A guide to Bolt Tensioning

Hydraulic bolt tensioners find more prevalent use in bolting applications across Europe compared to the United States. These tensioners are commonly used on fasteners with a diameter of 2 inches or larger, although they can also accommodate studs as small as ¾ inches in diameter.

Their application spans various industries, with notable usage in subsea both topside and underwater, wind turbine, and power generation sectors. While less commonly used in the oil & gas industry, tensioning tools prove beneficial for critical bolted joints such as those found in heat exchangers and large pressure vessels.

Achieving 50% or greater tensioner coverage yields more uniform and precise gasket compression compared to conventional torque wrench methods, including single or multiple torque wrench applications.

A key advantage of stud tensioners lies in their ability to eliminate the friction factor or k-factor between the stud and nut face. Furthermore, tensioners generally offer greater accuracy, boasting a rating of plus or minus 10%, in contrast to the 30% accuracy typical of other methods.

It is often claimed that hydraulic bolt tensioners allow for more frequent reuse of stud bolts by mitigating concerns of galling or frictional forces on the fastener. While laboratory findings support this assertion, it's important to consider real-world operating conditions, such as temperature and pressure variations, which can impact the longevity of fasteners over time.

[edit] Using Bolt Tensioners in Practical Scenarios

One challenge with tensioners lies in their requirement for increased spacing. While this isn't typically problematic with standard piping flanges, space constraints may arise when dealing with valves and specialised flanges, such as those found on heat exchangers.

Furthermore, the maximum stud preload stress achievable with tensioners typically ranges between 50,000 to 60,000 psi. If aiming for preload levels exceeding 60,000 psi, a specialised tensioner is likely necessary.

Additionally, using tensioners needs additional stud length. The rule of thumb dictates having one stud diameter protruding above the nut to enable the puller bar to grip the stud when pressure is applied via the load cell. Hence, to calculate the required size, one must add another diameter of the stud to the standard stud length used for torquing.

Avoid using washers with tensioners, as the tensioner's footprint may rest on the washer, causing it to bow up and potentially resulting in the tensioner becoming locked in place.

Another drawback of tensioners is their limited suitability in high-temperature environments. Consequently, while assembling the flange at ambient temperatures is feasible with tensioning, post-startup adjustments, commonly referred to as hot tension passes, are not viable as the seals may blow and fail to sustain pressure.

[edit] How Hydraulic Tensioning Works

Beginning from the base of the unit, the fundamental components of a hydraulic tensioner include:

• Nut: Positioned on the flange, while the bridge surrounds the nut. The bolt extends through the nut, requiring one diameter of the stud to protrude above it for proper installation.
• Load cell: Located above the nut, housing the hydraulic piston. A nipple connected to the load cell allows oil to enter at a specified hydraulic pressure from the tensioner pump.
• Puller bar: Situated on the piston, which is threaded onto the stud. The hydraulic pressure from the pump and the dimensions of the load cell determine the force exerted on the bolt.

Bolt tensioning involves covering 50% or 25% of the bolts within a flange.

- 50% coverage entails using one tensioner for every other bolt and is considered standard practice. While 25% coverage is possible, it is not recommended due to increased passes, greater difficulty for the assembler, and reduced accuracy.

[edit] Hydraulic Tensioning Safety

Safety in hydraulic tensioning is paramount. The primary concern revolves around the handling of high-pressure hydraulic fluid, which flows from the tensioner pump to the load cell and subsequently to other tensioners. The immense pressure of this fluid poses a significant risk of injury, underscoring the importance of adhering to proper storage, cleanliness, and safety protocols before commencing operations. Any signs of wear or damage on hoses or fittings should prompt immediate replacement.

Moreover, assemblers must avoid positioning themselves directly above the tensioner during operation. In the event of insufficient thread engagement, stripping, or other issues, a dislodged tensioner could potentially become a hazardous projectile. Therefore, always ensure that assemblers are positioned to the side of the tensioners, rather than directly above the puller bar.

[edit] Maintenance, Troubleshooting, and Calibration

Seals often emerge as the primary culprit for issues encountered with hydraulic tensioners. Similar to hydraulic torque wrench pumps, contaminants like oil, dirt, or other debris present in the hydraulic fluid can infiltrate the tools and deteriorate the seals, often causing the initial components to fail.

Beyond the seals, hydraulic tensioners typically require minimal maintenance. Another significant troubleshooting point is to inspect the couplers if a tensioner is malfunctioning. Regarding calibration, ensuring that the measure has been calibrated within the last 12 months is all that's necessary.

[edit] Related articles on Designing Buildings

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• Cramp.
• Crimp.
• Fittings.
• Fixture.
• Fixing v fastening.
• Nails - a brief history.
• Off-site prefabrication of buildings: A guide to connection choices.
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