ENVIRONMENTAL regulations continue to impact floating roof storage tanks, particularly when vapors escape while the tank is standing idle or during refilling.
These and other issues surrounding floating roof storage tank operations were on the program at the 2007 International Operating Conference and Trade Show sponsored by the Independent Liquid Terminals Association June 11-13 in Houston, Texas.
Issues impacting floating tank roof operations were addressed by Rob Ferry of The TGB Partnership, Keith Sheedy of the Texas Commission on Environmental Quality, Nancy Van Burgel of Kinder Morgan Inc, Matthew Kuryla of Baker Botts LLP, Merrill Kramer of Chadbourne and Parke LLP, John Lieb of Tank Industry Consultants, Ken Erdmann of Matrix Services Inc, Mike Doxey of HMT Inc, Paul Anderson of GEM Mobil Treatment Services Inc, Les Bente of Don Miller and Associates, and Charlie Stone of ConocoPhillips.
Emission causes
Ferry reminded terminal managers that they should be aware of the causes associated with fugitive emissions. Preventing emissions isn't just the law, it is environmentally sound and saves money, he added.
Sheedy discussed recent studies in the Houston area that revealed emission releases not previously detected from landing floating roofs, which prompted an initial voluntary “find it and fix it” program.
Kuryla noted that the volatile organic compounds (VOC) rates uncovered by the Texas survey of floating roof storage tanks were higher than those at refineries. As a result, states and the federal government are likely to pay more attention to storage facilities as a result of the Texas situation.
He added that storage and terminaling facilities should manage their emissions properly to avoid the federal government stepping in with mandates. He predicted that more efficient and cost-effective technology will be developed to capture and control emissions, and as more companies step up to the plate, others will join in.
As a result of the Texas study, terminals in the Houston area were requested to revise their reports for emissions due to landing and refilling emissions from floating roofs. Kinder Morgan, Oiltanking, Vopak, Intercontinental Terminals, and Teppco have committed to making reductions in the amount of emissions due to the landing causes, Sheedy said.
Van Burgel said Kinder Morgan launched a thorough review of its air emitting activities at two Texas facilities and took action to correct them. “It was a huge exercise that we had to put a lot of energy into,” she said.
The company informed its customers on the limits to landings and looked at three scenarios: maintenance, product changes, and emergencies. A decision was made to keep all emissions within the facility annual cap and to revise its permit to include landing loss emissions. “We did the right thing and we're happy with the results,” Van Burgel said.
Avoid failure
Turning to the subject of floating roof design and ways to avoid failures, Lieb said designers should calculate pontoon volume required to keep the roof afloat under specified loading conditions. Allowable stress criteria and effective pontoon section should be established for the design of the pontoon. Engineers should calculate the center deck deflection (sag) under rainfall and flooded deck conditions, as well as associated radial forces on the pontoon section.
The section properties of the effective annular pontoon section should be calculated, as well as the stresses, rotations, and deflections resulting from the radial deck pull forces applied to the annular pontoon section. Compare the stresses, rotations, and deflections to the allowable criteria established for the design. If calculated stresses exceed allowable values, reproportion the pontoon section and recalculate values.
Set roof support leg sleeve heights and other openings so that oil cannot overtop the deck openings under any loading conditions. The governing case is the roof floating on the minimum specific gravity liquid. Evaluate float depth for roof floating on actual liquid, which may, and typically does, exceed 0.7 specific gravity.
Even with these precautions, significant seismic events in the last few years have resulted in sunken floating roofs, Lieb said. The incidents have prompted the American Petroleum Institute (API) to form a task group to develop more prescriptive design rules for floating roofs, he added.
Cable suspensions
Another floating roof discussion centered on those made of aluminum equipped with cable suspensions. Erdmann said that the design preserves coatings and reduces bimetallic corrosion because there is no contact with the tank bottom. With the cable-suspended roof, no legs are in the way for future repairs, and the roof can be raised to a higher level to provide headroom for larger equipment.
In his presentation, Doxey offered suggestions for tank modifications or alternative designs that can increase the net working capacity of the tank. To achieve success, modifications or designs should begin with early planning, and completed projects should be inspected carefully.
He said a tank capacity can be increased by maximizing the distance between the minimum fill level and the normal fill level. For tanks without floating roofs, it is usually only feasible to lower the height of the suction nozzle or install an underbottom suction nozzle. For tanks with floating roofs, the “general rule of thumb” is to remove obstructions at the bottom of the roof's travel and at the top of the roof's travel.
Tank degassing
Anderson discussed tank degassing and vapor control. Degassing requires draining any remaining product and removing heel before beginning the process. A ventilation blower should be used for safe entry. Not until all care has been taken in tank degassing should a tank be entered for repairs. Safety considerations include proper grounding, continuous area monitoring, and use of personal protective equipment. Hot work permits should be in order and detonation/flame arrestors should be in place.
He pointed out that vapor control involves accounting for emissions points that can be at popup vents, pressure/vacuum relief vents, antirotation pipes, and/or floating roof seals.
Safety issues
Safety issues were addressed by Bente and Stone. Bente discussed tank entry safety precautions, noting that each year workers are killed or injured in confined space incidents, and more than 50% of the deaths are would-be rescuers. Of those, approximately a third are supervisors. He pointed out that accidents can occur in very small spaces as well as large.
To help prevent these accidents, he recommended an American Petroleum Institute (API) tank entry supervisor certification program. Bente said the API certification improves safety during cleaning, inspection, and repair of confined spaces. It provides a benchmark to assess and select contractors and results in better trained facility personnel.
API listed seven areas to address, including project planning, tank preparation, ventilation and atmospheric testing, entry for inspection (initial), tank cleaning, entry for repair and modifications (inspection), and return to service. Project planning should include operator responsibilities and contractor responsibilities.
The certification provides a baseline of professional competency, voluntary participation (except where required), and targets tank cleaning and inspection contractors, tank repair contractors, third party inspectors, tank lining contractors, and owner/operator in-house entry supervisors and inspectors.
Stone said ConocoPhillips has issued a Required Standard of Practice (RSP) that calls for an initial review of the tank work plan with contractors, a written safety plan before start of work, pre-entry tank inspection, and compliance with lockout/tagout, hot work, and confined space requirements. The RSP also includes installation of crib towers upon initial tank entry and review of the plans by a qualified engineer, as well as a health safety and environment (HSE) and certified tank entry supervisor.
ConocoPhillips requires contractors to be a certified tank entry supervisor during cleaning, floor replacement, leg replacement, and other heavy crafts that are conducted. Contractors also must participate in an initial review of the work plan, complete a site safety plan, and participate in the review of the safety and roof stabilization plan.
More information about the tank entry supervisor certification procedures, as well as many other API certification and qualification programs, can be found at api.org/icp.