Bolt Torquing vs Tensioning: How to Choose
Both methods put load into a bolt; they get there differently. Torquing turns the nut against friction, while tensioning stretches the stud directly. Here is how each works and when to use which.
What both methods are trying to do
A bolted joint seals because the studs are stretched slightly, like stiff springs, and that stretch clamps the gasket. The clamping force is called preload. Torquing and tensioning are two ways of creating that preload.
How bolt torquing works
A torque wrench turns the nut. As the nut climbs the thread, it stretches the stud. But most of the turning effort is spent overcoming friction in the threads and under the nut face, and only a small part ends up as stretch. That is why the lubricant, the thread condition and the nut face finish all change the preload you get from the same torque.
Torquing is fast, the tools are compact, and it works on standard stud lengths. With a calculated target and a controlled lubricant, it gives good results on most process flanges and equipment joints.
How bolt tensioning works
A hydraulic tensioner fits over the stud end above the nut, grips the thread and pulls the stud to a calculated stretch. With the stud held, the nut is run down by hand, then the tool is released. Because the stud is stretched directly, friction has much less influence on the result.
Several tensioners can be linked to one pump and pressurized together, so a set of studs is loaded at the same moment. On large flanges this gives a more even load than tightening one stud at a time.
Side by side
| Factor | Torquing | Tensioning |
|---|---|---|
| Load created by | Turning the nut | Stretching the stud |
| Effect of friction | Large | Small |
| Studs loaded at once | Usually one | Several or all |
| Stud length | Standard | Extra thread above the nut, about one diameter |
| Access needed | Room for the wrench head and reaction | Room for the tool over the stud end |
| Typical use | Most process flanges and equipment | Large, high-pressure and critical joints |
Load transfer loss
When a tensioner releases, the nut and threads settle slightly and some stretch is lost. The applied pressure is therefore set higher than the final load needed, using a load transfer factor, so the remaining preload meets the target. A good tensioning procedure always includes this allowance.
Which should you use?
- Choose tensioning for large-diameter studs, high-pressure or high-temperature service, joints with many studs that must load evenly, and critical joints where leaks are not acceptable.
- Choose torquing for most standard process flanges, valves and equipment, where studs are standard length and access is tight.
- Use both on the same shutdown. Most turnarounds mix the two, joint by joint.
Keep reading
Questions about this topic
Is tensioning more accurate than torquing?
Tensioning is less affected by friction, so the preload is usually more consistent. Torquing can still be accurate with a calculated target and a controlled lubricant.
Can any stud be tensioned?
Only if there is enough thread above the nut for the tool to grip and room around the flange for the tool. Otherwise the joint is torqued.
Why do torque values change with lubricant?
Lubricant changes the friction in the threads and under the nut. Less friction means more of the torque becomes stretch, so the same torque gives a higher preload.