When 300 degrees and high pressure hit a screw connection

The use case shows how thermal influences, settling processes, and assembly effects can be precisely evaluated in accordance with VDI 2230 and how specific measures for a secure, durable, and economical screw connection can be derived from this.

Bolted connections on turbines are among the most safety-critical elements in a power plant. They must withstand high internal pressures, thermal cycling, and long operating times. Preload force, material elongation, settlement losses, and assembly methods have a direct impact on operational safety. Even minor deviations can lead to premature failure.

After several operating cycles, abnormalities appeared on the bolts of the flange connection of a high-pressure turbine housing in a gas-fired power plant. The original design was based on internal factory standards, but not on a complete calculation according to VDI 2230. After individual bolts failed, the operator decided to perform a complete recalculation in order to clearly identify the causes and create a permanently safe solution. The calculation was performed with MDESIGN to ensure compliance with standards, verifiability, and reproducibility.

The connection under consideration consisted of a two-part high-pressure turbine housing, bolted together via a large-area flange connection.

Connection parameters:

• Screws: 32 × M24 in strength class 12.9

• Medium / load: high internal pressure → approx. 80 kN axial force per screw

• Operating temperature: approximately 300 °C

• Housing material: G-X12CrMoWVNbN10-1-1 (created as a separate material in the screw database module; more information)

The special feature: High operating temperatures lead to relative length changes between the screw and the housing and thus to losses in preload force, which must be taken into account in accordance with standards.

Source: MDESIGN

The permissible installation preload force was 349 kN, and the target preload was 310 kN. This meant that the bolt was subjected to high loads during operation, but remained within the VDI-compliant range. This high preload force is necessary to keep the flange permanently closed under pressure.

Source: MDESIGN

The thermal expansion differed significantly between the screw and the housing. A temperature difference of around 300 K resulted in an additional length change of 0.18 mm.
This seemingly small value already led to a loss of preload force of around 15 percent, or around 9000 N per screw.

High-temperature connections in particular demonstrate how sensitive the clamping force is to thermal effects and how important precise modeling is.

In addition, the rough cast surface and a functional coating influenced the settling behavior. With settling amounts between 40 and 60 µm, there was a further 5 to 7 percent loss of preload force.
The combination of thermal expansion and settling explained exactly the deviations in clamping forces measured during operation.

• FZ = 22,445 N   

It was precisely this dual loss mechanism that explained the deviations in clamping forces observed during operation.

After taking all influences into account, the screws were checked in accordance with standards:

Only after optimizing all boundary conditions did the connection fully meet the requirements of VDI 2230.

All input variables, safety factors, standard steps, and results are automatically documented. A must for operators, certifiers, and auditors.

Source: MDESIGN

MDESIGN temperature corrections and settling processes in accordance with standards and iteratively. This is an area in which manual table calculations quickly reach their limits. Especially in the case of complex screw connections, where thermal expansion, settling losses, and multiple load cases act simultaneously, classic homemade Excel sheets often reach their structural limits. Many users still work with their own tables, which have grown over the years, but this is precisely where simplifications, missing iteration steps, or incomplete standard parameters often occur.

For anyone who has been using Excel for calculations, it is therefore worth taking a look at the differences in accuracy, verifiability, and documentation.

Further technical article: "MDESIGN . Excel – Which solution is better for your technical calculations?"

The prestressing forces measured later using strain gauges deviated by only 3.2% from the calculated target value, which is a very good result for a high-temperature connection.

Compared to the table-based method used previously, MDESIGN was MDESIGN in:

• to ensure the accuracy of the evidence,

• to improve the assembly parameters in accordance with standards,

• and provide verifiable documentation.

The analysis led to several effective measures to stabilize the screw connection for high-temperature operation.

Source: MDESIGN

Switch to X6NiCrTiMo,
→ higher creep resistance at 300 °C
→ more stable preload force over the operating time.

→ More favorable elasticity ratios,
→ More stable stress distribution,
→ Lower prestressing force losses.

Source: MDESIGN

Instead of pure torque tightening, the torque-angle method was introduced. Supplemented by digital test reports, the assembly force became reproducible and fully traceable.

90% – Tighten with torque wrench: 

Source: MDESIGN

rotation anglecontrolled over-elastic tightening with joining torque  

Source: MDESIGN

Example of assembly instructions from MDESIGN 2026

Source: MDESIGN

cost efficiency

Calculations and consultation with the manufacturer made it possible to avoid the use of expensive special bolts.
→ 18% lower production costs.

The case study shows how important it is to have a VDI 2230-compliant calculation is, especially for high-temperature bolted joints . 

Only through verifiable documentationthat consideration of thermal effects and close coordination with the Hmanufacturer , a durable and economical solution was found.