For decades, well before the promulgation of NSPSs for storage tanks, internal floating roofs (IFR) have been used to reduce the evaporation of petroleum and chemicals during storage. The vast majority of this evaporation is due to “breathing” losses. This includes rim seal, deck seam and deck fitting losses. Inevitably, storage tank owners and operators wonder how they can reduce emissions to stay in compliance. Of course, additional controls could be used but a simpler answer would be to examine storage tank emissions and evaluate where reductions can be found with simple physical changes to the floating roof itself. Specifically, an analysis of rim seals, deck fittings, and deck seam emissions can yield surprising results.
Emissions from volatile liquid storage are estimated using the methods found in AP-42 Chapter 7 – Organic Liquid Storage Tanks. AP-42 provides emission factors for the variety of different floating roof appurtenances and identifies which deck fittings, deck seam configuration and rim seals are most commonly used in floating roof tanks. Using these defaults, an emissions simulation was run for both a default IFR tank and a “master” tank using equipment with the lowest emission factors. A comparison of these emissions provides us an understanding of how emissions can be reduced by usually small equipment changes.
To perform this analysis, I’ve modeled two standard tanks with equal shell diameter of 55 feet and 20,000-barrel storage capacity. Both tanks are in gasoline service.
The default IFR has a vapor mounted primary rim seal on the floating roof, whereas the master tank is equipped with liquid mounted primary and rim mounted secondary rim seals.
The default IFR has 5-foot wide bolted deck sheets. The master tank has welded deck seams.
The primary difference between the default deck fittings and the master deck fittings is that the master deck fittings are bolted and gasketed, where the default fittings are neither. Also, some deck fittings were eliminated as a result of the cable suspended floating roof. A list of the type and quantity of deck fittings on each tank are as follows:
The differences in emissions between these two tank scenarios are summarized in the table below.
With the properly selected rim seal, deck fittings, and deck seam configuration, total VOC emissions can be reduced by over 96%. Of course, upgrading every storage tank to this “master” tank would be prohibitive. However, smaller steps such as bolting and gasketing deck fittings when tanks come out of service are easily implemented.
CFR provides its clients with storage tank compliance expertise, including recommendations to reduce emissions that allow a facility the freedom it needs to grow. Contact CFR to learn more about how we can assist.