Flange Bolting-Up Practices
How bolt preload, torque accuracy, and gasket behavior affect flange sealing reliability
Flange joints are among the most common pressure‑retaining connections in industrial plants, yet they remain a frequent source of leaks. Most failures are not caused by the gasket itself, but by incorrect bolt preload, uneven tightening, thread friction, or loss of tension due to gasket creep. This article explains the fundamentals of bolt behavior, torque accuracy, gasket performance, and proper tightening sequences so technicians can achieve reliable, long‑lasting flange seals.
1. The Bolt as a Spring
A bolt is not just a bolt—it behaves like a long, stiff spring. When tightened, the bolt stretches slightly, creating a tension force that clamps the flange faces together. This preload must exceed the internal pressure forces trying to separate the joint.
For most industrial bolting applications, bolts are tightened to approximately 75–80% of their yield strength. This ensures adequate preload without causing permanent deformation.
Example: A bolt stretched by 0.01 mm (0.0004 in) may generate thousands of pounds of clamping force depending on its diameter and grade.
2. A Gasket Is Not Just a Gasket
When compressed between flanges, a gasket behaves both as a seal and as a spring. In this article, it is referred to as a gasket‑spring. Over time, the gasket creeps—gradually deforming under load—which reduces bolt tension. If preload drops too low, internal pressure can lift the flange faces and cause leakage.
This is why many procedures require a re‑torque after a short period, especially with fiber, spiral‑wound, or composite gaskets.
To prevent flange leaks, several conditions must be met. The following table provides guidance.
| WHAT IS REQUIRED | WHAT YOU CAN DO |
|---|---|
| The bolt‑spring shaft must be stretched to 75–80% of yield strength and exceed maximum bolt load. | Ask the bolt manufacturer for maximum torque and number of turns from snug, or use load‑indicating washers. As a last resort, use a high‑quality torque wrench to the manufacturer’s specification. |
| Bolts must have sufficient tensile strength for process and bolting forces. | Check bolt head forging marks against manufacturer standards to ensure bolts and nuts meet required load ratings. |
| Bolt tension must allow for cycling loads, shock loads, shear loads, and vibration. | Use higher‑strength bolts and tighten to proper preload. Ensure all bolts are the same grade and diameter. |
| Even, continuous torque application. | Lightly lubricate threads and follow the recommended bolting‑up sequence. |
| Clean, flat flange faces. | Check both flanges with a straightedge. Machine faces that are not flat, leaving enough thickness to meet pressure code requirements. |
| Use washers under bolt heads and nuts. | Washers distribute load evenly and eliminate high spots that cause uneven bearing. |
| Gasket must be suitable for the service and exhibit minimal creep. | Choose the thinnest gasket that meets seating pressure requirements. Always use a new gasket. |
| Gasket must fully cover the flange pressure face. | Cut a close‑tolerance gasket that completely covers the sealing surface. |
| Flanges must be aligned correctly. | Flange faces must be square, flat, and in‑line. Misalignment must be corrected. Flanges must line up within 0.01 mm (0.005 in). |
3. Tightening the Bolt and Nut
Tightening bolts by “feel” is the least accurate method, with errors up to ±35%. Torque wrenches improve accuracy, but friction still causes large variations. The most accurate method is measuring bolt stretch or bolt stress directly.
| Method | Typical Accuracy |
|---|---|
| Tightening by feel | ±35% |
| Standard torque wrench | ±25% |
| Turns from snug | ±15% |
| Load‑indicating washers | ±10% |
| Measuring bolt length change | ±5% |
| Measuring bolt stress directly | ±1% |
Torque is a twisting force. An 80 kg (176 lb) technician standing on a 1 m (39 in) wrench applies approximately:
- 800 N·m of torque (metric)
- 590 ft·lb of torque (US customary)
If the wrench is shortened to 500 mm (19.7 in), torque drops to 400 N·m (295 ft·lb). At 300 mm (11.8 in), torque drops to ~250 N·m (185 ft·lb). This demonstrates why short hand tools often cannot achieve required torque for larger bolts.
For bolts 20 mm (0.79 in) and larger, hydraulic tensioners or extension arms may be required to achieve proper preload.
4. Bolt Thread Surface Finish
Thread surface finish significantly affects the torque required to achieve proper bolt stretch. Rough or galvanized threads produce more friction than machined and lubricated threads. Torque values must be adjusted accordingly.
| Surface / Lubricant | Friction Coefficient Min | Friction Coefficient Max |
|---|---|---|
| Bare passivated surface | 0.10 | 0.25 |
| Baked‑on MoS₂ film | 0.04 | 0.10 |
| Molycote G Paste | 0.04 | 0.09 |
| Liquid Teflon (water‑based) on MoS₂ | 0.03 | 0.10 |
If using Teflon‑coated threads, torque must be reduced to avoid over‑stretching the bolt. Always consult the bolt supplier for correction factors.
In some cases, the bolt must be turned instead of the nut. Required torque will differ because the friction under the bolt head is not the same as under the nut.
5. Bolting‑Up Sequence
Flanges must be tightened in a cross‑pattern sequence to ensure even gasket compression. Uneven tightening can warp the flange, crush the gasket, or create leak paths. Typical tightening sequence for 4‑, 8‑, and 16‑bolt flanges.
Bolting Up Sequence
Hover over animation above for bolt tightening sequence chart.
Recommended procedure:
- Snug all bolts in a star pattern.
- Apply 30% of final torque in sequence.
- Apply 60% of final torque in sequence.
- Apply 100% of final torque in sequence.
- Perform a final circular pass at full torque.
6. Technician Checklist
| Item | Why It Matters |
|---|---|
| Correct bolt grade | Ensures proper preload without yielding. |
| Clean, lubricated threads | Improves torque accuracy and consistency. |
| Flat, undamaged flange faces | Prevents uneven gasket loading. |
| Correct gasket type | Matches pressure, temperature, and media. |
| Proper tightening sequence | Ensures uniform gasket compression. |
| Re‑torque if required | Compensates for gasket creep. |
When these fundamentals are followed, flange joints remain tight, reliable, and safe under pressure.
Adapted from original work by Mike Sondalini, Equipment Longevity Engineer.
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