Chemical or petroleum storage in a floating roof tank is the standard way to reduce air emissions. When floating roof tanks are taken out of service, the floating roof is landed on its legs and a vapor space is created between the floating roof and the liquid surface. As the tank draws down, the vapor space grows until the discharge pump breaks suction. When product is pumped back into the tank, the vapor space is squeezed until the roof is refloated. Most companies are aware of roof landing and refilling emissions and incorporate them into their air pollution inventories. However, the “in between” emissions from the degassing and cleaning of the tank are often overlooked.
Degassing emissions begin when the manways of the tank are opened and the vapor space is subject to forced ventilation in preparation for employee entry. The air being purged is typically monitored for both oxygen content and %LEL while employee entry will be prevented until acceptable conditions have been met. Quantifying the amount of product purged is straight forward if the air flows and %LELs are known.
After degassing the tank, employees can begin the process of sludge removal. During sludge removal, ventilation continues and volatile materials continue to evaporate. Sludge removal can take several days as part of a daily cycle of standing idle, vapor space purge, and sludge removal. The important variables in sludge removal are the total time for sludge removal, the concentration of the vapors and the molecular weight of the vapors. The first two of these variables should be available from the confined space monitoring.
Emissions from cleaning and degassing can be significant. In fact, we’ve seen operators who have imposed strict operating limits on the entire process that begins with the roof legs touching down on the tank floor and ending with the roof being refloated. Otherwise, degassing and sludge removal emissions could eat up any wiggle room between actual and permitted emissions. This makes it imperative to have accurate data when accounting for these emissions.
The key factors affecting degassing and sludge removal emissions are:
Number of days taken for sludge removal
Ventilation rate of air during degassing/sludge removal
Percent of the lower explosive limit during degassing/sludge removal
These factors should also be evaluated to reduce degassing and sludge removal emissions. Examples of ways to reduce emissions include:
Reduce the number of days taken for sludge removal. This approach will reduce overall time the tank is being ventilated and can be accomplished by using more workers. Each subsequent day of sludge removal also requires an additional initial vapor space purge and constant ventilation.
Route these emissions to a control device. Portable units are available if one is not currently at your location.
Avoid roof landing and tank cleaning during the hotter summer months to minimize evaporation.
When procedural changes aren’t effective, a permit amendment could be the answer. Many of our clients have already begun this permit amendment process. Here at CFR, we have working templates for degassing and sludge removal losses from any type of floating roof tank. Recognizing that these emissions may become a problem for your facility is the first step in becoming a more environmentally responsible business and minimizing the risk of future enforcement.
CFR has been working with tank operators for over 30 years. Call us today to see how we can help. CFR Environmental has a long track record of providing support for tank operators in the petroleum and chemical industry. Contact CFR to learn more about we can assist with your storage tank cleaning emissions program.