Bolt Tensioning vs. Torquing: Pros and Cons Explained
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.
What is Bolt Torquing and How Does It Work?
Bolt torquing is the process of applying a measured rotational force to a nut or bolt head to achieve the desired preload in the fastener. This preload creates a clamping force that holds the joint components together. In industrial settings, torque is applied using manual torque wrenches, pneumatic tools, or hydraulic torque wrenches, each offering different levels of precision and power.
However, it’s important to understand that up to 90% of the applied torque is lost to friction between the threads and the bearing surface, with only about 10% contributing to actual bolt elongation and preload (Nord-Lock). This means that surface condition, lubrication, and bolt material can significantly affect accuracy. While torquing bolts is a cost-effective and versatile method, it requires careful calibration and monitoring to ensure consistent results across all fasteners.
What is Bolt Tensioning and How Does It Work?
Bolt tensioning is a method of directly stretching a bolt to achieve the required preload without relying on rotational torque. This is typically done using hydraulic bolt tensioners, which apply high-pressure hydraulic force to a tensioner that grips the protruding threads above the nut. As the bolt elongates under controlled load, the nut is rotated down by hand to maintain the stretched length, then the hydraulic pressure is released, locking in the preload.
Unlike torquing, tensioning eliminates the majority of frictional losses, allowing for preload accuracy within ±10% (Hex Technology). This makes it ideal for critical joints in high-pressure vessels, flanges, and rotating machinery. The trade-off is that tensioning equipment is bulkier, requires more setup space, and is generally more expensive, but it offers superior consistency and reliability in safety-critical applications.
When to Choose Bolt Tensioning vs Torquing
The decision between tensioning vs torquing depends on factors such as joint criticality, accuracy requirements, access limitations, and project budget:
– Use bolt torquing for general-purpose applications, smaller bolts, or projects where cost and speed of deployment are primary concerns. It’s suitable for less critical joints where minor preload variations are acceptable.
– Choose bolt tensioning for high-pressure, high-temperature, or safety-critical connections where preload accuracy is essential, such as in heat exchanger flanges, pipeline flanges, and turbine casings.
– Consider accessibility: Torquing tools can be used in tighter spaces, whereas tensioners require sufficient clearance around the bolt and above the nut to fit the hydraulic cell.
By evaluating these parameters early in the planning stage, engineers can ensure that the selected bolting method supports long-term joint integrity and minimizes the risk of leakage or mechanical failure.
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 bolt tensioning and torquing services.
