Bolt tensioners for leak proof joints
Introduction
Leakages in Bolted flange joints have always been a problem. To resolve such painful issues, precision bolting tools like bolt tensioners 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.
1.
Need for Bolt Tensioning
A bolt or stud
tensioner
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.
2.
How Bolt tensioning is achieved?
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.
3. Steps
required for deploying Hydraulic tensioners
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
- The Bolt
tensioner is assembled on the stud
- Hydraulic
pressure is applied to the tensioner via hoses and Hydraulic cylinders
- Uniform
pressure will be applied to stretch the stud up to required pre-load.
- Once,
the bolt is tensioned, nut is rotated against the joint face using a tommy
bar for achieving final tightening.
- Pressure
is released and the Hydraulic bolt tensioner will be removed from the
setup.
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.
5. Determine the load transfer factor
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
6. Types of Hydraulic Tensioners
-
Standard
Bolt Tensioners: Standard
bolt tensioners are designed to fit API flanges.
-
Single
stage Tensioners: Single-stage
tensioners are multipurpose hydraulic
bolt tensioners suitable for applications like high load and tight
access locations. These are also best suitable for places where there are
overhead obstructions.
-
Multistage
Tensioners: Multistage
tensioners are perfect for high load and tight access applications like in wind
turbines.
-
Special
Tensioners: Special tensioners
are the right fit for Compact oil and gas flanges and supporting for higher
loads and reduced space restrictions.
7.
Advantages & Disadvantages of Bolt Tensioners
- Stability and ease of control
- Highly accurate
- More expensive than manual techniques
- Most often used on large bolts
- but not always practical
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