Can Existing Exhaust Pipework Be Reused During an RECD Retrofit?

Short answer: often, in part. Most RECD retrofits keep some of the existing exhaust pipework, replace some of it and modify the rest. Whether a particular section can stay depends on its condition, its size, its supports and how it fits with the new device. A careful inspection before the design is frozen usually saves money and avoids surprises during installation.

The rest of this article explains how to reach that answer for your own system. It sets out a simple green, amber and red grading for each section of pipe, lists the checks that feed into each grade, and describes the compatibility questions that decide whether sound pipe can still work with the new layout.

Why Owners Want to Reuse Pipe

There are good reasons to keep what is already there.

• Cost. Exhaust pipe, bends, flanges, supports and insulation add up, especially on long runs and large engines.
• Disruption. Replacing pipe that passes through walls, fire-rated shafts or occupied areas means more cutting, more hot work and more restoration.
• Time. Fabricating a complete new run takes longer than modifying an existing one.
• Approvals. Some buildings require fresh fire or structural approvals for new penetrations; reusing existing routes avoids them.

Our article on RECD cost in India breaks down where project money goes, and pipework is often one of the variable items.

Why Reuse Can Go Wrong

There are equally good reasons for caution.

• Hidden corrosion. Exhaust pipe corrodes from the inside, where acidic condensate collects, and from the outside under wet insulation. Neither is visible from a casual look.
• Soot and oil deposits. Years of smoky running leave deposits inside the pipe. When a new device is fitted downstream, loosened deposits can travel into it.
• Undersized or oversized pipe. The original pipe was sized for the original layout. Adding a device changes the backpressure budget.
• Tired supports. Hangers and brackets that have carried a pipe for years may be corroded, bent or no longer suitable for the new weight distribution.
• Mismatched flanges. The device’s flanges may not match the existing pipe’s flange pattern, diameter or pressure class.

A retrofit that reuses a bad section of pipe simply moves the failure point to a later date, usually an inconvenient one.

The Grading System

Divide the existing system into sections: each straight length, each bend, each flange pair, each support and the existing silencer. Inspect each and give it a grade.

Grade	Meaning	Typical action
Green	Sound, correctly sized, compatible with the new layout	Reuse as is, perhaps with cleaning and new gaskets
Amber	Usable with work: cleaning, new flanges, extra supports, insulation repair, local patching	Reuse after the specified work
Red	Unsafe, undersized, badly corroded, incompatible or in the wrong place	Replace

Paint or tag each section on site with its grade. Mark it on the drawing too. This gives the vendor, the fabricator and the owner a shared picture of what stays and what goes.

What Feeds Into the Grade

Check 1: External condition

Start with a walk along the whole run.

• Visible rust, flaking or holes, especially at the underside of horizontal runs and at the bottom of bends
• Exhaust staining or soot marks around joints, which show leaks
• Bulges, dents or flattening from impact or overheating
• Cracked welds at bends, branches and flange collars
• Damaged or missing insulation, and especially wet insulation

Any visible hole or crack is an immediate red for that section. Staining at joints usually means amber: new gaskets or a remade joint.

Check 2: Wall thickness

External appearance can mislead. A pipe that looks fine outside may have lost much of its wall inside. Ultrasonic thickness gauging measures the remaining wall from the outside without cutting. The ultrasonic thickness measurement article explains the principle.

Take readings at several points around the circumference and along each section, focusing on:

• The bottom of horizontal runs, where condensate collects
• The outside of bends, where flow erodes the wall
• Areas near drains and low points
• Sections that pass through walls, where inspection is otherwise difficult

Compare readings with the original nominal thickness. The acceptable loss depends on the material, pressure, temperature and the engineer’s judgement. Sections with significant loss are red; sections with modest, even loss may be amber with a shorter inspection interval.

Check 3: Internal condition

Remove a flange or the silencer and look inside. A borescope camera can reach further.

• Light, dry soot is normal and can be cleaned
• Thick, oily or tarry deposits suggest a history of wet stacking or oil burning; these must be removed before the device goes in
• Flaking scale from internal corrosion can break off and travel downstream
• Water or rust-coloured sludge at low points shows poor drainage

Thick deposits upstream of a new device are a real risk. Loose material can be carried into the device on the first loaded run. Our article on why a generator emits blue smoke explains oil-related deposits, and why a generator emits black smoke covers soot.

Check 4: Size

Measure the pipe’s internal diameter section by section. Compare it with the size recommended for the engine’s exhaust flow and the new total backpressure, including the device. Our guide on using OEM exhaust flow data to specify an RECD explains where to find the flow figure.

• Correct or larger than needed: green for size
• Slightly small, but total backpressure still within the engine maker’s limit with the device: amber; acceptable if the calculation confirms it
• Too small for the new total: red; replace with a larger size, at least for part of the run

Our explainer on how exhaust backpressure affects a DG set describes the consequences of getting this wrong.

Check 5: Supports

Inspect every hanger, bracket and guide.

• Corroded or bent supports are red
• Supports in the wrong place for the new layout, for example where the device’s weight will now sit, are amber; they may need moving or adding
• Rigid supports that should allow sliding are amber; they can be modified

The new device adds weight at one point. Existing pipe supports were not designed for it. The device should normally have its own supports, with the existing pipe supported independently on either side. The general role of supports is described in the pipe support article.

Check 6: Flanges and joints

Check each flange pair.

• Flange pattern and size: do they match the device’s inlet and outlet? If not, an adapter or new flange is needed
• Flatness and face condition: warped or pitted faces will not seal; machine, replace or use a suitable gasket
• Bolts: seized, corroded or mismatched bolts should be replaced as a matter of course
• Slip-on or socket joints: older systems sometimes use clamped slip joints that may not suit the new arrangement

Replacing gaskets and bolts at every reused joint is cheap insurance and normally part of an amber grade.

Check 7: Thermal movement provision

Look for the existing flexible bellows and any expansion joints. Old bellows are often near the end of their life, and their behaviour changes once the new device adds weight and changes the support layout. Treat the existing bellows as amber at best and inspect it closely; our companion article on flexible bellows explains what to look for. The general principles of expansion joints apply to the whole run.

Check 8: Position relative to the new device

Even sound pipe can be red if it is in the wrong place. If the device needs to sit where an existing bend is, that bend must go. If a section creates a water trap upstream of the device, it may need re-routing.

The Existing Silencer: A Special Case

The silencer is the most debated item in any pipework reuse discussion.

Situation	Common outcome
Silencer in good condition, backpressure budget allows device plus silencer	Keep it, usually downstream of the device
Silencer internally damaged, rattling or corroded	Replace or remove
Backpressure budget too tight for both	Remove the silencer if the device and remaining system meet noise needs; otherwise upgrade pipe size elsewhere
Silencer in the position the device needs	Relocate or remove

Removing a silencer is only acceptable if the resulting noise level still meets the applicable limits. Our article on whether an RECD can replace a generator’s exhaust silencer explains the trade-offs, and noise reduction strategies for diesel generators covers wider measures.

Compatibility Questions That Override Condition

A section can be in perfect condition and still unsuitable for reuse. Ask:

1. Does it fit the new backpressure budget? If not, its condition is irrelevant.
2. Does it put the device in a position that gives adequate exhaust temperature? A sound but long uninsulated run before the device may need insulation added.
3. Does it give the device enough straight pipe before the inlet? Many vendors want a straight length upstream for even flow; an existing bend right before the inlet may have to go.
4. Does it drain properly with the device in place? Low points that were harmless before may now drain into the device.
5. Is the material suitable? Galvanised pipe is unsuitable for hot exhaust sections because the zinc coating can degrade at exhaust temperatures; stainless or carbon steel is normal.
6. Will the joints be accessible? A reused joint buried inside a wall cavity cannot be inspected after the retrofit.

Cleaning Before Reuse

Sections graded green or amber should be cleaned before the device is connected, particularly those upstream of it.

• Mechanical cleaning with brushes or scrapers removes dry soot and loose scale
• Blowing out with compressed air clears loose material from long runs
• Running the engine at load before connection, with the device not yet installed, can burn off some soft deposits and blow out loose material, but this produces smoke and must be planned
• Wet or oily deposits may need solvent or specialist cleaning

Ask the vendor how they want the upstream pipe prepared. Some will insist on a cleaning step before commissioning to protect the filter.

Cost Comparison: An Illustrative Framework

Every project is different, but the comparison usually looks like this.

Item	Full replacement	Partial reuse
New pipe, bends and flanges	Whole run	Red sections only
Supports	All new	Red sections plus device supports
Insulation	All new	New on replaced sections; repair elsewhere
Cutting and hot work	Throughout	Only at changed sections
Wall and shaft penetrations	New or reworked	Usually reused
Inspection cost	Minimal	Thickness gauging and internal inspection
Cleaning	Not needed	Needed for reused sections
Risk of hidden defects later	Low	Moderate, managed by inspection
Programme time	Longer	Shorter

On a short, simple run, full replacement may cost little more than reuse after inspection. On long runs through occupied buildings, partial reuse often wins clearly, provided the inspection is honest.

A Walkthrough: IT Office Campus in Chennai

This is an illustrative case.

The site

• A 500 kVA set in a ground-floor DG room, about 14 years old
• Exhaust ran 4 metres inside the room, through a fire-rated wall, then 18 metres up an external shaft to the terrace
• Original residential silencer inside the room; uninsulated external pipe; coastal humidity

The inspection

Section	Findings	Grade	Action
Flexible bellows	Visible distortion of convolutions; outer braid frayed	Red	Replace
Room section, 4 m	Sound; light soot; supports good	Green	Clean; reuse
Silencer	Rattling internally; baffles partly detached	Red	Remove; RECD takes its place, noise verified
Wall penetration sleeve	Sound; fire seal intact	Green	Reuse
First external bend	Thickness loss on outer radius	Red	Replace
External riser, lower 6 m	Moderate even loss; no pitting	Amber	Reuse with insulation and annual thickness check
External riser, upper 12 m	Sound	Green	Reuse; add insulation
Riser supports	Two of six corroded	Amber	Replace two; reuse four
Terminal	No rain cap	Amber	Add rain cap
Flanges throughout	Mixed bolt types; old gaskets	Amber	New bolts and gaskets at all reused joints

The outcome

Roughly two thirds of the existing pipe stayed in service. The new device sat in the room where the silencer had been, with a new bellows upstream and new device supports. A condensate drain was added at the foot of the riser. The fire-rated penetration was reused, which avoided new fire approvals. Noise measured after commissioning stayed within the earlier figure, partly because the device and a new insulation layer on the riser reduced radiated sound.

Campuses with multiple sets face these questions repeatedly; our guide on RECDs for university campuses with distributed backup generators offers a planning approach. The 500 kVA DG set RECD page covers this rating.

Sections You Cannot See

Every exhaust system has stretches that no inspector can reach easily: pipe buried in a masonry shaft, a sleeve through a fire wall, a riser boxed in behind cladding. These sections need a deliberate decision rather than a hopeful assumption.

Hidden section	Ways to assess it	Fallback if it cannot be assessed
Pipe inside a closed shaft	Borescope from the nearest flange; thickness readings at the shaft entry and exit points	Treat as amber with a planned opening at the next major shutdown
Wall or slab sleeve	Inspect both faces; check the fire seal; borescope through the bore	Reuse only if both ends are sound and the seal is intact
Clad riser on a facade	Remove a few cladding panels at low points and bends	Grade by the worst exposed reading
Pipe under a floor trench cover	Lift covers and inspect along the length	Replace if covers cannot be lifted safely

The cautious rule is simple: a section is only as good as the evidence for it. Where evidence is thin, record the assumption, shorten the next inspection interval and make sure the maintenance team knows which stretch carries the risk.

A Quick Reuse Decision Flow

For each section of pipe, run through these questions in order and stop at the first “no”.

1. Is it free of holes, cracks and leaking joints?
2. Does the remaining wall thickness meet the engineer’s acceptance figure?
3. Is the inside free of heavy oily deposits and flaking scale, or can it be cleaned?
4. Does the total backpressure, with the device at its loaded condition, stay within the engine maker’s limit with this section in place?
5. Does it sit in a position that suits the device, without creating a water trap or a bend at the device inlet?
6. Are its supports sound and suited to the new layout?

Six “yes” answers give a green. A “no” at question 3 or 6 usually means amber, because cleaning or new supports can fix it. A “no” at questions 1, 2, 4 or 5 usually means red.

Should the Engine Be Checked Too?

Pipe condition often tells a story about the engine. Heavy oily deposits suggest oil consumption; thick dry soot suggests poor combustion; water at low points suggests frequent short, cold runs. An engine that fouls its exhaust will also load its new filter quickly. Our article on whether to complete an engine overhaul before fitting an RECD explains how to decide, and retrofitting an old DG set with an RECD weighs the wider economics.

Practical Tips for the Inspection Day

• Schedule it with the set cold and isolated, so inspectors can touch the pipe and open flanges safely
• Bring a borescope and a thickness gauge, not just a torch
• Photograph every section with its grade tag visible
• Record readings on a drawing, not just a list
• Note what you could not reach, such as pipe buried in a shaft, and decide how to treat it
• Share the results with the vendor before final design and pricing

Our guide to DG nameplate photos for an RECD enquiry explains which photographs help a vendor understand the engine; pipe photographs serve the same purpose for the exhaust.

Installing Without Cutting

On some sites, the device can be fitted between existing flanges with minimal cutting, which is attractive when the pipe is sound and the flanges are suitable. Our article on whether an RECD can be installed without cutting the existing exhaust pipe describes the approaches and their limits.

Compliance and Records

The device’s approval comes under CPCB’s System and Procedure for Emission Compliance Testing of RECDs. Keep the pipe inspection report, thickness readings, grading drawing and photographs with the installation records. If reused sections are graded amber with a shorter inspection interval, add that interval to the maintenance plan. Our environmental compliance checklist for generator owners puts these records in context.

Frequently Asked Questions

Is it normal to reuse exhaust pipe during an RECD retrofit?

Yes. Many retrofits keep sound sections and replace only those that are damaged, undersized or in the wrong place. The key is inspection before design.

How do I know if my exhaust pipe is too corroded?

Visible holes and cracks are obvious signs. Hidden wall loss is found by ultrasonic thickness gauging, especially at the bottom of horizontal runs and the outside of bends.

Does the existing silencer have to go?

Not necessarily. It can stay if the total backpressure allows it and it is in good condition. If it must go, the system must still meet noise limits.

Should I replace the flexible bellows during the retrofit?

It is usually wise. Old bellows are often near the end of their life, and the retrofit changes the loads they see. Inspect it carefully at the very least.

Can soot in old pipe damage the new RECD?

Loose deposits upstream can be carried into the device. Clean reused sections before connection, particularly those just upstream of the inlet.

Can I reuse galvanised pipe?

Galvanised pipe is not recommended for hot exhaust sections. Replace it with suitable carbon or stainless steel.

Reuse What Earns Its Place

Existing exhaust pipework is not automatically an asset or a liability. Each section has to earn its place in the new system by being sound, the right size, well supported, clean and in the right position. Grade it honestly, replace what fails, upgrade what can be saved, and record what you decide. A retrofit built on that basis costs less than a full replacement and lasts far longer than a careless reuse.

Aceget inspects and grades existing exhaust systems as part of RECD site surveys, so quotations reflect what can be reused and what must change. For an exhaust condition survey, contact the Aceget team, or review device ratings on the retrofit emission control device page.