Effect of pipe pressure on flange bolt-loading

Many of us have the impression that the bolt stress will go up when a flange pair in a pipe system is pressurized after assembly.

However the opposite is true. The video demonstrates

(1) first a fully rigid flange in which case the bolt load indeed will go up

(2) But the second part of the video show the real flange behaviour: the flange facing will rotate around the gasket and the bolt load will go down.

In the video the left side is the medium or internal side of the flange. The top two horizontal pieces are only hinges. The bottom horizontal piece is the gasket. The vertical top and bottom part are the (weld)neck of the flanges. The smaller horizontal parts at the right (with my thumbs) are the flange rings. The blue elastic bands are the bolts and brown ones are attached to simulate the flexibility between the flange ring and neck.

This bolt unloading phenomena is frequently misunderstood. The bolted flange assembly does not only have a force balance but also a moment balance. The result is that many bolt assembly forces, calculated according the Taylor Forge method, are perhaps too low.

(*) Written by Rutger Botermans (Red-Bag)