Engineering guide
This module calculates waisted (reduced-shank) studs and full-shank bolts to TRD 309, part of the German Technical Rules for Steam Boilers (TRD), for detachable, preloaded joints in steam boiler construction — typically gasketed flange and cover bolting. Starting from the forces to be transmitted from internal pressure and gasket seating, the required bolt cross-sections are determined for the assembly (bolt-up), operating and pressure test conditions.
Waisted studs with a necked-down shank accommodate cyclic loading and settlement losses of the joint far better than full-shank bolts; TRD 309 rewards this with different allowable stresses for the two designs. This makes it possible to decide whether a joint can be executed with full-shank bolts or whether waisted studs are required, for instance on boiler flanges subject to frequent start-up and shutdown cycles.
The TRD rules were withdrawn at the end of 2012; for existing boilers and the recalculation of existing bolted joints, TRD 309 remains the associated calculation basis. For new designs, the EN boiler standards (EN 12952/EN 12953) or, in pressure vessel construction, AD 2000-Merkblatt B 7 and the flange calculation to EN 1591-1 take over this role.
Scope
- Elongation bolts and full shank bolts
Typical workflow
Determine the bolt forces for each load condition
The total bolt forces are determined for the operating, test and assembly conditions: the pressure force from internal pressure acting on the effective gasket area, the gasket force needed to maintain tightness, and the gasket seating force required during bolt-up.
Select bolt design and material
It is decided whether waisted studs (necked shank) or full-shank bolts are used. The bolt material yields the strength values at room and design temperature; different safety factors and allowable stresses apply to the two designs.
Calculate the required root cross-section
For each load condition, the required root or waist cross-section follows from the bolt force, the number of bolts and the allowable stress; in addition, a design margin for bending and torsion during tightening is included. The most unfavourable condition determines the bolt diameter.
Verify the selected bolt
The selected bolt size, with its actual root or waist cross-section, is checked against all load conditions. Minimum dimensions — such as the waist diameter relative to the thread root — and the requirements for nut height must be observed.
Frequently asked questions
When are waisted studs required instead of full-shank bolts?
Waisted studs are more resilient: their necked shank reduces the additional force from operating loads and compensates better for gasket settlement and differential thermal expansion. For cyclic loading, high temperatures and large nominal pressures, the code therefore mandates waisted studs, or permits only lower utilization for full-shank bolts. For rarely opened, statically loaded joints with moderate parameters, full-shank bolts are sufficient.
Why is the assembly condition often governing?
During bolt-up, the gasket must be brought to its seating stress without any internal pressure relieving the joint; at the same time, room-temperature strength values apply, and tightening produces additional torsional stress. Especially with soft gaskets requiring a high seating force, the assembly condition can demand a larger bolt cross-section than the operating condition.
How is the bolt calculation linked to the gasket selection?
The gasket factors (seating stress and operating surface pressure) determine the bolt forces directly. A harder gasket increases the assembly force and hence the bolting requirement; too little preload leads to leakage in service. Bolt, flange and gasket calculations therefore always form one interdependent system and must be performed with the same characteristic values.
Technical content source: LV-Solved-Problems-Content 628d313247e7