- October 16, 2026
- Posted by: editor
- Category: RECD & Emission Control Devices
Most Indian sites did not buy their generators in one go. A factory may have a 500 kVA set bought last year, two 250 kVA sets from an expansion, an imported unit that arrived with a machine line, and a small 62.5 kVA set that nobody remembers buying. When the time comes to fit Retrofit Emission Control Devices (RECDs), that history becomes a planning problem. Each engine is different, the paperwork is uneven, and some sets are in better shape than others.
The temptation is to send one enquiry that says “RECDs for 9 DG sets” and let the vendor sort it out. That usually leads to rounds of questions, revised offers and, at worst, a device that does not suit the engine it is bolted to. A better approach is to start with an inventory, group the sets into families that genuinely share characteristics, and only then decide what can be specified together.
This guide sets out that inventory-first method in five stages: survey, sort, share, set aside and sequence. It ends with an illustrative nine-set fleet split into four families, with a phasing plan.
Why a Mixed Fleet Needs a Different Planning Approach
An RECD is not sized to a kVA number. It is sized to the engine behind the alternator: its displacement, exhaust gas flow, exhaust temperature profile and the way the set is actually run. Two 500 kVA sets from different makers can have different engines, flows and back-pressure limits, as our post on why same-kVA DG sets can need different RECDs explains.
Age adds another layer. An older mechanically governed engine with worn injectors produces more soot than it did when new. A particulate filter on that engine will load faster, regenerate less easily and raise back-pressure sooner. An engine already near its back-pressure limit because of a long, bent exhaust run has little margin left.
The CPCB requirement is at least 70% reduction in particulate matter (PM). That outcome depends on matching the device to the engine and duty. In a mixed fleet you cannot assume one specification will achieve it on every set, but you can build a common planning framework and apply it set by set.
Stage 1: Survey Every Set Before You Speak to Vendors
The survey is the foundation. Do it physically, set by set, with a camera, a torch and a printed form. Do not rely on the asset register alone, because registers drift after engine swaps, relocations and rebuilds.
The fields to capture
| Field | Why it matters for RECD fitment |
|---|---|
| DG set make and model | Identifies the packager and their exhaust data |
| Engine make, model and serial number | The engine, not the set, drives RECD sizing |
| Rating (kVA and kW) | Confirms duty class and approximate exhaust flow |
| Engine displacement (litres) | A primary sizing input for the device |
| CPCB stage label | Helps confirm age, engine family and consent history |
| Year of manufacture | Indicates likely condition and data availability |
| Mechanical or electronic governing | Affects soot output, load response and data available |
| Turbocharged or naturally aspirated | Changes exhaust flow, temperature and back-pressure sensitivity |
| Exhaust layout | Outlets, pipe route, silencer type, space for the device |
| Condition | Decides whether the engine is ready for a filter |
| Run hours | Shows wear and helps estimate future duty |
| Typical load and duty | Low or cyclic loads need careful regeneration planning |
| Documents available | Missing data adds time and may need site measurement |
Photograph the engine nameplate, set nameplate, CPCB label, exhaust outlet, silencer and the full route to the stack. Our guide to DG nameplate photos for an RECD enquiry shows what a usable photo looks like.
Run a quick condition check
This is not a full engine assessment, but a few simple checks flag sets that need attention:
- Smoke on start and under load. Persistent black smoke points to fuel or air intake problems. Blue smoke points to oil burning.
- Oil top-up frequency. High consumption means the filter will collect oil ash, which does not burn off.
- Crankcase blow-by. Heavy breather fumes suggest worn rings or liners.
- Exhaust leaks. Soot marks on joints and flanges will also distort later back-pressure readings.
- Existing back-pressure. If the maintainer can take a reading at a known load, record it. It shows how much margin remains before the device is added.
Record what you see, not what you hope. A set marked “good” by habit but smoking under load will cause trouble after fitment.
Collect the duty picture
Is the set a standby unit that runs 30 minutes a week on test and a few hours during grid outages? A prime set running most of the day? An oversized set idling at 20% load? The answers shape regeneration strategy and device choice. Our post on using DG load logs to assess RECD suitability shows how to turn controller data into a duty profile.
Stage 2: Sort the Sets into Families
Lay all the survey rows side by side and look for clusters. A family is a group of sets whose main engineering inputs are close enough that planning, specification wording and much of the evaluation can be shared.
Sorting keys, most important first
- Engine make and model. Sets with the same engine model are the strongest family candidates, even if the packagers differ.
- Displacement and rating band. Similar engines in the same broad size band often share device size ranges.
- Governing and aspiration. A mechanical, naturally aspirated engine behaves very differently from an electronic, turbocharged one. See our post on mechanically governed vs electronic engines.
- Condition and age. Engines needing work before fitment belong together for planning, even if their specifications differ.
- Duty profile. Short-run standby sets and long-run prime sets may need different regeneration approaches.
- Documentation status. Sets with complete OEM data move faster than those needing data recovery.
What a family is not
A family is a planning group, not a single part number. Two sets in the same family may still need different device sizes, inlet connections or mounting. The family tells you what can be done together: survey review, specification framework, vendor evaluation, test approach and often the installation crew.
Keep the number small. Four or five families is usually enough for ten or so sets. One family per set means your keys are too fine; one family for everything means they are too coarse.
Stage 3: Decide What Can Be Shared and What Cannot
The rule of thumb is simple: share the framework, never the sizing.
| Usually shared across a family | Always kept set-specific |
|---|---|
| Performance requirement wording and acceptance method | Device size and filter volume |
| Certification evidence per device range | Back-pressure budget against the OEM limit |
| Monitoring signals, alarm list and log format | Inlet and outlet connections, number of outlets |
| Documentation package template | Mounting and support arrangement |
| Warranty and service response terms | Regeneration strategy for the actual duty |
| Operator training, with set-specific notes | Cable routes and controller power supply |
On certification, ask for CPCB type-approval documents once per device range, then confirm each proposed device falls within the approved range for that engine. Our guide on how to verify a CPCB-tested RECD covers what to check. On sizing, our post on engine displacement and RECD selection explains why each engine needs its own check.
Where the line blurs
Sets with the same engine, duty and exhaust layout can sometimes share a device model and size. Treat that as a welcome result, not an assumption. If one “identical” set has a longer pipe run, an extra bend or a different silencer, it may need a different configuration. Ask the vendor for a back-pressure calculation for each set, not just for the family.
Stage 4: Set Aside the Special Cases
Every mixed fleet has sets that do not fit the normal flow. Give them their own track early so they do not hold up the rest.
Engines that need an overhaul first
A filter cannot fix an engine that burns oil or over-fuels. It will collect the extra soot and ash, load quickly and raise back-pressure. If the condition check shows heavy smoke, high oil use or blow-by, the engine should usually be repaired first. Our post on whether to complete an engine overhaul before fitting an RECD explains the decision. In the plan, these sets move to a later phase with the overhaul as a predecessor task.
Imported engines
Imported sets often come with foreign-language documents, data sheets based on different test conditions, or engines uncommon in India. An RECD can usually still be specified, but the vendor will need more information and may ask for site measurements. Our guide on retrofitting an RECD to an imported diesel generator lists the data to gather.
Also confirm gross mechanical power. The CPCB RECD certification framework covers engines up to 800 kW gross mechanical power. For larger engines, state boards may require a tested device with at least 70% reduction, so check your consent conditions with your State Pollution Control Board (SPCB) and the CPCB website for current guidance.
Sets with missing documents
Missing paperwork is common on older and second-hand sets. It slows the project rather than stopping it. Recover what you can from the engine maker or dealer using the serial number, then fill gaps with site measurements of exhaust temperature and back-pressure at known loads. Our post on specifying an RECD when original DG documents are missing sets out the steps.
Very old sets where replacement may be better
If a set is more than 15–20 years old, has had repeated major repairs, uses an unsupported model and has no reliable documents, ask whether replacement makes more sense. The comparison is not just device cost against new-set cost. It includes the overhaul needed, fuel efficiency, spare parts availability and remaining life. Our RECD vs new CPCB IV+ genset cost comparison walks through the trade-offs. Record the decision and who approved it.
Stage 5: Sequence the Work in Phases
Start where data is complete and risk is low, learn from the first installations, then apply those lessons to harder sets.
- Phase 1: the cleanest family. Complete data, sound engines, simple exhaust layouts. This tests your specification, vendor process and acceptance method.
- Phase 2: the next most similar sets. Refine the specification and installation method using Phase 1 lessons.
- Phase 3: data recovery or special engineering. Imported engines, multiple outlets and difficult layouts, once data is ready.
- Phase 4: sets needing an overhaul first. After the engine work is complete and the engine has been run in.
- Decision track: replacement candidates. Run in parallel. Retrofit decisions join Phase 4; replacements leave the RECD plan.
Never take more sets out of service than the site can afford. Use redundancy where you have it, and plan a rental set where you do not. Our guide on preserving backup power during an RECD retrofit covers this.
In Delhi NCR, GRAP stages can restrict which sets may run during high-pollution periods, so a critical set likely to be needed in winter may deserve an earlier phase. Confirm the current position with your SPCB.
Worked Example: A Nine-Set Mixed Fleet in Four Families (Illustrative)
This example is illustrative. The makes are anonymised and the figures are chosen to show the method, not to describe a real site.
A manufacturing campus in Haryana has nine DG sets bought over about 15 years. The facility team completes the survey in three days.
Survey summary
| Set | Make | kVA | Year | Governing / aspiration | Run hours | Exhaust | Condition | Documents |
|---|---|---|---|---|---|---|---|---|
| DG-01 | A | 500 | 2019 | Electronic, turbo | 2,400 | Single | Good | Complete |
| DG-02 | A | 500 | 2020 | Electronic, turbo | 1,900 | Single | Good | Complete |
| DG-03 | B | 500 | 2018 | Electronic, turbo | 3,100 | Twin | Good | Complete |
| DG-04 | C | 250 | 2016 | Mechanical, turbo | 5,200 | Single | Fair | Partial |
| DG-05 | C | 250 | 2017 | Mechanical, turbo | 4,800 | Single | Fair | Partial |
| DG-06 | D | 160 | 2012 | Mechanical, turbo | 9,800 | Single | Fair, some smoke | Partial |
| DG-07 | D | 125 | 2011 | Mechanical, NA | 11,500 | Single | Poor, blue smoke | Partial |
| DG-08 | E (imported) | 1,010 | 2015 | Electronic, turbo | 6,000 | Twin | Good | Partial, foreign language |
| DG-09 | F | 62.5 | 2004 | Mechanical, NA | Unknown | Single | Poor | None |
NA means naturally aspirated. Total installed capacity is 3 × 500 + 2 × 250 + 160 + 125 + 1,010 + 62.5 = 3,357.5 kVA.
Family grouping
| Family | Sets | Combined kVA | What is shared | What stays set-specific |
|---|---|---|---|---|
| F1: Electronic turbo, 500 kVA | DG-01, DG-02, DG-03 | 1,500 | Specification wording, certification check, alarms, training | DG-03 twin-outlet layout and back-pressure calculation |
| F2: Mechanical turbo, 250 kVA | DG-04, DG-05 | 500 | Most of the specification, probably the device size | Mounting, inlet connection, back-pressure check |
| F3: Older small mechanical | DG-06, DG-07, DG-09 | 347.5 | Condition-first approach, overhaul and replacement review | All technical choices, made after condition work |
| F4: Large imported | DG-08 | 1,010 | Monitoring and documentation standards | Data recovery, sizing, consent check above 800 kW |
The families hold 3 + 2 + 3 + 1 = 9 sets, and 1,500 + 500 + 347.5 + 1,010 = 3,357.5 kVA, matching the survey total.
Why each set landed where it did
F1. DG-01 and DG-02 share an engine model, making them the strongest pair. DG-03 is a different make with a similar engine type, so it shares the planning approach but not necessarily the device. Its twin outlets need a specific design, as covered in our post on RECD fitment on DG sets with multiple exhaust outlets.
F2. DG-04 and DG-05 are the same model, a year apart and 400 run hours apart (5,200 − 4,800). They will probably share a device size, but the partial documents mean the vendor needs displacement and back-pressure limits confirmed from the engine serial numbers.
F3. These sets share a problem rather than a specification. DG-06 needs a fuel system check. DG-07 burns oil and needs an overhaul before fitment. DG-09 is 22 years old (2026 − 2004), has no documents and an unknown run-hour history, so it goes on the replacement decision track.
F4. DG-08 is rated 1,010 kVA. At 0.8 power factor, that is 1,010 × 0.8 = 808 kWe. Gross engine power is higher than electrical output because of alternator and cooling fan losses, so the engine is very likely above 800 kW gross. That puts it outside the CPCB RECD certification framework, and the team must confirm with the state board what evidence is required. It also needs translated documents and probably site measurements.
Phasing plan
| Phase | Sets | Predecessor tasks | Notes |
|---|---|---|---|
| 1 | DG-01, DG-02 | None | Cleanest data, same engine, tests the process |
| 2 | DG-03, DG-04, DG-05 | Phase 1 lessons; engine data for DG-04 and DG-05 | Twin-outlet design review for DG-03 |
| 3 | DG-08 | Translated data, site measurements, SPCB confirmation | Longest lead time, so start data work in Phase 1 |
| 4 | DG-06, DG-07 | Fuel repair (DG-06), overhaul and run-in (DG-07) | Re-check smoke and back-pressure before fitment |
| Decision | DG-09 | Overhaul plus RECD cost vs replacement | Leaves the plan if replaced |
After Phases 1 and 2, five of nine sets are fitted: 1,500 + 500 = 2,000 kVA, or about 59.6% of installed capacity (2,000 ÷ 3,357.5 = 0.5957). Adding DG-08 in Phase 3 brings coverage to 3,010 kVA, about 89.7% (3,010 ÷ 3,357.5 = 0.8965). The remaining 347.5 kVA sits with the three older sets, where condition and replacement decisions come first.
Backup power is protected throughout. During Phase 1, DG-03 and DG-08 cover the campus while DG-01 and DG-02 are fitted one after the other.
Before You Send the Enquiry
Confirm these items for each family:
- Survey form and photographs complete for every set
- Engine serial number and displacement confirmed
- Exhaust layout sketched, with pipe sizes, bends and silencer type
- Baseline back-pressure at a known load, where available
- Condition result and repair needs noted
- Document gaps flagged, and consent status recorded
- Family and phase assigned
Send each family as a clear package so offers can be compared fairly. Our guide on how to write a technical RFQ for an RECD project explains the structure. Keep the survey in a shared location and update it after every overhaul, relocation or re-rating, because it becomes the baseline for acceptance testing and maintenance. For background on why engine condition and exhaust temperature matter so much to filter behaviour, technical references such as DieselNet are useful for non-specialist team members.
Frequently Asked Questions
Can one RECD model be used across every set in a mixed fleet?
Rarely across the whole fleet. Sets with the same engine, similar duty and similar exhaust layout may share a device model and size, but different engines, ratings or governing types usually need different sizes or configurations. Ask the vendor to confirm the selection set by set.
How long does a fleet survey take?
For eight to ten sets, a prepared team can usually finish the physical survey in two to four days. Recovering missing documents from engine makers or dealers can take several weeks, so start early and do not let it delay sets with complete data.
Must an old engine always be overhauled before an RECD is fitted?
No. An older engine with normal smoke and oil consumption may be ready. One that smokes heavily, burns oil or shows significant blow-by should usually be repaired first. Make the decision with your DG maintainer and the engine maker’s guidance.
What if a set is above 800 kW gross mechanical power?
The CPCB RECD certification framework covers engines up to 800 kW gross. For larger engines, state boards may require a tested device with at least 70% PM reduction. Check your consent conditions with your SPCB before finalising the specification.
Can one installation team handle every family?
Yes, and it often helps, because lessons carry from one family to the next. Give the team set-specific drawings and back-pressure checks for each installation rather than one generic method.
Related Reading
- How to Standardize RECD Specifications Across Multiple Factory Sites
- Scheduling RECD Work Around a Factory’s Annual Shutdown
- How to Rank DG Sets for a Phased RECD Rollout
- Retrofitting an Old DG Set with an RECD: Is It Worth It?
Plan Your Mixed-Fleet RECD Project with Aceget
A mixed fleet does not have to mean a messy project. Survey carefully, group sets into sensible families, put special cases on their own track and phase the work so early lessons improve later installations. Aceget supplies CPCB type-approved Retrofit Emission Control Devices for DG sets from 25 kVA to 2500 kVA. Review our certifications, and when your survey is ready, contact our team to discuss each set in your fleet.