Torquing and tensioning both end up with a tight bolt, but they get there differently — and on a critical joint that difference decides whether the flange leaks after you hand it back.
What torquing actually does
A hydraulic torque wrench turns the nut. The clamping force you end up with is a by-product of the friction in the threads and under the nut face. That is the catch: most of the effort goes into overcoming friction, and only a fraction converts into the load holding the joint together.
Friction is also the least predictable part of the process. Thread condition, lubricant, surface finish and even how many times the fastener has been used all shift it. Two bolts torqued to exactly the same figure can end up with noticeably different clamping loads.
What tensioning does instead
A bolt tensioner grips the end of the stud and stretches it directly, then the nut is simply wound down against the joint. Because the load is applied along the axis of the bolt, thread friction largely drops out of the equation.
Two practical consequences follow. Bolt-to-bolt consistency is better, and several bolts can be tensioned simultaneously — which matters on a large flange where tightening one bolt relaxes its neighbours.
Choosing between them
Torquing usually makes sense when:
- The joint is not highly critical, or the bolts are relatively small
- There is no room above the nut for a tensioner to sit
- The studs are standard length with no extra thread to grip
- You need a portable, quick setup
Tensioning tends to win when:
- The joint is critical — pressure containment, heat exchangers, large flanges
- Even load across many bolts matters more than raw speed
- The equipment manufacturer specifies a bolt load rather than a torque figure
- Studs are long enough, with enough exposed thread, to take a puller
Worth noting: tensioning needs the stud to be designed for it. If there is not enough protruding thread, that decision is already made for you.
Things that cause trouble either way
- Uncalibrated equipment. A torque figure means nothing if the gauge is out. Ask for calibration records — and if a contractor cannot produce them, that tells you something.
- The wrong sequence. Working around a flange in the wrong order distorts it. Follow the pattern the OEM specifies.
- Assumed friction. If a torque value was calculated for lubricated threads and the bolts go in dry, the actual clamping load can be far below target.
- Reused fasteners. Studs that have been loaded several times do not behave like new ones.
How we handle it on site
We supply Hi-Force hydraulic torque wrenches and bolt tensioners, and Enerpac high-force hydraulics. We also carry out the work: our own trained engineers and technicians attend site with calibrated tooling, and documentation is available on request.
Site jobs are planned around your shutdown window rather than ours. Tell us the window before you book us and we will say plainly whether we can commit to it.
What to tell us when you call
- Bolt size, stud length and how much thread is exposed
- How many bolts, and the joint type
- The OEM torque or bolt load figure, if you have it
- Your shutdown dates and how much access there is around the joint