Sizing a Blind Flange: PN Class per EN 1092-1 and Thickness Checks Around EN 13445
A blind flange looks simple: a round plate with a bolt circle. Yet several checks compete to set its thickness – and not always the one you expect. This post walks from operating pressure to recommended plate thickness step by step.
A German version of this article is available — switch the language to Deutsch.
:::info Tool The Blind Flange Calculator performs every step described here and runs entirely in the browser. :::
1. The PN class per EN 1092-1
EN 1092-1 specifies the connecting dimensions of steel flanges; the blind flange is flange type 05. For each combination of nominal size (DN) and pressure class (PN) it defines, among others:
- flange outside diameter D
- bolt circle diameter K
- number and size of bolts
The standard classes are PN 2.5 – 6 – 10 – 16 – 25 – 40 – 63 – 100 – 160 – 250 – 320 – 400. The calculator starts at PN 10 and selects the next class up that covers the operating pressure and is standardised for the chosen DN:
| Operating pressure | Selected class |
|---|---|
| 10 bar | PN 10 |
| 12 bar | PN 16 |
| 40 bar | PN 40 |
| 41 bar | PN 63 |
:::caution PN is not an allowable pressure The PN class designates connecting dimensions. The pressure actually allowed at a given temperature depends on the material and is governed by the pressure/temperature ratings in EN 1092-1. At elevated temperature a PN 16 flange may carry considerably less than 16 bar. :::
2. Allowable stresses
For non-austenitic steels, EN 13445-3 takes the nominal design stress as the lower of the yield- and tensile-based values, each with its safety factor:
f = min( R_p0.2/T / 1.5 ; R_m/20 / 2.4 )
f_test = R_p0.2/T_test / 1.05
The calculator simplifies to the yield-based term (R_p0.2/T / 1.5 in
operation, R_p0.2 / 1.05 for the test). For materials with a low tensile
strength relative to yield, check the tensile criterion by hand.
3. Which check sets the thickness?
The required thickness is the maximum of four criteria:
| Criterion | Idea | Load case |
|---|---|---|
| Strength (operation) | Bending moment from pressure force × lever arm to the bolt circle | operating pressure, f |
| Strength (test) | Same moment under test pressure | test pressure, f_test |
| Plasticity | Plate bending stress stays below yield | test pressure |
| Stiffness | Centre deflection ≤ 1 mm | operating pressure |
The corrosion allowance is added to the minimum thickness, which is then rounded up to the next commercially available plate thickness.
The stiffness criterion is easily underestimated: a plate can be strong enough and still deflect so much that the gasket unloads at its inner edge and leaks.
Worked example: circular plate under pressure
The plate formulas come from classical plate theory (simply supported circular
plate under uniform load). Inputs: pressure p = 1.6 MPa (16 bar), effective
radius R = 125 mm, thickness t = 20 mm, E = 200 000 MPa, ν = 0.3.
Maximum bending stress at the centre:
σ = 3 · p · R² · (3 + ν) / (8 · t²)
= 3 · 1.6 · 125² · 3.3 / (8 · 20²) = 77.34 MPa
Centre deflection, with plate stiffness D = E · t³ / (12 · (1 − ν²)):
w = (5 + ν) · p · R⁴ / (64 · D · (1 + ν)) = 0.170 mm
Both values are part of the calculator's automated tests and are published as a validation case on the documentation page.
4. Bolts and gasket
At low pressure and large DN, the bolt load is often governed not by the
operating pressure but by gasket seating – the load needed before any
pressure is applied. With the classical gasket model and factors m and y:
W_m1 = π · G · b · y
with effective gasket diameter G, effective gasket width b and seating stress
y. For G = 250 mm, b = 12 mm and y = 11 MPa (graphite):
W_m1 = π · 250 · 12 · 11 ≈ 103 673 N
The calculator compares the provided bolt area (count × stress area) with the largest requirement from seating, operation and test, and names the governing case.
Test pressure
For the pressure test per EN 13445-5 the minimum is
P_t = max( 1.25 · P_d · f_a / f_T ; 1.43 · PS )
with the nominal design stress at test temperature f_a and at design
temperature f_T. Enter the test pressure from your vessel specification in the
calculator when you have it – it takes precedence over the automatically derived
value.
Common mistakes
- Treating the PN class as the allowable pressure – a real safety risk at high temperature.
- Forgetting the corrosion allowance, or applying it to one side only.
- Checking only the operating case. Test pressure and gasket seating often govern.
- Rounding the standard thickness down instead of up.
:::caution Note The calculator supports preliminary design and does not implement EN 13445-3 in full. The current editions of EN 1092-1 and EN 13445 and the project specification govern. Release requires a qualified engineer. :::
Open the tool: Blind Flange Calculator · Documentation