Proper selection of bolts for flange connections is critical to ensuring safety and reliability in mechanical systems. This article explains the detailed steps to determine the appropriate bolt quantity, diameter, and length when designing or installing flange connections.
1. Understanding the Flange Standard
Flanges are manufactured according to various international standards, such as:
- ASME/ANSI B16.5 (American standard)
- DIN EN 1092-1 (German/European standard)
- JIS B2220 (Japanese standard)
- GB/T 9119 (Chinese standard)
Each standard specifies the number of bolt holes, bolt diameter, and bolt circle diameter based on the flange’s nominal diameter (DN) and pressure rating (PN or Class). Consult the relevant standard for the flange being used to retrieve this information.
2. Determining the Number of Bolts
The number of bolts required for a flange is determined by the flange’s nominal diameter and pressure rating. For example:
- A DN100 PN16 flange based on the ASME B16.5 standard typically has 8 bolt holes.
- Higher pressure ratings or larger diameters usually result in more bolt holes.
Use the flange specification table provided in the standard to find the exact bolt quantity.
3. Determining the Bolt Diameter
Each flange standard specifies the required bolt diameter based on the size of the bolt holes in the flange. For example:
- A DN100 PN16 flange might require M16 (metric) or 5/8-inch (imperial) bolts, depending on the standard.
Ensure that the selected bolts match the specified size to prevent loose fits or misalignments.
4. Calculating the Bolt Length
Bolt length depends on several factors:
- Flange Thickness: Measure the thickness of each flange being connected.
- Gasket Thickness: Include the gasket’s thickness as it contributes to the overall assembly height.
- Nut Thickness: Add the thickness of the nuts used on each side.
- Allowance for Protrusion: Allow an additional length (usually 5–10 mm) to ensure the bolt extends beyond the nut for proper engagement.
Formula for Bolt Length
Bolt Length=Flange Thickness 1+Flange Thickness 2+Gasket Thickness+Nut Thickness (2)+Protrusion Allowance
Example Calculation
- Flange Thickness: 18 mm (each flange)
- Gasket Thickness: 3 mm
- Nut Thickness: 16 mm (each nut)
- Protrusion Allowance: 5 mm
Bolt Length=18+18+3+16+16+5=76 mm
Choose the closest commercially available bolt length that exceeds this calculated value, such as an M16×80 mm bolt.
5. Material Selection
The material of the bolts should match the application environment:
- Carbon Steel Bolts: Suitable for general-purpose applications.
- Stainless Steel Bolts: Used in corrosive environments.
- High-Strength Bolts: Required for high-pressure or high-temperature conditions.
- Exotic Alloys: Necessary for extreme conditions like subsea or cryogenic applications.
Verify material compatibility with the medium and temperature of the piping system.
6. Installation Guidelines
Proper installation ensures the integrity of the flange connection:
- Align Bolt Holes: Ensure that the bolt holes of both flanges are perfectly aligned.
- Apply Lubrication: Apply anti-seize lubricant on the bolt threads to facilitate tightening and prevent galling.
- Use a Torque Wrench: Tighten bolts in a star or crisscross pattern to achieve uniform compression of the gasket.
- Verify Torque: Confirm the torque values match the specifications for the selected bolt grade and flange material.
7. Example Scenario
Problem: Determine the bolts for a DN150 PN16 flange connection.
Solution:
- Bolt Quantity: Check the standard table for DN150 PN16. According to GB/T 9119, it requires 8 bolts.
- Bolt Diameter: The standard specifies M20 bolts.
- Bolt Length:
- Flange Thickness: 20 mm (each flange)
- Gasket Thickness: 3 mm
- Nut Thickness: 18 mm (each nut)
- Protrusion Allowance: 5 mm
Bolt Length=20+20+3+18+18+5=84 mm
Choose M20×90 mm bolts to ensure proper fit and protrusion.
Conclusion
Selecting the correct bolts for flange connections involves consulting the appropriate standard, calculating the necessary bolt length, and considering the application environment. Accurate determination of these parameters ensures a secure and leak-free flange joint, contributing to the overall safety and efficiency of the system.