Blind Plate Thickness
Minimum thickness of a line blank, spectacle blind, or paddle blind for internal pressure per ASME B31.3 para 304.5.3.
When to use this calculator
Use when specifying a line blank for isolation or for a hydrotest boundary, and you need to prove the plate thickness against the pressure it will actually hold. It applies the flat circular blank rule of ASME B31.3 para 304.5.3 with the gasket-face diameter of the mating flange, and carries DN presets for raised-face bores so you do not have to look the diameter up.
Required inputs
- Inside gasket-face diameter d — DN presets for RF flanges, or a custom value
- Applied pressure — hydrotest pressure or MAWP, whichever governs
- Allowable stress S at design temperature (B31.3 Table A-1)
- Quality factor E — 1.0 for rolled plate, 0.80 for a Class 130 casting
Expected outputs
- Minimum required blank thickness t_m
- The S × E product used
- Fully substituted formula for the calculation record
Formula overview
SI: p and S in MPa, d and t_m in mm. E is dimensionless. Standard line-blank dimensions are given in ASME B16.48.
ASME B31.3 para 304.5.3, flat circular blank:
t_m = d × √( 3p / (16 × S × E) )
where d is the inside gasket-face diameter for a raised-face flange, or
the gasket pitch diameter for a ring-joint or fully retained gasket.Worked example
6" Class 300 RF blank tested at 5 MPa, ASTM A285 Gr.C plate:
d = 154.1 mm, p = 5.0 MPa, S = 103.4 MPa, E = 1.0
t_m = 154.1 × √( 3 × 5.0 / (16 × 103.4 × 1.0) )
= 154.1 × √( 15 / 1654.4 )
= 154.1 × 0.09522
= 14.67 mm
Order the next plate thickness up, and add any corrosion allowance
on top of this — the code equation does not include one.Common mistakes
- Using the bolt-circle or the flange OD for d. The rule wants the gasket-face diameter, which is much smaller — using the bolt circle inflates t_m substantially and is a costly over-design on large blanks.
- Expecting the result to include a corrosion allowance. It does not: t_m is the bare pressure requirement, so add c yourself and then round up to a commercial plate thickness.
- Sizing the blank on design pressure when it will sit in the line during the hydrotest. Test pressure is typically 1.5 times design, so the test case usually governs the plate.
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