FieldEngineersKitEngineering Reference & ASME Code Basis
1. Core Formula & Variable Definitions
ASME B31.3 (2022 Edition) · (Process Piping) · Para. 304.4.1 (Closure Plates) & Chapter VI (Inspection & Testing) · ASME BPVC Section VIII, Division 1 · UG-34 (Unstayed Flat Heads and Covers) · ASME B16.5 (Pipe Flanges & Flanged Fittings - Raised Face Dimensions) · ASTM A516 / ASTM A36 / ASTM A240 (Carbon & Stainless Steel Plate Specs)
Permanent Blind: tm = d · √(C · PS · E) + c (C = 0.30, c ≥ 3.0 mm)
Hydrotest Blank: tm = d · √(C · PtSamb · E) (C = 0.30, c = 0.0 mm)
ASME B31.3 Para. 304.4.1 & ASME BPVC Section VIII Div 1 UG-34 / ASME B31.3 Ch. VI
ASME B31.3 Para. 304.4.1 and ASME BPVC Section VIII Div 1 UG-34 define the minimum required design thickness for flat unstayed circular blind covers bolted with ring or full-face gaskets. The structural attachment factor C = 0.30 accounts for edge-clamping moments on bolted flanges with gasket reaction within the bolt circle. For permanent operating blinds, allowable stress S is evaluated at design temperature and corrosion allowance c is added (typically 3.0 mm). For temporary hydrotest blanks (ASME B31.3 Chapter VI), test pressure Pt is applied at ambient temperature with S_amb and corrosion allowance c = 0.0 mm.
- t_m (Required Minimum Thickness) — Minimum design thickness of the flat blind plate or temporary test blank (mm or in). Calculator safety margin compares recommended plate to t_m — not pipe t_nom_req.
- d (Gasket Contact / Reaction Diameter) — Mean gasket reaction diameter or ASME B16.5 Raised Face (RF) contact diameter (mm or in).
- P (Internal Design Pressure) — Maximum internal design pressure at operating temperature for permanent blinds (MPa or psi).
- P_t (Hydrotest Test Pressure) — Shop or field hydrostatic/pneumatic test pressure applied to temporary test blanks (MPa or psi).
- S (Design Allowable Stress) — Basic allowable stress of plate material at design temperature. Calculator A516-70 design preset ≈ 125 MPa / 18,100 psi.
- S_amb (Ambient Allowable Stress) — Ambient allowable for hydrotest blanks. Calculator binds A516-70 as 138 MPa / 20,000 psi (20.0 ksi).
- E (Quality / Joint Factor) — Weld joint efficiency factor (E = 1.00 for forged or seamless rolled plate stock).
- C (Attachment Factor) — Empirical structural attachment factor (C = 0.30 for bolted flat heads per ASME UG-34). Distinct from lowercase corrosion allowance c.
- c (Corrosion Allowance) — Added after structural pressure term (typically ≥ 3.0 mm / 0.125 in permanent; 0 for hydrotest).
2. Allowances, Tolerances & Standards
Permanent process blinds require full corrosion allowance and thermal stress derating, while temporary hydrotest blanks operate at ambient temperature with zero corrosion allowance.
For continuous plant operation, carbon steel blind plates must include the piping specification corrosion allowance (typically 1.5 to 3.0 mm) to protect structural integrity throughout the 25-year design life.
Per ASME B31.3 Chapter VI, temporary test blinds and spades installed solely for hydrostatic/pneumatic testing operate with non-corrosive test fluid for short durations; setting c = 0.0 mm significantly reduces required plate stock thickness.
Rolled carbon steel plate (ASTM A516 Gr. 70 / A36) has an allowable mill under-thickness tolerance of 0.3 mm. Selected commercial plate stock must exceed calculated t_m by at least 0.3 mm.
In flanged joints with ring gaskets, d is taken as the ASME B16.5 raised-face diameter or mean gasket contact circle. Using pipe ID is unconservative for bolting moments.
Quick Reference Lookup Table
| NPS | RF Dia d (mm) | Cl 150# Pt≈2.93 MPa tm (Plate) | Cl 300# Pt≈7.71 MPa tm (Plate) | Cl 600# Pt≈15.3 MPa tm (Plate) |
|---|---|---|---|---|
| 2" | 92.1 | 7.4 mm (8T) | 12.0 mm (14T) | 16.8 mm (18T) |
| 3" | 127.0 | 10.3 mm (12T) | 16.5 mm (18T) | 23.2 mm (25T) |
| 4" | 157.2 | 12.7 mm (14T) | 20.4 mm (22T) | 28.7 mm (30T) |
| 6" | 215.9 | 17.4 mm (18T) | 28.0 mm (28T) | 39.4 mm (40T) |
| 8" | 269.9 | 21.8 mm (25T) | 35.0 mm (35T) | 49.3 mm (50T) |
| 10" | 323.8 | 26.2 mm (28T) | 42.0 mm (45T) | 59.1 mm (60T) |
| 12" | 381.0 | 30.8 mm (32T) | 49.4 mm (50T) | 69.6 mm (70T) |
| 16" | 469.9 | 38.0 mm (40T) | 60.9 mm (65T) | 85.8 mm (90T) |
| 20" | 584.2 | 47.2 mm (50T) | 75.7 mm (80T) | 106.7 mm (110T) |
| 24" | 692.2 | 55.9 mm (60T) | 89.7 mm (90T) | 126.4 mm (130T) |
Hydrotest pressure Pt ≈ 1.5 × ambient rating (Group 1.1): 150# ≈ 2.93 MPa (29.3 bar), 300# ≈ 7.71 MPa (77.1 bar), 600# ≈ 15.32 MPa (153.2 bar). Calculated with C = 0.30, E = 1.00, S_amb = 138 MPa (A516-70), c = 0.0 mm. Parentheses show recommended commercial plate stock (next size ≥ t_m).
3. Material & Code Limitations
Selecting the correct plate grade determines structural safety. Permanent process blinds require high-temperature derating, while temporary test blanks prioritize cost and toughness.
| Material Group | Temperature Range | Allowable Stress / Limit | Engineering Notes |
|---|---|---|---|
| ASTM A516 Gr. 70 (PV Quality Carbon Steel) | -29 °C to 427 °C (-20 °F to 800 °F) | S_amb = 138.0 MPa (20.0 ksi) @ 20 °C; 125.0 MPa @ 200 °C | Industry gold standard for fabricated permanent blinds, spectacle blinds, and high-pressure temporary hydrotest blanks. |
| ASTM A36 / SS400 (Structural Carbon Steel) | -20 °C to 343 °C (-4 °F to 650 °F) | S_amb = 115.0 MPa (16.6 ksi) @ 20 °C; 100.0 MPa @ 200 °C | Widely stocked and economical for temporary hydrotest blanks; lower allowable stress requires ~10% thicker plate than A516-70. |
| ASTM A240 Gr. 304 / 304L (Austenitic Stainless) | -196 °C to 538 °C (-320 °F to 1000 °F) | S_amb = 138 MPa (20.0 ksi / 20,000 psi) @ 20 °C; 104 MPa @ 200 °C | Mandatory for stainless/alloy lines where carbon steel blanks could cause galvanic contamination or corrosion. |
| ASTM A240 Gr. 316 / 316L (Molybdenum Stainless) | -196 °C to 538 °C (-320 °F to 1000 °F) | S_amb = 138 MPa (20.0 ksi / 20,000 psi) @ 20 °C; 110 MPa @ 200 °C | Used in chloride, acid, and offshore marine process systems. |
Code Applicability & Safety Boundaries
- Prohibition of Simply-Supported Plate Factor: Using C = 0.1875 (3/16) instead of C = 0.30 dangerously underestimates bolt-prying bending moments, violating ASME B31.3 Para. 304.4.1 and leading to edge deformation.
- Temporary Blind Identification & Removal: All temporary hydrotest blanks must have an extended paddle handle painted bright red/yellow, stamped with size/class, and logged on the P&ID blind register for mandatory removal prior to commissioning.
- Gasket Sealing Surface Quality: Fabricated flat plate blinds must have a phonographic serrated finish (Ra 3.2 ~ 6.3 µm / 125 ~ 250 µin) in the gasket contact zone to prevent spiral-wound gasket blowout under test pressure.
4. Step-by-Step Worked Example
Field VerificationShowHide
Calculate the minimum required temporary test blank thickness and select standard commercial steel plate stock for shop hydrostatic testing of an NPS 6 (DN 150) Class 300 piping spool at a test pressure of Pt = 7.71 MPa (77.1 bar / 1,118 psi) using ASTM A516 Gr. 70 plate at ambient temperature (c = 0.0 mm). Pt matches ASME B16.5 Group 1.1 Class 300 hydrotest rating used by this calculator.
Determine Hydrotest Pressure & Material Parameters
Temporary test blanks use c = 0.0 mm since water test duration is brief and non-corrosive.
Calculate Dimensionless Stress & Pressure Ratio
Square-root factor represents elastic bending deflection under uniform hydrostatic load.
Calculate Minimum Required Test Blank Thickness (t_m)
Pure structural thickness required to resist edge bending moments at 7.71 MPa test pressure.
Apply Plate Mill Under-Tolerance Verification
Procured plate stock should account for manufacturing rolling tolerances when the next stock size is tight.
Select Commercial Plate Stock
Safety margin vs t_m = (t_plate / t_m − 1) × 100%. Stamp paddle 'TEST BLANK NPS 6 CL300'.
5. Frequently Asked Questions & Technical References
ShowHide
Permanent Blinds remain in continuous service throughout the plant life; they must account for elevated temperature stress derating (lower S) and include full corrosion allowance (c ≥ 3.0 mm). Temporary Hydrotest Blanks are installed solely during pre-commissioning testing with treated water; they are evaluated at ambient temperature (higher S_amb) with zero corrosion allowance (c = 0.0 mm), resulting in approximately 20% to 40% thinner plates.
The factor 3/16 (0.1875) applies only to ideal simply-supported circular plates under uniform load with zero edge restraint. In real bolted pipe flanges, bolt pre-tensioning creates intense edge-bending moments and prying forces. ASME Section VIII Div 1 UG-34 and ASME B31.3 mandate C = 0.30, which results in approximately 26% thicker plates to prevent catastrophic edge yielding.
Yes. ASTM A36 / SS400 plate is widely used for temporary test blanks due to high availability and low cost. However, because A36 has a lower allowable stress (S_amb = 115 MPa vs 138 MPa for A516-70), the calculated blank thickness will be approximately 10% thicker. Always verify calculated t_m against available plate stock.
Per ASME UG-34, d is the mean gasket reaction diameter (or the ASME B16.5 Raised Face outer diameter for standard RF ring gaskets). For screening purposes, using pipe ID is an acceptable initial check for pressure flexure, but the full gasket reaction diameter d must be used for final fabrication sizing.