Boltight at full stretch in the power business

25 July 2018

Across all power generation technologies, bolt fatigue, relaxation and corrosion are concerns for engineers. In particular, it is critical to ensure bolts are at the right tension or torque.

Without these measures in place, structural integrity is compromised and the results can be catastrophic. A new white paper from hydraulic bolt tensioning specialist Boltight, looking at trends in the power industry and implications for bolting technology, refers to the example of a V112 wind turbine at the Lemnhult wind farm in Sweden, which collapsed in 2015 due to a failure in the bolted flange between tower sections one and two. First reports suggested the quality of the bolts was to blame, yet further investigations concluded the fault lay with incorrect bolt pre-tensioning procedures.

Hydraulic bolt tensioning provides accurate and easy tightening of bolts to the correct pre-loads, and is far safer and more reliable than conventional bolt tightening methods, says Boltight. It does not use torque or require any forceful turning. Other methods cause friction which uses up around 80% of the torque energy applied to the bolt. Instead, with hydraulic bolt tensioning, tension is directly applied to the bolt to stretch it hydraulically. The nut can then be rotated freely on the thread to the required position.

The method applies a consistent force, meaning the applied load can be controlled far more accurately. It is uniform, stress-free and the load transfer factor is calculable.

Other benefits include leak free joints, even compression of the gasket and the opportunity for simultaneous tightening of several bolts, for example on a flange. 

As the white paper shows, with the rise of renewables, this sector has in recent years become an ever more integral part of Boltight’s business.

For instance, within the wind power industry Boltight tools are used on wind turbine tower base bolts, frames, bearings and blade bolted joints, with a variety of bolt tensioners, foundation bolt tooling and a compact electric 1500 bar pump to provide the required hydraulic pressures in the tight spaces often encountered in wind turbines.

The company offers a specialist hydraulic bolt tensioning tool for wind turbine applications. Called Typhoon, it is optimised for size, weight and load capacity, and enables the tightening of bolts to high loads quickly in confined spaces to precise measurements. Wind turbine applications include blade bolting, bearing to hub bolting, tower bolting, foundation bolting, outer frame bolting and hub to spindle bolting. The tools have been used on all the main wind turbine types, and are proving particularly popular with contractors providing aftermarket inspection and maintenance services.

As well as systems for wind turbines, Boltight offers compressor rotor bolt tensioning systems for gas turbines, gas and steam turbine casing bolt tensioners, steam turbine casing closure systems, systems for hydroelectric plants and hydraulic tensioning tool sets for reciprocating engines, which ensure accurate tensioning of critical bolts on cylinder heads, connecting rods and main bearings.

The company also has extensive experience in the nuclear sector, eg multi stud tensioners for reactor pressure vessels and steam generators. A recent reactor pressure vessel stud bolt project undertaken for Doosan Heavy Industries & Construction, for example, entailed a bolt preload of 14 500 kN with very limited space. The tensioning tools developed by Boltight for this application incorporated a hydraulic piston retraction function to enable the equipment to be reset quickly, reducing operating time. The tools also had to compensate for dynamic joint behaviour. Accordingly, a spherical reaction nut and piston interface was employed to accommodate any bending effects in the event of flange rotation. Also, a number of safety mechanisms were incorporated to protect both operators and tools. Pressure relief valves were installed to prevent the hydraulic return system from over-pressurising. In addition, a floating gearbox design was engineered, to avert drivetrain damage should nut misalignment occur. The end result was realisation of a cost effective, bespoke solution for a highly loaded joint, with large bolt diameters, large number of bolts and high bolt loads, combined with restricted access.

Since 2015 Boltight has been part of the Nord-Lock Group, enabling hydraulic bolt tensioning to be offered alongside Nord-Lock’s Expander System, wedge locking washers, and Superbolt mechanical tensioning systems. 

Hydraulic bolt tensioning in practice, power generation examples
Hydraulic bolt tensioning in practice, power generation examples
Nuclear reactor pressure vessel stud bolt tool developed for Doosan
Hydraulic bolt tensioning. The hydraulic tool, show in place on the bolt, stretches the bolt, allowing the nut to be freely turned to the required position. (Images are stills from recently produced Boltight animations)
Conventional bolting. Up to its elastic limit force exerted by the bolt is essentially proportional to extension (Hooke’s Law). Beyond this limit the bolt permanently deforms, leading to failure
Hydraulic bolt tensioning in practice, power generation examples
Hydraulic bolt tensioning in practice, power generation examples
Hydraulic bolt tensioning in practice, power generation examples
Hydraulic bolt tensioning in practice, power generation examples

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