Selecting identical bolts and then applying the same tightening load to each one sounds like a sensible approach to creating a safe bolted joint – but things are not as simple as they may seem…
Material and manufacturing differences between apparently identical bolts can cause large differences in the bolt tension achieved on installation because each bolt stretches to a different extent when put under load. In addition, frictional forces between the bolt head and the bolting surface and those along the thread will differ from bolt to bolt, making achieving uniform ‘tightness’ through the application of an identical force to each bolt something of a lottery.
- This chart demonstrates the wildly varying bolt stress achieved on the bolts in a 6” 900# RTJ flange when the same tightening force was applied to 12 ‘identical’ B16 bolts.
- 50% of bolts either exceeded maximum allowable flange stress or fell below the minimum advisable gasket stress values – not a good start to achieving a leak-free joint with long-term integrity.
Applying the same torque to all bolts in the flange resulted in a ‘scatter’ of bolt tension. This means that the joint is already subject to overload in some areas, whilst at other points there is insufficient tension across the joint resulting in non-uniform compression of the gasket.
Calibrated bolts will help to achieve a more consistent result but even then, only the tightening force applied, and not the achieved bolt tension, is known on installation unless some type of tension measurement fastener or system is utilised.
Measurement takes the guess-work out of creating leak-free bolted joints – Sealing flanges in high temperature and pressure applications