INGENIEUR
INGENIEUR
QUALITY EXECUTION ( Equipment and Technical Theory )
Since a pipe flange connection is effective for quick replacement , when a pipe section is found to be damaged or unacceptable , the unwanted pipe section can be quickly swapped with a prefabricated pipe spool to complete the change-out process . Some organisations , especially those with ISO 9001 , do have a degree of quality control established to ensure a good outcome , such as approved procedures , qualified personnel , record tracking , and leak tests . Moreover , the correct tightening technique does affect the overall quality outcome . Theoretically , during tightening , the gasket will be compressed to a certain degree , leading to cold flow that seals off the mating surface , including slight imperfections , ultimately preventing any leakage .
As easy as it sounds , there are some techniques for acquiring the ideal flange connection and bolt stress is one of the crucial factors to be emphasised . Bolt stress needs to be within the elastic region of the Stress- Strain curve , basically below the material Yield Strength , to provide a compression effect for better flange tightness . If the bolt is undertightened , the gasket cold flow ( deformation ) is insufficient to effectively seal off the mating surface . However , overtightening will cause the bolt to experience plastic deformation , losing its elastic characteristic . Both under-tightened and over-tightened bolts have the potential to leak when in service . Thus , ideal bolt stress is key .
The desired final torque value can be obtained from international codes that are equivalent to the flange specification . For ASME-type flanges , “ ASME PCC-1 Guidelines for Pressure Boundary Bolted Flange Joint Assembly ” has a sample formula that will be used for discussion . In theory , per “ T = KDF / 12 ”, when torque is applied on the nut during tightening , the bolt will be pulled , simultaneously forcing the bolt to be in tension . Both the “ Applied Torque ” of the nut and the “ Axial Load ” of the bolt are directly proportional to one another . However , the torque applied to the nut is not directly converted to the load experienced by the bolt , so a constant is introduced in the calculation .
As an alternative to torquing , the bolt tensioning method can be used in tightening flange connections . Bolt tensioning pulls the bolt to reach the desired stress value via a hydraulic system before tightening the nut manually . Our local industry normally considers the commonly
NORSOK STANDARD L-005 ( Rev . 1 , Sep . 2003 ) Compact flanged connections Table 8 - Final bolt tension and torque values
Stud bolt size ( in )
Applied Tension ( kN )
1 / 2 – UNC - 98 1 – UNC 244 816 1 1 / 2 – 8UN 623 2989 2 – 8UN 1177 7351 2 1 / 2 – 8UN 1904 14665
Applied Torque with lubricant , μ = 0,12 ( Nm )
Note : The symbol ” μ ” indicates the coefficient of friction . Most Bolts and Nuts used in the industry are coated for corrosion prevention . Reuse of bolt is not advisable . However , if the bolt condition is still good and without plastic deformation ( necking ), then it may be considered by the owner .
ASME PCC-1 ( 2013 ), Appendix K , Nut Factor Calculation of Target Torque ( Formula )
T = K D F / 12
Sample Flange Tightening Values from International Codes ( for discussion only )
Legends : T = Target Torque ( ft-lb ) K = nut factor ( dimensionless constant ) D = nominal diameter of the bolt ( in ) F = target bolt load ( lb )
Note : K ” is an experimental value related to the coefficient of friction , depending on the situation it can be around 0.20 and 0.16 for non-coated and coated bolts respectively .
52 VOL 94 APRIL-JUNE 2023