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TECHNICAL FEATURES l EMISSIONS

Storage tank vapour emissions have experienced significant reductions over the past 30

years. Regulatory requirements have driven a host of tank upgrades now available to the industry. Upgrades include internal floating roofs, external floating roofs, guide pole sleeves, double seals, liquid mounted mechan-ical shoe seals, closed loop sample systems, inert gas blanket pads and vapour recovery systems.

In addition, updated tank moni-toring systems, use of overfill alarms and operating procedures have further reduced vapour emissions by better product level monitoring, required scheduled seal inspections as well as minimising poor past practices like: emptying tanks for extended periods, routine product stripping and refilling operations, overfilling of tanks and so on.

Lowering of tank emissions from this point forward is a challenge as most of the larger emission sources have been addressed. To achieve fur-ther reductions will require examining

the remaining emission sources. Tank operations such as tank cleaning, initial filling and unscheduled maintenance activities can produce a sub-stantial amount of hydrocarbon vapours. In order to mitigate emissions from these activities conventional practices should be challenged and replaced with use of better technologies, operational and maintenance practices.

Due to the migration by the industry to API’s-653 internal inspection program, one could make an argument that increasing the frequency of internal tank inspections has actually increased emissions from tank desludging, cleaning and gas freeing operations. That being said no one would or should challenge the benefits to the environment by industries adherence to API-653 inspection and repair program.

In an effort to combat the increased frequency of tank cleanings, sev-eral US states have placed specific requirements on tank cleaning oper-ations in an effort to minimise vapour emissions. Use of vapour burning engines, thermal oxidisers, portable condensers, charcoal adsorptions packs and scrubbers have worked to some extent on specific clean product tanks where the tank atmosphere can be displaced rapidly, although less harmful emissions from the destruction of hydrocarbon (HC) vapors can still occur.

However minimising HC vapour emissions from black/crude oil storage tank cleanings can be much more difficult as bottom sludge continually emits (HC) vapours when disturbed during cleaning/mining operations that can takes weeks to complete. Due to the challenges, duration and cost of cleaning black/crude oil tanks, little headway has been made toward reduction of emissions from this activity.

MINIMISING EMISSIONSThe good news is, there are cleaning technologies, methods, practices, available to industry that can, and do, minimise black/crude oil tank emis-sions. Submerged jet mixing (SJM), solvent selection extraction (SSE) and chemical cleaning (CC) are a few proven technologies that are cost effective and work.

These cleaning methods typically do not require extended opening of the tank as the solids/sludge are diluted, dissolved and liquefied for pump out of the tank as a liquid product. Once the sludge has been liquefied and pumped from the tank a more typical cleaning can take place similar to clean product tanks. An added advantage of the SJM system is the jet mixers can be permanently installed for future use of routine desludging and scheduled cleaning without the need for opening up of the tank.

Lastly, unscheduled repairs and associated tank cleanings in many cases can be avoided all together by making in-service repairs. There are new proven cost effective repair technologies available to industry that allow for safely making engineered long term repairs to in-service tanks that have the integrity to keep the tank in service until it’s next scheduled API-653 internal inspection. Friction forge bonding (FFB) is

LOWERING STORAGETANK EMISSIONS

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TECHNICAL FEATURES l EMISSIONS

one such technology that success-fully bonds metal studs to tank roof or shell plate. Once the studs are installed an engineered gasketed lap patch plate is bolted in place. This in-service repair method restores integrity to the leaking or weakened area of the tank in a similar manner that a welded lap patch repair does only without need of conventional welding.

In summary it should be noted that reduction in tank emissions has come a long way due to regulatory driven innovative technologies over the years. The combination of drivers and innovation should continue as newer and more innovative technologies continue to evolve.

FOR MORE INFORMATIONThis article was written by Robert Buckley, tank T/A strategist and con-sultant at Buckley Ventures.

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Reduction in tank emissions

has come a long way due to regulatory

driven innovative

technologies

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EMISSION CONTROL SOLUTIONS