- October 8, 2026
- Posted by: Aceget
- Category: RECD & Emission Control Devices
At 7:58 in the morning the grid drops and the generator starts. At 7:59 the exhaust pipe is still at room temperature. By 8:05 the section nearest the turbocharger is several hundred degrees hotter than it was, and it is longer than it was a few minutes ago. Not by a lot per metre, but enough that a straight run between two rigid points now has somewhere between a few millimetres and a few centimetres of extra steel that must go somewhere.
When the run includes a retrofit emission control device, that extra length meets a heavy, firmly mounted object with flanges at both ends. If nobody planned where the growth would go, it finds its own path: into the turbocharger flange, into a weld, into the device’s inlet cone or into the bracket anchors. None of those were designed to take it.
This guide explains how much an RECD exhaust connection actually moves, where that movement should be allowed to go and how to set out anchors, guides, sliding supports and flexible elements so that the system heats and cools thousands of times without complaint.
Why Steel Gets Longer When It Gets Hot
Metals expand as their temperature rises. The amount depends on three things: the material, the length and the temperature change. Engineers express the material property as a coefficient of linear expansion, usually in millionths of a metre per metre per degree Celsius. The thermal expansion article sets out the physics.
The relationship is simple:
Growth = coefficient × length × temperature rise
For a DG exhaust, the temperature rise is large because the pipe goes from ambient to close to exhaust gas temperature. That is why exhaust systems need more attention to movement than almost any other pipe in a building.
Typical values for exhaust materials
The figures below are approximate, rounded values for planning conversations. Use the material supplier’s data for design.
| Material | Approximate coefficient (µm per m per °C) | Growth per metre for a 400 °C rise | Growth per metre for a 500 °C rise |
|---|---|---|---|
| Carbon steel | about 12 | about 4.8 mm | about 6.0 mm |
| Ferritic stainless steel | about 11 | about 4.4 mm | about 5.5 mm |
| Austenitic stainless steel (304 or 316 type) | about 17 | about 6.8 mm | about 8.5 mm |
Two points stand out. First, austenitic stainless, which is common in bellows, device housings and some pipe, grows roughly 40 percent more than carbon steel for the same temperature rise. Second, the numbers are per metre. A 10 metre straight section of carbon steel at a 450 °C rise grows by roughly 54 mm. That is more than five centimetres of movement every time the set comes up to temperature.
What Changes When an RECD Joins the Run
Before the retrofit, many DG exhausts were a pipe hanging from simple supports, free to bow and slide a little. Nobody calculated movement because the pipe found its own way. Adding an RECD changes that in four ways.
A heavy, fixed object appears in the middle
The device is usually mounted on a frame or bracket with firm saddles. It becomes a point that does not want to move. Pipe on either side now grows towards it or away from it, and the device’s flanges see the push or pull.
Materials become mixed
The pipe may be carbon steel, the device housing stainless, the bellows stainless and the support frame carbon steel. Each grows at a different rate. Where a stainless housing sits on carbon steel saddles, the housing grows more than the frame beneath it.
Temperatures shift along the run
A device near the engine sees hot gas and grows a lot. A device further along sees cooler gas but the pipe upstream of it grows over a longer length. Our guide on how exhaust backpressure affects a DG set also notes that a more restrictive system can run hotter, which adds to growth.
Regeneration can add heat
When the filter burns off collected soot, the device’s internal temperature can rise above the normal exhaust temperature for a period. The housing and outlet flange may grow more during that time than during ordinary running. Ask the vendor for the maximum housing temperature during regeneration, not just the normal operating figure. Our explainer on what an RECD is and how it works describes the filter and catalyst stages that produce that heat.
The Three Building Blocks of Movement Control
Every successful expansion plan uses the same three elements. The skill is in placing them.
Anchors: points that do not move
An anchor holds the pipe or device firmly in all directions. It divides the system into sections, so that growth in one section does not affect another. Typical anchors on an RECD run are the device’s fixed saddle, a fixed support at the foot of a riser and the structural support immediately downstream of the engine bellows.
Guides and sliding supports: points that move in one direction
A guide lets the pipe slide along its length while stopping it from moving sideways or lifting. A sliding saddle under a device does the same: it carries weight while letting the device grow along its axis. Low-friction pads such as PTFE-faced slide plates are often used so the pipe moves smoothly rather than in jerks.
Flexible elements: points that absorb movement
A flexible bellows, an expansion joint or a deliberate change of direction in the pipe absorbs movement between anchors. The expansion joint article describes the family. Our dedicated guide on flexible bellows in RECD exhaust systems explains how the engine-end bellows should be specified and installed.
A simple rule ties them together: between any two anchors there must be exactly enough flexibility to absorb the growth of the pipe between them, and the guides must make the pipe move towards that flexibility rather than anywhere else.
Drawing the Movement Map
Before choosing hardware, draw a movement map. It takes an hour with a printed layout and coloured pens, and it reveals most problems before fabrication.
Step 1: Mark the fixed points
Circle every point that cannot move: the turbocharger flange (which moves with the engine, not the building), the device’s fixed saddle, any anchor on a riser, the wall penetration if it is a rigid sleeve and the stack base.
Step 2: Divide the run into sections
Each section runs from one fixed point to the next. Label them A, B, C and so on.
Step 3: Calculate growth for each section
Use the length, the material and the expected maximum temperature rise for that section. Sections near the engine see the largest rise. Sections far down the run see less, especially on uninsulated outdoor pipe.
Step 4: Draw arrows
From each fixed point, draw an arrow along the pipe showing the direction it will grow. Where two arrows meet head to head, growth collides and something must absorb it. Where an arrow ends at a bend, the bend will swing sideways.
Step 5: Place the flexibility
For each section, decide what absorbs the growth: a bellows, an expansion joint or a bend with enough free length to deflect. Check that the guides direct movement into it.
Step 6: Check the edges
Look at every place the moving pipe passes something fixed: a wall sleeve, a roof penetration, a canopy opening, a cable tray, a beam. Make sure the clearance exceeds the movement, with a margin.
A Movement Budget Table
For each section, record the numbers in a table like this. The example values are illustrative.
| Section | From | To | Length | Material | Max temperature rise | Estimated growth | Absorbed by |
|---|---|---|---|---|---|---|---|
| A | Turbo flange (moves with engine) | Anchor after bellows | 0.6 m | Stainless bellows and spool | 480 °C | about 5 mm plus engine movement | Engine bellows |
| B | Anchor after bellows | Device fixed saddle | 3.2 m | Carbon steel | 460 °C | about 18 mm | Second bellows at device inlet |
| C | Device fixed saddle | Device outlet end (sliding saddle) | 2.1 m | Stainless housing | 480 °C during regeneration | about 17 mm | Sliding saddle |
| D | Device outlet | Riser anchor | 4.5 m | Carbon steel | 420 °C | about 23 mm | Bend at riser foot with guided offset |
| E | Riser anchor | Stack top | 9 m | Carbon steel | 380 °C | about 41 mm, upwards | Free end at top; guides along riser |
A table like this is the single most useful document in an expansion review. It turns a vague worry into a list of millimetres, each with a named solution.
Where the Growth Should Go Around the Device
The device is usually the most sensitive point in the run. These rules keep it out of trouble.
Fix one saddle, slide the other
The device should have one fixed saddle and one sliding saddle. The fixed saddle anchors it; the sliding saddle lets the housing grow. Which end to fix depends on the layout. Fixing the inlet end is common when the upstream pipe is short and well supported, because it keeps the inlet connection stable. Fixing the outlet end can suit layouts where the downstream pipe rises immediately into a stack. Our guide on horizontal vs vertical RECD mounting discusses how orientation changes saddle arrangements.
Do not fix both ends
Two fixed saddles on a stainless housing that grows 15 to 20 mm between them will either bend the saddles, crack welds or push the frame out of shape. This is one of the most frequent and most avoidable expansion errors.
Give the device’s own connections flexibility
If both the upstream and downstream pipes are anchored close to the device, the housing’s growth has nowhere to go except into the flanges. A bellows or expansion joint at one side of the device, usually the inlet, provides the relief.
Keep the sliding saddle sliding
A sliding saddle that rusts solid or fills with debris becomes a fixed saddle. Use low-friction pads, keep the slide surface clean and inspect it at every service. Our RECD maintenance guide should include a line for this.
Watch the sensor lines
Pressure tapping tubes and thermocouple cables run from the device to gauges and panels. As the device grows, they are pulled or pushed. Leave a loop or use flexible sections so they do not snap or loosen.
The Engine End: Movement From Two Directions
At the engine, two kinds of movement meet.
Thermal growth of the exhaust spool and pipe pushes away from the turbocharger.
Engine movement on resilient mounts swings the turbo flange as the engine rocks during start-up, load changes and shutdown. That movement can be larger than many people expect, particularly at start and stop.
The engine-end bellows must absorb both, and the anchor beyond it must stop pipe growth from reaching back into the turbo. Engine makers specify the maximum force and bending load allowed on the exhaust outlet. Exceeding it can crack turbo housings or loosen manifold studs. Our article on whether installing an RECD affects the generator’s OEM warranty explains why respecting those limits matters for warranty as well as reliability. The difference between engine types is covered in our guide to RECDs on turbocharged vs naturally aspirated engines.
Bends as Natural Expansion Absorbers
Not every section needs a bellows. A change of direction can absorb growth if the leg beyond the bend is long enough to flex and is not locked in place.
Picture an L-shaped pipe with an anchor at the far end of each leg. When the long leg grows, it pushes the bend sideways, and the short leg bends a little to accommodate it. If the short leg is long enough and free of rigid supports near the bend, the stress stays acceptable. If the short leg is too short, or clamped tight close to the bend, the stress concentrates and cracks follow.
Practical points:
- Leave the bend free. Do not place a rigid clamp at or close to a bend that is meant to absorb movement.
- Use guides further away to control the direction of movement without locking the bend.
- Check clearances around the bend; it will swing sideways when hot.
- Ask for a stress check on long runs or large diameters. Simple rules of thumb are useful for review, but pipe stress analysis is the proper tool for complex layouts.
Risers: Growth That Goes Up
Vertical runs bring their own expansion behaviour.
A tall riser grows upwards
A 12 metre carbon steel riser at a 400 °C rise can grow by roughly 55 to 60 mm. If it is anchored at the bottom, the top rises. If it is anchored at the top, the bottom pushes down on the horizontal pipe below.
Choose one anchor point
Most designs anchor the riser near its base, carry its weight there, and let it grow upwards through guides. The top must be free to rise: check the roof penetration, the stack cap and any bracing that might restrain it.
Protect the horizontal-to-vertical bend
The bend at the foot of a riser sees the riser’s weight and the horizontal run’s growth. It needs a support close by and enough flexibility in the horizontal section. Our guide to rooftop RECD installation covers stacks and roof structures, and the basement DG room planning guide covers risers climbing out of basements.
Penetrations: Where Moving Pipe Meets Fixed Building
Walls, slabs and roofs do not move. The pipe passing through them does.
| Penetration type | What can go wrong | Good practice |
|---|---|---|
| Rigid sleeve grouted tight to the pipe | Pipe growth cracks the grout or the wall | Use a sleeve larger than the pipe, with packing that allows sliding |
| Fire-rated wall | Fire seal damaged by repeated movement | Use a fire-stop system rated for movement and high temperature |
| Roof penetration | Flashing tears as the stack rises and falls | Use a sliding collar or storm collar that lets the stack move |
| Canopy roof outlet on a packaged set | Pipe rubs on the canopy edge | Leave clearance; fit a heat-resistant collar |
| Wall-mounted bracket near the device | Pipe growth drags the bracket | Use a sliding saddle on the bracket |
The penetration should never become an accidental anchor. If you cannot slide a gloved hand along the gap around a cold pipe, check whether the gap will close when the pipe is hot.
Heat-Up Rate and Cycling
Growth is not only about the final temperature. How often and how quickly the system cycles matters for fatigue.
Frequent starts
A set that starts several times a day goes through a full heat cycle each time. Every cycle bends the bellows, slides the saddles and flexes the bends. Over years, the number of cycles adds up. Our guide on RECD selection for DG sets with frequent short start-stop cycles explains the duty from the device’s side.
Cold starts
On a cold start, the gas side heats quickly while the outer surfaces and supports lag behind. For a short time, temperature differences across the system are large, which can create stresses that do not exist at steady temperature. Our article on whether an RECD can work reliably during cold starts looks at cold-start behaviour.
Mixed Materials: Small Differences, Real Consequences
When a stainless housing sits on carbon steel saddles, or stainless pipe bolts to a carbon steel flange, the two materials grow at different rates. Over a few metres, the difference is a few millimetres.
Practical consequences:
- Bolted flanges between materials can lose gasket stress as they cycle. Use gaskets and bolting suited to exhaust temperatures and check bolt tension after the first few heat cycles.
- Welded joints between dissimilar metals need the right filler and procedure. Ask the fabricator how they qualify them.
- Saddles should allow the housing to slide relative to the frame, with a pad that also separates the metals to reduce galvanic corrosion in damp locations.
Insulation and Expansion
Insulation keeps the pipe hotter, which increases growth slightly on uninsulated sections that would otherwise run cooler. It also hides movement from view. Two rules help:
- Never let insulation lock a guide or saddle. Cladding should stop short of sliding surfaces or be cut to allow movement.
- Leave expansion gaps in cladding at bends and long straights so the outer sheet does not buckle.
Our companion guide on RECD insulation and heat shields covers the site planning for insulation in detail.
A Walkthrough: Auto Component Plant in Bhiwadi
This is an illustrative case.
The site
- A 400 kVA set in an enclosed generator room, supplying an auto component machining line
- The RECD sat on a floor frame 2.8 m from the engine, connected by a short stainless bellows, then 3 m of carbon steel pipe with a 90 degree bend, then the device
- Downstream, 5 m of horizontal pipe ran to a wall sleeve and then a 10 m external riser
- The set ran through two shifts during frequent grid interruptions, with several starts per day
The symptoms
Within six months of the retrofit, the maintenance team reported three things: a ticking noise from the device frame after shutdown, a hairline crack at the weld between the device’s outlet flange and the downstream pipe, and grout cracking around the wall sleeve.
The movement map
The review team drew the map and found:
| Finding | Effect |
|---|---|
| Both device saddles were bolted tight | Housing growth was pushed into the frame and outlet flange |
| Downstream pipe was clamped rigidly at the wall sleeve | The wall acted as an anchor; growth between device and wall had nowhere to go |
| Riser was anchored at the top bracket | Riser growth pushed down onto the horizontal section and the bend |
| No flexibility between device outlet and wall | The outlet weld took the combined load |
| Engine bellows was fine | The engine end had been planned correctly; the problems were all downstream |
The correction
- The device’s outlet saddle was converted to a sliding saddle with a low-friction pad
- A stainless expansion joint was fitted just downstream of the device outlet
- The wall sleeve was opened up and refitted with a larger sleeve and high-temperature packing that lets the pipe slide
- The riser anchor was moved to its base, with guides above, so it grows upwards freely
- A rain collar at the stack top was replaced with a sliding storm collar
- The cracked weld was cut out and remade
The ticking stopped and no further cracking appeared at later inspections. Production-line sites often need retrofit work squeezed into shutdown windows; our guide on RECDs for auto component factories covers that planning. The 400 kVA DG set RECD page shows the rating used here.
Warning Signs That Expansion Was Not Planned
These signs usually point to restrained growth.
| Sign | Likely cause | First check |
|---|---|---|
| Ticking or banging as the system heats or cools | Pipe or housing sticking, then jumping | Sliding saddles and guides |
| Cracked welds at flanges or bends | Repeated stress from restrained growth | Anchor positions either side |
| Bellows visibly compressed or bowed sideways when hot | Too much growth or wrong guide positions | Movement budget for that section |
| Cracked plaster or grout around a sleeve | Pipe anchored by the wall | Sleeve clearance and packing |
| Roof flashing torn around the stack | Riser growth restrained at the roof | Riser anchor position; storm collar |
| Loose flange bolts after a few weeks | Thermal cycling between dissimilar materials | Gasket type; re-torque schedule |
| Frame members bent or base bolts loosened | Device pushing on its own supports | Saddle arrangement |
| Turbo outlet studs loosening | Pipe load reaching the engine | Engine bellows and first anchor |
Several of these appear on our list of common RECD installation mistakes.
Questions to Ask the Vendor or Fabricator
- What maximum exhaust and housing temperatures did you use for expansion, including regeneration?
- Can you show a movement map or table with growth for each section?
- Which device saddle is fixed and which slides? What pad is used?
- Where are the anchors and guides, and what does each one restrain?
- Where are the bellows or expansion joints, and what movement are they rated for?
- How are wall, slab and roof penetrations detailed for movement?
- Has a stress check been done on bends near the device and the engine?
- What should our maintenance team inspect, and how often?
Clear answers are a good test when choosing CPCB approved RECD vendors, and the movement plan should appear as a defined step in the RECD installation process.
Commissioning Checks for Movement
Expansion planning is proved on the first hot run. During commissioning:
- Mark reference points on the pipe, saddles and bellows with paint or tags while cold
- Measure their positions cold, then after the set has run at load long enough to reach steady temperature
- Compare movements with the movement budget; large differences mean something is restrained or the temperature assumption was wrong
- Listen during heat-up and cool-down for ticking or banging
- Check penetrations for contact or rubbing
- Re-torque flange bolts after the first few heat cycles, as the gasket supplier recommends
- Record everything in the commissioning file
Compliance and Records
Device approval is governed by CPCB’s System and Procedure for Emission Compliance Testing of RECDs. Expansion planning is not an approval item, but a cracked flange upstream of the filter lets untreated exhaust escape, which defeats the purpose of the retrofit and can show up during inspection as visible smoke or a smell. Keep the movement table, drawings and commissioning measurements with your environmental compliance records.
Frequently Asked Questions
How much does a DG exhaust pipe expand?
Roughly 5 to 6 mm per metre for carbon steel and 7 to 8.5 mm per metre for austenitic stainless steel at typical exhaust temperature rises of 400 to 500 °C. Use the supplier’s data and your actual temperatures for design.
Does an RECD need its own expansion joint?
Often, yes. If the pipe is anchored close to the device on both sides, a bellows or expansion joint at one side, frequently the inlet, gives the housing room to grow.
Why should only one device saddle be fixed?
Because the housing grows when hot. Fixing both ends stops that growth and forces the load into saddles, frames and flanges.
Is the engine bellows enough for the whole run?
Usually not. It handles engine movement and growth of the short section near the engine. Longer runs need their own anchors and flexible elements.
Can bends absorb expansion instead of bellows?
Yes, if the leg beyond the bend is long enough and not clamped near the bend. A stress check confirms whether the bend is adequate.
What about pipes passing through walls?
Use an oversized sleeve with packing that allows sliding, and a fire-stop rated for movement where required. A tightly grouted sleeve becomes an unplanned anchor.
Plan the Millimetres Before the Metal
Thermal expansion is predictable. The pipe will grow by an amount you can calculate, in a direction you can choose, every single time the generator starts. The difference between a system that lasts and one that cracks is whether someone decided in advance where each millimetre goes. Mark the anchors, divide the run, calculate the growth, place the flexibility, free the device’s sliding end and check every penetration. Then measure it on the first hot run and keep the numbers.
Aceget includes a movement map, saddle arrangement and expansion joint selection with every RECD layout, and checks them during commissioning. To have your exhaust connection reviewed, speak with the Aceget team, or browse the full retrofit emission control device range.