Engineering guide
Module 25V calculates flange joints to the German pre-standard DIN 2505 (January 1986 edition). The focus is on the force balance within the joint: from internal pressure, gasket geometry and gasket factors, the module determines the pipe force, the annular-area force, the gasket seating (pre-deformation) force and the required bolt loads for both the bolting-up and the operating condition. This makes it possible to calculate a flange connection and demonstrate that the sealing conditions are satisfied in all load cases.
The pre-standard DIN 2505 has been withdrawn; its successors are the European flange calculation to DIN EN 1591-1 and the design checks of DIN EN 13445-3 (Annex G and Clause 11). For re-rating existing plants, for comparative calculations and for assessing older design documentation, however, the DIN 2505 method remains highly relevant in pressure equipment practice, because countless flange joints in service were sized to it.
The module supports the common flange types such as weld-neck flanges, slip-on/plate flanges and loose (lap-joint) flanges with stub end, and covers soft gaskets as well as metallic and serrated (camprofile) gaskets — the latter via the number of serrations Z.
Scope
- Joint flanges
Typical workflow
Define gasket geometry and gasket factors
The starting point is the mean gasket diameter dD together with the gasket factors: the seating (pre-deformation) factor (k0·KD), the operating gasket factor k1, the deformation resistance of the gasket material and, for serrated camprofile gaskets, the number of serrations Z. These values are taken from the supplement to DIN 2505 or from manufacturer data.
Determine the pressure forces in the operating condition
From the design pressure, the module calculates the pipe force due to internal pressure (acting on the pipe bore cross-section), the annular-area force on the ring area between the pipe inside diameter and the gasket diameter, and the total internal pressure force. Additional external pipe forces are included in the total pipe force.
Calculate the gasket forces
For the bolting-up condition, the seating force is determined — the force the gasket needs to conform to the flange facing surfaces for the first time. For the operating condition, the operating gasket force follows from the gasket diameter, the factor k1, the safety factor SD and the internal pressure; it ensures that the residual surface pressure on the gasket is sufficient to seal.
Form the bolt loads for bolting-up and operation
The bolt load in the operating condition is the sum of the total pipe force, the annular-area force and the operating gasket force. The bolt load in the bolting-up condition must at least provide the seating force; for metallic gaskets, the crush resistance is checked in addition. In each case the larger value governs.
Check the sealing and crush-resistance conditions
Finally, the module verifies that the load-bearing capacity of the gasket is not exceeded in operation (limit-load factor, deformation resistance) and that all sealing conditions are met. This provides the basis for the subsequent strength calculation of the flange ring and bolts.
Frequently asked questions
Is it still acceptable to design flange joints to DIN 2505 today?
For new designs within the scope of the Pressure Equipment Directive, the harmonized methods of DIN EN 1591-1 or DIN EN 13445-3 are preferred. DIN 2505 was a pre-standard and has been withdrawn. In practice, however, it is still used for re-checking existing joints, for spare-part and modification assessments, and as a comparison method, because a very large number of joints in service are based on it and its force model is easy to follow.
What is the difference between the seating force and the operating gasket force?
The seating (pre-deformation) force is needed once, during bolt tightening, to plastically conform the gasket to the flange facing surfaces and level out surface roughness; it depends on the factor k0·KD. The operating gasket force is the residual compression permanently required in service so that the joint stays tight under internal pressure; it is determined from the factor k1, the safety factor SD and the pressure. In the bolting-up condition the seating force usually governs; in operation, the sum of the pressure forces and the operating gasket force governs.
What do the limit-load factor and the gasket crush resistance mean?
A gasket must not be over-compressed in either the bolting-up or the operating condition, otherwise the gasket material flows or crumbles and the joint leaks. The crush resistance (deformation resistance) limits the maximum permissible gasket force; the limit-load factor describes the utilization of this limit. If it is exceeded, a wider or stronger gasket must be chosen or the bolt load must be limited.
What role does the number of serrations Z play for camprofile gaskets?
With serrated camprofile gaskets, it is not the full gasket width that bears the load but only the serration tips. The effective contact area follows from the number of serrations Z and their bearing width. For all other gasket types, Z = 1 is used. An incorrect serration count leads directly to incorrect gasket and bolt forces.
Worked example
For a DN 150 flange joint with a soft gasket, the forces are to be determined to DIN 2505 in a worked example. The gasket factors are taken from manufacturer data. Required: the internal pressure forces, the operating gasket force, the bolt load in the operating condition and the seating force for the bolting-up condition.
Given
| Mean gasket diameter dD | 170 mm |
|---|---|
| Design pressure p | 1.6 N/mm² (16 bar) |
| Seating factor k0·KD | 400 N/mm |
| Operating gasket factor k1 | 25 mm |
| Safety factor SD (operation) | 1.2 |
| Number of serrations Z | 1 (soft gasket) |
Solution
Internal pressure force on the gasket circle
The pipe force and the annular-area force together act on the circular area up to the mean gasket diameter:
FRP + FF = (π/4) · dD² · p = (π/4) · 170² · 1.6 ≈ 36,317 N ≈ 36.3 kN
Operating gasket force
The gasket force required in operation follows from the operating gasket factor:
FDB = π · dD · k1 · SD · p = π · 170 · 25 · 1.2 · 1.6 ≈ 25,635 N ≈ 25.6 kN
Bolt load in the operating condition
FSB = (FRP + FF) + FDB = 36,317 + 25,635 ≈ 61,952 N ≈ 62.0 kN
Seating force (bolting-up condition)
FDV = π · dD · (k0·KD) = π · 170 · 400 ≈ 213,628 N ≈ 213.6 kN
The bolt load in the bolting-up condition must at least provide the seating force. Since FDV is considerably larger than FSB, the bolting-up condition governs the bolt and flange design here.
Result
| Internal pressure force (up to dD) | ≈ 36.3 kN |
|---|---|
| Operating gasket force FDB | ≈ 25.6 kN |
| Bolt load, operation FSB | ≈ 62.0 kN |
| Seating force FDV (governing) | ≈ 213.6 kN |
Technical content source: LV-Solved-Problems-Content 628d313247e7