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.
The recommended is 100% 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. Also the total time for the entire process is shorter compared to 50% or 25% tensioning, as you can tension all bolts at once.
However, all flange designs and geometry do not allow to use 100% tensioners. 50% tensioning means, we use 50% 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 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 give '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.