How Thermal Expansion Affects Pipe Supports
When piping systems operate at elevated temperatures, the pipe expands in both length and diameter. This thermal growth causes the pipe to move at its support points, changing the loading on supports and potentially displacing shims that were installed during cold construction.
Understanding thermal expansion is essential for proper shimming because pipe supports must be set at a cold position during construction that accounts for the expected movement when the system reaches operating temperature.
Cold Set vs Hot Operating Position
The cold set position is where the pipe support is installed during construction when the pipe is at ambient temperature. The hot operating position is where the support will be when the pipe reaches its design operating temperature.
For sliding supports, the shim height typically remains the same between cold and hot conditions because the pipe slides across the support. However, the horizontal position of the pipe on the support changes as the pipe expands axially.
For spring hangers, the cold set position is calculated by the spring manufacturer to achieve the correct hot load when the pipe moves to its operating position. Shims at spring hanger attachment points must be set precisely to the cold set elevation.
Thermal Expansion Rates for Common Pipe Materials
- Carbon steel (A106): Approximately 6.3 mm per 10 meters per 100°C rise
- Stainless steel (304/316): Approximately 8.6 mm per 10 meters per 100°C rise
- Chrome-moly (P11/P22): Approximately 6.2 mm per 10 meters per 100°C rise
- Copper: Approximately 8.3 mm per 10 meters per 100°C rise
- Duplex stainless: Approximately 6.5 mm per 10 meters per 100°C rise
Shimming Considerations for High-Temperature Systems
Shim Material Selection
High-temperature piping systems require shims that can withstand sustained elevated temperatures without losing strength or corroding at an accelerated rate. Carbon steel shims are suitable up to approximately 425°C (800°F). Above this temperature, alloy steel or stainless steel shims may be required.
Thermal Cycling Effects
Repeated heating and cooling cycles cause the pipe to expand and contract, which can work shims out of position over time. On systems subject to frequent thermal cycling, shims should be tack-welded to the support steel and the shim pack should be checked during scheduled maintenance intervals.
Friction and Sliding
At sliding pipe supports, the friction between the pipe shoe and the shim surface affects how the pipe moves during thermal expansion. Low-friction slide plates or PTFE pads are sometimes used on top of shims to reduce friction and allow free thermal movement.
Anchor Points and Fixed Supports
At anchor points or fixed pipe supports, the pipe does not move relative to the support during thermal expansion. All thermal growth is directed away from the anchor point toward the nearest expansion device. Shims at anchor points must be tightly secured because they experience the full thermal loading transmitted through the pipe.
Documenting Cold Set and Hot Position Data
For systems with significant thermal expansion, both the cold set shim thickness and the expected hot position should be documented. This data is critical for commissioning teams who need to verify that the pipe moves as predicted during initial startup.
ShimSheet allows field crews to record both the construction shim thickness and notes about expected thermal movement at each support, creating a complete record that bridges the gap between construction and commissioning.
Track Shim Data for Hot & Cold Conditions
Use ShimSheet to document shim dimensions at every pipe support, including cold set and hot operating position data for thermal systems.
Try ShimSheet FreeFrequently Asked Questions
Do shims need to be changed for hot operating conditions?
Shim thickness typically does not change between cold and hot conditions at sliding supports. However, the pipe position on the support shifts due to axial expansion. Spring hanger shims are set to a calculated cold position that accounts for thermal movement.
What temperature requires special shim materials?
Carbon steel shims are suitable up to approximately 425 degrees Celsius. Above this temperature, alloy steel or stainless steel shims should be considered based on the project material specification.
How do I account for thermal expansion when shimming?
Review the pipe stress analysis output for each support to determine the expected thermal movement. Set supports at the cold installation position specified by the stress engineer, and verify movement during startup.