Tensioning is a controlled way of tightening a bolt on applications within critical industries such as oil and gas, wind energy, construction and nuclear etc.This is done by stretching the bolt to the initial load. The nut is then rotated onto the flange face which keeps the load in the bolt once it has relaxed.
How can a bolt stretch?
Most materials have what we call an elastic region. This is where the material when put under tensile stress will go back to its original state once the stress has been removed.
If a material is put under too much tensile stress, then it will enter the plastic region which is the point of no return and where the material can no longer revert to its original form. This is the same case for metal.
Hydraulics are used to pressurize the tool which then transfers the pressure energy into tensile stress on the stud. When designing a tensioning tool, calculations are made based on the stud’s material specification to make sure the stress is not too great that it will damage the stud.
The first signs of damage is what is called necking. This is when the stud starts to deform and beyond this point, the stud will then break.
Most materials have what we call an elastic region. This is where the material when put under tensile stress will go back to its original state once the stress has been removed.
If a material is put under too much tensile stress, then it will enter the plastic region which is the point of no return and where the material can no longer revert to its original form. This is the same case for metal.
Hydraulics are used to pressurize the tool which then transfers the pressure energy into tensile stress on the stud. When designing a tensioning tool, calculations are made based on the stud’s material specification to make sure the stress is not too great that it will damage the stud. The first signs of damage is what is called necking. This is when the stud starts to deform and beyond this point, the stud will then break.
The main benefits of bolt tensioning are that tools can achieve extremely high loads. The process can be quicker by one operator using multiple tools simultaneously, it is very accurate the tools are extremely durable, and it can be used in multiple environments. Topside in extreme temperatures, and subsea at greater depths.
Most of the time bolt tensioners are used in sets, to have uniform compression of joints for proper sealing. General practice is to use 25%, 50% or 100% bolt tensioning.
100% tensioning
The recommended is 100% bolt tensioning. This means, that all bolts in the flange will be tightened simultaneously, achieving uniform compression using just one pressurization of the tensioners followed by a check pass.
This is more beneficial for the joint because it will deliver uniform compression, and not create any potential uneven tightening. The total time for the entire process is shorter compared to 50% or 25% tensioning, as you can tension all bolts at once.
50% tensioning
However, all flange designs and geometry do not allow to use 100% tensioners. 50% tensioning means, we use 50% bolt tensioners to bolt ratio. This means that every alternate bolt will have tensioners and would need to be tightened in two-stages.
First stage with pressure A, and second stage with pressure B. Bolts are marked as 1 and 2, and pressure A is applied on bolts marked one and pressure B on bolts marked 2. Pressure A will always be higher than pressure B. To compensate the bolt load loss that happens when a bolt is tensioned next to an already tensioned bolt.
This cross-loading bolt loss is very predictable in tensioning. To compensate for this predictable load loss, extra load is applied in the form of pump pressure A to the first bolt so that the relax down to the required load when pump pressure B is applied to the next bolts.
At times this pressure A being higher than pressure B may not be permitted to be applied to be on safe side of the bolt yield capacity. In this situation, only 100% bolt tensioning is possible. Unless the required bolt stress can be reduced. If space from one side of the flange does not permit, tensioners can be mounted from both sides on alternate bolts.
25% tensioning
25% tensioning means, 25% tensioners to bolt ratio. If you have eight bolts on the flange, then you use two tensioners. Mark bolts 1,2,3,4. You are given 'A' pressures to go on 1 and 2, and 'B' pressures to go on 3 and 4.
Tightening procedure and number of passes can vary based on application, type of gaskets, sizes etc. Use our Bolt Load Software to get pressure load values for these configurations on standard flanges.
For more information on bolt tensioning, contact Atlas Copco today and schedule a demo.
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