4.7 Minimum Required Thickness of Permanent Blanks

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):

$t_m = d \cdot \sqrt{\frac{3P}{16SEW}} + c$

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

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

$\sqrt{\frac{3P}{16SEW}} = \sqrt{\frac{3 \times 275}{16 \times 17{,}100 \times 1.00}} = \sqrt{\frac{825}{273{,}600}} = \sqrt{0.003015} = 0.0549$

Step 2 — Multiply by d

$t_m = 6.065 \times 0.0549 = 0.333 \text{ in.}$

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

1. Tag and position: Spectacle blind orientation — open vs closed — must match P&ID and permit status.
2. Rating: Blind material and flange class must meet design pressure-temperature per ASME B16.5 (or B16.47 for large sizes).
3. Thickness: UT the blind plate, especially behind gasket seating where corrosion is hidden.
4. Bolting: Full engagement, correct stud grade (B7/2H typical), no substituted non-rated fasteners.
5. Gasket: Proper material; blinded joints still require adequate gasket load.
6. Records: If a blind is permanently installed (e.g., dead-ended branch), thickness calculation should be in the piping file.

Common Exam Traps

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.