ASME B31.3 & UG-34 Blind Flange & Spectacle Blind Thickness Calculation Guide

Wrong plate thickness on a line blind, spectacle blind, or temporary hydrotest blank is a classic turnaround failure mode: too thin and the cover yields at the gasket reaction circle; too thick and you fight weight, gasket seating, and schedule. This guide walks through a field-ready blind flange thickness calculation using the same ASME B31.3 / Section VIII Div. 1 UG-34 attachment model implemented in FieldEngineersKit.

Open the live Blind Flange Thickness calculator to size permanent blinds and hydrotest blanks with ASME B16.5 raised-face diameter auto-fill.

Quick Summary (TL;DR)

• Takeaway: Required blind thickness is $t_m = d\sqrt{C P / (S E)} + c$ with bolted-cover factor $C = 0.30$ (not the simply-supported $3/16$).
• Governing relation: $t_m = d \cdot \sqrt{\dfrac{0.30\,P}{S\,E}} + c$ (permanent); hydrotest blanks often use $c = 0$ and $P_t$ at ambient $S$.
• Code / basis: ASME B31.3 Para. 304.4.1 (closure plates) + ASME VIII-1 UG-34; FEK RF $d$ from ASME B16.5.

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Blind flange thickness calculation in the field

Process plants isolate lines with spectacle blinds, spades/spacers, forged blind flanges, and temporary test blanks. Pressure design of the flat circular cover is governed by the same family of relations used for unstayed flat heads: diameter at the gasket reaction circle, design (or test) pressure, plate allowable stress, joint efficiency, and corrosion allowance.

Use the interactive Blind Flange Thickness tool for mode switching (permanent vs hydrotest), B16.5 NPS × class RF diameter, and commercial plate recommendation. Pair pressure class checks with the Flange Pressure-Temperature Rating calculator, and pipe wall selection with ASME B31.3 Pipe Thickness.

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Core formulas & parameter definitions

Permanent operating blind / spectacle plate

FEK implements the bolted flat-cover form used in UG-34 / B31.3 closure-plate screening:

$$t_m = d \cdot \sqrt{\frac{C \cdot P}{S \cdot E}} + c,\qquad C = 0.30$$

Equivalently:

$$t_m = d \cdot \sqrt{\frac{0.30\,P}{S\,E}} + c$$

$P$ and $S$ must share stress units (MPa with MPa, or psi with psi). $d$, $t_m$, and $c$ share length units.

Temporary hydrotest blank

For short-duration, non-corrosive hydro/pneumatic tests (B31.3 Chapter VI practice in FEK):

$$t_m = d \cdot \sqrt{\frac{0.30\,P_t}{S_{\mathrm{amb}}\,E}}$$

with $c = 0$ and ambient allowable stress $S_{\mathrm{amb}}$.

Why not $t = d\sqrt{3P/(16SE)}$?

The factor $3/16 = 0.1875$ is the classical simply-supported circular plate coefficient under uniform load. Bolted pipe flanges introduce edge clamping and prying. ASME UG-34 / FEK therefore use $C = 0.30$, which increases thickness by roughly $\sqrt{0.30/0.1875} \approx 1.26$ (about +26%) versus the $3/16$ internet form. Using $3/16$ under-sizes FEK/code screening blinds.

Parameter definitions

FEK material screening presets (engine)

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How to calculate spectacle blind thickness per ASME B31.3 (worked example)

Scenario

NPS 10″ Class 300 line isolation blank for screening (ASTM A105 / CS plate allowable taken as $S = 137.9\text{ MPa}$ / 20,000 psi, design pressure 50 bar = 5.0 MPa near Class 300 ambient rating, $c = 3.0\text{ mm}$, $E = 1.0$).

Step 1 — Confirm gasket reaction diameter $d$

From FEK B16.5 geometry for NPS 10 Class 300:

$$d = 323.8\ \text{mm (RF)}$$

Step 2 — Pressure (uncorroded) term

$$t = d \cdot \sqrt{\frac{0.30 \cdot P}{S \cdot E}} = 323.8 \cdot \sqrt{\frac{0.30 \times 5.0}{137.9 \times 1.0}} = 323.8 \cdot \sqrt{0.010877} = 323.8 \times 0.10429 = 33.77\ \text{mm}$$

Step 3 — Add corrosion allowance

$$t_m = t + c = 33.77\ \text{mm} + 3.0\ \text{mm} = 36.77\ \text{mm} \approx \mathbf{36.8\ mm}$$

Step 4 — Commercial plate & catalog check

Conclusion: Required screening thickness is $t_m \approx 36.8\text{ mm}$. Procure at least 38 mm plate (or a forged blind whose pressure design meets the same $t_m$). Confirm ASME B16.48 spectacle-blind paddle thickness on the current edition chart for the exact ring/spacer geometry — FEK does not embed a full B16.48 paddle table.

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Interactive tool CTA

Public reference matrices (ambient class rating and hydrotest plate capability) are also published on the calculator page for field lookup and image reference.

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Frequently Asked Questions (FAQ)

Q1. How to calculate spectacle blind thickness per ASME B31.3 Para. 304.5.3?

FEK screens flat blinds with Para. 304.4.1 / UG-34 using $t_m = d\sqrt{0.30 P/(S E)} + c$. Spec writers sometimes cite neighboring paragraphs for specialty closures; always match the attachment detail and $C$ factor to the actual blind (bolted RF cover vs other geometries). For spectacle assemblies, size the pressure-retaining paddle with this flat-cover screen, then confirm B16.48 paddle/spacer dimensions and ring drilling against the purchase standard—not against pipe ID alone.

Q2. What is the difference between ASME UG-34 unstayed flat heads and ASME B31.3 blanks?

UG-34 is the Section VIII Div. 1 flat-head / cover thickness framework (including attachment factor $C$). B31.3 process piping adopts the same bolted-cover logic for closure plates / blinds in the Para. 304.4 family and treats temporary test blanks under Chapter VI testing practice. FEK uses one engineering model ($C = 0.30$) with mode switches for corrosion and ambient vs design stress—so a hydrotest blank is usually thinner than a permanent blind at the same diameter and pressure class.

Q3. How do ASME B16.5 flange pressure-temperature ratings relate to custom blind thickness?

B16.5 P–T ratings limit the flanged joint’s allowable working pressure by material group and temperature. Blind thickness $t_m$ answers a different question: can the flat cover plate carry that pressure at diameter $d$? You need both: verify class rating (e.g. Class 300 ambient ≈ 51.1 bar in FEK) and compute $t_m$. A forged B16.5 blind flange brings a catalog face thickness; a fabricated plate blind / spectacle paddle must still meet $t_m$ even if the mating flanges are correctly rated.

Q4. Why does my spreadsheet give ~26 mm for NPS 10 / 5 MPa when FEK gives ~37 mm?

Two common causes: (1) using $C = 3/16 = 0.1875$ instead of $C = 0.30$, and/or (2) using a smaller $d$ than the B16.5 RF diameter (323.8 mm in FEK for NPS 10). FEK follows the bolted-cover factor and RF $d$ auto-fill—treat thinner $3/16$ results as non-conservative relative to this screening basis.

Q5. Can I use ASTM A36 plate for hydrotest blanks?

Yes, for temporary blanks, with a lower ambient allowable (FEK A36 preset $S_{\mathrm{amb}} = 115\text{ MPa}$), which increases $t_m$ versus A516-70. Stamp, tag, and register temporary blanks for mandatory removal before commissioning.