A tensioner stretches the bolt axially with a hydraulic cylinder and the nut is then run down against the joint. Torquing turns the nut and relies on the resulting twist to stretch the bolt. Tensioning applies load directly; torquing applies it through friction.

Because most of the applied torque is consumed overcoming friction under the nut face and in the threads, and that friction varies with lubrication, finish, coating and temperature. The relationship between torque and actual bolt load is uncertain by a wide margin, commonly cited as a quarter or worse.

Even gasket compression in a single operation. Tensioners on a flange are linked to one pump and operated together, so every bolt loads at the same moment. Sequential torquing means each bolt tightened relaxes those already done, needing several passes and still leaving scatter.

Enough exposed thread above the nut for the tensioner puller to engage, and enough space between adjacent bolts for the tool to fit. Joints either have to be designed for tensioning or fitted with longer studs, and both points are checked at design stage rather than discovered on site.

Pressure vessel and pipeline flanges, wind turbine foundations and towers, subsea connections, turbine casings and heavy machinery - anywhere joint integrity is critical and the consequence of an uneven or incorrect bolt load is a leak or a failure. Uneven bolt load is the failure mode all of them are guarding against.

The small loss of bolt load when the tensioner releases and the load transfers onto the nut face, as the threads and joint settle. It is predictable and is allowed for by tensioning to a slightly higher figure, and the manufacturer's procedure states how much.

Yes, and it is a genuine advantage on seized or heavily loaded joints. Stretching the bolt relieves the load on the nut so it can be turned off by hand, which avoids the shock loading and damage that comes from trying to break out a badly seized nut with impact tools.