Hydraulic tensioners are more commonly used for nut-tightening applications in Europe than in the United States. You will typically see hydraulic tensioners used on fasteners with a bolt diameter of 2 inches or more, but they can be used on dowels with bolt diameters as small as ¾ inch.
The hydraulic bolt tensioner rentals are commonly used for underwater applications, wind turbines, and power generation but are less widely used in the oil and gas industry. However, in the oil and gas industry, tensioners are a good use for fasteners in bolted joints, such as heat exchangers and other large pressure vessels.
What is Bolt Tensioning?
Bolt tensioning is the process of applying a load to some or all of the bolts in a flange simultaneously using a hydraulic nut tensioner or a removable bolt tensioner. It uses hydraulic pressure to apply the axial load directly to the dowels rather than rotating the nut to apply the load. Tensioning can be done on some or all of the dowels simultaneously.
How does Bolt Tensioning Work?
The bolt tensioning services uses hydraulic pressure to extend the studs axially. The nut is manually turned down until it contacts the flange again, and the hydraulic pressure on the tensioner is released to allow the nut to hold the applied tension.
Unlike bolt torque adjustment, which uses a nut rotational force through friction on the bolt threads, which causes the bolt to stretch, the tensioning process applies an axial force directly to the bolt without having to rotate the nut to adjust the tension, eliminating the need to predict the amount of friction required or applied.
Key Terms to Know:
Assembly Load or Preload:
Assembly load refers to the load placed on the bolt during assembly. However, this term is important to know prior to operation because the load placed on the bolt changes as the flange is operated. It is important to correctly calculate, apply, and verify the assembly load to ensure that the flange is loaded sufficiently to accommodate the movement in the system.
Working Load or Operating Load:
Working load refers to the load on the bolt during system operation. System operation causes fluctuations in temperature, pressure, and cable load, which can affect the change in load on the bolt from the original assembly load. The change in load from assembly to operation occurs regardless of the load applied. It is, therefore, important to correctly calculate and apply the load during assembly to overcome the known changes in operation that will happen in the system. Tensioning Terminology:
Applied Load:
The applied load is the load applied directly to the stud shaft while the hydraulic pressure is applied to the tensioner. It is also the load applied to the bolt during tensioning before the load is transferred from the hydraulic system to the bolt and before operation. This load is known to be 99% accurate as it is calculated as Force = Pressure x Area.
Loss of Load:
Loss of Load refers to the load lost during the transfer of the load from the hydraulic system to the nut. When the load on the stud is transferred from the tensioner using hydraulic pressure to the nut, a subsequent load loss occurs. This load loss is due to the multiple points of contact that loosen and deform elastically as the nut takes the load instead of the tensioner. This load loss only occurs during assembly and is known for most applications. Therefore, it is compensated for when the applied load is determined and used.
