Steel pipe grows when it gets hot. Plenty of designs go wrong because nobody allowed for it. A steam line, hot water main or exhaust duct can move tens or even hundreds of millimetres between cold and operating temperature. If that movement has nowhere to go, it turns into force on supports, flanges, pumps and building structure. This post covers how to estimate the growth, how designers deal with it, and what happens when they don’t.
The formula and a worked example
Linear thermal growth is simple:
ΔL = α × L × ΔT
- ΔL is the change in length
- α is the coefficient of linear thermal expansion for the material (per °C)
- L is the original length between fixed points
- ΔT is the temperature change from the installation (or cold) temperature to the operating temperature
As an example, take a 100 m straight run of carbon steel pipe installed at 20°C and operating at 120°C. Using α of about 12 × 10-6 per °C, the growth is 12 × 10-6 × 100 × 100 = 0.12 m, or about 120 mm. That is more than most people expect from a length of steel that looks completely rigid.
Note that L is the length between anchor points, not necessarily the whole pipe. And ΔT should cover the worst case you need to design for, which may include a cold-climate installation or a steam-out and shutdown cycle, not just normal running.
Typical coefficients
Approximate values for common materials, to be used for early estimates only:
- Carbon steel: roughly 11 to 12 × 10-6 per °C
- Austenitic stainless steel (such as 304 or 316): roughly 16 to 17 × 10-6 per °C
- Copper: roughly 17 × 10-6 per °C
- Aluminium: roughly 23 × 10-6 per °C
- Plastic pipe (PVC, polyethylene and similar): several times higher than steel, and it varies a lot between materials
[CHECK: coefficient ranges are typical textbook values; confirm or replace with the figures Ross normally uses, and confirm the plastics comparison wording]
Stainless grows noticeably more than carbon steel for the same temperature rise, which catches people out when they swap materials on a job without revisiting the supports. The exact coefficient also changes slightly with temperature, so for high-temperature work use values from the relevant piping code or a material data sheet.
If you would rather not do the arithmetic by hand, the free thermal growth calculator does it for you.
Letting the pipe move: loops, bends and joints
There are two broad approaches. You can absorb the movement, or you can restrain it and design for the resulting loads. Most systems use some of each. The common ways to absorb movement are:
- Expansion loops (U-bends): a section of pipe that flexes as the run grows. They are robust and have no moving parts, but they need space and add pressure drop.
- Natural changes of direction: L-bends and Z-bends in the routing give the pipe room to flex. Using the layout this way is often the cheapest option, provided the supports allow the pipe to swing.
- Bellows or slip-type expansion joints: useful where space is tight. They need correct guiding and anchoring, they have a service life, and pressure thrust on them must be dealt with.
- Flexible connections at equipment, which protect pumps, fans and tanks from pipe loads. In ductwork, fabric or flexible expansion joints do the same job.
Ducts follow the same rules as pipes. A long hot exhaust duct needs expansion allowance just as a steam line does, and thin sheet-metal ducts can buckle or tear at joints if it is ignored.
Anchors, guides and supports
Expansion allowance only works if the pipe is controlled. That is the job of anchors and guides:
- Anchors fix the pipe at a point so growth goes in a known direction. Each anchor has to resist the forces from the pipe, which include friction, spring-back from loops and any pressure thrust.
- Guides let the pipe slide along its axis but stop it wandering sideways, which is especially important next to expansion joints.
- Sliding or hanging supports carry the weight while letting the pipe move. A rigid support in the wrong place can act as an accidental anchor.
Anchor loads are often larger than people assume, and the structure behind them has to take those loads. Whether a building frame or slab can accept them is a question for the structural engineer, not something to assume.
What happens when the pipe cannot move
If a pipe is restrained at both ends and heated, it can’t grow, so it develops compressive stress instead. A rough estimate for fully restrained straight pipe is:
σ = E × α × ΔT
where E is the modulus of elasticity (about 200 GPa for steel). With the earlier numbers (100°C rise, carbon steel), that gives roughly 240 MPa. [CHECK: confirm figure and that comparison with typical carbon steel yield strength is fair to state] That is a large fraction of the strength of common carbon steel, and it appears with no change in length at all. It does not depend on how long the pipe is.
In practice the pipe will find somewhere to relieve that load. The result might be:
- buckling or bowing between supports
- overloaded flanges, leaking gaskets or sheared bolts
- damaged pump or valve nozzles, and misaligned equipment
- cracked welds, or failed supports and anchors
- fatigue damage over many heating and cooling cycles, even if no single cycle looks severe
The thermal stress calculator gives a quick figure for the restrained case so you can see whether you have a real problem. It is a screening tool. Real piping has bends, flexibility and combined loads, and high-temperature or high-pressure systems need proper pipe stress analysis under the applicable piping code.
Where to start
Begin with the basics. Work out the temperature range, the length between fixed points and the material, then calculate the free growth. If the number is small compared with the flexibility in your layout, you may be fine. If it isn’t, decide where the anchors go, which direction the pipe will grow, and whether a loop, a bend or an expansion joint suits the space.
Be honest about the limits of a quick calculation. Steam, hazardous fluids, high pressures or anything connected to sensitive equipment should be checked by a suitably qualified piping or stress engineer, and any structural loads from anchors should be checked by someone qualified to certify them. Pressure-equipment work may also have regulatory requirements that apply to your site. [CHECK: confirm how Ross wants to phrase the regulatory/registration caveat for pressure piping in Australia]
At Drafting Central we help with the mechanical side of this: laying out pipe and duct runs, producing the 3D models and workshop drawings that show anchors, guides and expansion provisions clearly, and building safety in design thinking into the layout early. You can see the services we offer, or if you have a system you’d like a second look at, send us the details for a quote.
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