Costs of Steel IFRs • Higher than aluminum for initial installation • More loss of capacity than aluminum • Heavier material displaces more liquid • Doorsheet escalates replacement costs • The doorsheet itself involves additional construction • A hydrotest is required when complete • To test the integrity of the doorsheet, and • To stress relieve the doorsheet welds Tanks Essentials 3/31/18 Page 51
Comparison of Aluminum IFRs • Sandwich panel type: • More expensive than skin and pontoon, • More length of deck seams (higher emissions), • May require doorsheet/hydrotest to replace • If closed cell core, inherent safety concerns • Appendix H states that this type is “permitted however…enclosed spaces within a module may result in undetectable combustible gas.” • This type, then, does not provide the safety implicit in H.4.1.7 Tanks Essentials 3/31/18 Page 52
Comparison of Aluminum IFRs • Skin and pontoon type: • Least expensive, • Lower emissions than sandwich panel, • Replace without doorsheet or roof hole, • Foam system must be designed for full surface • Skin and pontoon type decks cost less than other types both initially and for replacement, but if a foam system is required for fire protection the foam system will cost more for skin and pontoon aluminum than for steel pontoon or doubledeck. Tanks Essentials 3/31/18 Page 53
Aluminum Skin & Pontoon • If best, why are others even considered? • Seems to be clear choice from value engineering • Negative perceptions in the market: • Flexibility is disconcerting for personnel. • Reputation of poor service history. • The Question: Are they inherently flimsy, OR have they been underdesigned? Tanks Essentials 3/31/18 Page 54
Aluminum Skin & Pontoon – Negative Perceptions • Flexibility is disconcerting for personnel. • Workers feel uneasy walking on it • Safe, but sensation like walking on a huge waterbed • A moot point in light of Confined Space Entry • Workmen generally not allowed to enter any IFRT • Reputation for poor service history. • API 650 Appendix H has had a design loophole. • Only required to support 500 pounds. Tanks Essentials 3/31/18 Page 55
API 650, Appendix H pre-Revision • Design standard for all IFRs • Allowed waiver of uniform live load • If equipped with deck drains • Only design load was then 500 pound load • As a result, some manufacturers use few legs • ‘Typical’ tributary area assumed for emission estimates is about 400 square feet per leg (an area 40 ft x 10 ft) Tanks Essentials 3/31/18 Page 56
API 650, Appendix H post-Revision • All IFRs to support uniform live load: • Still 12.5 psf if no deck drains. • Now 5 psf with deck drains. • This translates to 2,000 lbs for a 400 sq.ft. area. • Most failures have had too few deck legs. • Legs on 40 ft spacing would now need 10 ft spacing, or • Support 4-times more load than previously. Tanks Essentials 3/31/18 Page 57
Aluminum Skin & Pontoon - Summary of Service History • Frequent failures occur with poor designs • Successful deck leg design has featured: • Limit on tributary area of about 200 square feet • Designed to support 1000 pounds {5 psf} • Designed for real world load conditions • (i.e., out-of-plumb, out-of-level, & subject to cyclic loads) • Properly designed skin & pontoon IFRs can last longer w/o major repairs than the tank Tanks Essentials 3/31/18 Page 58
Summary of IFR Types • Steel pontoon and double deck: • Most durable, lowest emissions, best fire safety • Most expensive • Aluminum skin-and-pontoon: • Best value for many applications IF well made • Quality varies greatly • Higher emissions • Other designs: • May suit limited or specialized applications Tanks Essentials 3/31/18 Page 59
Floating Roof Features • Points of emissions from floating roofs • Deck seams (if bolted), rim seals, deck fittings • Types of devices to control emissions • Features of common rim seals and deck fitting controls Tanks Essentials 3/31/18 Page 60
Points of Emissions from Floating Roofs • Deck Seams • Contribute to emissions if bolted; but not if welded • Rim Seals • This is the closure device between the deck and tank • Deck Fittings • Only those features that open through the deck to the stored liquid are considered to be deck fittings for emissions purposes Tanks Essentials 3/31/18 Page 61
Deck Seams – Sheet Construction Tanks Essentials 3/31/18 Page 62
Deck Seams – Panel Construction Tanks Essentials 3/31/18 Page 63
Rim Seals Tanks Essentials 3/31/18 Page 64
Rim Seal Vapor Space Tanks Essentials 3/31/18 Page 65
Rim Seal Emissions Tanks Essentials 3/31/18 Page 66
Types of Primary Rim Seals • Vapor-mounted: • Highest emissions - Fair service history • Liquid-mounted: • Lowest emissions - Poorest service history • Mechanical-shoe: • Fairly low emissions - Best service history • Note that, in California, a mechanical-shoe seal is considered a type of liquid-mounted seal, rather than a separate category of rim seal. Tanks Essentials 3/31/18 Page 67
Vapor-Mounted Rim Seal • Bottom is not in contact with the liquid surface • Annular vapor space beneath the rim seal • Between the rim of the deck and the shell of the tank • Two common types: • Flexible-wiper (blade) • Continuous wiper blade around the rim of the deck • Resilient-filled type • A foam log within an elastomeric-coated fabric envelope • The envelope protects the foam core from liquid & abrasion Tanks Essentials 3/31/18 Page 68
Vapor-Mounted Rim Seal – Flexible-Wiper Type Tanks Essentials 3/31/18 Page 69
Vapor-Mounted Rim Seal – Resilient-Filled Type Tanks Essentials 3/31/18 Page 70
Liquid-Mounted Rim Seal • Bottom of the seal contacts the liquid surface • The rim vapor space is virtually eliminated • Resilient-filled design • Typically a foam log in an elastomeric-coated fabric • Differs from resilient-filled vapor-mounted rim seal in its position relative to the liquid • i.e., whether or not the bottom of the seal contacts the liquid surface Tanks Essentials 3/31/18 Page 71
Liquid-Mounted Rim Seal Tanks Essentials 3/31/18 Page 72
Mechanical-Shoe Rim Seal • Metallic shoe - slides against the tank shell • A series of overlapping sheets form a ring inside the shell • Mechanical device - pushing the shoe outward • Regularly spaced devices hold the shoe against the shell • Primary seal fabric - to cover the annular space • An elastomeric-coated fabric closes the annular space between the metallic shoe and the rim of the deck • In California, mechanical-shoe is considered a type of liquid-mounted seal, rather than a separate category Tanks Essentials 3/31/18 Page 73
Mechanical-Shoe Rim Seal Tanks Essentials 3/31/18 Page 74
Strategies to Improve Rim Seal Effectiveness • Minimize gaps between rim seal and tank shell • Seek to keep the rim seal firmly against the tank shell • Minimize the rim space tributary to a gap • Fill the rim space or provide a baffle to isolate it • Provide secondary closure of the rim space • Equip the floating roof with a secondary rim seal Tanks Essentials 3/31/18 Page 75
Minimize Rim Seal Gaps Tanks Essentials 3/31/18 Page 76
Baffle-Type Controls Tanks Essentials 3/31/18 Page 77
Rim Seal Gaps – Mechanical-shoe Seal Tanks Essentials 3/31/18 Page 78
Eliminate the Rim Space (Filling It) Tanks Essentials 3/31/18 Page 79
Rim Seal Gaps – Liquid-mounted Seal Tanks Essentials 3/31/18 Page 80
Secondary Rim Seals • Purpose is to reduce emissions • Particularly effective for external floating roofs • Mounted above the primary seal • The secondary is the upper seal in a double seal system • May be added to any type of primary seal • Mounted sufficiently high to not interfere w/the primary • Generally results in loss of tank capacity • Particularly with internal floating roofs, where the fixed roof limits upward travel of the floating roof Tanks Essentials 3/31/18 Page 81
Secondary Rim Seals – Rim-Mounted Secondary; Mechanical Shoe Primary Tanks Essentials 3/31/18 Page 82
Secondary Rim Seals – Shoe-Mounted Secondary; Mechanical Shoe Primary Tanks Essentials 3/31/18 Page 83
Secondary Rim Seals – Wiper Secondary; Log Seal Primary Tanks Essentials 3/31/18 Page 84
Secondary Rim Seals – Wiper Secondary; Wiper Primary Tanks Essentials 3/31/18 Page 85
Deck Fittings – Common Features • Only create emissions if through to liquid • Deck opening may be round or square • Shape of opening is a matter of fabricator preference • Bottom of opening (for most) extends below the liquid surface • Noncontact decks require a well extension or skirt • If for a penetration, deck cover must be loose • The cover must be allowed to slide for out-of-plumbness Tanks Essentials 3/31/18 Page 86
Deck Fittings Tanks Essentials 3/31/18 Page 87
Not Deck Fittings Tanks Essentials 3/31/18 Page 88
Deck Fittings Tanks Essentials 3/31/18 Page 89
Deck Fittings – Basic Categories • Small opening (nozzle or pipe sleeve) type • e.g., deck support legs, gauge hatches, deck drains. • Vent devices • Not required to extend below the liquid surface • Large opening (well) type • e.g., manways, gauge float wells, column wells, and vertical ladders • Guidepoles–aka gaugepoles, gaugepipes, etc. • Important enough to be a category unto themselves Tanks Essentials 3/31/18 Page 90
Deck Fittings – Support Legs Tanks Essentials 3/31/18 Page 91
Deck Support Legs – Control Options Tanks Essentials 3/31/18 Page 92
Sample Ports Tanks Essentials 3/31/18 Page 93
Effectiveness of Slit Fabric/Slotted Membrane • There is no apparent benefit to 90% closure at 0 mph wind • By comparison, emission factor for an uncontrolled slotted guidepole at 10 mph is 4200 lb-mol/yr Tanks Essentials 3/31/18 Page 94
Deck Drains (that empty into stored liquid) Tanks Essentials 3/31/18 Page 95
Rim Space Vents Tanks Essentials 3/31/18 Page 96
Vacuum Breaker (Automatic Bleeder Vent) Tanks Essentials 3/31/18 Page 97
Vacuum Breaker (Automatic Bleeder Vent) Tanks Essentials 3/31/18 Page 98
Access Hatch (Manway) Tanks Essentials 3/31/18 Page 99
Gauge Float (Automatic Tank Gauge) Tanks Essentials 3/31/18 Page 100
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