The main factors affecting the bolt torque value include...

Proper tightening is essential for achieving leak-free connections.

The vast majority of bolted joint failures originate from improper assembly operations and incorrect bolt torque application. Therefore, ensuring proper tightening is crucial for achieving leak-free connections.

The torque applied to a bolt is primarily used to overcome the following two types of friction:
• Friction between the bolt and nut threads
• Friction between the nut head and the bearing surface (the outer surface of the flange)

Using lubricant to reduce friction

A flanged connection tightened with bolts works on a principle similar to that of a compressed spring (see Figure 1). In a bolted joint, approximately 50% of the applied torque is used to overcome friction between the nut and the bearing surface, about 35% is consumed by thread friction, and only 15% is converted into the desired bolt preload. When a lubricant is used to reduce thread friction, a higher bolt preload is achieved for the same applied torque. Therefore, special precautions must be taken when using lubricants, because the same torque can generate excessive preload, leading to yielding of the fastener and subsequent failure of the bolted connection.

Lubricant should be applied to both the bearing surface of the nut and the external threads of the bolt. The lubricant used must be chemically compatible with the bolt and flange materials. Special care should be taken to avoid using lubricants containing chemical components that may induce stress corrosion cracking. In addition, the lubricant must be capable of withstanding the expected maximum operating temperature.

Factors Affecting Torque Value

The factors affecting the torque value mainly include the flange pressure class, bolt size, gasket type, and the friction coefficient of the lubricant.

Applied torque T = kDF/1000, where:
T = torque (N·m)
F = bolt load (N)
D = bolt diameter (mm)

or

Applied torque T = kDF/12, where:
T = torque (ft-lb)
F = bolt load (pounds)
D = bolt diameter (inches)
k = nut factor (also called tightening factor or k-factor)

The k‑factor is not a friction coefficient; it is an empirically derived correction factor. The nut factor depends on many factors, including:
• Geometric factors – thread shape or type
• Friction between nut and bolt threads
• Friction between nut and flange bearing surface
• Bolt material
• Bolt diameter
• Assembly temperature

Due to the many factors affecting torque, even fasteners from the same batch may require different torque values to achieve the same bolt preload. The actual applied torque may vary by 20% to 30%.

How to Determine the Appropriate Bolt Load

The torque applied to a bolt ultimately converts into tensile load (i.e., tension value) on the bolt. The bolt tension must be kept within its elastic range so that the bolt can fully return to its original state after unloading. By definition, the proof load is the maximum tensile load that a fastener can withstand without permanent deformation. In the above formula, the acceptable bolt load (F) is typically taken as 40% to 75% of the proof load. This means the target bolt preload stress should be set at about 40% to 75% of the material's specified minimum yield strength.

Bolt load = 40% to 75% of specified minimum yield strength × tensile stress area of the bolt

According to the modulus of elasticity formula Y = stress / strain, if the required bolt preload is 70,000 psi and the modulus of elasticity Y of carbon steel is 29,000 ksi, the strain can be calculated as:
Strain ΔL/L = 70,000 / (29,000 × 1,000) = 0.0024 inches per inch

It can be seen that the longer the clamp length of the joint, the greater the total elongation required to achieve the predetermined preload.

Example of Bolt Torque Calculation

This example demonstrates how to calculate the minimum and maximum bolt torque for a 6‑inch Class 150 flange joint using a spiral wound gasket and ASTM A193 Grade B7 bolts. The nut factor is taken as 0.18, and the required gasket seating stress for the spiral wound gasket is 7,500 psi. The bolt yield strength of ASTM A193 B7 material is 105,000 psi. The maximum bolt design stress used in the calculation is 60,000 psi, which is 57% of the bolt yield strength and falls within the recommended range mentioned earlier.

1. Calculate bolt load and torque for gasket seating

   The first step is to calculate the bolt torque required to achieve a gasket seating stress of 7,500 psi.

   For a 6‑inch Class 150 spiral wound gasket, the inside diameter is 7.19 inches and the outside diameter is 8.25 inches. These values are taken from Table 9 of ASME B16.20.

   From the gasket ID and OD, the gasket seating area is calculated as (1): 12.854 in²

   Total gasket compression load is calculated as (2):
   Gasket seating stress × gasket seating area = 7,500 × 12.854 = 96,406 lbs

   For a 6‑inch Class 150 flange, according to ASME B16.5, the bolt size is 3/4 inch and the number of bolts is 8.

   The thread root area of a 3/4‑inch bolt is (3): 0.302 in² (taken from Table H‑1 of ASME B1.1).

   Therefore, the bolt load per bolt from gasket seating = total gasket compression load / number of bolts

   Bolt load per bolt for gasket seating (4) = 96,406 / 8 = 12,051 lbs

   Bolt stress (gasket seating) = bolt load per bolt (4) / bolt root area (3) = 12,051 / 0.302 = 39,903 psi

   Gasket seating torque (5) = kDF/12 = (0.18 × 0.75 × 12,051) / 12 = 136 ft‑lbs

   This is the minimum bolt torque required for a 6‑inch Class 150 flange joint using a spiral wound gasket and ASTM A193 B7 bolts.

2. Calculate bolt load and torque at design bolt stress

   The second step is to calculate the torque required to achieve a design bolt stress of 60,000 psi.

   The required bolt load to achieve a design bolt stress of 60,000 psi is calculated as follows:

   Bolt load per bolt at design bolt stress (6)
   = bolt design stress × bolt root area
   = 60,000 × 0.302 = 18,120 lbs

   Torque at design bolt stress (7)
   = kDF/12 = (0.18 × 0.75 × 18,120) / 12
   = 204 ft‑lbs

   This is the maximum bolt torque required for a 6‑inch Class 150 flange joint using a spiral wound gasket and ASTM A193 B7 bolts.

   Based on the bolt load of 18,120 lbs, it is necessary to verify that the gasket seating stress will not crush the gasket.

   The gasket seating stress calculated from the 18,120 lb bolt load is as follows (8):
   (Bolt load × number of bolts) / gasket seating area