Why is attachment factor C = 0.30 used instead of simply-supported 3/16 (0.1875)?

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.

Can structural steel plate (ASTM A36 / SS400) be used for hydrotest blanks?

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.

What diameter should be used for variable 'd' in the blind calculation?

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.

2" Class 150 Blind Flange Thickness | Engineering Screening & Sizing

Engineering 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: t_m = d · √(C · PS · E) + c (C = 0.30, c ≥ 3.0 mm)

Hydrotest Blank: t_m = d · √(C · P_tS_amb · 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 B31.3 304.4.1 C = 0.30Permanent Blind c ≥ 3.0 mmHydrotest Blank c = 0.0 mmSeamless Plate E = 1.00

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).
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, bar, or psi).
P_t (Hydrotest Test Pressure) — Shop or field hydrostatic/pneumatic test pressure applied to temporary test blanks (MPa, bar, or psi).
S (Design Allowable Stress) — Basic allowable stress of plate material at design temperature per ASME B31.3 Table A-1 (MPa or psi).
S_amb (Ambient Allowable Stress) — Basic allowable stress of plate material at ambient temperature (20 °C) for hydrotest blanks (MPa or psi).
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(c)(2)(b)).
c (Corrosion Allowance) — Specified corrosion allowance added after structural pressure term (c ≥ 3.0 mm for permanent, c = 0.0 mm 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.

Permanent Blind Corrosion Allowancec = 3.0 mm (0.125 in) Standard

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.

Temporary Test Blank Rulesc = 0.0 mm at Ambient Temperature

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.

Plate Mill Under-Tolerance (ASTM A20)-0.3 mm (-0.01 in) Allowance

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.

Gasket Contact Reaction Diameter (d)ASME B16.5 Raised Face (RF) OD

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

Hydrotest Temporary Blank Thickness Lookup: ASME B16.5 RF (c = 0 mm, A516-70 S = 138 MPa, C = 0.30)
NPS	RF Dia d (mm)	Cl 150# Pt=3.0 MPa tm (Plate)	Cl 300# Pt=7.7 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 (30T)	39.4 mm (40T)
8"	269.9	21.8 mm (25T)	35.0 mm (38T)	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 × P_rating @ ambient (150#: 3.0 MPa, 300#: 7.73 MPa, 600#: 15.32 MPa). Calculated with C = 0.30, E = 1.00, S_amb = 138 MPa (A516-70), c = 0.0 mm. Parentheses show recommended commercial plate stock.

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 = 137.9 MPa (20.0 ksi) @ 20 °C; 104.0 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 = 137.9 MPa (20.0 ksi) @ 20 °C; 110.0 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 Verification

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 = 7.73 MPa (77.3 bar / 1121 psi) using ASTM A516 Gr. 70 plate at ambient temperature (c = 0.0 mm).

Nominal Pipe Size:NPS 6 (DN 150)Flange Pressure Rating:ASME B16.5 Class 300 Raised FaceTest Condition:Temporary Hydrostatic Test (ASME B31.3 Ch. VI)Hydrotest Pressure (Pt):7.73 MPa (77.3 bar / 1,121 psi)Plate Material:ASTM A516 Gr. 70 (Ambient S_amb = 138.0 MPa)Joint Quality Factor (E):E = 1.00 (Seamless rolled plate)Attachment Factor (C):C = 0.30 (ASME UG-34 Bolted Flat Cover)Corrosion Allowance (c):0.0 mm (Temporary Test Blank)Gasket Contact Dia (d):215.9 mm (ASME B16.5 6" Class 300 RF OD)

Determine Hydrotest Pressure & Material Parameters

Hydrotest test pressure Pt = 1.5 × P_design = 1.5 × 5.15 MPa = 7.73 MPa (77.3 bar). For ASTM A516 Gr. 70 at ambient temperature (20 °C): allowable stress S_amb = 138.0 MPa (20.0 ksi). Joint efficiency E = 1.00, attachment factor C = 0.30, and corrosion allowance c = 0.0 mm.

Result:Pt = 7.73 MPa, S_amb = 138.0 MPa, E = 1.00, C = 0.30, c = 0.0 mm

Temporary test blanks use c = 0.0 mm since water test duration is brief and non-corrosive.

Calculate Dimensionless Stress & Pressure Ratio

Formula: \text{Ratio} = \frac{C · P_t}{S_{\text{amb}} · E}

Ratio = (0.30 × 7.73 MPa) / (138.0 MPa × 1.00) = 2.319 / 138.0 = 0.016804

Result:Ratio = 0.016804, \sqrt{\text{Ratio}} = 0.12963

Square-root factor represents elastic bending deflection under uniform hydrostatic load.

Calculate Minimum Required Test Blank Thickness (t_m)

Formula: t_{\text{m}} = d · \sqrt{\frac{C · P_t}{S_{\text{amb}} · E}} + c

t_m = 215.9 mm × 0.12963 + 0.0 mm = 27.99 mm (1.102 in)

Result:t_m = 27.99 mm (1.102 in)

Pure structural thickness required to resist edge bending moments at 7.73 MPa test pressure.

Apply Plate Mill Under-Tolerance Verification

Per ASTM A20, rolled carbon steel plate under-tolerance is 0.3 mm. Minimum plate thickness to procure = 27.99 mm + 0.30 mm = 28.29 mm.

Result:t_{\text{procure, min}} = 28.29 mm

Procured plate stock must account for manufacturing rolling tolerances.

Select Commercial Plate Stock & Compare with Permanent Blind

Standard commercial plate thicknesses available: 25 mm (25T), 28 mm (28T), 30 mm (30T), 32 mm (32T). Since 27.99 mm exceeds 28T with mill tolerance, select 30.0 mm (30T) or 32.0 mm (32T / 1-1/4") plate. For comparison, a permanent operating blind with c = 3.0 mm requires 31.0 mm (recommended 32T / 35T).

Result:Selected: 30 mm (30T) Plate (Safety Margin: +2.01 mm)

Temporary blank requires extended paddle handle stamped 'TEST BLANK NPS 6 CL300'.

Conclusion: For an NPS 6 Class 300 hydrotest at 7.73 MPa (77.3 bar), the ASME B31.3 / UG-34 calculated minimum required temporary test blank thickness is 27.99 mm. Selecting standard 30.0 mm (30T) or 32.0 mm (1-1/4") ASTM A516 Gr. 70 plate provides complete elastic structural safety.

5. Code Limitations & FAQ

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.