4.7 Minimum Required Thickness of Permanent Blanks

API 570 Piping Inspector Exam

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1 Chapter 1: Piping Inspection Planning & Circuits

1.1 Inspection Planning, History, and Safety 1.2 Piping Circuitization and Circuit Design 1.3 Condition Monitoring Locations (CMLs) & Coverage

2 Chapter 2: Inspection Intervals & Extent of Inspection

2.1 Piping Classifications (Classes 1, 2, 3, and 4) 2.2 Thickness Inspection Intervals (Half-Life & Visual Rules) 2.3 Owner-User QA and Roles (Inspector, Engineer, Examiner)

3 Chapter 3: Calculations & Remaining Life

3.1 Corrosion Rates (Short-Term, Long-Term, and Initial Estimates) 3.2 Remaining Life and Retirement Thickness Calculations 3.3 MAWP Calculations for In-Service Piping

4 Chapter 4: ASME Codes for Piping Design

4.1 Wall Thickness Calculations for Pipe under Pressure (ASME B31.3) 4.2 Flange Ratings & P-T Limits (ASME B16.5) 4.3 Pressure & Leak Testing Rules (Hydrostatic vs. Pneumatic) 4.4 Weld Joint Quality Factors (Ej) and Casting Quality Factors (Ec) 4.5 Impact Testing, MDMT Exemption, and Charpy Requirements 4.6 Thermal Expansion Calculations (ASME B31.3 Appendix C) 4.7 Minimum Required Thickness of Permanent Blanks

5 Chapter 5: Nondestructive Examination (NDE)

5.1 Visual, Penetrant, and Magnetic Testing (VT, PT, MT) per ASME Sec V 5.2 Radiographic and Ultrasonic Testing (RT, UT) per ASME Sec V 5.3 Examiner Selection, Calibrations, and Acceptance Standards 5.4 Leak Testing per ASME Section V Article 10

6 Chapter 6: Welding Procedures & Qualifications

6.1 Reviewing WPS and PQR per ASME Section IX 6.2 Welder Performance Qualification (WPQ) and Limits 6.3 Preheating & PWHT Rules and Controls 6.4 Welding Processes, Inspection, and Metallurgy (API RP 577)

7 Chapter 7: Valves, Components, Materials & PRDs

7.1 Piping Components and Valves (API RP 574) 7.2 Positive Material Identification (PMI) (API RP 578) 7.3 Pressure Relieving Devices (PRDs) (API RP 576)

8 Chapter 8: Damage Mechanisms & Repairs

8.1 Corrosion Under Insulation (CUI) and Ambient Mechanisms 8.2 Process-Driven Damage Mechanisms (API RP 571) 8.3 Temporary and Permanent Repair Methods (ASME PCC-2 & API 570) 8.4 Soil Corrosion, Galvanic Corrosion, and Boiler Water Condensate Corrosion 8.5 Piping Rerating and Alterations (API 570) 8.6 Inspection Records, Reports, and Documentation 8.7 Amine/Caustic SCC, HCl, Sour Water, and MIC (API RP 571)

4.7 Minimum Required Thickness of Permanent Blanks

Key Takeaways Explain This Topic AI

ASME B31.3 304.5.3 governs minimum thickness of flat permanent blanks (spectacle blinds, spades) using t = d√(3P/16SE).

Variable d is the inside diameter of the gasket seating surface — not NPS and not outside diameter unless the question defines d that way.

Solid monolithic blanks without welds typically use E = 1.00; welded blanks use the applicable joint efficiency.

Permanent blinds are designed for extended isolation in the line; temporary line blinds for maintenance may have different administrative and tagging requirements under plant procedures.

API 570 inspectors verify blinds left in service are rated, tagged, thickness-adequate, and not corroded below tm — an improperly thin blind is a loss-of-containment risk.

Last updated: July 2026

4.7 Minimum Required Thickness of Permanent Blanks

Blanks (also called blinds or spades ) provide positive isolation when a line must be sealed for extended periods. ASME B31.3 304.5.3 prescribes the minimum required thickness for flat permanent blanks — distinct from pipe wall thickness in 304.1.2. API 570 inspectors encounter blinds left in place for turnaround seasons or long-term decommissioning; an undersized or corroded blank is a pressure-boundary failure waiting to occur.

304.5.3 — The Permanent Blank Thickness Formula

For a flat, circular permanent blank (nozzles and manways have separate rules in 304.5.2): t m = d ⋅ 3 P 16 S E W + c t_m = d \cdot \sqrt{\frac{3P}{16SEW}} + c t m ​ = d ⋅ 16 SE W 3 P ​ ​ + c

• Variable Definition
• t_m Minimum required thickness of the blank (in.)
• d Inside diameter of the gasket seating surface (in.) — the bore against which the gasket seals
• P Internal design gage pressure (psig)
• S Allowable stress at design temperature from Table A-1 (psi)
• E Quality factor — 1.00 for solid plate without welds
• W Weld joint strength reduction factor (usually 1.00 below creep range)
• c Sum of mechanical, corrosion, and erosion allowances (para. 304.1.1)

The formula is a flat-plate pressure design relation. It assumes uniform pressure on the blinded side and standard flange pair geometry. Exam trap: using NPS or pipe OD for d — the code specifies gasket seating inside diameter , which for standard raised-face blinds closely tracks the pipe ID of the mating flange class.

Permanent Blanks vs Temporary Line Blinds

• Feature Permanent blank (304.5.3) Temporary line blind (maintenance)
• Design basis ASME B31.3 304.5.3 thickness formula Often same formula but administrative controls differ
• Duration May remain installed for months or years Installed for specific maintenance, removed before startup
• Documentation P&ID marked, spec sheet, thickness calculation on file LOTO tag, permit, shift turnover
• Hardware Spectacle blind, paddle blind with jack screws Spade with slip blind on tee, figure-8

Inspector focus Corrosion, thickness ≥ t_m, rating stamp, bolting Verify removal before pressurization, tag discipline

API 570 does not replace B31.3 design rules but requires inspectors to confirm pressure-retaining blanks meet design and are fit for continued service. A blind left in the closed position through multiple cycles must be treated like any pressure component: measure thickness, check for corrosion, verify rating matches flange class.

Worked Example — NPS 6 Class 300 Blind

Given:

• NPS 6 raised-face blind isolating a line
• d = 6.065 in. (gasket seating ID matching NPS 6 Schedule 40 pipe bore)
• P = 275 psig design
• Material: ASTM A516 Grade 70 plate at 300°F, S = 17,100 psi (Table A-1)
• Solid plate, E = 1.00
• No corrosion allowance stated (add per spec if given)

Step 1 — Compute the radical 3 P 16 S E W = 3 × 275 16 × 17,100 × 1.00 = 825 273,600 = 0.003015 = 0.0549 \sqrt{\frac{3P}{16SEW}} = \sqrt{\frac{3 \times 275}{16 \times 17{,}100 \times 1.00}} = \sqrt{\frac{825}{273{,}600}} = \sqrt{0.003015} = 0.0549 16 SE W 3 P ​ ​ = 16 × 17 , 100 × 1.00 3 × 275 ​ ​ = 273 , 600 825 ​ ​ = 0.003015 ​ = 0.0549

Step 2 — Multiply by d t m = 6.065 × 0.0549 = 0.333 in. t_m = 6.065 \times 0.0549 = 0.333 \text{ in.} t m ​ = 6.065 × 0.0549 = 0.333 in.

Step 3 — Select standard plate

Order minimum 3/8 in. (0.375 in.) plate unless a corrosion allowance adds to t_m. If c = 1/16 in. , then t_m + c = 0.396 in. → still 3/8 in. plate minimum, but verify after corrosion per project spec.

Step 4 — Compare to field measurement

An inspector measures 0.310 in. on a blind after 12 years' service. 0.310 < 0.333 → blind is below minimum required thickness → replace or de-rate line per API 570 engineering evaluation.

Effect of Pressure and Diameter

Thickness scales with √P and d :

Doubling P increases t_m by √2 ≈ 1.41 — not linearly.

Doubling d doubles t_m — large blanks on big flanges grow thick quickly.

Higher flange class on the same NPS often has the same bore d but higher P — thickness rises with pressure, not with flange rating letter alone.

Welded Blanks and Large Openings

A blank welded to a ring or with hand holes uses reduced E on the weld and may require reinforcement per 304.5.3 notes. Monolithic forged spectacle blinds in a flanged union are the straightforward exam case: solid plate, E = 1.00 .

API 570 Inspection Points for Blinds in Service

Tag and position: Spectacle blind orientation — open vs closed — must match P&ID and permit status.

Rating: Blind material and flange class must meet design pressure-temperature per ASME B16.5 (or B16.47 for large sizes).

Thickness: UT the blind plate, especially behind gasket seating where corrosion is hidden.

Bolting: Full engagement, correct stud grade (B7/2H typical), no substituted non-rated fasteners.

Gasket: Proper material; blinded joints still require adequate gasket load.

Records: If a blind is permanently installed (e.g., dead-ended branch), thickness calculation should be in the piping file.

Common Exam Traps

• Trap Correct approach
• Using pipe OD (6.625 in.) for NPS 6 instead of gasket ID Use d = 6.065 in. or value given
• Forgetting E on welded blank plate Apply weld joint efficiency per 302.3.4
• Confusing blank formula with pipe formula 304.1.2 Blanks use 304.5.3 flat-plate form
• Assuming temporary blind can be thinner Design pressure still governs; administrative type ≠ lower pressure
• Relation to Positive Isolation (API 570 Context)

Chapter 1 positive isolation requires a rated barrier. A paddle blind cut from scrap plate without calculation is a serious deficiency. The 304.5.3 calculation proves the blank survives P at S and T — the inspector's role is confirming field hardware matches that design basis and retains adequate thickness after corrosion.