents are manufactured of aluminum, power coated cast iron and have FKM seals.
Inspectors and local authorities having jurisdiction are looking for UL listed emergency vents. UL 2085 tanks are required to be shipped from the fabricator with a UL Listed emergency vent on the primary and the interstitial vent openings. It is important to select an emergency vent with a UL 2583 Listing. The vent should be permanently labeled with the UL label and include the certified SCFH vent capacity.
The following is an example of the permanent labeling on a Morrison Model 244 emergency vent:
Labeling on a Morrison Model 244 Emergency Vent
Many of the emergency vents sold today are set with an eight ounces per square inch opening pressure. Morrison’s “rule of thumb” for vent selection has been to choose a vent with a lighter opening pressure setting for a heavier product such as diesel fuel or select a vent with a heavier pressure setting for lighter products such as gasoline. A heavier setting will conserve more vapors resulting in a cost savings through a reduction of fuel evaporation. The Morrison “rule of thumb” suggests selecting a normal or working vent with a setting less than the emergency vent so the normal vent will perform the day to day breathing while the emergency vent remains closed.
Morrison Lightweight Emergency Vent
Vents with the eight ounce per square inch settings are available with either weighted covers or lightweight spring-assisted covers. The sixteen ounce per square inch vents are available in the weighted cover style. From a shipping, handling, and installation perspective, the lightweight vents are a popular choice.
Morrison EVR approved emergency vent
Vents with screens are available in smaller size emergency vents. The screen option is considered a theft deterrent for individuals attempting to steal fuel through the emergency vent opening.
In some markets, there are additional requirements for a California Air Resources Board’s (CARB) Enhanced Vapor Recovery (EVR) listing. EVR-approved vents have been tested to California’s certification process, CP-206, and have been approved for use on tanks dispensing gasoline in the State of California. Inspectors will be looking for the EVR marking on the vent cover.
Morrison 244 Emergency Vent
One additional consideration in vent selection is compatibility. If the stored product is not compatible with the vent seals or materials of construction, select a vent that is. Incompatibility may cause vapors to escape the tank or may hamper the vent’s ability to open or close as designed.
Most experts consider the emergency vent the number one safety device on an aboveground storage tank. Selecting the best vent for this critical function is important.
