3.12 (Addition) Floating Roof Design 3.12.1 The design of floating roof tanks with specified flush type nozzles shall permit the outer pontoon to reach a point within 380 mm of the tank bottom, unless limited by obstructions such as mixers, roof drains, etc. 3.12.2 Tank roof design for tanks larger than 18 m in diameter shall be as follows a. Roofs shall be of the low deck (minimum vapor space) type. b. Center decks greater than 45 m in diameter shall have channel stiffeners on the under side of the deck. Stiffeners shall be 6in, 8.2 lbs/ft, or 160 mm, 13.3 kg/m minimum and shall be installed as concentric rings with a maximum radial spacing of 6m. In addition, the underside deck plates shall have continuously welded lap joints at a radial distance of 12 m from the center of the roof. 3.12.3 Roofs for tanks greater than 46 m in diameter shall be designed for elastic stability against “gross out of plane” buckling and “local” buckling of the outer pontoon, due to the radial load imposed by deflection of the center deck. This radial load shall be determined from the 250 mm of rainfall loading condition as defined in API 650, Appendix C.3.4. or punctured center deck loading condition, whichever governs. For prevention of gross out of plane buckling, the following relations shall be satisfied: a. For “Fully stiffened” pontoons: N< 7.5 EIx /R3 b. For “Partially Stiffened” pontoons:
N < 5.0 EIx /R3 Where: N = Design radial inward load, lb./in. E = Modulus of elasticity, psi. R = Mean radius of pontoon ring, in. Ix = Moment of inertia of full pontoon cross section with respect to horizontal axis through its centroid, in4. Or: a. For “Fully Stiffened” pontoons: N < 7.5 x 10–3 EIx / R3 b. For “Partially Stiffened” pontoons: N < 5.0 x 10–3 EIx / R3 Where: N = Design radial inward load, Newtons per millimeter. E = Modulus of elasticity, Kpa. R = Mean radius of pontoon ring, mm. Ix = Moment of inertia of full pontoon cross section with respect to horizontal axis through its centroid, mm4. c. The radial unsupported width of “Partially Stiffened” plates shall not exceed 3m. 3.12.4 The inner rim of “Partially” and “Fully Stiffened” pontoons shall have the following minimum thickness: TANK DIAMETER RIM THICKNESS 60 to 75 m 16 mm 75 to 90 m 19 mm 3.12.5 For tanks over 60 m in diameter the following calculations and tests are required
to substantiate the elastic stability of the roof pontoon design: a. Where the roof design has not been previously approved by PDVSA, the vendor shall submit test data of a roof of similar diameter, to validate the design. b. Where validation has not been made, the vendor shall conduct a proof test on the largest tank to be supplied. The proof test shall be based on the most critical design load condition. Calculations shall be submitted to PDVSA for approval. 3.12.6 Tanks greater than 90 m in diameter shall have the following roof construction: a. The roof shall be double deck type. b. The top edge of the inner rim plates in the first two central compartments shall be welded with continuous single fillet welds. c. All circumferential compartments other than the outer compartments shall be provided with a minimum of four radial partition plates. 3.13 (Addition) Floating Roof Seals 3.13.1 Roof seals shall contact the liquid level at the shell for at least 90% of the circumference of the tank. The maximum permissible gap between the primary seal and the tank shell is 6 mm. 3.13.2 Toroidal type seals shall be equipped with a weather shield unless secondary seals are specified. Liquid mounted toroidal seals are preferred. 3.13.3 Floating roofs with a flexible steel shoe type primary seal shall be equipped with an abrasion resistant, weather and product proof, flexible seal in order to close
the space between the seating ring and the rim of the floating roof and shall be designed to avoid any jam between the shell and the shoe when the roof is moving. 3.13.4 Secondary seals shall be installed if required by PDVSA in the inquiry documents and the design shall be approved by PDVSA. 3.13.5 The design of primary seals, secondary seals and weather shields shall permit installation and removal from the top of the roof. 3.14 (Additions) Floating Roof Supports and Drains 3.14.1 Supports shall be made of carbon steel pipe, schedule 80 minimum thickness. They shall be provided with a in hole at the bottom for drainage. 3.14.2 The clearance between the support and the roof support sleeve shall be 3 mm nominally at all points. 3.14.3 Supports shall be adjustable to two positions: a. The lower position shall permit the roof to reach 75 mm below the specified lowest operating position, without interference with any internal accessory or roof seal mechanism. b. The upper position shall provide a clearance of 2 meters between the roof and the tank bottom, for cleaning purposes. 3.14.4 Bearing plates shall be centered under each support, welded to the shell bottom by a 5–mm continuous fillet weld. Plate dimension shall be 24 in square by 3/8 in thick. 3.14.5 The length of the leg support sleeves shall be such that any opening will be above
the liquid level when the deck is deflected by the 250–mm rainfall or by a punctured center deck loading condition. The height of the support sleeves for a single deck pontoon roof shall never be less than the tank diameter divided by 60. 3.14.6 Emergency drains shall not be provided when the pontoon area is less than 50% of the roof area. Only for double deck type roofs may credit is taken in the roof design when emergency drains are furnished.
