Most failures in flanged bolted connections are due to poor assembly and incorrect application of bolt torque. Therefore, it is very important to ensure that the bolts are properly tightened to ensure that the joints are leak-free. You can usually refer to the flange bolt torque table to query the corresponding data.

Now, TorStark will show you how to calculate the tightening torque of flange bolts.

Flange bolt torque calculation formula

Bolt torque is the torsional or rotational force applied to tighten the nut on a bolt. Using a calibrated torque wrench (manual or hydraulic), flange bolt torque can be measured during flange assembly. This torque creates an axial force in the bolt. The more torque you apply, the more the nut stretches the bolt and the load on the washer increases.

Use the formula below to calculate the bolt torque for flange assemblies.

Applied torque, T= (k∙f∙d)/12 (FPS units)

T = Torque (ft-lbs)
K = dimensionless nut factor or tightening factor
F = axial force (lbs)
D = Bolt Nominal Diameter (inches)

Applied torque, T = (kdf)/1000

T = torque (Nm)
f = bolt load (N)
d = bolt diameter (mm)
K = dimensionless nut factor or tightening factor

Nut Factor in Flange Bolt Torque Calculations

The nut factor or tightening factor (k) is the “corrected” coefficient of friction. It is an empirically derived correlation coefficient including the effect of friction. The nut factor depends on a variety of factors, including:

Geometric Factors – Shape or Type of Thread
The friction force between the nut and flange bearing surface
Friction between nut and bolt threads
Bolt diameter
Bolt material
Assembly temperature, etc.

Due to many factors, the torque applied between two fasteners always varies between 20-30%. A small change in the nut factor/tightening factor will result in a large change in the gasket load. A nut factor of 0.1 produces twice as much axial force as a nut factor of 0.2 for the same torque value.

Calculation case of flange bolt tightening torque

Now take the flange bolt connection widely used in the petrochemical industry as an example.

In engineering construction, in order to increase the safety of bolted connections, it is necessary to apply a suitable pre-tightening force, and the application of pre-tightening force is realized by a certain tightening torque. If the tightening torque of the flange bolts is too large, the gasket will be damaged or even the bolts will be twisted off to cause leakage. If the tightening torque is too small, the residual compression force of the gasket after compression will not reach the sealing-specific pressure, thus causing the sealing surface to leak.

The following is a brief calculation and analysis of the force on the inlet and outlet flanges of the air cooler. Calculate the tightening torque required for each bolt of the flange before construction, so as to avoid leakage of the flange sealing surface due to too small tightening torque during construction, and damage to gaskets and bolts caused by excessive tightening torque.

1. Calculation of tightening torque of flange bolts

1.1 Known data

The design pressure of the air cooler p=15.0 MPa
The flange pressure rating is 15.0 MPa
The nominal diameter is DN250 mm
Design temperature 100°C
The bolt material is 35CrMoA
The bolt specification is M36X3
Number of bolts 16

Flange—Bolt Gasket Connection Schematic Diagram

1.2 Calculation

(1) Determine the basic sealing width of the gasket b0

For metal ring gaskets,b0=w/8= 15. 88/8=1.985(mm).

(2) Determine the effective sealing width of the gasket b

When b0≤6.4 mm, b=b0

when b0>6.4 mm, b=2.53b01/2

b0=1. 985 mm≤6.4 mm in the text

take b=b0=1. 985 mm

(3) Determine the diameter DG of the center circle of the gasket pressing force

When b0≤6.4 mm, the value of DG is the average diameter of the gasket contact surface; and when b0>6.4 mm, the value of DG is equal to the outer diameter of the gasket contact surface minus 2b. Since b0 is equal to 1.985 mm≤6.4 mm, DG takes the average diameter of the gasket contact surface, which is 323.85 mm.

(4) The total axial force F of the bolt caused by the internal pressure in the operating state

F=∏DG2p/ 4 =1234.90 kN

(5) The minimum gasket pressing force Fp necessary in the operating state

Fp=2∏DGbmp=393.61 kN

(6) The minimum total bolt load Wp required in the operating state

Wp=F+Fp =1628.51 kN

(7) The minimum total bolt load Wa required under the pre-tightening state

Wa=∏DGby=361.92 kN

(8) The total design load of the bolt under the pre-tightening state Wy


In the pre-tightened state, the minimum total area of bolts required is Aa=Wa/[σ]b=1587.37 mm2

In the operating state, the minimum total area of bolts required is Ap=Wp/[σ]bt=7905.39 mm2

(9) Total bolt area Am

Because Ap>Aa, Am= Ap=7 905.39 mm2

(10) Actual area A of a single bolt

A=Π/4×(d1-P1/6)2=883.65 mm2.

(11) Actual bolt total area Ab

Ab=nA =14138.4 mm2, because Ab>Am, Ab is what is sought.

(12) The total design load of the bolt is W

W=Wy=(Am+Ab)[σ]b/2=2513.00 kN.

(13) Tightening torque T required for each bolt

T=kWdB10-3/n=735.05 N·m.

In the end

From the above calculations and cases, it can be seen that it is very important to know the tightening torque of flange bolts accurately. It prevents damage to gaskets, bolts, and joint leaks. TorcStark has extensive experience in flange bolting. At the same time, our engineers can provide you with reasonable solutions at any time. Help you solve the problems of bolt installation and removal in engineering.