Last updated: March 2026
This blind flange thickness calculator helps you determine the minimum required thickness of blanks (spades/paddle blinds) and blind flanges used for hydrostatic testing isolation in piping systems. During pressure testing, temporary closure plates must be correctly sized — too thin and the plate could catastrophically fail; too thick wastes material and adds unnecessary weight. This guide provides an interactive thickness calculator based on the ASME B31.3 formula for blanks, explains the complete calculation with worked examples, covers the ASME Section VIII UG-34 approach for bolted blind flanges, and provides ready-to-use thickness charts.
📋 Table of Contents
- Blind Flange vs. Blank (Spade) vs. Spectacle Blind
- When Do You Need Isolation Blanks for Hydrostatic Testing?
- Blind Flange Thickness Calculator (Online)
- Thickness Calculation Formula — ASME B31.3
- Bolted Blind Flange Thickness — ASME VIII UG-34
- Step-by-Step Calculation Examples
- Blank Thickness Chart for Hydrotest
- Allowable Stress Values for Common Materials
- How to Calculate Hydrostatic Test Pressure
- Important Safety Considerations
- Frequently Asked Questions
1. Blind Flange vs. Blank (Spade) vs. Spectacle Blind
Although the terms "blind flange" and "blank" are often used interchangeably in the field, there are distinct engineering differences. Understanding them is important because each type has different design rules and thickness calculation methods.
| Feature | Blind Flange | Blank (Spade / Paddle Blind) | Spectacle Blind |
|---|---|---|---|
| Description | Solid disc with its own bolt holes, bolted to a companion flange to cap the pipe end | Flat plate inserted between two existing flanges at a bolted joint; has a handle for identification | Figure-8 shaped plate combining a blank and a spacer ring; rotated to switch between open and closed positions |
| Standard | ASME B16.5 / B16.47 (standard), ASME VIII UG-34 (custom) | ASME B31.3 Clause 304.5.3 | ASME B16.48 |
| Bolt holes | Yes — has its own | No — uses the joint bolts | Yes — has its own |
| Common use | Permanent pipe end closure | Temporary hydrotest isolation | Permanent switching isolation |
| Thickness formula | UG-34 (accounts for bolt moments) | B31.3 §304.5.3: Tm = dg√(3P/16SE) + C | Per ASME B16.48 tables |
ℹ️ What this calculator covers: This blind flange thickness calculator primarily uses the ASME B31.3 formula for blanks and spades — which is the most common need for temporary hydrotest isolation. For standard bolted blind flanges, see Section 5 (ASME VIII UG-34) below.
2. When Do You Need Isolation Blanks for Hydrostatic Testing?
Temporary test plates or blanks are required in the following situations:
- Equipment isolation: To protect equipment (vessels, heat exchangers, pumps, compressors) rated for a lower pressure than the hydrotest pressure
- Instrument protection: To isolate instrument connections, control valves, relief valves, and safety devices
- Section testing: To test individual loops or sections of a piping system independently
- Tie-in isolation: To isolate existing operating plant from new construction being tested
- Maintenance isolation: During plant turnaround activities requiring positive mechanical isolation
- Confined space entry: For safety isolation before vessel or pipeline entry
ℹ️ Why is the test pressure higher than design pressure?
The hydrostatic test pressure per ASME B31.3 is typically 1.5 times the design pressure. This elevated pressure often exceeds the rating of connected equipment, making properly designed isolation blanks essential for safe testing.
3. Blind Flange Thickness Calculator (Online)
Use this blind flange thickness calculator to instantly determine the minimum required blank or spade thickness for hydrostatic testing per ASME B31.3 Clause 304.5.3. Select your pipe size, enter the test pressure, choose a material, and get results with a full calculation breakdown.
🔢 Blind Flange & Blank Thickness Calculator
Auto-fills gasket ID for Class 150 RF
From gasket drawing or ASME B16.20
Use 0 for temporary test blanks
4. Thickness Calculation Formula — ASME B31.3
The minimum required thickness of a flat blank (spade or paddle blind) inserted between flanges is determined using the equation from ASME B31.3 Process Piping Code, Clause 304.5.3.
📝 Scope of this formula: This formula applies to unstayed flat blanks (spades/paddle blinds) inserted between flanges. This is the standard practice for temporary hydrotest isolation. For bolted blind flanges designed as pressure vessel components, see Section 5 — ASME VIII UG-34.
Tm = dg × √(3P / 16SE) + C
Where:
| Symbol | Description | Unit |
|---|---|---|
| Tm | Minimum required plate thickness | mm |
| dg | Internal diameter of the gasket (at the gasket load reaction point) | mm |
| P | Internal design pressure (or test pressure) | N/mm² (MPa) |
| S | Allowable stress of the plate material (from ASME Sec II Part D) | N/mm² |
| E | Quality factor (1.0 for seamless/solid plate) | — |
| C | Corrosion allowance (0 for temporary blanks) | mm |
💡 Key Notes on the ASME B31.3 Blank Formula:
- SE is often used as a combined value. For solid (seamless) plate stock, E = 1.0, so SE = S.
- The gasket internal diameter dg is the critical dimension. Using a larger dg gives a more conservative (thicker) result.
- For raised-face flanges with spiral wound gaskets, dg is approximately equal to the pipe outside diameter.
- The pressure P must be in N/mm² (MPa). Convert from bar by dividing by 10 (1 bar = 0.1 MPa).
- When designing blanks specifically for hydrostatic testing, P = test pressure.
5. Bolted Blind Flange Thickness — ASME Section VIII UG-34
When designing a custom bolted blind flange (one with its own bolt holes, as opposed to a blank inserted between flanges), the thickness calculation must follow ASME Section VIII, Division 1, Paragraph UG-34. This method accounts for bolt loading and gasket seating forces, which the simpler B31.3 blank formula does not address.
t = d × √(Cp × P / SE) + additional bolt-load terms
The UG-34 calculation involves:
- d = diameter measured per the applicable attachment sketch in UG-34(a)
- Cp = a constant depending on the attachment method (typically 0.33 for flat heads)
- P = design pressure (or test pressure)
- S = allowable stress at design temperature
- E = joint efficiency
- Additional checks for bolt loading (W) and gasket seating per Appendix 2
⚠️ When to use UG-34 vs. B31.3 §304.5.3:
- Use B31.3 §304.5.3 (the calculator above) for flat blanks/spades inserted between existing flanges.
- Use UG-34 for custom-designed bolted blind flanges that have their own bolt holes.
- For standard ASME B16.5 blind flanges, no calculation is required — verify that the flange pressure-temperature rating meets the test pressure.
6. Step-by-Step Calculation Examples
Example 1: Carbon Steel Spade for 10" Pipe
📘 Example 1 — 10" NPS, Class 150, Carbon Steel Blank
Given Data:
- Pipe Size: 10" NPS (DN250)
- Flange Class: 150
- Gasket ID (dg): 264.0 mm
- Design Pressure: 15 bar
- Test Pressure (P): 1.5 × 15 = 22.5 bar = 2.25 MPa = 2.25 N/mm²
- Material: SA-516 Gr.70 plate (SE = 137.9 N/mm²)
- Quality Factor (E): 1.0 (solid plate)
- Corrosion Allowance (C): 0 mm
Solution:
Step 1: Apply the formula Tm = dg × √(3P / 16SE) + C
Step 2: Calculate the term under the square root:
3P / 16SE = (3 × 2.25) / (16 × 137.9) = 6.75 / 2206.4 = 0.003059Step 3: Take the square root:
√0.003059 = 0.05531Step 4: Multiply by gasket ID and add corrosion allowance:
Tm = 264.0 × 0.05531 + 0 = 14.60 mm✅ Minimum Thickness = 14.60 mm → Use 16 mm standard plate
7. Blank Thickness Chart for Hydrotest
The following quick-reference chart shows calculated minimum blank thickness for common pipe sizes at various test pressures. Material is SA-516 Gr.70 (SE = 137.9 N/mm²), corrosion allowance = 0 mm. Gasket IDs are based on ASME B16.20 spiral wound gaskets for Class 150 RF flanges.
| Pipe Size (NPS) | Gasket ID dg (mm) | 15 bar (1.5 MPa) | 22.5 bar (2.25 MPa) | 30 bar (3.0 MPa) | 45 bar (4.5 MPa) | 60 bar (6.0 MPa) |
|---|---|---|---|---|---|---|
| 2" | 54.1 | 2.4 | 3.0 | 3.5 | 4.2 | 4.9 |
| 3" | 79.2 | 3.6 | 4.4 | 5.1 | 6.2 | 7.2 |
| 4" | 102.1 | 4.6 | 5.6 | 6.5 | 8.0 | 9.2 |
| 6" | 149.4 | 6.7 | 8.3 | 9.5 | 11.7 | 13.5 |
| 8" | 196.3 | 8.9 | 10.9 | 12.5 | 15.4 | 17.7 |
| 10" | 264.0 | 11.9 | 14.6 | 16.9 | 20.7 | 23.8 |
| 12" | 314.7 | 14.2 | 17.4 | 20.1 | 24.6 | 28.4 |
| 14" | 349.3 | 15.8 | 19.3 | 22.3 | 27.3 | 31.5 |
| 16" | 400.1 | 18.1 | 22.1 | 25.6 | 31.3 | 36.1 |
| 18" | 450.9 | 20.4 | 24.9 | 28.8 | 35.3 | 40.7 |
| 20" | 501.7 | 22.7 | 27.7 | 32.0 | 39.2 | 45.3 |
| 24" | 603.0 | 27.2 | 33.4 | 38.5 | 47.2 | 54.5 |
⚠️ Important: These values are minimum calculated thicknesses. Always round up to the next available standard plate thickness.
8. Allowable Stress Values for Common Materials
The allowable stress value (S) is a critical input in the blind flange thickness calculation. The table below provides commonly used values at ambient / test temperature (≤ 40°C / 100°F) from ASME Section II Part D.
| Material | Specification | Grade / Type | S (N/mm²) | S (psi) | Product Form |
|---|---|---|---|---|---|
| Carbon Steel Plate | SA-516 | Gr. 70 | 137.9 | 20,000 | Plate (blanks) |
| Carbon Steel Plate | SA-516 | Gr. 60 | 118.0 | 17,100 | Plate (blanks) |
| Carbon Steel Forging | SA-105 | — | 137.9 | 20,000 | Forgings (bolted blind flanges) |
| Structural Steel | A36 / S275JR | — | 165.0 | 23,900 | Plate (temp. blanks only) |
| Stainless Steel Plate | SA-240 | Type 304 | 138.0 | 20,000 | Plate (blanks) |
| Stainless Steel Plate | SA-240 | Type 316 | 138.0 | 20,000 | Plate (blanks) |
ℹ️ Notes on Material Selection:
- SA-516 Gr.70 is the preferred ASME-listed plate material for blanks and spades.
- SA-105 is a forging specification — it applies to forged blind flanges and fittings, not to flat plate blanks cut from sheet.
- Structural steel (A36, S275JR) is commonly used for temporary hydrotest blanks because it is readily available and cost-effective. The allowable stress is conservatively taken as 2/3 × minimum yield strength.
9. How to Calculate Hydrostatic Test Pressure
The hydrostatic test pressure is the pressure at which the piping system is tested to verify its integrity. Per ASME B31.3 Paragraph 345.4.2, the minimum test pressure is calculated as:
PT = 1.5 × Pd × (ST / S)
Where:
- PT = Minimum hydrostatic test pressure
- Pd = Design pressure of the piping system
- ST = Allowable stress at test temperature
- S = Allowable stress at design temperature
💡 Simplified Case: When the test temperature and design temperature are both at ambient conditions, the formula simplifies to: PT = 1.5 × Pd. When using this blind flange thickness calculator, enter PT (the test pressure) — not the design pressure.
10. Important Safety Considerations
Hydrostatic testing involves stored energy in compressed water. Follow these critical safety guidelines:
- Independent verification: All calculations should be verified by a qualified piping engineer.
- Material certification: Ensure the plate material has a valid Mill Test Certificate (MTC / EN 10204 3.1).
- Visual inspection: Inspect all blanks for laminations, cracks, or defects before installation.
- Gasket selection: Use an appropriate gasket between the blank and the flanges.
- Exclusion zone: Establish a safety exclusion zone around the test section during pressurization.
- Pressure relief: Provide a pressure relief device set at or below 110% of the test pressure.
⚠️ Critical Warning: Never re-use a temporary blank that has been visibly deformed, scratched, or damaged. Never drill holes through temporary blanks for pressure connections.
11. Frequently Asked Questions
What is the formula for blind flange or blank thickness calculation?
The blank thickness formula per ASME B31.3 (Clause 304.5.3) is: Tm = dg × √(3P / 16SE) + C. For bolted blind flanges with their own bolt holes, use ASME Section VIII Division 1, UG-34 instead.
What code governs blank and blind flange thickness for hydrostatic testing?
ASME B31.3 Process Piping Code (Clause 304.5.3) provides the primary formula for calculating flat blanks. ASME Section VIII Division 1 (UG-34) is used for bolted blind flanges.
Can I use a standard ASME B16.5 blind flange for hydrostatic testing?
Yes, standard ASME B16.5 blind flanges can be used if their pressure-temperature rating at the test temperature equals or exceeds the test pressure.
What is the difference between a blind flange, a spade, and a spectacle blind?
A blind flange is a solid disc with bolt holes. A spade (paddle blind) is a flat plate inserted between two flanges at an existing bolted joint. A spectacle blind combines a blank disc and a spacer ring in a single piece that can be rotated to switch between open and closed positions.
Should I use test pressure or design pressure in the blank thickness formula?
If the blank is being sized specifically for hydrostatic testing, use the test pressure (typically 1.5 × design pressure) as the input P in the formula. If it will remain installed during normal operation, use the design pressure.
⚖️ Disclaimer: This blind flange thickness calculator and reference material are provided for educational and preliminary estimation purposes only. All calculations must be verified by a qualified piping or mechanical engineer in accordance with the applicable design code (ASME B31.3, ASME VIII) before fabrication. The authors accept no liability for errors arising from the use of this tool.