Flange Bolt Torque calculation – What Is Piping: All about Piping Engineering

Proper bolt loading is essential for ensuring proper sealing of the flanged joint to avoid leakage problems. This bolt load for gasket sealing and flange seating is provided by Flange Bolt Torque. So Flange Bolt Torque calculation should be done with utmost care for proper functioning of the flanged joints. However, there is no direct method of measuring this load on the gasket, but the applied torque on the flanged bolts can be measured and controlled. Equation for Flange Bolt Torque Calculation Bolt Torque is the twisting or turning force applied to tighten the nut on a bolt. Using a calibrated torque wrench (Manual or Hydraulic Torque Wrench), flange bolt torque can be measured during flange assembly. This torque creates an axial force in the bolt. More torque is applied the nut stretches the bolt more and the load on the gasket increases. Bolt torque is calculated for a flanged assembly using the following equation. Applied Torque, T= (k∙f∙d)/12 in FPS Unit Where: T=Torque in ft-lbk=Dimensionless nut factor or tightening factorf=axial force in poundsd=Nominal bolt diameter in inches In the Metric System, Torque Applied, T = (k.d.f)/1000 where T = Torque in N-mf = Bolt load in Nd = Bolt diameter in mmk=Dimensionless nut factor or tightening factor Nut Factor on Flange Bolt Torque calculation The nut factor or tightening factor (k) is a "modified" friction factor. It is an empirically derived correlation factor that includes the impact of friction. The nut factor depends on various factors including the following: Geometric factor - shape or type of threadsThe friction of the nut against the bearing surface of the flangeFriction between threads of nuts and boltsBolt diameterBolt materialAssembly temperature, etc. Because of so many factors, the applied torque between the two fasteners always varies between 20-30%. A small change in the nut factor/tightening factor results in large changes in the gasket load. For same torque values with a 0.1 nut factor would produce twice the axial force as a 0.2 nut factor. That's why well-lubricated bolts, nuts, and washers are always preferred. Pipe Flange Bolt Torque Chart Even though flange blot toque calculation is possible, they are normally selected from the pipe flange bolt torque chart. The entire bolt pattern shall be tightened at least three times around the flange at 30%, 70%, and 100% of the torque value. For bolt diameters greater than 1.25”, Hydraulic Tensioning is recommended to achieve more uniform gasket stress. The following images provide some typical Pipe flange bolt torque charts. Typical pipe flange bolt torque chart for RTJ Gaskets Typical pipe flange bolt torque chart for Spiral Wound Gaskets Typical pipe flange bolt torque chart for Spiral Wound Gaskets with Inner and Outer Rings Typical Torque values for Line Seal gaskets Typical toque values for API 6A Wellhead Connection Flanges Typical Torque Values for Metallic Lined Pipes Typical Torque Values for HDPE Flange Adapters Hot Torquing When the tightening of all bolts of a flanged joint is performed at theoperating temperature, the process is known as Hot Torquing. For flanges that are known to leak at elevated temperatures due to gasket relaxation, the Hot Torqueing method is applied. It is normally performed when the temperature of the flange or the bolts is between 150°C and 230°C, or within 24 hours of a unit start-up if the joint temperature remains below 150°C. Flange Bolt Torquing Guidelines The following guidelines should be followed Flanges and Gaskets must be inspected prior to torque application.All working surfaces must be cleaned and lubricated properly.Gaskets must be new, Re-use is normally not permitted.Flange Bolt torquing must be done following the appropriate flange bolt tightening sequence. To know more about flange bolt tightening sequence, Read: Flange Bolt tightening Procedure/Bolt Tightening Steps