- October 26, 2026
- Posted by: editor
- Category: RECD & Emission Control Devices
The plant closed in March for a line relocation. The 320 kVA DG set, fitted with a retrofit emission control device the year before, was switched off, the battery isolator turned, and the room locked. Now it is October, production restarts in three weeks, and somebody has asked the obvious question: can we just press start?
You can, but you probably should not. Restarting an RECD-equipped DG set after months of inactivity is a small recommissioning job, not a routine start. Seven months is long enough for water to settle in fuel, for condensation to collect in a cold exhaust line, for rodents to find warm cable trays, and for the soot layer inside the RECD to absorb moisture. None of these are dramatic on their own. Together, they are how a perfectly good engine and emission control device get damaged in the first ten minutes.
Below is a chronological recovery protocol. Each phase ends with a go/no-go gate so the team knows when to proceed and when to stop. Wherever your engine OEM’s recommissioning instructions or your RECD supplier’s manual are more specific, follow them.
How long is “long”? Idle bands and depth of checks
A set that missed three weekly test runs is different from one that sat through a monsoon with the canopy door open. Use this generic table as a starting point, and defer to the engine OEM’s recommissioning guidance where its thresholds differ.
| Idle period | What typically changes | Recommended depth of checks |
|---|---|---|
| Under 1 month | Little. Battery may have dropped slightly; some condensation possible in humid weather | Normal pre-start checks, battery voltage, quick look at RECD drain points, standard loaded test run |
| 1–3 months | Battery self-discharge, early water in fuel tank bottoms, dust on air intake, oil drained back from upper engine | Phases 0–3 in brief, drain fuel water, check battery under load, confirm RECD sensors read plausibly at rest, loaded warm-up |
| 3–6 months | Fuel water and possible microbial growth, dried belts, rodent activity, moisture in exhaust and RECD housing, corroded terminals | Full Phases 0–5. Sample fuel, inspect exhaust and RECD for water, check insulation, record a fresh baseline |
| 6–12 months | Stale diesel, gasket and hose hardening, battery possibly beyond recovery, wet insulation, sensor connector corrosion | Full protocol plus engine OEM recommissioning procedure, likely fuel polishing or replacement, oil and filter change, supplier inspection of RECD advisable |
| Over 12 months | Everything above, plus coolant degradation, seized components, possible consent or registration lapses | Treat as a recommissioning project. Engage the engine service provider and the RECD supplier before any crank attempt; check compliance paperwork first |
Scale depth with conditions as well as time: four months in a dry room may need less than two months in a flooded basement.
Phase 0: Paperwork and safety before restarting an RECD-equipped DG set
Find out why it was stopped
Was it a planned lay-up, a fault, a fuel issue, or sealing by an authority? If your site keeps a structured RECD shift-handover log, the final entries should show the last load, the last differential pressure (ΔP) reading and any open alarms. An unresolved fault is still there; months of standing will not have fixed it.
Check that compliance paperwork is still valid
Confirm that your State Pollution Control Board consent still covers the set, that registration is current and that RECD documentation is on file. In Delhi NCR, check the current position under CAQM directions, because permitted DG use changes with GRAP stages. Our explainer on GRAP rules for DG sets in Delhi NCR gives background, and the Commission for Air Quality Management publishes the directions. If unsure, confirm with your SPCB.
Lockout/tagout and permits
Isolate the battery and auxiliary supplies, including any separately fed RECD controller or heater supply. Apply locks and tags and record who holds keys; the OSHA guidance on controlling hazardous energy sets out the principles, though your site permit system takes precedence. Cover work at height if the RECD or stack is elevated.
Go / no-go gate: Proceed only when (1) the reason for shutdown is known and any recorded fault has been addressed, (2) consent, registration and RECD documents are confirmed valid for operation, (3) the set is isolated under LOTO with a permit raised, and (4) the last known baseline readings have been located. If any of these is missing, stop and resolve it first.
Phase 1: Static engine checks for a DG set restart after long shutdown
Fuel: water, microbial growth and stale diesel
Tanks breathe moist air as temperatures swing, and water settles at the bottom.
- Drain the tank sump into a clear container until clean fuel appears.
- A dark, slimy layer at the fuel-water interface signals microbial growth, which can block filters within minutes.
- Hazy, dark or foul-smelling diesel should be tested or polished before use.
- Replace fuel filters and water separators and prime the system as the OEM describes.
Poor fuel also hurts the RECD: rough combustion means more soot for the device to capture.
Engine oil
Milky oil suggests water ingress and must be investigated before cranking. Past six months idle, many owners change oil and filter anyway, as moisture and acids accumulate; follow the OEM recommendation.
Coolant
Check level, colour and leaks. Rusty coolant or visible sediment may need flushing to the OEM specification.
Batteries
Charge and load-test the battery, and clean terminals. If a charger was meant to maintain it, confirm it actually worked; a failed charger is a common hidden fault.
Air intake, belts and hoses
Check the air filter and ducting for debris and nesting material, hoses for hardening and belts for cracks.
Rodents and nests
Look for droppings, chewed cable sheaths and nests in the alternator and canopy insulation. Note every damaged cable, including any running to the RECD.
Go / no-go gate: Proceed only when (1) fuel is free of water and visible contamination, with filters replaced, (2) oil and coolant are at correct levels with no signs of cross-contamination, (3) batteries hold voltage under load, (4) air intake is clear, and (5) no chewed wiring remains unrepaired. Milky oil, contaminated fuel that cannot be cleaned, or damaged control wiring are all no-go.
Phase 2: Static exhaust and RECD checks after long storage
Rain cap, stack and nests
Confirm the rain cap or flapper is intact and seating. A stuck cap over a monsoon admits a lot of water. Remove any bird nests from the outlet.
Water in the exhaust line and RECD housing
Open every drain on the exhaust line and RECD housing, where fitted, and note volume and colour. Sooty water is common; large volumes indicate a rain ingress route to fix, as covered in protecting an RECD from rainwater and condensation. Never crank into an exhaust holding standing water.
Flanges, gaskets and bellows
Surface rust is normal; flaking, pitting or weeping stains around a gasket are not. Check bolts, and inspect bellows for cracks.
Insulation and heat shields
Wet lagging insulates poorly, traps moisture against steel and steams heavily on heat-up. Dry or replace it per supplier guidance.
Sensor cables, connectors and controller
If your RECD has sensors or a controller, inspect cable runs for chewing and heat damage, and junction boxes for moisture. Pressure lines, where used, can hold condensate or insects. With the engine still locked out, power up the controller alone under permit and note fault codes and whether any self-test completes.
Sensor plausibility at rest
With no flow, ΔP should read near zero and temperatures near ambient. Anything else suggests a blocked line, drift or wiring fault. Record the at-rest values.
Go / no-go gate: Proceed only when (1) the rain cap and stack are clear and intact, (2) all drains have been opened and standing water removed, (3) no flange or gasket shows active leakage or serious corrosion, (4) insulation is dry or has been dealt with, (5) RECD wiring is undamaged, and (6) the controller, if fitted, powers up with no active faults and sensors read plausibly at rest. An unexplained controller fault or a ΔP reading well off zero at rest is a no-go until the supplier has advised.
Phase 3: First crank and idle
Keep the team small and agree who can stop the set.
Priming and pre-lubrication
Bleed fuel and pre-lubricate as the OEM specifies, so bearings and turbocharger are not dry. Cranking with fuel shut off to build oil pressure is done only if the OEM approves.
Short cranks, then start
Crank in short bursts. If it will not fire after a few attempts, stop and diagnose rather than draining the battery and overheating the starter.
Oil pressure, leaks and noises
Oil pressure must rise promptly; if not, shut down and see fixing low oil pressure. Walk round for leaks and listen for knocks.
What smoke is normal on a first run after inactivity?
Some white vapour in the first minutes is normal: condensation steams off as the line heats, and should thin quickly. Persistent white smoke with a fuel smell points to incomplete combustion or coolant ingress; see why a generator emits white smoke. Black smoke signals fuel or air problems (black smoke causes); blue smoke signals oil burning (blue smoke causes).
Because the RECD captures much visible particulate, the stack may not tell the full story. Watch smell and ΔP trend too.
Keep idling short
Idle only long enough to confirm oil pressure and check for leaks, then move to Phase 4.
Go / no-go gate: Proceed only when (1) oil pressure is in the normal range, (2) there are no fuel, oil, coolant or exhaust leaks, (3) there are no abnormal noises, (4) any white vapour is clearly thinning, and (5) the RECD controller shows no new alarms. Persistent heavy smoke of any colour, low oil pressure or a new RECD alarm is a no-go: shut down and investigate.
Phase 4: Loaded warm-up without wet stacking
Why long idling is the wrong way to warm up
At light load, a diesel runs cool, burns fuel incompletely and never dries its exhaust, leaving oily, sooty deposits known as wet stacking. For an RECD this is the worst case: sticky soot loads quickly while the conditions that burn it off are absent. Our post on choosing an RECD for a generator running at low load explains more.
Step the load up
Use a load bank or staged site load: roughly 25%, then 50%, then the target load, checking readings at each step, unless your OEM recommends otherwise.
Reach working exhaust temperature
The system must reach normal working temperature at load to dry the housing and soot layer. Note how long it takes.
Watch ΔP behaviour
Expect ΔP above baseline at first, because damp soot packs densely. It should fall as the device dries. A falling or stable trend is reassuring; a steadily rising one is not. See how exhaust backpressure affects a DG set for why sustained high backpressure harms the engine.
Regeneration, where the design uses it
Some designs regenerate passively, some actively via a controller, some not at all. A filter carrying damp, heavy soot can regenerate unevenly, so ask your supplier whether a controlled regeneration, a manual clean or a sustained loaded run is appropriate. DieselNet explains particulate filter regeneration principles.
Take the first readings at a stated load
Record readings at a stated load in kW, ideally matching your last good baseline.
Go / no-go gate: Proceed only when (1) exhaust temperature has reached its normal working range at load, (2) ΔP has peaked and is falling or stable rather than climbing, (3) engine temperatures and pressures are normal, and (4) no RECD alarms are active. A ΔP that keeps rising at constant load, or that reaches the supplier’s alarm threshold, is a no-go: reduce load, shut down safely and contact the supplier.
Phase 5: Verification run and new baseline
When restarting an RECD-equipped DG set, the job ends only when you have shown the system is back to normal, or documented how it differs.
Compare like with like
Run at baseline load until readings stabilise (often one to two hours), then compare:
- ΔP across the RECD
- Exhaust temperature at the RECD inlet and outlet, if measured
- Engine coolant temperature and oil pressure
- Any controller status values the design provides
- Visible stack condition
Small differences are normal; large unexplained ones need a reason.
Record the new baseline
Log results with an unambiguous ISO 8601 date (2026-10-17), load, ambient temperature, running hours and initials.
Go / no-go gate: Return the set to service only when (1) readings at matched load are within the supplier’s stated tolerance of the last good baseline, or the difference is understood and accepted by the supplier, (2) the new baseline is recorded, and (3) all temporary covers, open drains and permits are closed out.
Phase 6: The first 30 days watch-list
- Check fuel water separators weekly; residual contamination often shows up after a few runs.
- Watch ΔP at matched load at each run. A slow upward drift may mean soot is loading faster than before.
- Look at drain points after rainy days.
- Check for oil or coolant consumption that was not there before the lay-up.
- Re-inspect any repaired wiring and connectors for heat damage once the system has done a few hot cycles.
- Confirm the battery charger is maintaining the battery.
If staff changed during the closure, run a refresher based on RECD operator training skills before handover.
Troubleshooting table: symptoms after a long idle
| Symptom | Likely cause after idle | What to do |
|---|---|---|
| Engine cranks but will not fire | Air in fuel system, flat battery, blocked filter from contaminated fuel | Bleed fuel system, check battery under load, replace filters, check fuel quality |
| White vapour that clears in a few minutes | Condensation in exhaust and RECD housing turning to steam; wet insulation steaming | Normal; continue to loaded warm-up and confirm it clears |
| White smoke that persists with fuel smell | Incomplete combustion, injector or timing issue, possible coolant ingress | Shut down; check oil for milkiness, check coolant level, involve engine service provider |
| Heavy black smoke upstream of RECD | Restricted air intake (nest, dirty filter), fuel system fault | Shut down; inspect intake and fuel system; do not keep loading the RECD with soot |
| Blue smoke | Oil burning, possibly glazed bores or seals hardened during idle | Monitor oil consumption; consult engine OEM service |
| ΔP high at first, falls as system heats | Moisture in soot layer drying out | Normal; continue loaded run and confirm ΔP stabilises near baseline |
| ΔP high and rising at constant load | Damp soot packing, filter overloaded, blocked outlet, or regeneration not occurring | Reduce load, shut down safely, contact RECD supplier |
| ΔP near zero at load | Sensor line disconnected or blocked, sensor failed, or internal damage allowing bypass | Do not ignore; contact supplier for inspection |
| Controller fault on power-up | Moisture in enclosure, corroded connectors, damaged cables | Dry and inspect enclosure; check cables; contact supplier for fault code meaning |
| Exhaust leak at flange | Gasket hardened or corroded during idle, loose bolts | Shut down, let cool, replace gasket per supplier specification |
| Black water from RECD drain | Condensation mixed with soot | Drain fully; investigate rain ingress if volumes are large |
What if the RECD itself is the problem?
High ΔP from damp soot that does not clear. If ΔP stays high after a full warm-up, soot may have consolidated or ash accumulated; ash in particulate filters does not burn off. Do not keep running at high backpressure.
Controller fault. Moisture in an enclosure causes intermittent faults. Note the exact code and timing.
Sensor drift. Implausible readings may mean drift, damage or a blocked line. For replacements, see specifying replacement RECD sensors.
In each case, contact your RECD supplier with readings and photos. Never remove, bypass or blank off the RECD to keep the set running: it is a compliance risk that can lead to sealing and penalties, and may affect warranty. If it looks like a warranty matter, document from the first observation using what to record when raising an RECD warranty claim.
Illustrative example: a 320 kVA set idle for seven months
This example is illustrative. The figures are invented to show the method; your own readings, tolerances and decisions will depend on your engine, your RECD design and your supplier’s guidance.
A manufacturing unit runs a 320 kVA DG set with an RECD that has ΔP and inlet temperature sensors. The set was laid up for seven months. The supplier’s handover document asks the site to investigate if ΔP at matched load is more than 20% above baseline after a full warm-up.
Load used for comparison: about 70% of rating. At a power factor of 0.8, 320 kVA × 0.8 = 256 kW. 70% of 256 kW = 179.2 kW, so readings were taken at roughly 180 kW.
| Reading at ~180 kW | Before shutdown | After restart, 15 min at load | After restart, 2 h at load |
|---|---|---|---|
| ΔP across RECD | 8.0 kPa | 10.8 kPa | 8.6 kPa |
| RECD inlet temperature | 440 °C | 395 °C | 435 °C |
| Coolant temperature | 84 °C | 82 °C | 84 °C |
ΔP difference at 15 minutes: 10.8 − 8.0 = 2.8 kPa. As a percentage of baseline: 2.8 ÷ 8.0 × 100 = 35%.
ΔP difference at 2 hours: 8.6 − 8.0 = 0.6 kPa. As a percentage of baseline: 0.6 ÷ 8.0 × 100 = 7.5%.
Interpretation. The early reading was 35% above baseline, which on its own would be a concern. But inlet temperature was still 45 °C below normal (440 − 395 = 45), which is consistent with a system still drying out. Over two hours at load, inlet temperature recovered to within 5 °C of baseline and ΔP fell to 7.5% above baseline, well inside the 20% investigation trigger.
Decision. The set is returned to service. The new baseline of 8.6 kPa at about 180 kW is recorded with date, ambient temperature and running hours. The site flags ΔP for weekly comparison during the first 30 days. If ΔP at the same load drifts back towards 9.6 kPa (8.0 × 1.20 = 9.6 kPa, the 20% trigger), the site will contact the supplier before it gets there.
Had ΔP stayed at 35% above baseline after two hours, the decision would have been different: shut down, share data with the supplier, and arrange inspection.
Prevention: laying up a DG and RECD so the next restart is easier
A planned lay-up takes an hour or two and saves days later.
- Finish with a hot, loaded run so the RECD is left dry.
- Record a final baseline of ΔP, temperatures, load and hours.
- Fill or drain the fuel tank as the OEM advises.
- Protect the stack outlet, using a supplier-approved cover tagged so nobody starts with it fitted.
- Check drains monthly.
- Maintain the battery on a working charger.
- Seal cable entries against rodents.
- Choose monthly loaded runs or full isolation. A short unloaded run is the worst of both.
For a device not yet fitted, see how to store an RECD before it is installed. Sites with long gaps between runs may also read whether an RECD suits a DG set that runs only occasionally.
FAQ
Can I just start a DG set that has been idle for six months?
You can start it, but you should not do so without checks. At six months, fuel water, flat batteries, moisture in the exhaust and RECD, and rodent damage are all likely. Work through static checks first and follow your engine OEM’s recommissioning procedure.
Is white smoke normal when restarting an RECD-equipped DG set?
Some white vapour in the first few minutes is normal, because condensation in the exhaust and RECD turns to steam. It should thin and clear as the system warms under load. Persistent white smoke with a fuel smell is not normal and needs investigation.
Why is RECD backpressure higher after a long shutdown?
Soot inside the device can absorb moisture during idle months and pack more densely, raising resistance to flow. In many cases ΔP falls back towards baseline once the system reaches working temperature at load. If it keeps rising, shut down and contact your supplier.
Should I idle the generator for a long time to warm it up?
No. Extended light-load running keeps the exhaust cool, increases unburnt fuel and soot, and can cause wet stacking. Confirm oil pressure and leaks at idle, then step the load up to reach working temperature.
Do I need my RECD supplier involved in a restart?
For short idle periods with normal readings, usually not. For idle periods beyond six months, any controller faults, implausible sensor readings, or ΔP that will not settle, involve the supplier before returning the set to service.
Can I bypass the RECD if it is causing problems after restart?
No. Bypassing or removing the RECD is a compliance risk that can lead to sealing and penalties, and it may affect warranty. Shut down safely and contact the supplier instead.
Bringing your RECD back into service with Aceget
If your DG set has been standing and you are planning a restart before the winter season, Aceget can help you interpret RECD readings, plan a recommissioning run and decide whether inspection or cleaning is needed. Our retrofit emission control devices are CPCB type approved for DG sets from 25 kVA to 2500 kVA, and the approval documents are available on our certifications page. To discuss a restart, a fault or a new installation, contact the Aceget team with your DG rating, the length of the idle period and your last recorded readings.