C-P Systems
What Is a Spring Hanger in Piping Engineering?
What Is a Spring Hanger in Piping Engineering?
A spring hanger is a type of pipe support that carries the weight of a pipe while allowing it to move vertically as it expands and contracts with temperature changes. It uses a helical coil spring inside a housing to provide a supporting force that accommodates vertical movement without transferring excessive load to connected equipment or adjacent supports. Spring hangers are specified on high-temperature and cryogenic systems where rigid supports would either lift off the pipe during thermal expansion or impose unacceptable loads on equipment nozzles.
Why Spring Hangers Are Needed
A rigid support works well when the pipe stays in the same position in both cold and hot conditions. Many high-temperature lines, however, move significantly in the vertical direction as the system heats up from ambient to operating temperature. Where a pipe moves upward at a support point, a rigid support becomes inactive and the load transfers entirely to adjacent supports and equipment nozzles. Where a pipe moves downward, a rigid support imposes an increasing reaction force as the pipe pushes down onto it. Either condition can overstress the pipe or damage sensitive rotating equipment. A spring hanger resolves both problems by maintaining a supporting force throughout the full range of vertical movement.
Relationship to Pipe Stress Analysis
Spring hanger selection is driven directly by pipe stress analysis. The stress engineer calculates the vertical thermal displacement at each proposed spring hanger location and the pipe weight load that the support must carry in the operating condition. These two values, the operating load and the thermal travel, are the primary inputs to the spring selection process. The stress analysis model includes the selected spring with its spring rate so that the effect of the varying support load on pipe stresses and equipment nozzle loads can be verified across the full range of operating conditions.
Applications in Piping Engineering
Variable Spring Hangers
A variable spring hanger generates its support force through direct spring compression. As the pipe moves vertically, the spring compresses or extends, and the support force changes in proportion to the displacement. This change is characterised by the load variability, which is the difference between the hot load and the cold load expressed as a percentage of the hot load. MSS SP-58 limits load variability to 25 percent for standard variable spring hangers. This limit ensures that the support force does not deviate so far from the operating value during startup and shutdown that pipe stresses or equipment nozzle loads become unacceptable. Variable springs are suitable for most process plant applications where thermal movement is moderate.
Constant Spring Hangers
A constant spring hanger uses a cam-and-lever mechanism that compensates for spring stiffness as the pipe moves. The cam geometry is designed so that the moment arm increases as the spring compresses, exactly offsetting the increase in spring force. The result is a constant support force throughout the travel range regardless of the pipe’s vertical position. MSS SP-58 permits a tolerance of plus or minus 6 percent in the constant load through the working range. Constant spring hangers are specified where thermal movement is large, where the maximum allowable load variability would be exceeded by a variable spring, or where extremely sensitive equipment nozzles require the support force to remain essentially unchanged between the cold and hot conditions.
Hot Load, Cold Load, and Spring Setting
The hot load is the pipe weight that the spring must support in the operating condition, calculated from the pipe stress analysis for the sustained load case. The cold load is the spring force in the installed, ambient-temperature condition before thermal movement occurs. For a variable spring hanger, the cold load equals the hot load plus or minus the product of the spring rate and the thermal displacement. Fabricators preset the spring to the cold load during manufacture so that when the plant heats up and the pipe moves to its operating position, the spring carries exactly the designed hot load. The cold load is marked on the spring travel indicator as the installation position.
Travel Stop During Hydrostatic Testing
Spring hangers must be locked during hydrostatic pressure testing. The hydrotest fills the pipe with water, significantly increasing the total pipe weight beyond the operating load. If the spring is not locked, this extra weight compresses the spring far beyond its designed travel range, potentially yielding the spring coil or overloading adjacent supports. Fabricators install a travel stop, also called a shipping pin or locking pin, through the spring casing before dispatch. The installer removes this pin after the hydrotest and before the plant is commissioned for thermal operation. Leaving the travel stop in place during operation locks the spring rigid and eliminates all its thermal accommodation function.
Selection Based on Spring Rate and Travel
The spring rate is the force per unit deflection of the spring coil, expressed in newtons per millimetre or pounds per inch. A lower spring rate produces less load variation for the same thermal movement, making it preferable where nozzle loads are sensitive. A higher spring rate provides a stiffer support that limits pipe displacement more effectively at locations where pipe sag or sway must be controlled. Engineers select the spring size from vendor catalogues by entering the hot load and the designed thermal travel. The catalogue identifies spring sizes whose travel range encompasses the required displacement and whose load capacity suits the hot load. Engineers then verify that the resulting load variability falls within the 25 percent limit for variable springs or within the constant tolerance for constant springs.
Benefits of Spring Hangers
Continuous Weight Support Through Thermal Cycles
A spring hanger ensures that the pipe is always supported at the design location throughout the full thermal cycle from cold startup to full operating temperature and back. Rigid supports that lose contact when the pipe moves upward leave the pipe weight unsupported at that location, transferring load unexpectedly to other supports and equipment. Spring hangers eliminate this transfer by maintaining a positive, predictable support force at every stage of the thermal cycle.
Protection of Sensitive Equipment Nozzles
High-temperature piping connected to turbines, centrifugal compressors, and other rotating equipment imposes thermal forces and moments on equipment nozzles as the pipe expands. Spring hangers near equipment connections reduce these nozzle loads by carrying a portion of the pipe weight continuously, including during the thermal transient when expansion forces are highest. Constant spring hangers are particularly effective in this role because they do not impose load changes on the nozzle as the pipe moves between its cold and hot positions.
Accommodation of Large Thermal Movements
On high-temperature steam, hot oil, or cryogenic piping with large thermal movements, rigid supports cannot accommodate the required displacement without imposing prohibitively high restraint forces or losing contact with the pipe. Spring hangers absorb the vertical component of thermal movement elastically, maintaining support without restraining the movement. Constant spring hangers extend this capability to movements of several hundred millimetres where variable springs would exceed their travel range or variability limit.
Limitations to Consider
Load Variability in Variable Springs
Variable spring hangers cannot maintain a constant support force. The support force changes with every degree of temperature change as the pipe moves between its cold and hot positions. At startup and shutdown, the support force may deviate significantly from the operating value, temporarily increasing loads on adjacent supports and equipment nozzles. Where this transient load variation is unacceptable, constant spring hangers must be specified instead, at significantly higher cost and with more complex installation requirements.
Sensitivity to Incorrect Installation
A spring hanger that is set to the wrong cold load, or whose travel stop is left in place after commissioning, fails to perform its design function. An incorrectly set spring may over-support or under-support the pipe in the operating condition, producing unanticipated stress distributions and equipment nozzle loads that differ from the pipe stress analysis predictions. Consequently, spring hanger installation requires careful verification of the cold load setting and removal of travel stops, documented in the pre-commissioning checklist for the piping system.
Maintenance and Inspection Requirements
Spring hangers require periodic visual inspection to confirm that they are operating within their designed travel range, that the load indicator is positioned correctly, and that no corrosion or mechanical damage has affected the spring coil or casing. A spring that has bottomed out, topped out, or corroded solid no longer provides flexible support and must be replaced. On operating plants with large numbers of spring hangers, a systematic inspection programme is necessary to identify failing units before they allow pipe stress or nozzle load exceedances that could damage equipment or the piping system itself.
Space Requirements
Spring hangers are physically larger than equivalent rigid hangers and require more vertical clearance above and below the pipe to accommodate their casings and the pipe’s thermal travel range. Constant spring hangers are particularly bulky because of their cam-and-lever mechanism. On congested pipe racks or at locations with limited headroom, the space required for a spring hanger may exceed what is available. Engineers must verify spatial clearance for each spring hanger location during piping layout and support design, as discovering insufficient clearance during construction forces costly design changes or compromises in support arrangement.
Spring Hanger FAQ
What is a spring hanger in piping engineering? A spring hanger is a pipe support device that carries the pipe weight at a given location while allowing the pipe to move vertically due to thermal expansion or contraction. It uses a helical coil spring enclosed in a steel casing to generate the support force. As the pipe moves, the spring extends or compresses, maintaining support throughout the movement range. There are two main types: variable spring hangers, where the support force changes with pipe displacement, and constant spring hangers, where the support force remains steady throughout the travel range.
When should a constant spring hanger be used instead of a variable spring hanger? Constant spring hangers are specified when the thermal movement at a support location is so large that a variable spring hanger would exceed its travel range or its 25 percent load variability limit. They are also used when the piping connects to extremely sensitive equipment nozzles that cannot tolerate any variation in support force between the cold and hot conditions. Constant springs are more expensive than variable springs and physically larger, so engineers use them only where variable springs cannot satisfy the load variability or travel requirements of the pipe stress analysis.
What is the purpose of the travel stop on a spring hanger? The travel stop, also called a shipping pin or locking pin, locks the spring hanger rigid during hydrostatic pressure testing. Hydrostatic testing fills the pipe with water, which is much heavier than the operating fluid in most process services. If the spring were free during hydrotest, the extra water weight would compress it far beyond its designed travel range and potentially yield the spring coil. After the hydrotest is complete, the installer removes the travel stop before commissioning to restore the spring’s thermal accommodation function. Leaving a travel stop in place during normal operation eliminates the spring function entirely and turns the hanger into an unintended rigid support.
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