Ensure flange faces are parallel
Avoid pulling or pushing the flange into assembly position
– The resulting piping forces will affect the joint’s performance
Be sure the gasket is properly placed and centered
– The gasket must not be creased, twisted, bunched, etc.
Uniform bolt tightening assures uniform seating of the gasket and accurate application of the calculated design loads in the flange.
Bolt tensioning devices achieve an accurate and pre-determined bolt loading in a single operation.
The most common form of bolt tensioning essentially provides a hydraulic load which acts directly upon the stud bolt. The hydraulic load is transmitted to the bolt by the “puller”. This force stretches the bolt, and the extension, or strain, is retained by engaging the nut. Since strain and stress are proportional in the elastic range, the applied extension results in a known applied bolt load equal to the force applied by the “puller”.
The hydraulic load, commonly called the applied load, is retained by the bolt when the hydraulic tensioner is released. The magnitude of the load is determined to provide the necessary gasket seating and sealing stresses.
Bolt tension indicating washers are another reasonably accurate method of insuring a known bolt force. These washers are bowed up in the center. They are designed to flatten under a known force.
When the bolt is tightened, the washer flattens when this force is reached. Use of the proper washer then indicates when the required bolt force is present.
Other methods of bolt tightening develop much less predictable bolt tensions. Use of a torque wrench, or measurement of the applied torque, is especially unreliable. The angle of the threads, friction along the threads, type of lubricant, and friction between the nut and outer surface of the flange are all variable factors that resist the applied torque without resulting in any load being applied to the gasket. Use of a torque based method for providing bolt tension typically results in a 30 percent (or greater) variation in bolt stress around the flange. Bolt tensioning methods normally result in a variation of about 10 percent.
EDS-2004/FL-101
Flange Assembly
(continued)
Do not overtighten the bolts
– Bolts may yield, the gasket may be crushed, or the flange may be distorted (e.g., the outer portion squeezed together, causing a rotation tending to unload some of the gasket)
Tighten the bolts uniformly
– Bolt tensioning devices are the most reliable method
– Torquing is common, but unreliable
The objective is to close the flange at a uniform and constant rate, bringing all portions of the flange around the full circumference together at the same rate. Uneven compression of the gasket around the circumference must be avoided. Uneven gasket compression from tightening one side before the other cannot be effectively recovered.
Use of a flange cement may even glue the gasket to both flanges, meaning the gasket must be torn apart to separate the flanges. Gasket remains may remain stuck to the flanges. The glue can also damage, perhaps etch, the flange sealing surface, promoting current or future leakage.
Bolt lubricants are often recommended. Lubricants reduce the unknowns related to friction when bolts are tensioned via torquing. The type and amount of lubricant are chosen so that the nut does not “back off” or loosen.
Bolts tend to “cross talk”, or be affected by what happens to other bolts. Each time a new bolt is tightened it will reduce the strain, and stress, on the previously tightened bolts. This is why the bolts are tightened in stages and why after each stage the gap between the flanges is measured and evened by adjusting individual bolt tightnesses before proceeding with the next round of tightening. After three stages, further adjust bolts in a circumferential pattern to even the flanges.
Thick, large diameter flanges with large diameter bolts may also need a disassembly procedure. Usually this is to back-off to 50 - 70% of the bolt load, even the flange gap, drop to 20 - 30% of the bolt load, even the gap, then remove the nut.
EDS-2004/FL-102
Flange Assembly
(continued)
The order of bolt tightening is very important
– Tightening in circumferential order will not achieve a uniform seal (recent evidence suggests this may not be as bad as previously thought)
– Tighten bolts in a diametrically opposed pattern
– Do not fully tighten the bolts in one step
– Proceed through the tightening sequence several times (usually three) to achieve the desired bolt force
– After each step, adjust to an even gap between the flanges
– Refer to ASME PCC-1 2000, “Guidelines for Pressure Boundary Bolted Flange joints Assembly”
Avoid use of flange “cements” on the gasket -they tend to damage the flange surface and are difficult to remove without causing further damage to the sealing surface
EDS-2004/FL-103
This is a graphical representation of the recommended sequential and rotational order of bolt tightening for uniform sealing of a flanged joint with 20 bolts. It can be seen that two diametrically opposed bolts are tightened followed by two more on a diameter approximately 90ofrom the previous set. This procedure continues until each bolt has been tightened once, then is repeated. This approach is applicable to any similar situation of bolt tightening, e.g., automobile tires. Usually three cycles are used to fully tighten the bolt. Do not tighten any bolts fully on the first two cycles because the flange system is still loose enough that flange rotation and unrecoverable uneven gasket compression can (will) result.
B16.5 (Annex F) requires the bolt threads to be engaged to the surface of the nut.
B16.5 also says that the points on the bolt are not part of the bolt’s overall length.
B31.3 permits an underengagement of one thread. Standard practice is for at least two threads of the bolt to be exposed beyond the nut. This ensures full engagement and the ability to fully develop the bolt’s strength. More extension is wasteful, may create clearance problems, and exposes more of the bolt to rust or damage, complicating disassembly. Under engagement may reduce the bolt’s capacity below that required. If below the surface of the nut, a depression is formed that can collect dirt, water, oil, etc. This can complicate disassembly and contribute to bolt corrosion (rust).
EDS-2004/FL-104
Sequential Order 1-23-4 5-67-8 11-129-10 13-14 15-16 17-18 19-20 21-22 23-24
Rotational Order 19
175 1321 113 197 1523
102 186 1422 124 208 1624
PPF-R00-30
This figure illustrates the sequence for tightening a flange with 24 bolts.
Radial scratches or corrosion are one of the most common causes of leakage. The scratch provides a direct, preferential leak path under the gasket. Scratches are often too deep and/or too narrow for the gasket to effectively seal.
Take precautions to prevent damage and protect flanges during any Post Weld Heat Treatment. Corrosion, oxidation, etc., may interfere with the ability to seat the gasket and its subsequent ability to maintain a seal against pressure.
Flanges out of parallel or containing a wavy surface finish (not flat) can cause a non uniform seating of the gasket.
EDS-2004/FL-105