What Is a Constant Spring Support?
A constant spring support (also called a constant load hanger or constant effort support) is a mechanical device that provides a nearly uniform supporting force to a pipe throughout its full range of vertical thermal movement. Unlike a variable spring hanger where the load changes with displacement, a constant spring maintains within +/-6% of the set load regardless of pipe position.
When Constant Springs Are Required
Constant spring supports are specified in the pipe stress analysis whenever:
- The vertical thermal movement at a support point exceeds the capacity of a variable spring hanger
- The load variation of a variable spring would exceed 25% (the MSS SP-58 limit)
- The support is located near critical equipment nozzles (turbines, compressors, reactors) where load consistency is required
- The pipe operates at high temperature (above 350 C / 660 F) and undergoes large vertical displacement
These conditions are common in power plant steam piping, refinery heater outlet lines, and high-temperature process piping in petrochemical plants.
Operating Specifications
| Parameter | Typical Range | Notes |
|---|---|---|
| Load range | 0.5 kN to 2,000 kN (110 lbf to 450,000 lbf) | Per manufacturer catalog series |
| Travel range | 25 mm to 400 mm (1 in to 16 in) | Longer travel than variable springs |
| Load accuracy | +/-6% of set load over full travel | Per MSS SP-58 requirement |
| Operating temperature | Up to 650 C (1,200 F) | With high-temperature spring materials |
| Design standard | MSS SP-58 / MSS SP-69 | Materials, design, selection |
| Typical mechanism | Helical spring + cam or lever arm | Cam compensates for spring rate variation |
Working Principle
A constant spring uses a helical coil spring combined with a geometrically shaped cam (or bell crank lever) to offset the spring’s natural load variation. As the pipe moves and the spring deflects, the cam changes its mechanical advantage to compensate. The result is a nearly constant output force across the entire travel range.
The load accuracy of +/-6% means that a constant spring set for 50 kN will deliver between 47 kN and 53 kN at any point during its travel. This is a significant improvement over variable springs, which can vary by up to 25%.
Constant Spring vs. Variable Spring
| Feature | Variable Spring Hanger | Constant Spring Support |
|---|---|---|
| Load variation | Up to 25% | +/-6% |
| Travel range | 5-250 mm | 25-400 mm |
| Mechanism | Simple coil spring | Coil spring + cam/lever |
| Cost | 1x (baseline) | 3x to 8x (depending on size) |
| Size/weight | Compact | Larger and heavier |
| Maintenance | Minimal | Periodic inspection of cam and linkage |
| When to use | Load variation under 25%, moderate travel | Load variation over 25%, large travel, critical nozzles |
Selection and Installation
The selection process for constant springs follows the same general procedure as variable spring hangers:
- Pipe stress analysis determines the hot load, cold load, and vertical travel at each support point
- The engineer selects a constant spring where the required load falls within the catalog range for the needed travel
- The spring is factory-set to the specified operating load and locked with a travel stop pin
- During installation, the spring is attached to overhead structural steel (on pipe racks or pipe bridges) or base-mounted on support steel
- After the piping system reaches operating temperature, the travel stop pin is removed and the travel indicator is checked against the predicted displacement
Constant springs are high-value items with lead times of 8-16 weeks. They are typically procured as part of the pipe support package in an EPC project and require mill test certificates for the spring material and load testing documentation.
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