Double Deck Floating Roof [PDF]

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Horton Double Deck Floating Roof Designs Red Book Number: 9105-5 Revised: 16/01/2002 Issuing Authority: Ned A. Bacon/Plainfield/CBI Maintenance & Review: Plainfield Plate Structures Engineering 0.1 Scope: This standard furnishes detail and dimensional requirements for Horton Double Deck Floating Roofs up to and including 400'. The designs comply with API Standard 650 Appendix C.



A Horton Double Deck Floating Roof consists of two complete decks (top and bottom) joined by a series of concentric rims forming buoyant annular bays. The outer annular bay is divided into smaller compartments by radial bulkheads. The Horton Double Deck Floating Roof is designed to float directly on the product, thus reducing evaporation loss, corrosion, and fire hazard. The bottom deck is substantially flat when constructed, but due to roof flexibility, a small amount of temperature generated condensable vapor can be contained towards the center of the roof. Lower temperatures will condense the vapor back to liquid. A seal is used to close the space between the floating roof and the shell. API Standard 650 Appendix C requires all floatation compartments to be "liquid tight". This means continuous welding on one side of outer bay radial bulkheads and all circumferential rims to the top deck and "waterstop" welding where top deck lap joints cross bulkheads and rims is required. In this standard the diameter is the nominal tank diameter in ,which the roof will be installed.



1.1 Specification: API Standard 650, Appendix C "External Floating Roofs" and CBI requirements. The designs in this standard generally meet the requirements of BS 2654. See 3000-2-1Notes Link for specific differences. 1.2 Material: Designs are based on material with a minimum yield strength of 30 ksi. 1.3 Design Loads: The dead weight is the metal weight (theoretical) plus the weight of the accessories. a. Roof on Supports: Members and attachments are designed to support the dead weight and a uniform live load of 25 psf. b. Roof Floating: The roof is designed to float under following loading conditions: 25 psf uniform live load. Loss of buoyancy as in 1.4. Collection of water on top deck to elevation of emergency drains. c.



The roof is designed to be air raised during construction.



1.4 Floatation of Roof on 0.7 Specific Gravity Liquid: Sufficient pontoon volume is required to keep the roof floating with any two compartments punctured (a compartment is identified as an outer bay compartment or any annular ring) with no water or live load.



1.5 Design Procedure: The designs in this standard have been developed using computer program 50959A. 1.6 General Notes: Designs in this standard are based on 3/16" plate except where noted. Consult Plainfield Plate Structures for special thicknesses or where corrosion allowance is specified. Consult Plainfield Plate Structures for roofs floating on liquids of specific gravity lower than 0.7. 1.7 Shrinkage Allowance: Provide sufficient deck material (to allow for weld shrinkage) by calculating deck layout dimensions based on the following increase in theoretical deck radius: Tank Diameter 100' - 150' 151' - 200' 201' - 250' 251' - 300' 301' - 350' 351' and over



Radius Increase 1/2" 3/4" 1" 1 1/2" 2" 2 1/2"



The radius shown on deck drawing shall include the above shrinkage allowance. 1.8 Deck Weld: The top and bottom deck plates are lapped 1" and welded with continuous fillet welds on the top side. On the bottom side of the bottom deck, all seams within 1'-0 of rims, bulkheads, supports and other rigid members are fillet welded 2"-10". See Figures 3.2b, 3.2c and 3.2d for other weld details, All overhead welding is to be done with E7018 electrode. 1.9 Rafter Design: The maximum spacing for all rafters except in the outer bay is 2 π feet. The bending stress in the rafters is calculated by using a 25 psf live load with an allowable bending stress of 20 ksi. For the load condition for retention of water to top of emergency drains, the allowable bending stress used is .75 Fy = 27 ksi (A36). For angle rafters, the longer leg is vertical and perpendicular to the deck. 1.10 Deck Plate Layout: The usual maximum length of top and bottom deck plates is 24'-0. For longer plates, consult with Construction-Assigned. The top and bottom deck layouts should have a closing seam between the No. 2 and No. 3 rims as shown in Figure 1.10.



Figure 1.10 Typical Deck Layout



Figures 2.0a and 2.0b show typical views of Horton Double Deck Floating Roofs. Figure 2.0a is typical of the center draining roof designs given in 3.1. A single slope roof is one in which the top deck has a constant slope (minimum slope equals 1/4:12) from the outer rim to the center of the roof. Tables 3.1b and 3.1c give designs for single slope roofs up to and including 190' diameter. Figure 2.0b is a typical view of a reverse slope type roof. Designs for the reverse slope type roof for diameters greater than 190' up to and including 400' are given in Table 3.2a. The reverse slope roof has a top deck coned up in the center. This prevents the outer rim from becoming extremely high, thus allowing the use of mechanical shoe seals on large diameter double deck roofs. For general section details, use Std Form Drawings 3225-1 through 3225-10. Use Std Dwg 6B3 for miscellaneous roof welding details.



Figure 2.0a - Typical Single Slope Roofs



Figure 2.0b - Typical Reverse Slope Roof 2.1 Roof Supports: The Double Deck roof is furnished with adjustable roof supports. The roof supports are to clear all fittings and rolling ladder runway angles by a minimum of 1'-0. Roof supports shall clear moving parts of pipe drain by at least 24" and other parts of the pipe drain by at least 12". 9108-6Notes Link gives sizes, design procedures, and permissible loads for adjustable supports. The roof supports are located on Double Deck roofs so that the sleeves are attached to either bulkheads or circumferential rims by means of vertical shear plates. See Figure 3.2d. See Figures 2.0a and 2.0b for location of supports. See Tables 3.1 b, 3.1c and 3.2a for the number of supports. 2.2 Drainage Systems: A closed drainage system is generally used as the primary drain to remove water from the roof. Open drains can be used as the primary drain but do not meet the intent of API Standard 650 Appendix C. Locate the primary drain at the center of the roof for single slope roofs. For reverse slope roofs, locate primary drains 1'-4 inside of the low rim. Locate one drain near the rolling ladder runway. See Figure 2.0b.



See 9202Notes Link for number and size of primary drains. Emergency overflow drains (see 9202Notes Link) are also required. 2.3 Roof Manholes: For all center draining type roofs (see Figure 2.0a) provide 1-24" diameter manhole through both decks near the center of the roof. Where the roof diameter exceeds 250', provide an additional roof manhole for each shell manhole (maximum of 4) at the periphery of the roof. When a pipe drain is used, furnish a 30" diameter manhole (40" diameter for 6" pipe drain) in place of the 24" diameter manhole at the center to permit drain removal. For reverse slope roofs (see Figure 2.0b) provide a minimum of 2-24" diameter manholes about 4'-0 from the low rim and oriented approximately diametrically opposite each other. It is preferable to orient these manholes as close as possible on a radial line to the shell manholes. When pipe drains are used, make one of these manholes 30" diameter (40" diameter for 6" pipe drain) to permit drain removal. For roofs over 250' diameter provide one additional roof manholes for each shell manhole (maximum of 2) at the periphery of the roof. Locate these additional manholes perpendicular to the diametric line through the 2 manholes which are near the low rim. 2.4 Deck Manholes: 20" diameter vented manholes are required to permit access into each compartment of the Double Deck roof. Each manhole cover must be secured by a positive hold-down device. See Std Dwg B13B. For the number of deck manholes, see Tables 3.1b, 3.1c and 3.2a. The top edge of each manhole neck should be at least 6" higher than the maximum water level at the emergency drains. 2.5 Rim Vent: A 6" rim vent is required for each roof with a mechanical shoe type seal where the bottom of the shoe seal extends into the product more than approximately 4". See Std Dwg G6 for details. Locate the rim vent approximately 180° from the inlet line. Do not furnish this vent on roofs with foam filled seals. 2.6 Gage Hatch: One 8" diameter gage hatch is supplied with each roof. See Std Form Dwg 3200-R19. This gage hatch may be attached to a gage pipe as optional equipment when specified by the Purchaser. 2.7 Gage Well: A gage well is furnished only when the Purchaser specifies an automatic gage. Both automatic gage and gage well are optional. 2.8 Automatic Bleeder Vents: The automatic bleeder vent permits the release of air vapor mixture from under the roof while the tank is being filled and the roof is resting on its supports. The vent automatically closes after the roof floats. When the tank is emptied, the vent opens before the roof lands. It is fully open when the roof is on its supports allowing air to enter and preventing a vacuum from forming under the roof. See 9208-4Notes Link for capacities of automatic bleeder vents. See Figures 2.0a and 2.0b for locations of these vents. For reverse slope roofs, a minimum of 3 vents shall be installed. All vents must clear fittings and the rolling ladder runway angle by a minimum of 1 '-0. Pressure-Vacuum relief vents are strongly discouraged as a substitute for automatic bleeder vents. Furnish these relief vents only when the purchaser insists and only after providing the following reasons for concern. 1) Product vapor and residue collection on the vent pallet can restrict the relief vent from opening or can change the opening pressure. Adhesion "sticking" of the pallet to the seat can cause pressures far beyond design conditions. Failure of the vent to control vacuum



can cause significant roof damage during product withdrawal. Frequent maintenance of the vent is essential, however, accepted health and safety practices prohibit or restrict access to the floating roof while in service. 2) Gas pressure under a floating roof during initial filling operations must be controlled to prevent the roof from lifting. Relief vent settings must therefore be set well below a pressure equal to the average weight per unit area of the total floating roof structure. During tank operation, temperature and barometric pressure changes can result in gas formation under the floating roof. Gas formation occurs at pressures greater than the vent set pressure and will result in emissions. Design and sizing of pressure-vacuum relief vents may be made by the purchaser or by CBI using purchaser supplied information. Refer to 9208-2Notes Link for contract conditions and information required from the Customer. The minimum pressure relieving capacity (set pressure plus accumulation pressure) must be developed at a pressure not exceeding a pressure equivalent to the corroded average weight per unit area of the total floating roof structure. The minimum external pressure (vacuum) relieving capacity (set pressure plus accumulation pressure) must be developed at a pressure not exceeding the roof design live load of 12.5 Ibs/ft2 (2.4 in H2O). Refer to 9208-2-5Notes Link for vent sizing. As a minimum, API 2000 states the following rules for determining venting capacities: For emptying a tank, sufficient venting area must be provided for 560 cu ft of free air per hour for each 100 bbl per hour maximum emptying rate. For filling a tank, sufficient venting area must be provided for 1200 cu ft of free air per hour for each 100 bbl per hour maximum filling rate. Specific maintenance instructions for pressure-vacuum relief vents must be provided in the O&M manual. The set points of the vents must be confirmed by CBI at time of installation. See 761-1-2Notes Link. Provide a stilling well which reduces gas discharge velocity at the product surface to reduce the likelihood of entrained liquid particles going through the vent. See Std. Dwgs 3200-G10 and 3200-G11. 2.9 Guide Device: A guide pole prevents the floating roof from rotating in the tank during use and while resting on its supports. See 9201-6Notes Link for details. 2.10 Rolling Ladder and Rolling Ladder Runway: See 9203-3Notes Link. A rolling ladder is used to provide access to the roof from the gager's platform. Furnish the deck runway for the rolling ladder. See 9203-3Notes Link (6.1). Note: If the rolling ladder runway passes over a change in slope of the top deck, Engineering-Assigned shall detail the runway angle bend. It is acceptable to omit the rolling ladder if the Purchaser specifies, but the floating roof cannot be certified as meeting API Standard 650. 2.11 Seal Between Roof and Tank Shell: See 9204Notes Link. Nominal rim space is 8". Standard roofs over 320' diameter require a 12" rim space. However, if the tank height exceeds 56', tanks over 270' diameter will require a 12" rim space, unless they are built on a ringwall foundation. 2.12 Shell Inlet Nozzles: Provide a 5' minimum inner extension.



2.13 Rims: See 9105-0-20Notes Link for rim plate configuration. 2.14 Reinforcing Bar: Reinforcing bars must penetrate the bulkheads and be welded in accordance with Figure 3.2b. Welding around the end of the bar must be on the same side of the bulkhead as the bulkhead to bottom deck weld.



The roof weights listed in the following tables are theoretical and include both decks, rafters, rafter clips. bulkheads, rims, reinforcing bars, gager's platform, emergency drains and all items in 2.1 and 2.3 through 2.10. The weights do not include the seal or the primary drains. Weight given is for the maximum diameter. Nominal rim space is measured from the centerline of the shell plate to the outside of the outer rim plate. This dimension is used for design and details of the floating roof. EngineeringAssigned will show the actual space dimension between the inside of the bottom shell ring and the outside of the outer rim plate on the contract drawings. This dimension will be used by Construction-Assigned. Designs for both single and reverse slopes can accommodate up to 1/16" corrosion allowance on the bottom deck and outer rim without any other detail changes. English tables in this standard can be directly converted to metric using either 4.8 mm or 5.0 mm plate. 3.1 Single Slope Roof Designs: Table 3.1b gives standard designs for single slope roofs in tanks 50' diameter and less. Table 3.1c gives designs for single slope roofs in tanks over 50' through 190' diameter. Table 3.1a - Code for Rafter Sections Letter



Rafter Section



A B C D E G



3 x 2 x 1/4 C4 x 5.4 C5 x 6.7 C6 x 8.2 C7 x 9.8 C8 x 11.5



Section Modulus Metric Equivalent (in3) Size 1.93 3.00 4.38 6.08 8.14



UPN 100 UPN 120 UPN 140 UPN 160 UPN 180



Table 3.1b - Requirements for Single Slope Double Deck Floating Roof