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 of a flat blank is governed by bending stress under uniform pressure. For line blanks in piping, ASME B31.3 §304.5.3 provides the governing formula (the basis of the API 590 and B16.48 blank tables); the closely-related ASME VIII UG-34 flat-cover formula gives a more conservative result. 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 edge for clamped edges, or at the center 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 B31.3 §304.5.3)
Required Thickness (ASME B31.3 §304.5.3):
t = d × √(3P / (16 × S × E)) + c = d × √(C × P / (S × E)) + c
Where:
t = Minimum required thickness (inches)
d = Blank inside (gasket) diameter (inches)
C = Edge coefficient: 0.1875 (= 3/16) line blank; 0.30 UG-34 flat cover; 0.33 welded
P = Design pressure (psig)
S = Allowable stress from ASME II-D (psi)
E = Joint efficiency (1.0 for solid plate)
c = Corrosion allowance (inches)
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. B31.3's 3/16 corresponds to a clamped
plate (the bolted flange restrains the rim); UG-34's 0.30 is the more
conservative simply-supported case.
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 the edge for clamped edges; at the center for simply supported
Maximum Bending Stress (Clamped Edge):
σ_max = (3 × P × r²) / (4 × t²) [at the plate edge]
→ equivalent to C = 0.1875 (= 3/16) — the ASME B31.3 line-blank value (the bolted flange clamps the rim)
Maximum Bending Stress (Simply Supported):
σ_max = 3 × (3 + ν) × P × r² / (8 × t²) [at the plate center]
→ equivalent to C ≈ 0.30 for ν = 0.30 — the conservative ASME VIII UG-34 flat-cover value
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 Design Bases]
Three diagrams showing: (1) Line blank clamped at the rim by the bolted flange joint — ASME B31.3, C=0.1875, (2) Flat cover treated as simply supported — ASME VIII UG-34, C=0.30, (3) Welded/integral edge, C=0.33.
C-Factor Values by Design Basis
| C-Factor | Edge Condition | Description | Typical Application |
|---|---|---|---|
| 0.1875 | Line blank (ASME B31.3 §304.5.3) | The purpose-built blank formula, t = d·√(3P/16SE) + c (C = 3/16). The bolted flange joint restrains the rim, so the blank is modeled as a clamped circular plate. This is the basis for the API 590 and ASME B16.48 manufactured-blank thickness tables. | Standard paddle blind, spectacle blind, line blank (default, most common) |
| 0.30 | Flat cover, simply supported (ASME VIII UG-34) | Treats the blank as a simply-supported circular plate (free to rotate at the rim) — the conservative pressure-vessel flat-cover basis. About 26% thicker than the B31.3 line-blank value. | Conservative design, or vessel flat covers |
| 0.33 | Welded / integral edge | Fillet-welded edge — the weld provides little rotational restraint and carries a weld penalty, so UG-34 assigns a slightly higher (conservative) factor. | Welded spades, welded flat covers |
Determining the Correct C-Factor
The C-factor depends on which design code governs and how the edge is restrained. Stiffer restraint lowers the bending stress and the C-factor.
C = 0.1875 (= 3/16) — the default line-blank value:
- ASME B31.3 §304.5.3 blank formula, t = d·√(3P/16SE) + c
- The basis for the API 590 and ASME B16.48 manufactured-blank tables
- The bolted flange joint restrains the rim → modeled as a clamped plate
- Use for any paddle/spectacle blind or line blank in piping
C = 0.30 — conservative flat-cover basis:
- ASME VIII UG-34 bolted flat cover, treated as simply supported
- ~26% thicker than the B31.3 value; use when extra margin is wanted
C = 0.33 — welded / integral edge:
- Fillet-welded at the perimeter (weld gives little rotational restraint + weld penalty)
- Welded spades, welded flat covers
Impact on Required Thickness
The C-factor has a dramatic effect on required plate thickness:
| Diameter | Pressure | Line blank B31.3 (0.1875) | UG-34 cover (0.30) | Welded (0.33) |
|---|---|---|---|---|
| 6" | 1000 psig | 0.62" | 0.79" | 0.82" |
| 12" | 1000 psig | 1.24" | 1.57" | 1.65" |
| 24" | 500 psig | 1.76" | 2.22" | 2.33" |
Calculated with S = 17,500 psi (A516 Gr 70). Values rounded up to nearest standard plate thickness.
Design practice: For a standard paddle/spectacle blind or line blank, use the ASME B31.3 §304.5.3 value C = 0.1875 (= 3/16) — this matches the API 590 and B16.48 manufactured-blank tables and applies whether the gasket is a ring gasket or full-face. For extra margin, the ASME VIII UG-34 simply-supported flat-cover basis (C = 0.30) gives a ~26% thicker plate. Use C = 0.33 for a welded/integral edge. (The legacy 0.13 value some tools used is below even the B31.3 clamped coefficient and under-sizes the blank — do not use it.)
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
Line blank per ASME B31.3 §304.5.3 (basis of the API 590 / B16.48 tables)
→ C = 3/16 = 0.1875
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 × √(0.1875 × 1480 / 17,500)
t = 12 × √(277.5 / 17,500)
t = 12 × √(0.01586)
t = 12 × 0.1259
t = 1.511 inches
Step 4: Select standard thickness
Next standard plate: 1.625" (1-5/8")
Step 5: Verify safety factor
Max pressure = (t/d)² × S / C = (1.625/12)² × 17,500 / 0.1875
Max pressure = 0.01834 × 93,333 = 1,712 psig
Safety factor = 1,712 / 1,480 = 1.16
Result: Use 1-5/8" thick A516 Gr 70 line blank
(For extra margin, the UG-34 flat-cover basis C = 0.30 would give 1.911" → 2" plate)
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 line blank per ASME B31.3 §304.5.3 (gasket type does not change the basis)
→ C = 3/16 = 0.1875
Step 2: Diameter
6" Class 2500 ID ≈ 5.761" (heavy wall)
Use d = 6" (conservative)
Step 3: Calculate thickness
t = 6 × √(0.1875 × 2500 / 17,500)
t = 6 × √(0.02679)
t = 6 × 0.1637
t = 0.982 inches
Step 4: Standard thickness
Next standard: 1.0" (1")
Step 5: Spectacle blind total length
For figure-8 spectacle blind per ASME B16.48:
- Blind section: 1.0" thick solid plate
- Ring spacer section: Same thickness, matching flange
- Handle between sections for rotation
- Total length ≈ 2.5 × flange OD
Result: 1" thick spectacle blind
Example 3: Line-Blank vs Conservative Basis
Given:
Application: 24" Class 150 blind flange
Design pressure: 285 psig (hydrostatic test)
Material: A516 Gr 70 (S = 17,500 psi)
Step 1: Line-blank basis (default)
ASME B31.3 §304.5.3 → C = 3/16 = 0.1875
t = 24 × √(0.1875 × 285 / 17,500)
t = 24 × √(0.003054)
t = 24 × 0.05526
t = 1.326 inches → next standard 1.375" (1-3/8")
Step 2: Conservative flat-cover basis
ASME VIII UG-34 simply supported → C = 0.30
t = 24 × √(0.30 × 285 / 17,500) = 1.678" → next standard 1.75" (1-3/4")
Conclusion: The B31.3 line-blank basis (default, matching API 590 / B16.48 tables) gives 1-3/8"; the conservative UG-34 flat-cover basis gives 1-3/4" (~27% thicker). The gasket type (ring vs full-face) does not change the C-factor.
Common Design Errors
- Using pipeline allowable stress: B31.8 allowable (F × SMYS) is NOT the same as ASME VIII S
- Wrong C-factor: Using the legacy C = 0.13 (below the B31.3 3/16 value) under-sizes the blank — use C = 0.1875 for a line blank, or 0.30 for the conservative UG-34 basis
- 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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