Key takeaways
- A support’s job is defined by the directions it restrains: vertical, lateral, axial or all of them.
- Rests carry weight, guides stop sideways movement, line stops stop axial movement and anchors stop everything.
- Spring supports carry weight while letting the pipe move vertically, which is essential near hot equipment.
Think in directions
Every support can be described by which of the three directions (and three rotations) it restrains and which it allows. Stress engineers model them this way, and designers should think the same way.
The common types
Rest (resting support, shoe). Carries the weight of the pipe vertically. The pipe can slide along and across the beam. Shoes are welded or clamped to the pipe so insulation isn’t crushed and the pipe slides on the shoe, not on its wall. Low-friction slide plates (for example PTFE or graphite pads) reduce friction where large movements occur.
Guide. Stops sideways (lateral) movement but lets the pipe slide axially. Guides keep long runs straight on racks and keep expansion going where it is meant to go, such as into a loop.
Line stop (axial stop). Stops movement along the pipe axis but allows lateral movement. It is used to split a long run into sections and control where expansion goes.
Anchor. Restrains all directions (and usually rotations). Anchors divide a system into independent sections for analysis. They take large loads, so the steelwork must be designed for them.
Spring hanger or spring support. Carries weight while allowing vertical movement. Variable springs change their load a little as they move; constant springs keep nearly the same load across their travel and are used where vertical movement is large.
Rigid hanger (rod hanger). Carries weight from above. It allows some horizontal swing.
Dynamic restraints. Snubbers and sway braces resist sudden loads (earthquake, slug, water hammer) while allowing slow thermal movement.
Attachments
- Trunnions and dummy legs support a pipe at an elbow.
- U-bolts and clamps are cheap holding devices for small lines, but avoid them on hot lines and lines with large movements.
- Welded attachments need checking for local stress in the pipe wall.
Span
Supports are spaced so that the pipe’s sag and bending stress stay acceptable. Projects publish span tables by size, schedule and contents (gas or liquid filled). Hydrotest weight often governs for large gas lines.
Onshore
Onshore racks usually have frames every 6 m or so, which sets the support spacing for most lines.
Offshore
Offshore, support positions follow the module steel layout, and supports must also be checked for motion and blast loads.
What to take back to your desk
For each support you place, say out loud what it stops and what it allows. If you can’t, the stress engineer can’t either, and the support will be redesigned later.
Educational content. Always design to your project specification.
