The Pros and Cons of Bolt Tensioning and Torquing
Both torquing and tensioning are used to create a calculated clamping force, but how they are performed is very different. This article will discuss the advantages and disadvantages of bolt tensioning and torquing.
Bolt tensioning is performed by hydraulically stretching a stud to a calculated pre-load value and then hand-tightening a nut.
Bolt torquing is done by using a wrench to turn a nut against a combined resistance created by friction and material elongation. The torque causes the bolt to stretch and creates the required pre-load on the stud.
The Main Difference
The main difference is how torquing and tensioning are performed to achieve the desired pre-load. Torquing requires a specific rotational force (torque) to achieve a calculated pre-load, whereas tensioning creates a direct pre-load via hydraulic load cells.
Advantages of tensioning
– Consistent and repeatable results
– Bolting surface finish and lubrication are not a factor in achieving the desired pre-load
– Suitable for larger diameter bolting
Disadvantages of tensioning
– More expensive than torquing
– Requires specialized equipment
– Requires specifically trained workers
Advantages of torquing
– Can be performed with common tools
– Torquing procedures are well understood in the industry
– Typically required on legacy equipment that does not have clearances for hydraulic tensioning
Disadvantages of torquing
– Requires proper lubrication of studs and bolts
– Torque values (friction) are sensitive to the surface condition of the studs and nuts
– Less accurate and less consistent than tensioning
– May require significant torque loads as bolting increases in diameter, making it impractical for use.
Tensioning Step-by-Step
1. Clean and inspect the surfaces of the flanges, bolting, and gaskets.
2. Apply a liberal coat of anti-seize compound to the threads of the stud and nut-bearing surface.
3. The number of hydraulic load cells can be 25%, 50%, or 100% depending on various factors. Mark the bolting “A”, “B”, “C”, or “D”, for 25% and mount the tensioner on every 4th bolt.
4. Apply pump pressure to the tensioners and tighten down the nuts with even hand pressure.
5. Repeat Step 4 two (2) more times for the first series of bolting, and then repeat for “B”, “C”, and “D” bolting.
Torquing Step-by-Step
1. Clean and inspect the surfaces of the flanges, bolting, and gaskets.
2. Number the bolts and apply a liberal coat of anti-seize compound to the threads of the stud and nut-bearing surface.
3. Hand-tighten all nuts.
4. In a defined sequence, torque bolting to ⅓ of the defined torque value. Wait 15 minutes for the bolt strain to equalize.
5. Repeat the torquing sequence to ⅔ torque value, wait 15 minutes, and then repeat the sequence again to the final calculated torque value.
Let Altex Handle Your Complex Bolt Tensioning and Torquing
Whether you have an air-cooled or shell and tube heat exchanger, proper bolt-up will ensure leak-free performance at start-up that lasts to the next scheduled maintenance. Altex has the expertise and experience to properly tension and torque all of your critical bolting applications.
Contact us today for more information about our services.
