Advanced Bolting Solutions Pvt. Ltd. (ABS) is a leading multinational company providing Precision Bolting and Machining Solutions through consultative sales approach, on-site controlled bolting and machining services across multiple locations in India and the Middle East.
October 30 2020
Leakages in Bolted flange joints have always been a problem. To resolve such painful issues, precision bolting tools like bolting tool and torque wrenches are used to achieved joint integrity between flanges. In this article, we will understand how a bolt or stud tensioner achieves joint integrity and prevents flange leakages.
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A bolt or stud tensioners stretches the bolt axially to achieve the required preload. This replaces a high mechanical effort delivered to achieve the desired torque with hydraulic pressure that requires significantly less effort. Also, a uniform load can be applied on multiple bolts simultaneously to achieve equal tensioning. Therefore, the preload is equal to applied torque minus frictional losses.
Bolt Tensioning requires a longer bolt length and enough seating space on the assembly around the nut. A minimum length equal to 1-1.5 times the diameter of the stud/bolt is necessary to safely conduct bolt tensioning operation.
Hydraulic bolt Tensioners offer simultaneous tightening of multiple bolts of a joint. In this process, the tensioners are arranged and connected in sequence through a high-pressure hose assembly from a single pump unit.
This ensures that each hydraulic stud tensioner develops the exact and equally distributed same load and provides a uniform clamping force on the joint. This is particularly important for pressure vessels in chemical industries requiring evenly distributed gasket compression get the sealing affect to avoid leakages. Hydraulic bolt tensioners provide the utmost level of accuracy and safety on critical joints across all the industry.
Before installing the bolt tensioners, ensure that there is enough shank length above the nut. This ensures proper engagement of the puller of the hydraulic stud tensioner on the nut allowing it to apply accurate load on the fastener. As a rule, a minimum of two thread pitches should extend beyond the top surface of the nut.
The Nuts are numbered for identification and for better control in 50% tensioning which will be covered in detail, in next section of this article.
Steps to follow for Bolt tensioning
In this process, the bolt behaves like a solid spring and when the pressure is released, the bolt is under tension and attempts to contract, creating the required clamping force across the joint.
4. What is 50% Tensioning
All applications may not allow simultaneous tensioning of all bolts due to complications like space constraints. in such cases, 50% of the bolts are tensioned first and then repeated on the other bolts. To achieve the accuracy offered by hydraulic bolt tensioners while keeping costs low, an organization may choose to tension at 10%, 25%, or 33%. In some special applications, 100% tensioning may be nexessary.
Load losses are determined from a direct loss of stud elongation. Some other factors that can be taken into consideration are radial deflection of nut, friction, etc.
To avoid such losses, it is important that a load factor and operating pressure is calculated to deliver the desired tension on the bolt.
To obtain a known residual bolt load, an extra load must be applied so that required load is transferred for bolt tension. The load transfer factor can be obtained from the formula which is directly related to the clamp length and the nominal diameter of the bolt.
Formula for Load Transfer Factor = 1.01 + (D / C)
Where D = Nominal Thread Diameter in mm or in Inch and C = Bolt Clamp Length in mm or in Inch
Content Sources :- ABS Group
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