Engineering task and calculation objective
Piping must be supported and guided — and every attachment welded directly to the pipe introduces its forces locally into the pressure-bearing pipe wall. This module calculates integral attachments on metallic industrial piping to DIN EN 13480-3, clause 11, in particular support and guide trunnions, as commonly used on pipe racks, along pipeline routes and on vertical lines.
The core of the verification is the superposition of the local stresses from the load introduction with the stresses already present from design pressure and design temperature. Forces and moments from support reactions, friction or guide forces generate local membrane and bending stresses in the pipe wall at the welded attachment — and these limit the load capacity long before the trunnion itself would fail.
In practice, this calculation is needed in the detailed design of pipe supports: whenever a trunnion is placed directly on the pipe instead of introducing the load via clamps or loose saddles. The module verifies that the pipe wall and the attachment can safely carry the introduced loads in combination with the internal pressure.
Standard and calculation basis: DIN EN 13480-3/11: 2017-12
Calculation workflow
- Enter the operating data of the pipe: The starting point is the design pressure and design temperature of the line as well as pipe diameter, wall thickness and material. These yield the allowable stress at design temperature and the basic loading of the pipe wall already used up by the internal pressure.
- Define the geometry of the attachment: For the support or guide trunnion, the diameter or cross-section, wall thickness, projection length and, if applicable, a reinforcing plate between trunnion and pipe wall are defined. The dimensions of the welded connection determine the area over which the loads are introduced into the pipe wall.
- Compile the loads on the attachment: The forces and moments from the pipe stress analysis are applied: vertical support reaction from dead weight and contents, friction and guide forces from restrained thermal expansion, and dynamic components where applicable. From the force and lever arm of the trunnion, the longitudinal and circumferential moments acting on the pipe wall are obtained.
- Determine the local stresses in the pipe wall: The module calculates the local membrane and bending stresses in the pipe shell at the load introduction point and superimposes them on the pressure stresses. The stress combination is compared with the allowable values of the code for local loadings.
- Verification of trunnion and weld: Finally, the trunnion cross-section itself (bending, shear) and the connecting weld are assessed. If the load capacity is insufficient, a reinforcing plate can be added, the trunnion diameter increased, or the load distribution of the support arrangement adjusted.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Nominal design strength | ReHt | N/mm² |
| Temperature | tC | °C |
| Half cross section area of attachment | Am | mm² |
| Cross section area of attachments | At | mm² |
| Total weld area | Aw | mm² |
| Inside diameter of tube-shaped attachment | di | mm |
| Outside diameter of attachment | d0 | mm |
| Outside tube diameter | D0 | mm |
| Nominal wall thickness of tube | en | mm |
| Nominal wall thickness of attachment | en,t | mm |
| Design stress in creep range | fcr | N/mm² |
| Allowable stress at maximum metal temperature | fh | N/mm² |
| Half length of attachment in circumferential tube direction | L1 | mm |
| Half length of attachment in longitudinal tube direction | L2 | mm |
| Longitudinal bending moment in attachments | ML | N·mm |
| Circumferential bending moment in attachments | MN | N·mm |
| Torsional moment in attachments | MT | N·mm |
| Calculation pressure | Pc | MPa |
| Lateral load of attachments in circumferential direction | Q1 | N |
| Lateral load of attachments in longitudinal direction | Q2 | N |
| Mean radius of tube | Rm | mm |
| Axial loads of attachments | W | N |
| Section modulus of tube | Z | mm³ |
| Section modulus of attachment | Zt | mm³ |
Calculation options
Selection of type of weld
Fillet weld · full penetration weld · partially welded
Shape of attachment
Tube · Rectangle
Welded sides
two or three sides · all four sides
Frequently asked questions
How do support trunnions and guide trunnions differ in the verification?
A support trunnion carries vertical loads from weight and contents as intended, usually permanently and with a clearly defined lever arm. A guide trunnion limits lateral movements and is loaded mainly by friction and impact forces from thermal expansion — frequently shock-like and in alternating directions. For the verification, this changes the load direction, the load case combinations and the governing moments at the load introduction point.
When is a reinforcing plate under the trunnion required?
Whenever the local stresses in the pipe wall exceed the allowable values, an intermediate welded plate distributes the load introduction over a larger area. This is typically needed for thin-walled pipes, large projection lengths or high guide forces. Attention must be paid to the temperature compatibility of the plate and to ensuring it does not impair the inspectability of the circumferential weld beneath it.
Does this verification replace the flexibility analysis of the piping?
No, the two complement each other. The flexibility analysis to clause 12 of DIN EN 13480-3 delivers the global section forces and hence the support reactions, which serve as input values for the local verification at the attachment. The verification to clause 11 then checks whether the pipe wall can locally sustain these forces at the load introduction point — an aspect the global analysis does not cover.
Why are welded attachments more critical than clamps or loose supports?
Clamps and saddles distribute the load over a large circumferential region and can be installed without interfering with the pressure-bearing wall. A welded trunnion, by contrast, concentrates the load on a small area, generates bending stresses in the shell there, and at the same time constitutes a weld on the pressure wall — with consequences for heat treatment, inspection scope and fatigue. That is why the code requires the explicit local stress verification.