Flangebolts for Gas Turbine Exhaust
A Norwegian Patent, developed to optimize the relation between tension lenth and the flange with gasket thickness.
Gas Turbines are increasingly being used in industrial applications, as in onshore and offshore oil and gas industry, ship propulsion, and land based power generation.
Common for the installations is that plant downtime leads to significant economic consequences, thus high demands to availability and reliability are set forth.
Leakages from turbine exhaust ducting are a common operational problem, often caused by loose duct flange bolt connections. Leakages in gas turbine exhaust ducting may occur after only few hours of operation, and may cause damages and unwanted conditions such as:
- Exposure of hot exhaust gas to zone classified areas.
- Damages to expansion bellows.
- Flange gasket damage.
- Duct structural steel deformations due to temperature gradients.
- Turbine alarm and trip, due to high temperature in turbine enclosure.
- Damage to thermal insulation.
- Cosmetic damages.
- Deviation from HMS regulations.
Cause of problem
In a typical flange bolt connection, the bolt clamping length is equal to the thickness of the flange being bolted. During the turbine start sequence the duct temperature will rise and result in a growth in the flange thickness.
Having a time differential in temperature rise compared to the duct flange the bolt will be exposed to a stretch equal to the difference in thermal growth of the flange and the bolt. The bolt resultant stress, being the bolt prestress and the additional stress due to thermal stretch, may cause plastic deformation in the bolt material.
When the duct system is in stable operating temperature, the plastic deformation of the bolt plus the bolt thermal growth results in bolt relaxation and reduced gasket compression.
Short bolt connection with clamping length equal to flange thickness:
- Cold exhaust duct.
- Thermal growth of the duct and flange. Bolt is stretched.
- Thermal growth of the bolt.
- Cold duct, with permanent elongation of the bolt.
By using Thermobolt, a more slender bolt connection with increased clamping length is achieved. Design and material selection is customized for each specific application.
Thermobolt with long clamping length reduces thermal stretch:
- Cold exhaust duct.
- Thermal growth of the duct and flange. The stretching of Thermobolt does not result in yield stress.
- Thermal growth of Thermobolt. Prestress is maintained.
- Cold duct. No plastic deformation of the Thermobolt.
Typical design parameters for Thermobolt are:
- Material Selection
- Operating temperature
- Temperature gradients
- Static and dynamic loads
- Flange geometry
- Number of bolts
- Bolt diameter
- Gasket properties
- Bolt clamping length
- Bolt stretch
- Bold resultant load
- Embedment relaxation
- Material Creep
Delivery program for Thermobolt Exhaust Duct Bolting:
Thermobolt Material Selection
- Grade 310