1. Overview & Applications
A skillet (paddle blind) is a solid flat plate inserted between pipe flanges to provide positive isolation. Unlike valves which can leak, a blind provides absolute shutoff for maintenance, testing, or decommissioning.
[IMAGE: Paddle Blind Assembly]
Cross-section showing paddle blind inserted between raised-face flanges with gaskets on both sides, bolts providing clamping force.
Types of Blind Flanges
Paddle Blind (Skillet)
Single solid plate
Inserted between flanges for positive isolation. Has handle for identification and removal.
Spectacle Blind
Figure-8 design
Two sections (blind + ring spacer) permanently installed. Rotate to isolate or open flow.
Line Blank
Per ASME B16.48
Standard dimensions for Class 150-2500 flanges. Includes ring spacer matching flange thickness.
Spade
Permanent installation
Welded into piping system as permanent positive isolation point.
When to Use Blind Flanges
| Application | Purpose | Typical Blind Type |
|---|---|---|
| Turnaround isolation | Positive isolation for vessel entry, equipment maintenance | Spectacle blind (pre-installed) |
| Hydrostatic testing | Contain test pressure at pipe ends | Paddle blind or test flange |
| Future connections | Cap unused nozzle until future tie-in | Blind flange (standard per B16.5) |
| Process isolation | Separate incompatible fluids, prevent cross-contamination | Double block and bleed with spectacle |
| Decommissioning | Permanent isolation of abandoned lines | Welded spade |
Thickness calculation method: The minimum thickness for a flat blind flange is governed by bending stress under uniform pressure. ASME Section VIII UG-34 provides the governing formula, which also applies to flat heads on pressure vessels. The critical factors are internal pressure, unsupported diameter, edge support condition (C-factor), and material allowable stress.
2. Flat Plate Theory
A circular flat plate under uniform pressure experiences bending stress that varies with position. Maximum stress occurs at the center for clamped edges, or at the edge for simply supported conditions.
[IMAGE: Flat Plate Stress Distribution]
Diagram showing circular plate with uniform pressure P, deflected shape under load, and radial/tangential stress distribution from center to edge.
Governing Formula (ASME VIII UG-34)
Required Thickness:
t = d × √(C × P / (S × E))
Where:
t = Minimum required thickness (inches)
d = Inside diameter or unsupported span (inches)
C = Attachment factor (0.13 to 0.33, see table)
P = Design pressure (psig)
S = Allowable stress from ASME II-D (psi)
E = Joint efficiency (1.0 for solid plate)
Derivation Basis:
Based on plate bending theory for circular plates under uniform pressure.
Maximum bending stress: σ_max = k × P × (d/t)²
Where k depends on edge constraint. The C-factor incorporates both the
bending coefficient and the ASME safety factor.
Stress Distribution in Flat Plates
Under uniform internal pressure, a circular flat plate develops:
- Radial stress (σ_r): Tension on pressure side, compression on opposite
- Tangential stress (σ_t): Circumferential bending stress, varies with radius
- Maximum stress location: At center for clamped edges; at edge for simply supported
Maximum Bending Stress (Clamped Edge):
σ_max = (3 × P × r²) / (4 × t²) [at plate center]
Maximum Bending Stress (Simply Supported):
σ_max = (3 × P × r²) / (8 × t²) × (3 + ν) / (1 + ν) [at plate center]
Where:
r = Plate radius (inches)
t = Plate thickness (inches)
P = Uniform pressure (psi)
ν = Poisson's ratio (0.30 for steel)
The ASME UG-34 formula consolidates these relationships into the C-factor.
Deflection Considerations
While thickness is governed by stress, excessive deflection can cause gasket leakage or operational issues.
Maximum Deflection (Clamped Edge):
δ_max = (3 × P × r⁴ × (1 - ν²)) / (16 × E_m × t³) [at center]
Where:
E_m = Elastic modulus (29 × 10⁶ psi for steel)
ν = Poisson's ratio (0.30)
Rule of Thumb:
For flanged applications, deflection should be limited to prevent gasket unloading.
Typical limit: δ < d/1000 or 0.030", whichever is smaller.
ASME approach: UG-34 uses a unified formula with the C-factor to account for different edge conditions. This eliminates the need for separate stress calculations. The C-factor values have been validated by decades of service experience and incorporate appropriate safety margins.
3. C-Factor Selection
The C-factor (attachment coefficient) accounts for edge support conditions. Lower C-factors apply to stiffer edge constraints that reduce bending stress.
[IMAGE: C-Factor Edge Conditions]
Three diagrams showing: (1) Raised-face bolted flange with gasket inside bolt circle, (2) Full-face gasket to flange edge, (3) Welded attachment. Label C=0.13, 0.30, 0.33 respectively.
C-Factor Values per ASME VIII UG-34
| C-Factor | Edge Condition | Description | Typical Application |
|---|---|---|---|
| 0.13 | Bolted, raised face | Gasket entirely within bolt circle; bolts provide significant edge moment restraint | Standard paddle blind between RF flanges (most common) |
| 0.20 | Bolted, limited gasket | Gasket extends beyond bolt circle but not to edge; partial restraint | Large-bore flanges with wide gaskets |
| 0.30 | Bolted, full face | Full-face gasket extends to plate outer edge; minimal edge restraint | Full-face gasket applications (Class 150 cast iron) |
| 0.33 | Welded attachment | Plate welded at edge; plate acts as simply supported | Welded spades, vessel flat heads |
Determining the Correct C-Factor
For bolted connections, the key question is: where does the gasket seating load occur relative to the bolt circle?
C = 0.13 applies when:
- Gasket OD ≤ Bolt circle diameter
- Ring gasket (spiral wound, RTJ, etc.)
- Raised-face or ring-joint flange facing
C = 0.30 applies when:
- Gasket extends to plate edge
- Full-face gasket (flat sheet)
- Typically Class 150 with flat-face flanges
C = 0.33 applies when:
- Plate is welded at perimeter
- Corner-welded or butt-welded flat heads
- Permanent spade installations
Impact on Required Thickness
The C-factor has a dramatic effect on required plate thickness:
| Diameter | Pressure | C = 0.13 | C = 0.30 | C = 0.33 |
|---|---|---|---|---|
| 6" | 1000 psig | 0.52" | 0.78" | 0.82" |
| 12" | 1000 psig | 1.03" | 1.57" | 1.65" |
| 24" | 500 psig | 1.46" | 2.22" | 2.33" |
Calculated with S = 17,500 psi (A516 Gr 70). Values rounded up to nearest standard plate thickness.
Conservative practice: When edge conditions are uncertain, use C = 0.33 for maximum conservatism. For standard raised-face flange connections with ring gaskets, C = 0.13 is appropriate and well-established.
4. Materials & Allowable Stress
Material selection affects allowable stress (S), which directly impacts required thickness. Per ASME II-D, allowable stress for most applications is approximately SMYS/4 or UTS/4, whichever is lower.
Common Materials for Blind Flanges
| Material | Specification | SMYS (psi) | S @ 100°F (psi) | Application |
|---|---|---|---|---|
| Carbon Steel Forging | A105 | 36,000 | 17,500 | Most common for pipeline fittings; flanges, blinds |
| Low-Temp CS Forging | A350 LF2 | 36,000 | 17,500 | Cold service to -50°F; impact tested |
| High-Yield Forging | A694 F60 | 60,000 | 17,100 | High-pressure gas transmission pipelines |
| High-Yield Forging | A694 F65 | 65,000 | 18,600 | High-pressure gas transmission pipelines |
| Carbon Steel Plate | A516 Gr 70 | 38,000 | 17,500 | Pressure vessel plate; excellent weldability |
| Carbon Steel Plate | A516 Gr 60 | 32,000 | 15,000 | Lower strength; used where impact toughness critical |
| Stainless Forging | A182 F304 | 30,000 | 18,800 | Corrosive service; pipeline fittings |
| Stainless Forging | A182 F316 | 30,000 | 16,700 | Sour service, chloride environments |
Temperature Effects on Allowable Stress
Allowable stress decreases with increasing temperature. ASME II-D provides temperature-dependent values.
| Temperature | A105 / A516 Gr 70 | A182 F304 | A694 F60 |
|---|---|---|---|
| 100°F | 17,500 psi | 18,800 psi | 17,100 psi |
| 200°F | 17,500 psi | 17,800 psi | 17,100 psi |
| 400°F | 17,500 psi | 15,000 psi | 16,600 psi |
| 600°F | 16,600 psi | 13,300 psi | 16,600 psi |
| 800°F | 12,000 psi | 11,700 psi | 14,800 psi |
Values from ASME BPVC Section II-D, Tables 1A/1B. Use exact values from current edition for design.
Allowable Stress Basis
ASME Section VIII Division 1 (UG-23):
For most carbon and low-alloy steels below 500°F:
S = min(SMYS/3.5, UTS/4)
Note: This is different from ASME B31.8 pipeline code which uses:
S = F × E × T × SMYS
where F = 0.40 to 0.72 (design factor)
Important: Use ASME VIII allowable stress (S) for blind flanges,
not the pipeline allowable stress. The blind is a pressure vessel
component, not a piping component.
Material Selection Considerations
- Match pipe material: Blind should match or exceed pipe material strength for consistency
- Impact toughness: For cold service (below 0°F), specify Charpy impact testing
- Corrosion allowance: Add corrosion allowance to calculated thickness if applicable
- Weldability: A516 Gr 70 is standard for welded applications; avoid A36 for pressure welding
- Availability: Standard plate thicknesses per ASTM A6 are readily available
A105 forging is the standard choice for pipeline flanges and blinds, offering S = 17,500 psi at 100°F. For fabricated paddle blinds cut from plate, A516 Gr 70 provides the same allowable stress. For high-pressure gas transmission (X60/X65 pipe), use matching A694 F60/F65 forgings.
5. Design Examples
Example 1: Standard Pipeline Isolation
Given:
Pipeline: 12" NPS, Class 600, raised-face flanges
Design pressure: 1480 psig
Material: A516 Gr 70 (S = 17,500 psi)
Service: Natural gas transmission
Step 1: Determine C-factor
Class 600 RF flange with ring gasket (spiral wound, OD within bolt circle)
→ C = 0.13
Step 2: Determine diameter
12" NPS ID = 11.938" (Schedule 80, typical for Class 600)
Use d = 12" (conservative, to ID of gasket contact)
Step 3: Calculate minimum thickness
t = d × √(C × P / S)
t = 12 × √(0.13 × 1480 / 17,500)
t = 12 × √(192.4 / 17,500)
t = 12 × √(0.01099)
t = 12 × 0.1049
t = 1.259 inches
Step 4: Select standard thickness
Next standard plate: 1.375" (1-3/8")
Step 5: Verify safety factor
Max pressure = (t/d)² × S / C = (1.375/12)² × 17,500 / 0.13
Max pressure = 0.01316 × 134,615 = 1,772 psig
Safety factor = 1,772 / 1,480 = 1.20
Result: Use 1-3/8" thick A516 Gr 70 paddle blind
Weight ≈ 82 lbs (for 16" OD flange facing)
Example 2: High-Pressure Spectacle Blind
Given:
Application: 6" spectacle blind for compressor station
Design pressure: 2500 psig (Class 2500)
Material: A516 Gr 70 (S = 17,500 psi)
Flange: Ring-joint facing
Step 1: C-factor for RTJ
Ring-joint gasket is entirely within bolt circle
→ C = 0.13
Step 2: Diameter
6" Class 2500 ID ≈ 5.761" (heavy wall)
Use d = 6" (conservative)
Step 3: Calculate thickness
t = 6 × √(0.13 × 2500 / 17,500)
t = 6 × √(0.01857)
t = 6 × 0.1363
t = 0.818 inches
Step 4: Standard thickness
Next standard: 0.875" (7/8")
Step 5: Spectacle blind total length
For figure-8 spectacle blind per ASME B16.48:
- Blind section: 0.875" thick solid plate
- Ring spacer section: Same thickness, matching flange
- Handle between sections for rotation
- Total length ≈ 2.5 × flange OD
Result: 7/8" thick spectacle blind
Example 3: Full-Face Gasket Application
Given:
Application: 24" Class 150 blind flange
Design pressure: 285 psig (hydrostatic test)
Full-face gasket extends to flange edge
Material: A516 Gr 70
Step 1: C-factor for full-face gasket
Gasket extends to plate edge, minimal edge restraint
→ C = 0.30
Step 2: Calculate thickness
t = 24 × √(0.30 × 285 / 17,500)
t = 24 × √(0.00489)
t = 24 × 0.0699
t = 1.678 inches
Step 3: Standard thickness
Next standard: 1.75" (1-3/4")
Comparison with C = 0.13:
If raised-face with ring gasket were used:
t = 24 × √(0.13 × 285 / 17,500) = 1.10"
Standard: 1.125" (1-1/8")
Conclusion: Full-face gasket requires 55% thicker plate
Common Design Errors
- Using pipeline allowable stress: B31.8 allowable (F × SMYS) is NOT the same as ASME VIII S
- Wrong C-factor: Using C = 0.13 for full-face gasket underestimates thickness
- Ignoring temperature: High-temperature service reduces allowable stress significantly
- Using SMYS instead of S: Allowable stress is approximately SMYS/4, not SMYS
- Forgetting corrosion allowance: Add corrosion allowance after calculating minimum thickness
- Neglecting deflection: Very large blinds may need thicker plates to limit deflection
Standard Plate Thickness Reference
| Decimal (in) | Fraction | Decimal (in) | Fraction |
|---|---|---|---|
| 0.1875 | 3/16" | 1.000 | 1" |
| 0.2500 | 1/4" | 1.125 | 1-1/8" |
| 0.3125 | 5/16" | 1.250 | 1-1/4" |
| 0.3750 | 3/8" | 1.375 | 1-3/8" |
| 0.4375 | 7/16" | 1.500 | 1-1/2" |
| 0.5000 | 1/2" | 1.750 | 1-3/4" |
| 0.6250 | 5/8" | 2.000 | 2" |
| 0.7500 | 3/4" | 2.500 | 2-1/2" |
| 0.8750 | 7/8" | 3.000 | 3" |
Standard plate thicknesses per ASTM A6. Always round UP to next standard size.
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