Why Proper Flange Bolting Matters

Flange bolting is one of the most critical operations in piping assembly. Incorrect torque values, improper bolt loading sequences, or inadequate gasket seating can lead to joint leaks, environmental hazards, safety incidents, and costly downtime. This guide provides comprehensive bolting procedures, torque values, and best practices for 2026 piping projects.

Understanding Flange Bolting Fundamentals

The Purpose of Bolting

Flange bolts serve to compress the gasket to create a leak-tight seal between two flange faces. The bolt load must be sufficient to:

• Seat the gasket properly during initial assembly
• Maintain gasket compression under operating pressure and temperature
• Resist internal pressure forces that attempt to separate the flanges
• Account for thermal expansion differentials between bolts and flange material

Key Variables Affecting Torque

• Bolt material and grade: B7, L7, B16 (carbon/alloy steel), B8 (stainless steel)
• Bolt diameter: From 1/2″ to 4″ and larger
• Gasket type: Soft (fiber, rubber), semi-metallic (spiral wound), metallic (ring joint)
• Flange face finish: Raised face (RF), flat face (FF), ring joint (RTJ)
• Lubrication: Affects friction coefficient and torque-to-tension relationship

Torque Values by Bolt Size and Grade

Standard Torque Table (ASME PCC-1 Reference)

The following torque values assume standard lubrication (anti-seize compound) and spiral wound gaskets on raised face flanges:

Note: Values are for reference. Always consult the specific gasket manufacturer’s recommendations and applicable engineering standards.

Bolting Procedures

ASME PCC-1 Guideline for Bolted Joint Assembly

ASME PCC-1 (Pressure Technology – Guidelines for Pressure Boundary Bolted Flange Joint Assembly) is the definitive standard for flange bolting procedures.

Step 1: Joint Preparation

• Inspect flange faces for damage, scratches, or corrosion
• Clean bolt holes using compressed air or wire brush
• Verify gasket is correct type, size, and in good condition
• Lubricate bolt threads and nut bearing surfaces with appropriate compound

Step 2: Gasket Installation

• Center the gasket precisely on the raised face
• Ensure no debris or foreign material on gasket surfaces
• For spiral wound gaskets, verify centering ring is properly seated

Step 3: Bolt Insertion

• Insert all bolts hand-tight first
• Ensure bolts pass freely through holes without binding
• Verify correct number and size of bolts per flange specification

Step 4: Nut Tightening Sequence

Use the star pattern (cross-pattern) tightening sequence to ensure uniform gasket compression:

• Round 1 (Snugging): Tighten all bolts to 30% of final torque in star pattern
• Round 2: Tighten all bolts to 60% of final torque in star pattern
• Round 3: Tighten all bolts to 100% of final torque in star pattern
• Round 4 (Final): Go around all bolts at 100% torque in circle (sequential) pattern

Step 5: Verification

• After final pass, check each bolt at target torque value
• Record all torque values as part of QA documentation
• For critical services, consider using hydraulic tensioning or bolt tension indicators

Torque Methods Comparison

1. Manual Torque Wrench

• Most common method for general piping
• Accuracy: ±4% for calibrated click-type wrenches
• Requires proper technique and calibration

2. Hydraulic Torque Wrench

• For large diameter and high-torque applications (2″ and above)
• Higher accuracy: ±2-3%
• Requires hydraulic power pack

3. Hydraulic Bolt Tensioning

• Most accurate method: ±1% load accuracy
• Directly measures bolt elongation rather than torque
• Essential for critical applications: high-pressure, high-temperature, nuclear
• Simultaneous tensioning of multiple bolts for uniform loading

4. Thermal Tensioning

• Uses hydraulic nut or induction heater to elongate bolts
• Used for very large bolting (4″ and above)
• Less common but highly effective

Gasket-Specific Considerations

Spiral Wound Gaskets

• Require controlled compression — over-torque can crush the filler
• Use inner and outer rings for proper centering and blowout prevention
• Typical seating stress: 10,000-15,000 psi

Ring Joint Gaskets (RTJ)

• Require higher bolt loads for metal-to-metal seal
• Oval gaskets require lower torque than octagonal
• Verify ring fits properly in groove before bolting

Soft Cut Gaskets (Non-Asbestos Fiber)

• Lower torque requirements due to compressible filler
• Prone to creep relaxation — may require retorquing
• Use flat face flanges only (not raised face)

Flange Facing and Surface Finish

Flange face finish directly affects sealing performance:

• Stock finish (125-250 AARH): Standard for most raised face applications
• Spiral serrated (250-500 AARH): For harder gasket materials
• Smooth finish (125 AARH max): For flat gaskets and low-pressure service

Common Bolting Problems and Solutions

2026 Best Practices

• Digital torque documentation: Bluetooth-enabled torque wrenches that automatically log values to cloud databases
• Bolted joint integrity programs: Per ASME PCC-1, including training, certification, and auditing
• Flange management software: Tracking every joint’s assembly data, torque values, and maintenance history
• Qualified bolting technicians: Industry trend toward requiring certified personnel for critical joints

CoreMetal Steel: Flanges, Bolts, and Gaskets

CoreMetal Steel supplies ASME B16.5 and B16.47 flanges, stud bolts per ASTM A193/A194, and gaskets in all standard types. Available in carbon steel, stainless steel, and alloy grades with full material certification.

Contact: Tracy | tracy@coremetalsteel.com | +86 18291910632

Conclusion

Proper flange bolting is essential for leak-free piping systems. Following ASME PCC-1 procedures, using correct torque values, and implementing proper tightening sequences ensures joint integrity. Visit CoreMetal Steel Blog for more technical guides.