smartTensioner HydraulicTensioningUnit HTU


smartTensioner HTUs are used instead of conventional hexagon nuts. Multiple HTUs are often mounted on a multi-stud flange connection.


How does it work?
All HTUs are pressurized using the very small smartTensioner HydraulicPump HP. Hydraulic pressure (up to 300 MPa) presses an annular piston against the flange and leads to a defined elongation of the bolt.


The innovation:
The HTU uses two different hydraulic media simultaneously separated by a splitter: while one is of high viscosity but still pumpable, the other medium is extremely viscous and not pumpable at all. Its volume stays constant over the lifetime of the nut. It is pressed against the sealings on the annular piston.

No leakage
Because of the oil's high viscosity in combination with the smartTensioner sealing, leakages do not occur. Since all other openings of the HTU are equipped with metallic sealings, leakage of any kind is impossible. The pressure remains locked in place over infinite time.


After reaching the targeted pressure, the HTU pressure valve seals the pressure space within the nut.

HydraulicTensioningUnit HTU Technical Data
HTU 04RZ.pdf
Adobe Acrobat Document 2.4 MB

Advantages of hydraulic locking compared to conventional tensioning nuts

The bolt is loaded with tensile stress only. There is no torque stress and virtually no shear stress. The bolt material’s load bearing capacity is used to its full extent for the bolted connection.

The hydraulic pressure within the HTU can easily be measured. Any unexpected changes in the flange pressure, e.g. due to flange leakages, are detected at once.

No pump or hydraulic aggregate is needed for the removal of the HTU. Any discharging hydraulic medium can easily be absorbed.

Due to the lack of mechanical locking, after reaching the target pressure the force flow within the HTU remains unchanged.

The nut body can be fully used for straining the bolt. There is no need for mechanical locking. The result is an extremely high tensioning force and very small contact area, usable even with threads as small as M8.

HydraulicPistonReturn available.

Permanent Flange Pressure Monitoring (PFPM)

With smartTensioner HTUs, high hydraulic pressure is used to stress the bolts of a flange connection and therefore to press the flange parts against each other. The higher and the more even the bolt forces, the tighter and more reliable the flange. Both can be ensured using HTUs pressurized by smartTensioner hydraulic pumps that are electronically synchronized and ensure even pressures and bolt forces on all bolts.


However, even in this case flange connections might fail. The pressure between the flange parts might be reduced because of:

  • Permanent plastic strain of the flange material
  • (Local) thermal impact
  • Sealing failure due to geometric tolerances
  • Sealing failure due to unexpected chemical reaction

Safety first

HTUs allow the operator to permanently measure the internal hydraulic pressure using smartTensioner sensors that communicate via cable or wirelessly. The internal pressure is directly proportional to the applied bolt force and flange pressure. Any failure is detected at once. Leakages due to flange failure can be minimized or excluded.

Advantages of PFPM:

  • Reduced risk of
    environmental damage
  • Reduced risk of plant or
    process shutdown
  • Simplified diagnostics in the
    event of process problems
  • Supports comprehensive
    quality assurance

HTU specifications