Marine MARPOL Annex VI NOx Tier III SCR & EGR Differential Pressure, Urea Dosing & NECA Changeover Guide

Marine NOx Tier III SCR and EGR fault symptoms, primary causes and immediate engine-room verification actions

1. MARPOL Annex VI Reg. 13 NECAs, NOx Technical Code 2008 & Tier II/III Changeover Control

Under MARPOL Annex VI Regulation 13, Nitrogen Oxides (NOx) emission limits apply to each installed marine diesel engine with a power output above 130 kW (excluding engines used solely for emergencies under Regulation 13.1.2.1, with non-identical or steam-system replacement engines governed by Regulation 13.2.2 as amended by IMO Res. MEPC.385(81)). Outside a NOx Tier III Emission Control Area, ships constructed on or after 1 January 2011 operate to the Tier II standard (14.4 g/kWh below 130 rpm, 44.0 * n^(-0.23) g/kWh from 130 to under 2,000 rpm, and 7.7 g/kWh at 2,000 rpm and above). Inside a designated NOx Tier III ECA (NECA), applicable ships must meet the Tier III limit (3.4 g/kWh below 130 rpm, 9.0 * n^(-0.2) g/kWh from 130 to under 2,000 rpm, and 2.0 g/kWh at 2,000 rpm and above), roughly 80 per cent below the Tier I limits.

Chief Engineers and Technical Superintendents should verify vessel construction dates on the IAPP Supplement against the exact statutory applicability of each NECA. Tier III applies inside the North American ECA and United States Caribbean Sea ECA (IMO Res. MEPC.190(60) and MEPC.202(62)) to ships constructed on or after 1 January 2016, and inside the Baltic Sea ECA and North Sea ECA (IMO Res. MEPC.286(71)) to ships constructed on or after 1 January 2021. Under IMO Resolution MEPC.392(82), which entered into force on 1 March 2026 and expanded Appendix I Section 1.3 of the IAPP Supplement to record building contract, keel-laying and delivery dates, two additional NECAs are designated: the Canadian Arctic ECA (Tier III applicable to ships constructed on or after 1 January 2025) and the Norwegian Sea ECA (Tier III applicable to ships contracted on or after 1 March 2026, or in the absence of a contract, keel laid on or after 1 September 2026, or delivered on or after 1 March 2030, while the 0.10% m/m SOx fuel limit in both areas applies from 1 March 2027, after the first-year exemption in Regulation 14.7). By contrast, the Mediterranean Sea ECA (designated by IMO Res. MEPC.361(79), with its 0.10% m/m sulphur limit applying from 1 May 2025) is a SOx and PM ECA only, so Tier II NOx continues to apply there.

Statutory certification rests on the NOx Technical Code 2008 (IMO Res. MEPC.177(58), as amended). Every regulated engine requires an Engine International Air Pollution Prevention (EIAPP) Certificate (NOx Technical Code 2008, 2.1.1.1), which supports the ship's IAPP Certificate (MARPOL Annex VI Regulation 6), an Administration-approved Technical File retained on board for the life of the engine, and a Record Book of Engine Parameters logging every replacement or setting change affecting NOx-critical components. Chapter 6 defines three onboard NOx verification routes: the Engine Parameter Check method (Section 6.2), the Simplified Measurement method (Section 6.3), and the Direct Measurement and Monitoring method (Section 6.4). Superintendents planning SCR retrofits or multi-profile engine tuning should also account for IMO Res. MEPC.398(83) (in force 1 September 2026, governing re-certification of existing engines after substantial modification or Tier upgrade) and IMO Res. MEPC.397(83) (entering into force 1 March 2027, adding Chapter 8 on multiple engine operational profiles, which applies to new individual or parent engines with such profiles no later than 1 January 2028).

On ships to which Regulation 13.5.1 applies, engines certified to both Tier II and Tier III, or to Tier II only, are subject to operational changeover logging under MARPOL Annex VI Regulation 13.5.3. At every entry into and exit from a NOx Tier III ECA, and whenever an engine tier or on/off operating status changes inside that area, the crew must record each engine tier and on/off status, date, time and ship position (latitude and longitude) in the engine-room logbook or an Administration-approved electronic record book meeting IMO Res. MEPC.312(74). Port State Control officers check these tier and on/off records (IMO Res. A.1206(34), Appendix 18, 2.3.2), and A.1206(34) Appendix 2 lists missing records, or an engine not meeting Tier III where it is required, among deficiencies that may warrant detention, at the inspector's professional judgement.

• Verify the ship building contract, keel-laying and delivery dates on the updated IAPP Supplement (MEPC.392(82)) against each voyage NECA before passage planning.
• Confirm the EIAPP Certificate, approved Technical File and Record Book of Engine Parameters are on board for every diesel or dual-fuel engine above 130 kW and match installed component ID markings.
• Establish a bridge-to-engine-room pre-arrival call well ahead of every NECA boundary so SCR catalyst warm-up or EGR gas-loop stabilisation finishes before crossing into the regulated zone.
• Record exact date, UTC/ship time, latitude, longitude and individual main and auxiliary engine tier and on/off status at every NECA entry, exit and internal engine changeover under Regulation 13.5.3.

2. SCR Reactor Differential Pressure (Delta P), Sulphate Blinding, Sootblowing & Urea Dosing

Marine Selective Catalytic Reduction systems are certified together with the parent engine under a single EIAPP Certificate and Technical File in accordance with the NOx Technical Code 2008, taking into account the 2017 SCR Guidelines (IMO Res. MEPC.291(71) as amended by IMO Res. MEPC.313(74)), while Administrations are invited to apply the superseding 2025 SCR Guidelines (IMO Res. MEPC.399(83)) to SCR systems on ships with keels laid on or after 1 November 2025, or SCR systems delivered to an earlier ship on or after 1 May 2026 (contractual delivery date, or actual delivery if there is none). Certification follows either Scheme A (section 5: the engine and SCR system tested together on a test bed) or Scheme B (sections 6 and 7, which the 2017 Guidelines reserve for cases where that is not possible for technical and practical reasons, while the 2025 Guidelines let the applicant choose the scheme as appropriate: the engine tested and the SCR chamber, full-size or scaled, validated separately, then an onboard confirmation test before entry into service as close as possible to 25%, 50% and 75% of rated power under paragraph 7.3; the IAPP initial survey is not completed until that test shows compliance).

Exhaust gas temperature management is the primary defence against irreversible catalyst fouling. When aqueous urea is atomised into hot exhaust gas, it thermolyses and hydrolyses into ammonia (NH3) and isocyanic acid/carbon dioxide before reducing NO and NO2 over vanadium-titanium or metal-exchanged zeolite honeycomb blocks into nitrogen (N2) and water vapour. If the reactor inlet temperature drops below the fuel-sulphur-dependent minimum specified in the approved SCR Technical File, sulphur trioxide (SO3) in the exhaust reacts with ammonia and moisture to condense sticky liquid ammonium bisulphate ((NH4)HSO4) and solid ammonium sulphate ((NH4)2SO4) inside the microscopic catalyst pores. Never apply generic fleet temperature rules of thumb: always enforce the exact sulphur-versus-temperature curve in the ship-specific SCR Technical File, use low-sulphur compliant fuel or LNG gas mode where approved, and verify that exhaust bypass, turbine-bypass or cylinder thermal-management valves hold inlet temperature above the dosing permissive during low-load manoeuvring.

Reactor differential pressure (Delta P) should be trended against engine load and exhaust mass flow rather than read as an isolated static number, because pressure loss rises roughly with the square of gas velocity. A steady upward drift in load-corrected Delta P indicates honeycomb face plugging by diesel soot, cylinder lubricating oil ash (calcium deposits masking active catalytic sites) or sulphate salt condensation, while an unexplained drop in Delta P can indicate bypassed gas seals or eroded/collapsed catalyst blocks. Keep compressed-air sootblowers dry and sequenced per maker intervals so wet service air never thermal-shocks hot ceramic monoliths, and purge Delta P transmitter impulse pipes regularly so acidic exhaust condensate cannot falsify high-pressure or low-pressure readings.

Clean reductant handling prevents both injector nozzle blockage and downstream ammonia slip. Where the SCR Technical File specifies marine aqueous urea solution AUS 40 in accordance with ISO 18611-1 (about 40% m/m urea in demineralised water; check the concentration range on the supplier's certificate), verify each bunker batch certificate of analysis, sample tank clarity and refractometer concentration regularly, and keep storage tank ventilation and temperature within maker limits to avoid biuret formation or hydrolysis loss. Whenever urea injection stops at NECA exit or engine shutdown, confirm that the automatic compressed-air or demineralised-water line purge completes fully so residual urea cannot bake into hard white cyanuric acid crystals inside the lance tip. Under the 2025 SCR Guidelines (MEPC.399(83), paragraph 3.2.8), catalyst condition should be verified either by an installed NOx measurement device (3.2.8.1) or by calibrated NOx spot checks after installation and at least once every 12 months (3.2.8.2, at least at 50% of rated power and preferably 75% for propulsion engines), with results recorded in the Record Book of Engine Parameters; systems certified under the 2017 Guidelines follow the verification procedure in their approved Technical File.

• Plot SCR reactor Delta P against engine load (% MCR) and exhaust receiver pressure weekly, comparing readings directly against the clean commissioning curve in the SCR Technical File.
• Verify compressed-air sootblower moisture traps, air receiver pressure and solenoid firing sequence before entering an NECA, ensuring zero liquid water enters the hot catalyst chamber.
• Check each AUS 40 delivery against its ISO 18611-1 quality certificate and verify automatic post-dosing air purge on every urea lance to prevent crystal build-up and droplet impingement.
• Archive NOx measurement device calibrations or the 12-monthly catalyst NOx spot-check records (MEPC.399(83) 3.2.8.1 / 3.2.8.2, or the approved Technical File procedure) in the Record Book of Engine Parameters alongside catalyst block ID records for IAPP annual and renewal surveys.

3. EGR Loop Differential Pressure, Acid Neutralisation, Cooler Fouling & MEPC.307(73) Bleed-Off Water

Exhaust Gas Recirculation achieves Tier III NOx compliance inside the combustion chamber rather than in the uptake by routing a controlled fraction of cooled, cleaned exhaust gas back into the scavenge air path. Replacing part of the fresh scavenge oxygen with higher-heat-capacity carbon dioxide and water vapour lowers peak adiabatic flame temperature and suppresses thermal NOx formation. Depending on engine architecture, high-pressure EGR loops draw gas upstream of the turbocharger turbine through a pre-spray venturi scrubber, EGR gas cooler, water mist catcher (WMC) and electrically driven high-pressure EGR blower back to the scavenge receiver, whereas low-pressure EGR loops draw gas downstream of the turbocharger turbine and return cooled gas ahead of the compressor wheel.

Gas-side differential pressure (Delta P) trending across each stage of the EGR string reveals fouling well before scavenge oxygen control is lost. Rising Delta P across the EGR scrubber and gas cooler indicates wet soot, unburnt hydrocarbon film or sulphate scale narrowing the finned heat-exchanger passages, which simultaneously degrades gas cooling, elevates scavenge air temperature and pushes the EGR blower towards its surge line. Conversely, an abnormal pressure drop or flooding alarm across the water mist catcher warns of plugged demister drain traps or damaged chevron vanes. Carry-over of acidic water droplets into the scavenge receiver strips the cylinder oil film, accelerates cold sulphuric corrosion on two-stroke liners and damages high-pressure blower impellers or low-pressure compressor wheels.

Chemical control of the closed EGR washwater recirculation loop is critical because recirculated exhaust gas transfers sulphur oxides, nitric acid condensate and particulate soot into the scrubbing water. Engine automation injects neutralising alkali (typically sodium hydroxide solution controlled to the engine maker water-quality specification) to hold recirculation loop pH neutral and prevent acid attack on stainless-steel cooler tubes, blower casings and scavenge spaces. Never guess alkali dosing rates or override low-pH alarms: verify dosing pump stroke, chemical tank level, pH/conductivity probe calibration and coagulant/clarifier skimming strictly in accordance with the engine Technical File and maker water-treatment manual.

Surplus condensed water generated from combustion moisture is managed as EGR bleed-off water under the 2018 Guidelines for the Discharge of Exhaust Gas Recirculation (EGR) Bleed-Off Water (IMO Res. MEPC.307(73)), which apply to engines whose EIAPP Certificate was first issued on or after 1 June 2019. Under paragraph 3.2.1, when the engine operates on fuel oil complying with MARPOL Annex VI Regulation 14 and the ship is en route outside polar waters, ports, harbours and estuaries, bleed-off water may be discharged when its oil content, continuously monitored and recorded by a meter meeting IMO Res. MEPC.107(49), does not exceed 15 ppm. When the engine operates on fuel oil that does not comply with Regulation 14 (paragraph 3.1), the washwater discharge criteria of the EGCS Guidelines apply (MEPC.259(68) as may be amended, now superseded by MEPC.340(77)), including a PAH limit of 50 µg/L PAHphe above the inlet water level, normalised at 45 t/MWh. In polar waters, ports, harbours and estuaries the conditions in paragraph 3.2.2 apply, and bleed-off water may be retained in a holding tank (paragraphs 3.1.2 and 3.2.3). The Polar Code (Part II-A, 1.1.1) prohibits any discharge of oil or oily mixtures into the sea in Arctic waters, and MARPOL Annex I regulation 15.4 does the same for the Antarctic area. Under paragraph 4, EGR Water Treatment System (WTS) residues should be delivered ashore to adequate reception facilities, not discharged to the sea or incinerated on board, with their storage and disposal recorded in the EGR record book (paragraph 4.2); bleed-off discharges are evidenced by the oil-content meter record (3.2.1) or under the EGCS Guidelines (3.1).

• Blow through and drain EGR gas cooler, scrubber and water mist catcher Delta P transmitter impulse lines at maker intervals so trapped condensate cannot mask real gas-path fouling.
• Inspect water mist catcher drain traps, float switches and scavenge receiver drain sight-glasses daily during Tier III operation to confirm zero liquid carry-over into the cylinders.
• Calibrate EGR water-loop pH, conductivity and dissolved-oxygen sensors against reference buffers and verify neutralising alkali pump delivery before commencing NECA transit.
• Test the EGR bleed-off 15 ppm oil-content meter (MEPC.107(49)) and the maker's discharge stop or diversion valve before NECA entry, check which MEPC.307(73) condition applies (compliant fuel en route, non-compliant fuel, or polar waters, ports, harbours and estuaries), make no oily bleed-off discharge in Arctic or Antarctic waters, and land WTS residues ashore with their disposal recorded in the EGR record book.

4. NECA Malfunction Contingency, ISM Code 10.3 Critical Systems & MSC.581(110) Reactor Entry Safety

When an SCR or EGR fault occurs inside a NOx Tier III ECA, the safety of the ship comes first: MARPOL Annex VI regulation 3.1.1 excepts any emission necessary to secure the safety of a ship or save life at sea. If an SCR catalyst chamber suffers a sudden differential-pressure spike, urea dosing/crystallisation lock-out, or an EGR blower trips during restricted pilotage waters, many systems are designed by the maker to revert the engine to a Tier II or bypass mode to preserve manoeuvrability rather than causing a dead-ship blackout. Never tamper with NOx Technical File parameters, defeat alarm interlocks or run unlogged on bypass: immediately record the exact fault timestamp, coordinates and engine tier and on/off status in the Regulation 13.5.3 logbook, capture alarm history screenshots and troubleshoot the root cause per the Technical File onboard verification procedure.

Under ISM Code section 10.3, the Company should identify equipment and technical systems whose sudden operational failure may result in hazardous situations. A Company may list Tier III abatement changeovers, exhaust reactor isolation dampers, EGR shut-off valves and propulsion power-limitation interlocks among them, with regular functional tests recorded in the Planned Maintenance System (PMS). If a Tier III system cannot be restored while operating inside an NECA, report it to the Company (DPA or technical superintendent) as the SMS requires (ISM Code section 9.1) and follow the company procedure and any Flag or port State instructions, documenting spare-part mobilisation and corrective action before port arrival.

Fire safety around high-temperature Tier III exhaust ducting starts with SOLAS Chapter II-2 regulation 4.2.2.6.1 (applied to pressure lubricating oil by 4.2.3.1). High-pressure SCR reactors, pre-turbocharger exhaust receivers, electric or burner-assisted exhaust vaporisers and EGR hot-gas take-off pipes routinely operate at elevated exhaust temperatures, and any surface hotter than 220 deg C that fuel could spray onto if a fuel system fails has to be properly insulated; keeping that insulation intact and oil-resistant, and shielding nearby joints, is good practice. After any catalyst module inspection, thermocouple replacement or sootblower valve overhaul, verify with an infrared thermographic camera at service load that every disturbed insulation blanket and flange jacket has been reinstated before signing off the job.

Internal inspection, vacuum cleaning or element replacement inside an SCR reactor vessel, EGR scrubber tower, exhaust receiver or AUS 40 storage tank is a high-risk enclosed-space operation addressed by IMO Resolution MSC.581(110) (adopted June 2025, Revised Recommendations for Entering Enclosed Spaces Aboard Ships), SOLAS Regulation XI-1/7 (requiring ships to carry an appropriate portable atmosphere testing instrument capable of measuring at least oxygen, flammable gases or vapours, hydrogen sulphide and carbon monoxide prior to entry, together with suitable means on board for its calibration) and SOLAS regulations III/19.3.3 and 19.3.6 (enclosed space entry and rescue drills at least every two months for crew with those duties). Before entry, crews should isolate and lock out engine starting air, turning gear, exhaust dampers, compressed-air sootblowers, alkali pumps and AUS 40 dosing lines, allow full thermal cooldown, ventilate mechanically, and test the atmosphere at multiple levels with ventilation stopped to confirm 20.9% oxygen by volume, less than 1% LFL flammable vapour, less than 0.5% carbon dioxide, and toxic gases (including ammonia, NO, NO2, SO2, CO and H2S) below 50% of the occupational exposure limit under an entry permit whose validity should not exceed 8 hours and becomes immediately void if the space is left unattended, remembering that EEBDs are strictly for escape and never for entry. For ship-specific SCR/EGR Delta P diagnostics or NECA PSC readiness reviews, Marvel Solutionz provides remote engineering consultancy at $350 USD for a 1-hour consultancy (+ 5% sales tax) alongside the LNG, Dual-Fuel (DF) & Spark-Gas (SG) Academy at $395 USD self-paced (+ 5% sales tax) or $3,500 USD (+ 5% sales tax) for Corporate Team Training for up to 10 engineers.

• Test SCR reactor/bypass dampers, EGR shut-off valves and Tier II/III bridge-engine changeover sequences outside congested waters and log functional checks under ISM Code Section 10.3.
• In the event of an unrecoverable Tier III abatement fault inside an NECA, log position, time and engine tier and on/off status under Regulation 13.5.3 immediately, report it to the Company (DPA or superintendent) and follow the company procedure and any Flag or port State instructions.
• Scan reinstated SCR reactor, vaporiser and EGR hot-pipe insulation jackets under load with a thermal camera to verify zero exposed impingeable surfaces above 220 deg C (SOLAS II-2/4.2.2.6.1).
• Apply full LOTO of sootblowers, urea lances and exhaust dampers, verify the SOLAS XI-1/7 portable atmosphere testing instrument and onboard calibration means, and apply multi-level gas testing plus an entry permit whose validity should not exceed 8 hours (immediately void if left unattended) per IMO Res. MSC.581(110) before opening any SCR or EGR casing.

Frequently Asked Questions

Related Marvel Solutionz & SeaReady Resources

Regulatory & Technical Sources

• IMO - Nitrogen Oxides (NOx) Regulation 13 & Emission Control Areas (MARPOL Annex VI)
• IMO Resolution MEPC.177(58) - Revised NOx Technical Code 2008
• IMO Resolution MEPC.392(82) - 2024 Amendments to MARPOL Annex VI (Canadian Arctic and Norwegian Sea ECAs)
• IMO Resolution MEPC.291(71) - 2017 Guidelines on Marine Diesel Engines Fitted with Selective Catalytic Reduction (SCR) Systems
• IMO Resolution MEPC.313(74) - Amendments to the 2017 SCR Guidelines (MEPC.291(71))
• IMO Resolution MEPC.399(83) - 2025 Guidelines on Marine Diesel Engines Fitted with Selective Catalytic Reduction (SCR) Systems
• IMO Resolution MEPC.307(73) - 2018 Guidelines for the Discharge of Exhaust Gas Recirculation (EGR) Bleed-Off Water
• IMO Resolution MEPC.340(77) - 2021 Guidelines for Exhaust Gas Cleaning Systems (supersede MEPC.259(68))
• IMO Resolution MSC.581(110) - Revised Recommendations for Entering Enclosed Spaces Aboard Ships