Pipes change length with temperature
Almost every solid material gets longer as it warms and shorter as it cools. In a piping system this is not a curiosity but a design input. A line that is installed cold and then carries hot fluid will lengthen. A line that carries cold fluid outdoors will shrink in winter and grow in summer. Because pipes are long, even a small change per meter adds up.
The size of the change depends on three things: the length of the run, the temperature change, and a property of the material called its coefficient of thermal expansion. Different materials differ widely. Plastics generally expand much more than steel for the same temperature change, and stainless steels often expand somewhat more than carbon steel.
A hypothetical example
The numbers here are hypothetical and rounded, to show the idea. Carbon steel expands by roughly 12 millionths of its length for each degree Celsius. Take a straight, unrestrained run of 100 meters that warms by 100 degrees Celsius.
- Change in length is about 12 x 0.000001 x 100 m x 100 = 0.12 m.
- That is roughly 120 millimeters, or a little under 5 inches.
Five inches of growth is easy to overlook until a line is trying to push that far against anchors, equipment or a building structure. Real design uses material data and codes, so treat this only as an illustration of scale.
What happens when movement is blocked
If a pipe cannot lengthen freely, the expansion turns into force. Rigid anchors at both ends of a heated line can push very large loads into the pipe, the supports and whatever equipment is attached. The results can include bowed pipe, leaking flanged joints, overloaded supports, damaged pump or turbine connections and, in the worst cases, ruptured components. Cold lines can pull in the same way as they shrink.
For this reason designers work out how a line will move, decide where it should be fixed and where it can slide, and then provide a way for the movement to be absorbed.
Letting the layout do the work
The simplest approach uses the geometry of the pipe itself. Changes of direction make a line flexible, because a bend or elbow can flex a little and give the straight sections room to move. Common forms include:
- Natural flexibility from the normal turns in a layout, where the route is designed so that existing bends absorb the movement.
- Expansion loops, which are U-shaped or offset sections added deliberately. They take extra space but have no moving parts and need little maintenance.
- Offsets and Z-bends, where a short jog in the route provides the flexibility.
Where space and budget allow, these options are often preferred precisely because they are simple and robust.
When an expansion joint is used
Sometimes there is no room for a loop, or the movement is too great for the layout, or the line is short and connected to sensitive equipment. In that case a purpose-built expansion joint can be installed. Several types exist:
- Metal bellows joints use a corrugated flexible metal section that compresses and extends. They are common in steam and process lines.
- Rubber or fabric joints are flexible sleeves used on many water, air and low-pressure lines, and they also help isolate vibration and noise.
- Slip-type joints allow one pipe section to slide inside another, with packing or seals to retain the fluid. They need guiding and regular attention.
- Flexible connectors at pumps and other equipment help accommodate small movement and misalignment, although they do not substitute for good support.
Each type has limits on how much it can move, in what direction, and at what pressure and temperature. A joint chosen for axial movement may not tolerate sideways movement, and the manufacturer’s data should guide the choice.
Anchors and guides make joints work
An expansion joint is only one part of the answer. It has to be paired with anchors and guides. An anchor fixes a point on the line so that movement goes where it is intended. A guide lets the pipe slide in one direction while keeping it from wandering sideways. Without them, pressure in the line can push a bellows joint to extend or buckle, and a poorly guided joint may twist or fail early.
The way supports allow or restrict movement is discussed further under pipe supports, and a poorly chosen rigid support is a classic cause of damage on hot lines. The same awareness applies where flanged joints sit close to anchored equipment.
Installation and inspection points
Expansion joints are often shipped with temporary restraints that keep them at the right length during installation. These are usually meant to be removed before service, and leaving them in place defeats the joint. Pre-setting a joint to allow for the temperature at which it was installed is another detail that is easy to miss.
In service, joints are typically inspected for cracks, distortion, corrosion, leaks, worn or torn flexible material, and signs that the pipe is not moving as designed. Heat trace and insulation should be arranged so they do not hamper movement, and covering a joint with insulation requires attention, which ties into pipe insulation practice. Anything involving a hot or pressurized joint calls for depressurizing, cooling and lockout tagout steps before work begins, performed by trained people.
Thermal movement beyond heat-up
Movement also arises at start-up, shutdown and during upsets. A line that heats quickly can see temperature differences along its length, and sudden changes in flow temperature can add stress. Steam lines and the related steam traps need good drainage so cold condensate does not cause shock and uneven movement.
This article is general information. Piping flexibility analysis and joint selection are specialist tasks, and they should follow applicable codes and be performed by qualified engineers.