12-20Equipment Component and Compliance Flange Leakage/Stress Calculations The Flange Leakage/Stress Calculations are started by selecting the Main Menu option ANALYSIS-FLANGES. There have been primarily two different ways to calculate stress and one way to estimate leakage for flanges that have received general application over the past 20 years. The stress calculation methods are from the following sources: ASME Section VIII ANSI B16.5 Rating Tables The leakage calculations were also based on the B16.5 rating table approach. Leakage is a function of the relative stiffnesses of the flange, gasket and bolting. Using the B16.5 estimated stress calculations to predict leakage does not consider the gasket type, stiffness of the flange, or the stiffness of the bolting. Using B16.5 to estimate leakage makes the tendency to leak proportional to the allowable stress in the flange, i.e. a flange with a higher allowable will be able to resist higher moments without leakage. Leakage is very weakly tied to allowable stress, if at all. The CAESAR II Flange Leakage Calculation is COADEs first attempt to improve upon the solution of this difficult analysis problem. Equations were written to model the flexibility of the annular plate that is the flange, and its ability to rotate under moment, axial force, and pressure. The results compare favorably with three dimensional finite element analysis of the flange junction. These correlations assume that the distance between the inside diameter of the flange and the center of the effective gasket loading diameter is smaller than the distance between the effective gasket loading diameter and the bolt circle diameter, i.e. that (G-ID) < (BC-G), where, G is the effective gasket loading diameter, ID is the inside diameter of the flange, and BC is the diameter of the bolt circle. Several trends have been noticed as flange calculations have been made: The thinner the flange, the greater the tendency to leak. Larger diameter flanges have a greater tendency to leak. Stiffer gaskets have a greater tendency to leak. Leakage is a function of bolt tightening stress. Input for the Flange Module is broken into four sections. The first section describes flange geometry. Chapter 12 Equipment Component and Compliance Flange Analysis 12-21 . 12-22 The second section contains data on the bolts and gasket. Bolts and Gasket Equipment Component and Compliance . Chapter 12 Equipment Component and Compliance The third section is used to enter material and stress-related data. Material and Stress Data 12-23 . is used only for the flexibility/leakage determination. For this reason. The resulting value is however often not related to the actual tightening stress that appears in the flange when the bolts are tightened. Imposed Loads Note on Bolt Tightening Stress This is a critical item for leakage determination and for computing stresses in the flange.12-24 Equipment Component and Compliance The fourth section contains the imposed loads. The ASME Code bases it's stress calculations on a prespecified. Bolt Initial Tightening Stress. CAESAR II uses the value . The value for the bolt tightening stress used in the ASME Flange Stress Calculations is as defined by the ASME Code: Bolt Load = Hydrostatic End Force + Force for Leaktight Joint If the Bolt Initial Tightening Stress field is left blank. the initial bolt stress input field that appears in the first section of data input. fixed equation for the bolt stress. as it represents the ratio of the pressure required to prevent leakage over the line pressure.000 and 400. This table is reproduced in the help screens.1 in the ASME Section VIII code. When the ASME required stress is entered into the Bolt Initial Tightening Stress data field. Errors on the high side when estimating this value will lead to a more conservative design. Unique input cells are discussed as follows: Leak Pressure Ratio This value is taken directly from Table 2-5. It is interesting to compare this value to the bolt stress printed in the ASME stress report (also in the output). This is a rule of thumb tightening stress that will typically be applied by field personnel tightening the bolts. a comparison of the leakage safety factors can be made and the sensitivity of the joint to the tightening torque can be ascertained. The higher the modulus the greater the tendency for the program to predict leakage. This is a very important number for leakage determination. It is not unusual for the rule-of-thumb tightening stress to be larger than the ASME required stress.000 psi for spiral wound gaskets. Effective Gasket Modulus Typical values are between 300. Users are strongly encouraged to play with these numbers to get a feel for the relationship between all of the factors involved. and is termed the Gasket Factor in the ASME code. This computed value is printed in the output from the flange program. . These parameters include Flange Inside Diameter Flange Thickness Bolt Circle Diameter Number Of Bolts Bolt Diameter Effective Gasket Diameter Uncompressed Gasket Thickness Effective Gasket Width Leak Pressure Ratio Effective Gasket Modulus Externally Applied Moment Externally Applied Force Pressure The help screens (press [F1] or ? at the data cell) are very useful for all of the input items and should be used liberally here when there are questions.Chapter 12 Equipment Component and Compliance 12-25 where 45.000 psi is a constant and d is the nominal diameter of the bolt (correction is made for metric units). Using the CAESAR II Flange Modeler Only the following input parameters are required to get a leakage report. This value is more commonly recognized as m. Example output that the user will get upon entering the flange rating is shown as follows: EQUIVALENT PRESSURE MODEL Equivalent Pressure (lb. (Of course both flanges would have essentially the same flexibility tendency to leak. As mentioned above. the minimum and maximum rating table values from ANSI and API were stored and are used to print minimum and maximum leakage safety factors that would be predicted from this method./sq. Division 1 material library that is accessed from the pull-down list.5 and API 605 temperature/pressure rating tables as a gauge for leakage. and were not intended for leakage prediction.) The following input parameters are used only for the ASME Section VIII Division 1 stress calculations: Flange Type Flange Outside Diameter Design Temperature Small End Hub Thickness Large End Hub Thickness Hub Length Flange Allowables Bolt Allowables Gasket Seating Stress Optional Allowable Multipliers Flange Face & Gasket Dimensions The flange type can be selected from the icons on the first spreadsheet. Material allowables may be acquired from the Section VIII.00 This output shows that leakage. To give the user a feel for this old practice. the leakage predictions that resulted were a function of the allowable stress for the flange material.85 ANSI/API Min Equivalent Pressure Allowed 1080. modulus of elasticity of the flange.12-26 Equipment Component and Compliance Flange Rating This is an optional input. but results in some very interesting output. Because these rating tables are based on allowable stresses. occurred if a carbon steel flange was used.in. . according to this older method.e. it has been a widely used practice in the industry to use the ANSI B16. i.) 1639. and not the flexibility.00 ANSI/API Max Equivalent Pressure Allowed 1815. and leakage did not occur if an alloy flange was used. ) 400.in.000 Gasket Seating Stress [y](lb./sq.) Facing Sketch 1.) 30.) 0.000 Facing Column 2.000 Bolt Allowable Stress Multiplier (VIII Div 2 4-1411.) Effective Gasket Diameter [G] (in.) 36.500.in.) 24.in.000 Flange Outside Diameter [A](in.500.in.000 The following inputs are required only if the user wishes to perform stress calcs as per Sect VIII Div.000./sq.000 Externally Applied Force (optional)(lb.) 6.000 Gasket Inner Diameter (in.) 1.) 3.000.000 Small End Hub Thickness [g0](in.) 34.560 4.) 25./sq.620 Flange Allowable @Design Temperature(lb.000 Disable Leakage Calculations (Y/N) N 12-27 .) 33.888 Uncompressed Gasket Thickness (in.) 38.000.000 Nubbin Width (in.) Flange Thickness [t](in.000 Disable Leakage Calculations (Y/N) N Flange Face OD or Lapjt Cnt OD(in.440 Hub Length [h](in.in./sq.000 Bolt Diameter (in.) 25.690 Large End Hub Thickness [g1](in./sq./sq.) 33.in.700.500 Bolt Initial Tightening Stress(lb.) 41.) 17.000.000.000 Bolt Circle Diameter (in./sq.) 17.in.060 Flange Rating (Optional) 300.lb.000 Gasket Outer Diameter (in.279E+08 Bolt Allowable @Design Temperature(lb.Chapter 12 Equipment Component and Compliance An input listing for a typical flange analysis is shown below: CA E S A R I I MISCELLANEOUS REPORT ECHO Flange Inside Diameter [B](in.) 0.375 Leak Pressure Ratio [m] 2./sq.279E+08 Flange Modulus of Elasticity @Ambient(lb.) 1.000 Pressure [P](lb.) 3.500 Design Temperature°F 650. 1 Flange Type (1-8.000 Flange Allowable Stress Multiplier 1.500 Number of Bolts 32./sq.500 Flange Face ID or Lapjt Cnt ID(in.in.000 Externally Applied Moment (optional)(in.) 0./sq.) 0.in.) 1.) 33.750 Effective Gasket Modulus(b. see ?-Help or Alt-P to plot) 1.063 Basic Gasket Width [b0] (in.000 Flange Allowable @Ambient Temperature(lb.000 Bolt Allowable @Ambient Temperature(lb.in.000 Flange Modulus of Elasticity @Design(lb.) 300.