Why a riser needs flexibility
A steel riser grows when it warms and shrinks when it cools. Over a long run the change adds up to tens of millimetres. The PNGRB guidelines ask for suitable mechanisms in the riser design to take up thermal movement, so the pipe is not forced to resist it.
Expansion loops
A loop is a bend or offset built into the pipe so that the pipe itself flexes as it grows. It has no moving parts to wear out, which makes it popular, but it needs space in the shaft and adds fittings and welds.
Bellows and expansion joints
A bellows is a short flexible section that absorbs movement in a small length. It saves space where a loop will not fit. It must be fitted correctly, guided and anchored, and it is a part that needs inspection over the life of the building. Check what the gas distribution company accepts before choosing one.
Anchors and guides
Anchors fix the pipe at chosen points so that movement goes where the design intends. Guides hold the pipe in line while letting it slide along its length. Without proper anchors and guides, loops and bellows do not work as designed.
Choosing between them
The choice depends on the height of the riser, the space in the shaft, the amount of movement, the number of floors between anchors and the requirements of the gas company. The stress analysis tests each option and shows which one keeps the stresses and support loads within limits.
Plan it early
Space for a loop, access to a bellows and fixing points for anchors have to be agreed with the architect and the structural designer. Adding them after the shaft is built is difficult, so the flexibility layout belongs in the early design.
To have the flexibility of a riser checked before it is built, send us the layout and the building height. See our PNG riser stress analysis service.