Issue link: https://geospatial.trimble.com/en/resources/i/1415432
50 Spring 2019 The RVRU system addresses this important safety concern and is available at comparable cost to the flaring units. A few plants and terminals on the Texas and Louisiana coasts have established the RVRU as the best practice for storage tank degassing for the safety advantages. The RVRU has no ignition source, but instead draws the VOC vapour from the tank, cools it to the point of condensation and recovers the hydrocarbon in liquid phase for return to the operating process or sale. Liquid nitrogen is generally used as a refrigerant and the whole system often has only one moving part, the non-sparking vapour blower. Therefore, the safety concerns related to combustion-based degassing are eliminated. Some RVRUs can even be used in a classified area, allowing work to be done in tight terminal settings. The US Coast Guard (USCG) has even certified RVRU systems for work on flammable barges and ships without a distance requirement from the vessel in the flammable range. Additionally, the RVRU can operate in an inert gas atmosphere – it can degas a tank vapour space that is completely inert (no oxygen) and since nitrogen gas is a byproduct of the refrigeration, the RVRU can pad the tank with more inert gas during the degassing operation, maintaining an inert tank vapour space. Emissions The RVRU addresses another key concern: emissions inventory. The implementation of a temporary or portable recovery system during storage tank turn arounds eliminates the emissions associated with combustion. RVRUs are often operated in a 'closed loop' by circulating the tank vapour though the RVRU then back to the tank, filtering the VOC out of the background gas (either clean air or nitrogen gas) by condensation. The vapour is collected as liquid condensate and pumped into totes at the job site. In this case there is no exhaust to atmosphere – no exhaust to calculate and add to operating or MSS emissions. The VOC saving can be documented, resulting in a considerable reduction in yearly operating emissions that can equate to real money in tradable or bankable VOC emissions as well as valuable recovered hydrocarbons. Close to 6000 t of VOC vapour is flared annually in degassing applications in the Houston area alone – much of this source of VOC emission can be eliminated with the RVRU. Recovering storage tank operating emissions can be another benefit of using a permanent RVRU. Some hydrocarbons targeted by environmental regulators are easily liquefied by a low cost, electric powered RVRU. Vapours can be leaned out above a floating roof during the heat of the day to reduce or eliminate emissions while filling a tank. Figure 1. Typical site layout of RVRU storage tank degassing project. Figure 3. Capturing benzene vapour in an internal floating roof (IFR) to reduce operating emissions. Figure 2. Generic 'closed loop' layout of RVRU storage tank degassing project.