During marine tank vessel loading and other operations that introduce cargo or liquid into vessel cargo tanks, vapours, mostly volatile organic compounds (VOC), emit from the cargo tanks because of displacement and vapour growth. Under the authority of US Clean Air Act Amendments of 1990 (CAAA 90), the US Environmental Protection agency (EPA) has issued two sets of national standards for marine loading facilities designated as major sources to collect VOC and hazardous air pollutants displaced from marine tank vessels during loading and to reduce the captured vapours by 95 per cent to 98 per cent by weight.
These standards do not address vessel unloading operations. The EPA also requires collection of vapours displaced from marine tank vessel cargo tanks during loading of cargoes containing 70 per cent or more benzene by weight. Many US States have adopted the EPA standards and issued regulations, similar with EPA standards, requiring major terminals to control vapours emitted from vessel cargo tanks during various marine tank vessel operations. Also under the authority of CAAA 90, the US Coast Guard in 1990 issued safety regulations for marine vapour control systems (VCSs). The majority of these regulations are contained in 33 CFR part 154, subpart E (for facilities) and 46 CFR part 36 (for vessels). US Coast Guard Marine VCS requirements for transfer facilities
Currently, marine terminals in 23 US states have about 250 VCSs installed and certified as meeting the US Coast Guard requirements for facility marine VCSs. The majority of these systems are located in the States of Texas, Louisiana, California, and New Jersey. Most of these systems use a flare for vapour destruction. Tank vessels visiting these facilities are required to have a vessel VCS meeting the US Coast Guard requirements for transferring vapours emitted from cargo tanks to the facility’s VCS for processing.
A typical facility marine VCS consists of a vapour collection system and a vapour processing unit. In most cases, vapours emitted from a tank vessel’s cargo tanks are collected by the tank vessel’s vapour collection system and sent to the facility’s VCS for processing. Methods used to process the captured vapours include recovery (absorption, adsorption, refrigeration), combustion (open flair, incinerator), or balancing (returning the vapours to facility storage tanks being emptied). A facility VCS may also contain these major equipment: pressure and temperature sensors, a relief valves, an emergency shutoff valve, detonation arresters and flame arresters, a liquid knock out pot, vapour moving devices, oxygen or hydrocarbon analysers, a liquid seal, and a vapour enriching, inerting, or diluting system.
Controlling flammable and combustible cargo vapours presents numerous hazards including cargo tank overfill, overpressure or vacuum, cargo spillage, and fire and detonation of the flammable vapours. Because the EPA standards apply to marine facilities rather than to marine vessels, facilities are required to protect tank vessels from those hazards. 33 CFR 154 subpart E addresses the safe design and operational aspects of a transfer facility VCS used to control vapour of crude oil, gasoline blends, or benzene emitted from vessel cargo tanks. It focuses on the following areas:
• Liquid overfill protection
• Overpressure and vacuum protection
• Fire, explosion and detonation protection
• Requirements for inerting, enriching and diluting systems
• Vapour compressors and blowers
• Vapour recovery and vapour destruction systems
• Personnel training and operating requirements
• Design, performance, and testing standards for detonation and flame arresters