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By StaticEngineer.com
Published: May 1, 2025
Last Updated: May 1, 2025

Series Overview: This is the final installment in our 3-part series on ASME VIII Div 1, Mandatory Appendix 2. In Parts 1 and 2, we covered flange fundamentals, types, components, and design procedures. Now we’ll work through practical calculation examples and troubleshooting for pressure vessel flange design.

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ASME Flange Design Examples, Troubleshooting, and Advanced Considerations

Introduction to Practical Flange Calculations

Welcome to the final installment of our comprehensive guide to ASME VIII Division 1, Mandatory Appendix 2. In Parts 1 and 2, we covered flange fundamentals, types, components, and design procedures. Now, we’ll work through practical calculation examples, discuss common issues encountered in the field, and explore advanced design considerations.

Calculation Example: ASME Welding Neck Flange Design

Let’s work through a complete calculation example for an integral welding neck flange.

Given:

• Nominal pipe size: 10 inches (254 mm)
• Design pressure: 250 psi (1.72 MPa)
• Design temperature: 400°F (204°C)
• Material: SA-105 (Carbon Steel)
• Gasket: Spiral wound with flexible graphite filler, 1/8″ thick
• Bolt material: SA-193 B7

Step 1: Determine Basic Dimensions

• Flange outside diameter (A): 17.5 inches
• Flange inside diameter (B): 10.02 inches
• Hub thickness (g₁): 0.593 inches
• Flange thickness (t): 1.5 inches (initial guess)
• Bolt circle diameter (C): 15.0 inches
• Bolt size: 1-1/8″ diameter, quantity: 16

Step 2: Determine Gasket Parameters

• Gasket type: Spiral wound with graphite filler
• From Table 2-5.1:
  • Gasket factor (m): 3.0
  • Minimum seating stress (y): 10,000 psi
• Gasket outside diameter: 12.5 inches
• Gasket inside diameter: 10.5 inches
• Gasket width: 1.0 inch
• Effective gasket width (b): 0.5 inch (calculated per Appendix 2)
• Gasket diameter at location of gasket reaction (G): 11.5 inches

Step 3: Calculate Required Bolt Loads

For Operating Condition:

• Hydrostatic end force (HD): HD = 0.785 × G² × P = 0.785 × (11.5)² × 250 = 25,944 lbs
• Gasket load (HG): HG = 2 × b × π × G × m × P = 2 × 0.5 × π × 11.5 × 3.0 × 250 = 27,053 lbs
• Total operating bolt load (Wm₁): Wm₁ = HD + HG = 25,944 + 27,053 = 52,997 lbs

For Gasket Seating Condition:

• Wm₂ = π × b × G × y = π × 0.5 × 11.5 × 10,000 = 180,356 lbs

Governing condition: Since Wm₂ > Wm₁, the gasket seating condition governs.

Step 4: Calculate Required Bolt Area

• For operating condition: Am₁ = Wm₁/Sb = 52,997/20,000 = 2.65 in² (where Sb = allowable bolt stress at operating temperature)
• For gasket seating: Am₂ = Wm₂/Sa = 180,356/25,000 = 7.21 in² (where Sa = allowable bolt stress at ambient temperature)
• Required bolt area: 7.21 in²
• Actual bolt area (16 bolts, 1-1/8″ diameter): 14.3 in² (adequate)

Step 5: Calculate Flange Moments

After determining the required bolt loads, we now calculate the moments acting on the flange:

First, determine the moment arms:

• hD = Moment arm for hydrostatic end force
  • For an integral flange: hD = 0.5(C – G) = 0.5(15.0 – 11.5) = 1.75 inches
• hG = Moment arm for gasket load
  • For an integral flange: hG = 0.5(C – G) = 1.75 inches
• hT = Lever arm for bolt load
  • For an integral flange: hT = 0.5(C – B) = 0.5(15.0 – 10.02) = 2.49 inches

Next, calculate the individual moments:

• MD = Moment due to hydrostatic end force = HD × hD
  • MD = 25,944 × 1.75 = 45,402 in-lbs
• MG = Moment due to gasket reaction = HG × hG
  • MG = 27,053 × 1.75 = 47,343 in-lbs
• Total Moment (MT) for operating condition = MD + MG
  • MT = 45,402 + 47,343 = 92,745 in-lbs

For the gasket seating condition:

• MT for gasket seating = W × hT = 180,356 × 2.49 = 449,086 in-lbs

The gasket seating condition produces the larger moment (449,086 in-lbs) and will govern the flange design.

Step 6: Calculate Flange Stresses

To calculate flange stresses, we need to first determine various stress factors from the geometry:

1. Calculate Shape Constants
   • K = A/B (ratio of outside diameter to inside diameter)
   • K = 17.5/10.02 = 1.746
   • Based on K value, using the charts in Appendix 2, determine:
     • T factor = 1.91
     • U factor = 8.32
     • Y factor = 6.43
     • Z factor = 5.84
2. Calculate Hub Stress (SH):
   • SH = (f × MT)/(L × g₁²)
   • Where f is a hub stress factor based on geometry
   • L is a hub length parameter
   • g₁ is the hub thickness at small end
   • With f = 1.0, L = 2.0, g₁ = 0.593:
   • SH = 1.0 × 449,086/(2.0 × 0.593²) = 638,382 psi
   • This value exceeds allowable stress and indicates redesign is needed
3. Calculate Radial Stress (SR):
   • SR = (1.33 × te × MT)/(L × t²)
   • Where te is the effective thickness parameter
   • With te = 1.2, t = 1.5:
   • SR = 1.33 × 1.2 × 449,086/(2.0 × 1.5²) = 119,756 psi
4. Calculate Tangential Stress (ST):
   • ST = (Y × MT)/(t² × B) – Z × SR
   • ST = 6.43 × 449,086/(1.5² × 10.02) – 5.84 × 119,756
   • ST = 128,085 – 699,374 = -571,289 psi (compressive)
5. Check Stress Combinations:
   • SH + SR = 638,382 + 119,756 = 758,138 psi
   • SH + ST = 638,382 + (-571,289) = 67,093 psi
   • SR + ST = 119,756 + (-571,289) = -451,533 psi
6. Compare with Allowable Stress:
   • For SA-105 at 400°F, allowable stress = 20,000 psi
   • Each combined stress should be ≤ 20,000 psi
   • Each individual stress should be ≤ 1.5 × 20,000 = 30,000 psi
   The calculated stresses exceed allowables, so the flange design needs revision.

Step 7: Verify Design Acceptance

Based on the excessive stresses calculated above, we need to:

1. Increase flange thickness (t) to 2.5 inches
2. Increase hub thickness (g₁) to 0.75 inches
3. Recalculate all stresses with the new dimensions

With these adjustments, let’s recalculate (abbreviated calculations shown):

• New SH = 252,423 psi (still exceeds allowable)
• Further increase thickness to t = 3.0 inches and g₁ = 0.9 inches
• Final recalculation yields:
  • SH = 18,521 psi
  • SR = 11,236 psi
  • ST = 15,642 psi
  • Combined stresses:
    • SH + SR = 29,757 psi
    • SH + ST = 34,163 psi (slightly exceeds 30,000 psi)
    • SR + ST = 26,878 psi

We would need one more iteration to arrive at a fully compliant design, but this demonstrates the iterative nature of flange design calculations

Calculation Example: Blind Flange

Given:

• Nominal diameter: 24 inches
• Design pressure: 150 psi
• Design temperature: 300°F
• Material: SA-516 Gr. 70
• Gasket: Full-face non-asbestos

[Similar calculation steps would follow]

Common Pitfalls and Troubleshooting

Design Issues to Avoid

1. Incorrect Gasket Factors Using incorrect m and y values is surprisingly common. Always verify these values from Table 2-5.1 or manufacturer’s data. Using incorrect values can lead to either leakage (if too low) or unnecessarily thick flanges (if too high).

2. Temperature Effects Material strength decreases with temperature. Always use the material properties at design temperature. Differential thermal expansion can also create unexpected loads in the bolted joint.

3. Bolt Preload Problems

• Too low: Gasket may not seal properly
• Too high: May yield bolts
• Uneven: Creates leak paths

4. Excessive Flange Rotation If the flange is too flexible, it can rotate under load, causing uneven gasket compression and leakage at the outer edge.

5. Hub Transition Issues In welding neck flanges, an improper hub transition can create stress concentrations that lead to fatigue failure.

Troubleshooting Field Issues

Leaking Flange Joints Common causes and solutions:

Problem	Possible Causes	Solutions
External leakage	Insufficient bolt preload	Retorque bolts using proper sequence
	Gasket damage/compression set	Replace gasket
	Flange face damage	Repair or replace flange
	Misalignment	Check alignment before tightening
Bolt failures	Over-torquing	Use calibrated torque tools
	Incorrect material for temperature	Verify bolt material suitability
	Stress corrosion cracking	Select appropriate bolt material

Advanced Design Considerations

Non-Circular Flanges

While Appendix 2 specifically addresses circular flanges, modified approaches can be used for rectangular or oval flanges by:

• Using equivalent diameters
• Applying appropriate shape factors
• Following the same fundamental principles with adjustments

High-Temperature Considerations

For services above 750°F:

• Creep becomes a significant factor
• Bolt relaxation increases dramatically
• Special materials may be required
• Retorquing protocols may be necessary
• Thermal expansion must be carefully evaluated

Cryogenic Service

For very low temperatures:

• Material toughness becomes critical
• Thermal contraction must be considered
• Special gasket materials are required
• Bolt preload should account for contraction

External Loads

When significant external loads or moments are applied to a flange:

• Convert external moments to equivalent pressure
• Consider combined loading effects
• Evaluate localized stress concentrations
• More detailed FEA analysis may be warranted

The Future of Flange Design

Modern developments in flange design include:

• Finite Element Analysis: More accurate stress distributions
• Advanced Gasket Materials: Better performance in extreme conditions
• Alternative Joint Designs: Including metal-to-metal contact joints
• Smart Bolting Technology: Real-time monitoring of bolt preload
• Computational Fluid Dynamics: For analyzing potential leak paths

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Expert Tips: Advanced Flange Engineering Applications

Expert Tip #7: For services with frequent thermal cycling, consider using a ‘live-loading’ system for critical flange connections. These systems use Belleville spring washers to maintain relatively constant bolt load despite thermal expansion and contraction.

Expert Tip #8: When working in low-temperature applications, be aware that gasket recovery properties can change dramatically. A gasket that performs well at ambient temperature may be too rigid at cryogenic temperatures to maintain a seal during thermal cycles.

Expert Tip #9: For difficult sealing applications, consider surface finish carefully. ASME standards allow various surface finish options, but specifying a smoother finish (especially for RTJ faces) can dramatically improve sealing performance with minimal cost impact.

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Frequently Asked Questions: Advanced ASME Flange Design

Q: When is it appropriate to use finite element analysis (FEA) rather than Appendix 2 calculations?

A: FEA should be considered in these situations:

• When the flange geometry deviates significantly from standard designs
• For high-temperature applications (above 900°F) where creep effects are significant
• When external loads and moments are substantial
• For critical service where detailed stress distribution information is needed
• When designing compact flanges or other non-standard joint types

Remember that if using FEA in an ASME Section VIII vessel, the results must still satisfy the code’s basic requirements for safety factors.

Q: How should flange connections be designed for cyclic service?

A: For cyclic service:

• Consider using welding neck flanges due to their superior fatigue resistance
• Select gaskets with good recovery properties
• Design for a higher bolt preload to minimize joint movement
• Be particularly cautious about stress concentrations
• Consider applying a fatigue analysis using ASME Section VIII, Division 2, Appendix 5
• Use controlled bolt tensioning methods to ensure uniform preload

Q: Are there any special considerations for flanges in hydrogen service?

A: Yes, hydrogen service presents unique challenges:

• Hydrogen can cause embrittlement in many metals, including bolt materials
• Select appropriate bolt materials (often A193 B7M or B16 are preferred over standard B7)
• Consider lower allowable stresses to provide margin against hydrogen effects
• Use gaskets specifically tested for hydrogen service
• Be particularly careful about potential leak paths, as hydrogen molecules are very small
• Follow industry standards like ASME B31.12 for additional guidance

Q: How do I properly specify a flange for a project?

A: A complete flange specification should include:

• Flange type (welding neck, slip-on, etc.)
• Size (nominal pipe size)
• Pressure class or design pressure
• Material specification
• Facing type (raised face, flat face, RTJ, etc.)
• Surface finish requirements
• Gasket type and material
• Bolt material
• Any special requirements (NACE compliance, positive material identification, NDE, etc.)

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Conclusion: Mastering ASME Pressure Vessel Flange Design

Mastering the design of bolted flange connections according to ASME VIII Division 1, Mandatory Appendix 2 requires understanding not just the calculations, but also the underlying principles and practical considerations that affect joint integrity.

From the basic flange types we explored in Part 1, through the components and design procedures in Part 2, to the practical examples and troubleshooting guides in this final installment, we’ve covered the essential knowledge needed to successfully apply these code requirements.

Remember that proper flange design is critical to pressure vessel safety and reliability. When in doubt, consult with experienced engineers, refer to the code directly, or seek guidance from industry experts.

We hope this three-part series has provided valuable insights into this important aspect of pressure vessel design. If you have questions or would like to suggest topics for future articles, please leave a comment below.

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About the Author:
StaticEngineer.com specializes in pressure vessel design and code compliance, with extensive experience in implementing ASME standards for industrial applications.

References:

1. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1
2. ASME B16.5 – Pipe Flanges and Flanged Fittings
3. “Pressure Vessel Design Manual” by Dennis Moss
4. “Companion Guide to the ASME Boiler & Pressure Vessel Code” edited by K.R. Rao
5. “ASME Section VIII: Division 1 – Design & Fabrication of Pressure Vessels” by Will J. Carter

By StaticEngineer.com
Published: May 1, 2025
Last Updated: May 1, 2025

Series Overview: This is the second installment in our 3-part series on ASME VIII Div 1, Mandatory Appendix 2. In Part 1, we covered flange fundamentals and types. Now we’ll examine design procedures and critical components required for code-compliant pressure vessel flange design.

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Flange Design Procedures and Critical Components for ASME Compliance

Introduction to ASME Flange Design Calculations

Welcome to the second installment of our comprehensive guide to ASME VIII Division 1, Mandatory Appendix 2. In Part 1, we explored the fundamentals of flange design and the various types of flanges covered by the code. Now, we’ll delve into the critical components that make up a functioning flange assembly and the procedures for proper design.

Flange Facing Types and Gasket Selection for Pressure Vessel Design

The interface between mating flanges is as important as the flanges themselves. Various facing types exist, each designed for specific applications:

Common Flange Facing Types in ASME Applications

Raised Face (RF) The most common type, with a raised area around the bore that confines the gasket. The raised portion typically extends from the bore to a point just inside the bolt holes.

Flat Face (FF) The entire flange face is in one plane, often used when connecting to equipment with fragile materials (like cast iron) that could crack if a raised face were used.

Ring Joint (RTJ) A precision metal gasket groove designed for high-pressure and high-temperature applications. The metallic ring gasket deforms to create a seal.

Tongue and Groove Provides improved gasket containment with one flange having a raised ring (tongue) that fits into a depression (groove) on the mating flange.

Male and Female Similar to tongue and groove but with different proportions, providing excellent gasket containment.

Gasket Types and Properties

The selection of the correct gasket is critical to flange joint performance. Gaskets are broadly categorized as:

Non-metallic

• Compressed fiber materials
• PTFE (polytetrafluoroethylene)
• Rubber compounds
• Graphite

Semi-metallic

• Spiral-wound gaskets
• Metal-jacketed gaskets
• Corrugated metal with non-metallic filler

Metallic

• Solid metal gaskets
• Ring-joint gaskets

Each gasket material has specific properties defined by two critical parameters:

• Gasket Factor (m): Used for calculating operating condition loads
• Minimum Gasket Seating Stress (y): The compression required to conform the gasket to flange imperfections

These factors are provided in Table 2-5.1 of Appendix 2 and are essential for proper flange design calculations.

Bolt Design and Selection

Bolts are the force-applying elements of the flange assembly, creating and maintaining the gasket seal while resisting the pressure forces trying to separate the flanges.

Bolt Selection Criteria

Material Selection The bolt material must be suitable for the design temperature and environmental conditions. Common materials include:

• SA-193 B7 (for temperatures up to 800°F)
• SA-193 B8 Class 1 (stainless steel)
• SA-193 B16 (for higher temperature applications)

Size and Quantity The size and number of bolts are determined by calculations to ensure:

• Sufficient strength to resist pressure forces
• Adequate compression of the gasket
• Proper bolt spacing to maintain gasket compression

Thread Series Most pressure vessel applications use coarse threads (UNC), but fine threads (UNF) may be used for special applications requiring more precise tensioning.

Bolt Area Requirements

Two key parameters must be calculated:

1. Required Bolt Area (Am): The cross-sectional area needed to handle the bolt loads
2. Actual Bolt Area (Ab): Based on the selected bolt size and quantity

For design acceptance, Ab must equal or exceed Am for both operating and gasket seating conditions.

Calculation Methodology

Basic Loads Calculation

The design approach centers around calculating the forces acting on the flange and the resulting moments and stresses. The primary forces are:

1. Hydrostatic End Force (HD) The force due to pressure acting on the area within the gasket: HD = 0.785G²P Where:

• G = Diameter at gasket load reaction
• P = Design pressure

2. Gasket Load (HG) The force required to maintain the gasket seal: HG = 2b × π × G × m × P Where:

• b = Effective gasket seating width
• m = Gasket factor

3. Total Bolt Load – Operating Condition (Wm₁) Wm₁ = HD + HG

4. Gasket Seating Condition (Wm₂) Wm₂ = π × b × G × y Where:

• y = Minimum gasket seating stress

The larger of Wm₁ and Wm₂ determines the minimum required bolt area.

Show Image Figure 6: Forces acting on a bolted flange connection

Flange Stress Calculations

For each design condition (operating and seating), three types of stresses must be calculated and checked:

1. Longitudinal Hub Stress (SH) The stress acting parallel to the vessel axis at the hub-flange junction.

2. Radial Stress (SR) The stress acting in the radial direction of the flange.

3. Tangential Stress (ST) The stress acting around the circumference of the flange.

These stresses are calculated using specific formulas that incorporate geometric factors and the total moment (MT) acting on the flange.

The calculated stresses must satisfy these conditions:

• SH + SR ≤ allowable stress
• SH + ST ≤ allowable stress
• SR + ST ≤ allowable stress

Additionally, no individual stress (SH, SR, or ST) may exceed 1.5 times the allowable stress.

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Expert Tips: Advanced Flange Design Considerations

Expert Tip #4: The integrity of a flange joint is only as good as its weakest component. Always evaluate the entire joint system—flange, gasket, and bolting—as an integrated unit rather than individual components.

Expert Tip #5: For critical services, use a controlled bolt-tightening sequence with calibrated torque wrenches or hydraulic tensioners. A star pattern with multiple passes gradually increasing to final torque values can dramatically improve joint reliability.

Expert Tip #6: When selecting gasket material, don’t just consider pressure and temperature. Also evaluate chemical compatibility, thermal cycling frequency, flange rotation under load, and required service life. A gasket that works perfectly in one application may fail rapidly in another with identical pressure and temperature but different chemical exposure.

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Frequently Asked Questions: ASME Flange Design Calculations

Q: How do I determine the correct bolt torque for my flange application?

A: While Appendix 2 doesn’t directly provide bolt torque values, you can calculate them using the formula: T = K × D × P

Where:

• T = Torque (ft-lbs)
• K = Torque coefficient (typically 0.15-0.20 for lubricated bolts)
• D = Nominal bolt diameter (inches)
• P = Desired bolt preload (lbs)

The desired bolt preload is derived from the gasket seating load (Wm₂) divided by the number of bolts. For critical applications, consider consulting the gasket manufacturer for their specific recommendations.

Q: Can I mix bolt materials in a single flange joint?

A: No, all bolts in a single flange assembly should be of the same material, size, and grade. Mixing materials can lead to uneven load distribution due to differences in thermal expansion and elastic properties, potentially causing leakage or joint failure.

Q: How do I account for external loads (like pipe reactions) in flange design?

A: Appendix 2 allows for the consideration of external moments and forces through an equivalent pressure approach. The external moment is converted to an equivalent pressure, which is then added to the design pressure. The formula is: Pe = 4Me/(πG³)

Where:

• Pe = Equivalent pressure
• Me = External moment
• G = Gasket reaction diameter

Q: What’s the difference between “effective” and “actual” gasket width?

A: The actual gasket width is the physical dimension of the gasket, while the effective gasket width (b) is the width used in calculations to account for the non-uniform distribution of gasket pressure. For narrow gaskets, b equals the actual width, but for wider gaskets, b is calculated using formulas provided in Appendix 2 that account for the width reduction due to gasket deformation patterns.

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Coming next in Part 3: ASME Flange Design Examples, Troubleshooting, and Advanced Applications

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By StaticEngineer.com
Published: May 1, 2025
Last Updated: May 1, 2025

Series Overview: This comprehensive 3-part guide breaks down the complex requirements of ASME VIII Div 1, Mandatory Appendix 2 on bolted flange connections. Whether you’re preparing for certification exams or designing pressure vessels in the field, this resource provides the knowledge you need to properly apply the code requirements for safe, compliant flange design.

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The Fundamentals of Pressure Vessel Flange Design

Introduction to ASME Flange Design Requirements

When it comes to pressure vessel design, few components are as critical yet challenging as bolted flange connections. These mechanical joints must maintain their integrity across widely varying pressures and temperatures while preventing leakage of potentially hazardous substances.

Mandatory Appendix 2 of ASME VIII Division 1 provides the comprehensive rules and calculations required for the proper design of bolted flange connections. In this first part of our series, we’ll explore the fundamental concepts and types of flanges covered by this important appendix.

What is Mandatory Appendix 2?

Mandatory Appendix 2 establishes standardized methods for determining the appropriate dimensions and ratings of various types of flanges. It uses an analytical approach known as the “Taylor Forge method” that considers:

• Rigidity of the flange under load
• Distribution of gasket contact pressure
• Effects of bolt preload and operating conditions
• Impact of differential thermal expansion
• Multiple loading scenarios (pressure, external loads, moments)

This design methodology helps ensure that pressure vessel flanges maintain their structural integrity and sealing capabilities throughout their service life.

Scope and Applicability of ASME Flange Design Rules

Before diving into the details, it’s important to understand what Appendix 2 covers:

• Circular bolted flange connections with gaskets entirely within the bolt circle
• Both integral and loose type flanged connections
• Determination of required flange thickness, bolt size, and bolt quantities
• Various flange types including welding neck, slip-on, threaded, lap joint, and blind flanges

Types of Pressure Vessel Flanges Covered in ASME Code

Integral Type Flanges

Welding Neck Flanges
These flanges are attached to the vessel or pipe with a full penetration weld. Their gradual transition from flange thickness to vessel wall thickness provides excellent load distribution and fatigue resistance, making them ideal for severe service conditions.

Slip-On Flanges
Slip-on flanges slide over the pipe or nozzle and are welded both inside and outside. They’re generally less expensive than welding neck flanges but have lower pressure and fatigue resistance.

Socket Welding Flanges
Used primarily for small diameter piping, these flanges have a socket that receives the pipe before welding.

Threaded Flanges
Connected via threads rather than welding, these flanges are limited to lower pressure applications and situations where welding isn’t possible.

Loose Type Flanges

Lap Joint Flanges
These are used with stub ends and allow for easy alignment and bolt hole positioning. They’re commonly used with corrosive fluids since the flange can be made from less expensive material than the stub end.

Backing Flanges
Used with special connections and typically not in direct contact with the process fluid.

Special Flanges

Blind Flanges
Used to close openings in piping systems or vessels, these solid flanges can withstand full system pressure.

Reverse Flanges
Used for special design conditions where the flange faces outward rather than inward.

Design Philosophy and Approach

Appendix 2 is based on an elastic analysis approach that considers the flange as a simplified structural element. The design must be calculated for two conditions:

1. Operating Condition: With design pressure and temperature applied
2. Gasket Seating Condition: Initial assembly condition (no pressure)

The more severe of these two conditions governs the final design.

Key Design Parameters and Terminology

Understanding the terminology is critical before proceeding to calculations:

• Design Pressure (P): Maximum allowable working pressure
• Design Temperature: Maximum or minimum temperature in service
• Bolt Circle Diameter (C): Diameter through the centers of the bolts
• Flange Outside Diameter (A): Extreme outer diameter of the flange
• Flange Inside Diameter (B): Inner diameter at the hub intersection
• Hub Thickness (g₁): Thickness of the hub at its small end
• Flange Thickness (t): Thickness of the flange ring
• Gasket Factor (m): Factor for determining operating gasket load
• Gasket Seating Stress (y): Minimum stress required for initial gasket seating

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Expert Tips: Flange Design Fundamentals

Expert Tip #1: “When selecting between flange types, remember that welding neck flanges distribute stress better than slip-on flanges, particularly in cyclic applications. The additional cost is often justified by the extended service life.”

Expert Tip #2: “Always verify your gasket factors (m and y) carefully. Using incorrect values is one of the most common errors in flange design and can lead to either leakage or unnecessary over-design.”

Expert Tip #3: “For critical services, consider specifying 100% radiography of flange-to-nozzle welds, even when code minimum requirements don’t mandate it. The cost of inspection is minimal compared to the cost of leakage in hazardous service.”

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Frequently Asked Questions: ASME Flange Design Basics

Q: Can I use standard ASME B16.5 or B16.47 flanges without performing Appendix 2 calculations?

A: Yes, standard flanges manufactured to ASME B16.5 or B16.47 specifications do not require recalculation under Appendix 2 when used within their rated pressure-temperature conditions. However, you still need to verify that the material is suitable for the design temperature and that the flange rating meets or exceeds the vessel design pressure.

Q: How do I choose between an integral flange and a lap joint flange?

A: Consider these factors:

• Integral flanges (especially welding neck) offer better fatigue resistance
• Lap joint flanges allow for material transition and easier alignment
• Maintenance requirements (lap joints are easier to replace)
• Cost considerations (lap joints can be more economical when exotic materials are needed)

Q: Are there any services where threaded flanges should never be used?

A: Threaded flanges are generally not recommended for:

• Lethal services
• High pressure applications (typically above 400 psi)
• Severe thermal cycling
• Services where leakage would create significant safety hazards
• Applications requiring full radiography

Q: How does Appendix 2 relate to ASME B16.5 flange standards?

A: ASME B16.5 provides standardized flange dimensions and pressure-temperature ratings for flanges already designed and proven. Appendix 2 provides the calculation methodology to design custom flanges or to verify existing designs. Standard B16.5 flanges are considered pre-qualified and do not require Appendix 2 calculations when used within their rated conditions.

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Coming next in Part 2: ASME Flange Design Procedures, Gasket Selection, and Bolt Requirements

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