For standard flange connections (EN 1092-1), the design code for industrial piping (EN 13480-3) in clause 6.6 of this standard has created the possibility of providing proof of strength in a simple way by applying an additional equivalent pressure load to additional external loads (force and moments).
EN 13480-3 in clause 6.6 stated:
A standard steel flange connection in accordance with defined material requirements , giving the maximale allowable pressure with regards to the flange and the design temperature , may be used with the construction of piping subjected to internal pressure, without the necessity of carrying out a calculation to verify its resistance when the following conditions are met:
- a) for each normal working condition, the design pressure P shall not exceed the maximal allowable pressure PS specified
- at design temperature TS = max. 50°C: PN ≥ PS ≥ P
- at higher design temperature TS > 50 °C: Pp/T ≥ PS ≥ P
- b) for conditions where the flange connection is simultaneously subjected to internal pressure, axial load and bending moment, the equivalent design pressure , Peq, according the equation below shall not the limits specified in a);
Pp/T > Peq = P + 4•F / π•G2 + 16•|M| / π•C•G2
- c) the gasket types, for each PN, are specified in EN 1514-1 to EN 1514-8
- d) the strength of bolting for the flange connections, for each PN, shall be as indicated in EN 1515-1 to 4;
- e) the difference of temperature between the flanges and the bolting shall not exceed, 50 °C in any case;
- f) if the design temperature is ≥ 120 °C, the thermal expansion coefficient of the flange material shall not exceed the thermal expansion coefficient of the bolt material by more than 10%;
Symbols in above equation:
| Symbol | Description | Unit |
|---|---|---|
| Peq | Equivalent design pressure | MPa |
| P | internal calculation pressure (usual PS) | MPa |
| Pp/T | Pressure p-T rating at higher design temperature | MPa |
| F | Pulling axial force (to be a positive value in equation) | N |
| M | External bending moments | N•mm |
| G | Diameter of gasket load reaction | mm |
| C | Diameter of bolt pitch circle | mm |
Note: If specific requirements on tightness for flange connection are specified, calculation in accordance to EN 1591-1 and EN 1591-2 is required. Recommend gaskets are specified in EN 13480-3 Annex P.
Example
Design Condition:
Flange DN 200 / PN 25 EN 1092-1 Flange Face B2
Material: 1.4404 according EN 10222-5;
Gasket: EN 1514-1
Internal design pressure PS = P = 18,5 bar = 1,85 MPa;
Design Temperature: max. 180 °C
External Loads: Axial Force: F = 4450 N; Bending M = 8500000 Nmm;
Step 1: For flange PN = 25, material 1.4541, design temperature TS = 180°C;
flange pressure rating Pp/T = 22,6 bar = 2,26 MPa ;
Step 2: Flange DN 200/PN25
w = 58mm; b0=w/2 =29mm
when b0 > 6,3mm: b = 2,52• √b0 = 2,52• √29 = 13,6mm ; G = outside gasket contact face – 2•b = 278 – 2•13,6 = 250,8mm;
Step 3:
Peq = P + 4•F / π•G2 + 16•|M| / π•C•G2 =
1,85 + 4•4450 / π•250,82 + 16•850000 / π • 310 • 250,82 = 1,85 + 0,09 + 0,22 = 2,16 MPa = 21,6 bar
Step 4: Validation
Peq = 21,6 bar < 22,6 bar = Pp/T
Flange connection is acceptable.
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