Engineering guide
The 250S module calculates the bolts of flange connections to DIN 2505 in the 1990 draft edition. This German standard has been withdrawn; new designs today follow DIN EN 1591-1 or the German AD 2000 code, Merkblatt B 7. For re-assessing existing equipment whose flange joints were designed to DIN 2505, the method remains the consistent calculation basis – particularly because the associated gasket parameters are matched to this calculation model.
Beyond the pure strength verification of the bolts for the seating, operating and test conditions, the module considers the elastic behaviour of the preloaded joint: the spring rates of bolts (waisted bolt or full-shank bolt), flanges and gasket determine how the bolt load changes on pressurization, on temperature change, with differential thermal expansion of bolt and flange, and with gasket creep and settlement. The spring rate of the gasket follows from its modulus of elasticity, the seating area from the mean gasket diameter and gasket width, and the gasket height.
This makes it possible to judge whether, after settlement losses and in the hot operating condition, sufficient residual gasket load remains – the central question during start-up and shut-down of equipment with significant temperature transients.
Scope
- Bolts
Typical workflow
Determine the required bolt loads per load case
From the hydrostatic end force and the required gasket load, the bolt loads for the seating condition (initial gasket deformation), the operating condition and the test condition are determined; the most unfavourable case sizes the required bolt cross-section.
Verify bolt cross-section and material
With the allowable stress of the bolt material at the associated temperature, the required root cross-section is calculated and compared with the selected thread (waisted bolt or full-shank bolt); scatter from friction and tightening method must be covered by margins.
Determine the spring rates of the clamped parts
The elastic compliances of bolt, flange rings and gasket are calculated. For the gasket, the spring rate follows from c_D = E_D·A_D/h_D with the seating area A_D from the mean gasket diameter and the gasket width.
Track load changes in operation
Pressurization, gasket settlement and temperature differences between bolt and flange (differential thermal expansion) change the bolt load along the joint load diagram; the module calculates the elastic compression of the gasket and the resulting residual gasket load.
Check tightness and limits
The remaining gasket load in the most unfavourable condition must reach the minimum gasket load required for the operating condition; otherwise larger bolts, a stiffer gasket, extension sleeves, or retightening after the first start-up must be provided.
Frequently asked questions
Why are waisted bolts advantageous for hot-running flange connections?
Waisted bolts have a lower spring rate than full-shank bolts. At the same preload they store more elastic elongation, so gasket settlement and differential thermal expansion cause a smaller load loss. The residual gasket load in operation therefore stays higher – for cyclically operated equipment often the decisive criterion.
What is gasket settlement and how does it enter the calculation?
Settlement is the plastic deformation portion of the gasket, the thread contacts and the joint interfaces after tightening and during the first temperature cycle. The settlement amount shortens the clamped stack and reduces the bolt load in proportion to the overall stiffness of the joint. Soft gaskets with a large settlement amount can leak in operation if this effect is neglected, even though the static verification is satisfied.
What is the spring rate of the gasket used for?
It describes how much the gasket yields elastically when the load changes: c_D = E_D·A_D/h_D. A low spring rate (soft, tall gasket) buffers load losses but demands more elastic compression; a high spring rate transmits load changes almost entirely to the bolts. The gasket's modulus of elasticity depends on contact pressure and temperature – for accurate calculations, manufacturer data for the relevant pressure range should be used.
Is the bolt verification per DIN 2505 sufficient as a tightness verification by today's standards?
Only to a limited extent. DIN 2505 secures the force balance with lump-sum gasket parameters but makes no statement about a specific leak rate. If a defined tightness class (e.g. per TA Luft / VDI 2290) has to be demonstrated, the calculation must be performed to DIN EN 1591-1 with leak-rate-based gasket parameters per EN 13555.
Worked example
For the gasket of a flange connection, determine the spring rate and the elastic compression under the seating gasket load – a worked example of the gasket spring model. The modulus of elasticity of the gasket is given as a manufacturer value for the relevant contact pressure range (example assumption).
Given
| Mean gasket diameter dD | 250 mm |
|---|---|
| Gasket width bD | 20 mm |
| Gasket height hD | 2 mm |
| Modulus of elasticity of gasket ED | 500 N/mm² (assumption) |
| Gasket load FD | 200 kN |
Solution
Seating area of the gasket
AD = π · dD · bD = π · 250 · 20 = 15,708 mm²
Spring rate of the gasket
cD = ED · AD / hD = 500 · 15,708 / 2 = 3.93 · 106 N/mm
Elastic compression under the gasket load
ΔhD = FD / cD = 200,000 / 3,926,991 = 0.051 mm
This elastic deformation is available as a load reserve against settlement losses and differential thermal expansion; plastic settlement amounts of the gasket come on top and must be applied separately.
Result
| Seating area AD | 15,708 mm² |
|---|---|
| Spring rate cD | 3.93 · 10⁶ N/mm |
| Elastic compression ΔhD | 0.051 mm |
Technical content source: LV-Solved-Problems-Content 628d313247e7