Engineering guide
The bolts of a flanged joint must fulfil two duties in every condition: hold the joint together against the pressure forces and compress the gasket far enough that it remains tight. If you want to calculate bolts to AD 2000 — the German pressure vessel code — you therefore first determine the minimum bolt loads for the bolting-up, operating and test conditions to AD 2000-Merkblatt B7 and from them size the required bolt cross-section.
The B7 module calculates these minimum bolt loads for circular, rectangular and arbitrary bolt arrangements — with internal gaskets as well as full-face gaskets. In the operating condition, the forces are composed of the hydrostatic end force (pressure force on the enclosed cross-section), the ring area force and the gasket load; in the bolting-up condition, the gasket seating load governs, which can be reduced in accordance with the code if necessary. The gasket factors for standard gaskets are built in and can be supplemented via the DICH module.
From the minimum loads, the module determines the required bolt dimensions for waisted (reduced-shank) bolts and full-shank bolts. The results are at the same time input quantities for the flange calculation to B8 and for bolted plates to B5 — B7 is thus the first step of every flanged joint calculation to AD 2000.
Scope
- Bolts
Typical workflow
Select the arrangement and gasket type
First, the arrangement (circular, rectangular or arbitrary bolt positions) and the gasket type are defined: an internal gasket in the force bypass or a full-face gasket across the entire flange width. For standard gaskets the factors are built in.
Determine the forces in the operating condition
From the design pressure and the gasket geometry follow the hydrostatic end force on the cross-section enclosed by the gasket diameter, the ring area force on the pressurized annular area up to the gasket, and the gasket load that maintains the required residual surface pressure in operation. Their sum is the minimum bolt load in the operating condition.
Check the bolting-up condition with the seating load
In the bolting-up condition, the gasket must be plastically conditioned with the seating load so that it conforms to the flange faces. If this force is disproportionately large, it can be reduced in accordance with the Merkblatt (switch "Reduce F_DV"); however, the minimum bolt load at bolting-up must not fall below the operating requirements.
Evaluate the test condition
For the pressure test condition, the calculation is repeated with the test pressure and the corresponding reduced safety factors; the most unfavorable of the three conditions governs the bolt design.
Size the bolts
From the governing bolt load, the number of bolts and the nominal design stress of the bolt material at the associated temperature follows the required root cross-section, distinguished between waisted bolts and full-shank bolts with their respective design allowances.
Frequently asked questions
How do internal and full-face gaskets differ in the calculation?
With an internal gasket (force bypass), the entire bolt load acts on the narrow sealing face; gasket load and lever arms are clearly defined. With a full-face gasket, the surface pressure is distributed across the entire flange width, beyond the bolt circle — the effective sealing area and lever arms are determined by separate rules of the Merkblatt, which generally results in higher bolt loads.
When may the seating load F_DV be reduced?
For gaskets with a high required seating deformation (such as wide soft gaskets), the calculated bolting-up force can far exceed the operating force and make bolts and flanges unnecessarily large. The Merkblatt permits a reduction in certain cases, provided tightness in operation is still ensured. The reduction is a deliberate engineering decision and should be agreed with the gasket manufacturer.
Why is the bolting-up condition often governing for the bolts?
In the bolting-up condition there is no pressure relief, and the gasket demands its full seating load while the bolt material is cold. Especially at low operating pressures, the bolting-up force clearly exceeds the operating force. That is why all conditions — bolting-up, operation, test — must always be calculated and the most unfavorable one used.
What is the difference between a waisted bolt and a full-shank bolt?
Waisted (reduced-shank) bolts have a narrowed shank and absorb fluctuations of the operating load elastically — they are preferable under cyclic loading and large temperature cycles. Full-shank bolts are stiffer and cheaper, but receive a larger allowance on the required diameter in the AD calculation. The module reports both variants.
Technical content source: LV-Solved-Problems-Content 628d313247e7