Design Calculations of Elevated Cylindrical Flare Stack

DESIGN CALCULATIONS OF ELEVATED CYLINDRICAL FLARE STACK Page : 1 of 24 Designed by :Eng. Abdel Halim Galala, Design General Manager(Assistant) REV. : 0 Project:Design & procurement of Flare StackDate :4.4.2000 Job Name: Propylene Recovery Unit Location :Gulf of Seuz Dwg. No. : Client :ABB/OPCStack Type :Self supported & Multiple DiameterItem : X-06-02 ELEVATED FLARE STACK SELF SUPPORTED & MULTIPLE DIAMETER Design General Mgr. BY Eng. Abd El Halim Galala Design General Manager Assistance 1st issue Dated : 6.6.1985

Contents Page A. Wind Loads as Computed in Accordance with ANSI A58.1. 3 B. Allowable Shell Buckling (Compression) Stress. 5 C. Shell Plate Thickness, Design Procedure. 7 1.TotalUncrodded Stack Weight. 7 2. Computation of the Projected Area. 8 3. Computation of the Wind Loads. 9 4. Computation of the Wind Moments. 11 5. Required Shell Plate Thickness. 12 6. Anchor Bolt Chair. 13 7. Width of Base Ring. 14 8. Base Plate Thickness. 15 9. Top Plate Thickness. 15 D. Vibration Analysis. 16 1. Cantilever Vibration.16 - Analyzing Technique. 17 - Static Deflection. 18 - Dynamic Deflection. 19 2. Ovaling Vibration.20 - Design of Ovaling Ring : 22 - Critical Wind Velocity. 22 - Required Ring Section Modulus. 22 E References

A.WIND LOADS AS COMPUTED IN ACCORDANCE WITH ANSI A58.1-1955. The procrdure for calculation of the minimum design wind load normal to the surface is as follows : 1. The geographical area of the job site (Gulf of Suez&Mostorod) is located on the wind pressure map, see Table-1. The basic wind pressure p is selected. 2. The wind design pressure p z, corresponding to tha basic wind pressure p, for various heightzones above the ground are given inTable 1. 3 . To calculate design wind forces from wind pressures, shape factor B shall be used.The shapefactor for round objects is equal to 0.6 and is applied to the design pressure p z. 4.If the windward surface area projected on the vertical plane normal to the direction of the windis A ft 2, then the resultant of the wind pressure load over the areap w is assumed to act at thearea centroid and is given by : Pw = A B pz, lb The wind pressure forces are applied simultaneously, normal to all exposed windward surfaces of thestructure. The minimum net pressure B*pz in the above formula for cylindrical vertical vessels is notless than : For L/D <= 10 13PSF and forL/D >= 10 18PSF

Where L is the overall tangent-to-tangent length of the vessel, and D is the vessel nominal diameter. Table 1.Design Wind Pressure of the Job Site, p. H e i g h t z o n e B a s i c w i n d above grade pressure, p Ft MPSFKg/m2 30 915.361275 3 2 t o 4 6 1 0 t o 1 4 20.4816100 4 9 t o 6 2 1 5 t o 1 9 25.602 125 o v e r 6 5 o v e r 2 0 30.7224 150 1 Kg/M2 = 0.2048159 lb/Ft 2 Computation of the Projected Area,A. An approach to computingAwhich is often used and is recommended here is to increase the vesseldiameterDto the so called effective vessel diameter to approximate the combined design wind load :De = (Vessel OD + Twice insulation Thickness) x KdThe coefficient Kd is given inTable 2. The required projected area A will then be equal to : A = De Lswhere Ls = length of the shell section in the zone of the uniform wind velocity.

E. References. 24