1. Flange leakage check to be carried out for the below-mentioned cases:
i. Hydrogen/flammable fluid.
ii. Category M fluid service.
iii. Flanges #600 or more.
iv. Flanges #300 and size greater than xx”.
v. Flanges that experience very high bending moment.
vi. Flanges in jacketed piping.
2. The below-mentioned factors need to be considered for custom flange design.
i. Internal pressure.
ii. Flange rotation due to external moment (flange rigidity to be evaluated
based on design P).
iii. Stud stress
iv. Residual gasket stress during operation.
3. Stud load to be calculated for 2 design cases; gasket operating stress & gasket seating stress and the most severe case shall govern.
4. For heat exchanger flanges, stud stress shall be in between xx and xx% of the minimum ambient bolt yield strength.
5. Stud loads should be adjusted to obtain a targeted gasket operating stresses of xx ksi under MAWP condition, with a maximum of xx ksi and a minimum of xx ksi for
graphite faced gasket.
6. B7M studs have a xx ksi yield stress for all studs less than x” diameter.
ForB8 studs, refer to Code as its yield strength significantly changes based on the stud
diameter.
7. Hydraulic tensioning is typically used on fasteners with 2″ bolt diameter or greater.
The main advantage of stud tensioners is that it eliminates friction factor on stud and
nut face. Tensioners are more accurate (+/- xx% accuracy) in achieving rating
compared to hydraulic torqueing (+/-xx% accuracy). Accuracy of tensioning decreases
for stud stress < xx% of stud yield.
8. Design considerations:
i. Max. permissible stud stress: xx to xx% of ambient stud yield stress.
ii. Min. permissible stud stress: typically xx to xx ksi.
iii. Max. permissible stud stress for flange: flange rotation should be determined
iv. Maximum assembly gasket stress at room temperature should be advised by
gasket manufacturer. Typically xx ksi.
v. Minimum assembly gasket stress at room temperature should be advised by
gasket manufacturer. Typically xx ksi.
vi. Minimum gasket operating stress should be advised by gasket
manufacturer.
vii. Calculations shall be based on full contact width of gasket at design
pressure. Total pass partition rib area to be included in this calculation.
9. Bolt torqueing of flanges shall be carried out between xxx °C & xxx °C, after unit
startup. It shall not be performed > xxx°C as lubricant burns off and stud/nut friction
factor increase substantially. Accuracy of stud loads will be reduced.
10. As the number of tube passes increases, the temperature difference between tubeside inlet & outlet increases. A temperature gradient will form around the circumference of tubeside girth flange and tubesheet, and will increase the tendency of fluid leakage.
11. Differential thermal expansion of flanges increases flange bowing/opening.
The following guidelines can be adopted to mitigate flange opening.
i. Reduce allowable flange rigidity index to xx.
ii. Provided/required bolt area ratio should be xxx% of design, and full
bolt-load calculations should be performed
iii. Reduce allowable stresses for flanges and tubesheet to xx% of Sect II Part D.
iv. Increase the required thickness of flanges/tubesheet by xmm,
if t <= xxxmm, or by xxmm if t >xxxmm.
Future machining is excluded from extra thickness.
v. Target bolt and gasket stresses are determined by PCC-1.