Pipe Support Engineering Guide: Selecting Rigid Struts, Variable Springs, and Constant Effort Hangers

Pipe supports perform three primary engineering functions: carrying sustained dead weight loads (pipe, fluid, valves, insulation), restricting thermal displacement directions, and absorbing dynamic transient vibrations. Supports are classified into: 1. **Restraints (Guides, Line Stops, Anchors):** Constrain specific translational or rotational degrees of freedom while permitting movement along permitted axes. 2. **Standard Gravity Supports (Shoes, Saddles, Racks):** Transfer downward gravitational loads directly into civil foundations or structural beams. 3. **Flexible / Resilient Supports (Variable & Constant Springs):** Maintain continuous upward support force while yielding elastically to vertical thermal movements.

When a hot steam or process pipe moves vertically upward ($Delta Y > 0$), a rigid hanger causes zero-load lift-off. Conversely, downward thermal growth overloads a rigid rod. Variable spring hangers bridge this behavior through helical compression springs. The critical parameter in variable spring engineering is the **Variability Index**:

// Variability Calculation per MSS SP-58
Variability (%) = [ |Hot Load - Cold Load| / Hot Load ] * 100%

Where:
Hot Load   = Calculated operating support reaction from CAESAR II
Travel     = Vertical thermal displacement (mm)
Spring Rate = Spring stiffness constant (N/mm)
Cold Load  = Hot Load + (Travel * Spring Rate)

Trunnions (dummy legs) welded directly to piping elbows or straight runs are widely used on pipe rack turns and vertical riser bases. However, uncalculated trunnions risk local shell punch-through or excessive bending moments. Engineers must verify: - **Bending Stress on Dummy Pipe:** Under cantilever dead weight plus frictional lateral loads. - **Local Stresses in Parent Run Pipe:** Evaluated using Kellogg or WRC methods. The maximum localized longitudinal stress must remain within code allowable limits ($1.5 S_h$). - **Drainage Hole Provision:** All vertical dummy legs must feature a bottom 12 mm weep hole to prevent internal rainwater accumulation and subsequent freezing rupture.