Environmental Regulations Drive Changes In Floating-Roof Designs for Storage Tanks
May 1, 2001 12:00 PM
Environmental regulations continue to be a driving force for change in the way storage tanks are managed by the petroleum industry. Though recent regulations don't affect the main characteristics of floating roofs for storage tanks, they do impact details such as rim seals.
The US Environmental Protection Agency (EPA) in the January 14, 2000, edition of the Federal Register announced a regulation that will require industry action, says Terry Gallagher, product design manager for Chicago Bridge & Iron Company, Plainfield, Illinois.
The announcement reaffirms that slotted guide poles for petroleum and volatile organic liquid storage tanks are subject to the “no visible gap” clause of EPA Regulations on Standards of Performance for New Stationary Sources Title 40, Part 60, sub-parts K, Ka, and Kb. These sub-parts comprise the regulations for storage tanks.
The opening in the storage tank roof through which the guide pole passes, as well as the slots in the guide pole, constitutes “visible gaps: that must be maintained in a closed position at all times except when the device is in actual use.
“We expect that there will be a need to modify many of the external floating roof tanks currently in operation in the US refinery and terminal facilities over the next few years,” Gallagher says. “New equipment such as low-loss slotted guide poles is available. They have been available for several years and have proven performance records. In many cases, these components can be implemented economically and without removing the tank from product service.”
Floating-roof tanks use a significant amount of mechanical equipment to ensure safe and efficient operation, he says. Floating-roof primary and secondary seals, adjustable roof supports, mixers, swing-lines, special fitting details, and rolling ladders all must be designed to operate with the floating roof without compromising the integrity of the storage tank. Rolling ladders, roof drains, and fire foam systems must be designed and installed so that the floating roof remains in a balanced operating condition.
Floating-roof tanks are not maintenance free, Gallagher adds, but with the proper roof selection and use of well-designed rim seals, fittings, and related equipment, the new floating roof can provide many years of trouble-free operation. Many tanks built more than 45 years ago are still in operation.
Periodic visual inspections of the floating roof will help eliminate surprises, Gallagher says. Operators should look for indications of product on the deck, unusual water ponding, deck ballooning, manhole covers out of place, shunts contacting the tank shell above the secondary seal, trash in the drain, out-of-place roof support and bleeder vent locks, and bird nests in foam chambers.
The initial cost of a floating-roof storage tank does not represent the true cost of operation, Gallagher notes. A life cycle cost analysis represents the true cost and should be the primary consideration when selecting a storage system. When properly designed and constructed, a floating-roof tank should provide from 15 to 30 years of service with a minimum level of preventive maintenance.
Though many floating-roof tanks provide long life, problems with the service history of internal floating tanks has prompted an American Petroleum Institute subcommittee on pressure vessels and tanks to revise the design standard for internal floating roofs (IFRs), says Robert Ferry, The TGB Partnership, Hillsborough, North Carolina.
“The recently approved revisions to the API 650 Appendix H standard will require all IFRs, including the skin-and-pontoon type, to be designed to support a uniformly distributed live load,” Ferry says. “This will effectively require the deck leg attachment to be designed to support a significantly greater load than the 500 pounds previously specified.”
At the 2000 Independent Liquid Terminals Association annual meeting, Ferry updated a presentation he made at a 1987 ILTA session suggesting that design specifications developed by Colonial Pipeline Company appeared to be adequate for an acceptable service life of a skin-and-pontoon IFR. The paper cited as a case history an IFR that had been in service for eight years at that time.
In the early 1960s, Colonial had steel external floating roofs in their tank farms, Ferry says. By 1978, Colonial had evaluated several roof replacement options and devised a replacement program. An aluminum geodesic dome roof with an aluminum internal floating deck was adopted as the company's standard roof design for roof replacements and new tanks.
Rigorous Tank Duty
One of the aluminum internal floating roof designs installed by Colonial early in the company's replacement program was a skin-and-pontoon type manufactured by Conservatek. Colonial installed the system in a 180-ft-diameter tank built in 1979 to store distillate products.
“This tank is filled and emptied through a 36-inch line opening into a center sump,” Ferry says. “A deflector plate mounted above the sump is designed to prevent the incoming flow from surging upward into the floating roof deck, but there is no diffuser to reduce the speed of the flow. Incoming liquid directed along the tank floor by the deflector plate rushes outward until it meets the tank shell. Thus, the deck support legs can be knocked about by the torrent of incoming product.”
Besides that abuse, the deck support legs also are subject to being pushed laterally when they land on the sloped bottom, he added. The slope of the floor is such that the legs near the center are nearly two feet longer than the legs near the tank shell.
In the 20 years since the tank was put in service, the floating roof has had 1,000 to 2,000 landings, Ferry says. The repeated exposure of the space under the landed floating roof to moist air created an extremely corrosive environment that eventually compromised the integrity of the tank.
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