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Storage Tank Floating Roof Landing Emissions: Managing and Reducing VOC and HAP Emissions

August 16, 2019

 

 Today everyone should have an inventory to track emissions such as criteria pollutants, hazardous air pollutants (HAPs) and greenhouse gases (GHGs) on a routine basis. The development and use of the inventory is also a necessary planning tool to manage emissions as operational conditions evolve. For a bulk liquid terminal, emissions associated with floating roof landings can make up a surprisingly large percentage of emissions.

 

This is of particular importance for bulk liquid distribution terminals that are synthetic minor sources as these sources typically avoid Title V permitting by restricting an operational parameter (such as truck loading rack throughput).  These operational restrictions are often set to limit facility-wide emissions to less than 100 tons/year. In some cases, we’ve seen storage tank operators set the operational limit to reduce potential-to-emit to just below the major source threshold, up to 99.9 tons/year. This is problematic if every source of VOC emissions are not accounted for, especially floating roof landings. In this case, an overlooked source of emissions will result in the terminal inadvertently being subject to permitting as a major source.

 

As a refresher, floating roof landings occur when a storage tank liquid level pumped down until the roof rests on its legs (the floating roof is no longer floating). These floating roof landings often occur in gasoline tanks in order to reduce RVP to comply with USEPA limits on the summer volatility. 

 

Alternatively, floating roof landings also occur when storage tanks are taken out of service, usually for inspection or repair.  Floating roof landings are now being scrutinized by regulatory agencies, leaving many storage tank operators wondering how to reduce emissions to maintain compliance. While reducing the number of floating roof landings is one obvious solution, another way to lower emissions would be to alter the physical characteristics of the tank. Parameters such as leg height and heel type are critical in estimating roof landing emissions.

 

Floating roof landing emissions include standing idle and filling losses. Standing idle losses occur once the floating roof has landed and a vapor space of gasoline is created between the liquid surface and the floating roof. These vapors will breathe as a result of temperature changes similar to the breathing losses of a fixed roof tank. Filling losses occur when the floating roof is re-floated. As liquid enters the tank, the vapor space under the floating roof shrinks, forcing vapors from under the floating roof.

 

One way of controlling these vapors is to reduce the floating roof leg height. Landing a floating roof on low legs is an excellent and easy way to lower emissions during RVP landings. This decreases the size of the vapor space and corresponding losses. Another option to consider includes reconfiguring the tank bottom to eliminate the presence of a “heel” after the discharge pump loses suction. This “drain-dry” type of tank has significantly less emissions because there is no liquid in the tank to replenish the vapor space (clingage, only). 

 

CFR conducted an analysis of these emissions and found significant reductions can be achieved by using these techniques. The first tank is the “default tank”, which is a representation of the standard IFR storage tank. The default tank has a full liquid heel and a high leg setting of 6 feet. The second tank is the “master tank,” with a drain-dry tank bottom and a low leg height of 3 feet.

As presented in the table, floating roof landing total VOC emissions can be reduced by up to 63.5% with simple changes to the tank.

 

CFR has been assisting floating roof tank operators with their environmental needs for over 30 years. Contact CFR to learn how we can assist you in your emissions reporting and permitting needs.

 

 

 

 

 

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