Open Loop Scrubber
medium
2 / 2 models (Inspector)
An open loop scrubber sprays untreated seawater through the exhaust stream to absorb sulphur oxides, then discharges the wash water overboard after treatment, trading a simple process for a discharge that a growing number of ports now restrict or ban.
Read more — Open Loop Scrubber explained ▾
What makes this type distinct
An open loop scrubber draws seawater directly from the sea chest, sprays it through a tower against the rising exhaust gas, and discharges the wash water overboard after settling and monitoring, with no recirculation and no chemical dosing beyond what the seawater's own natural alkalinity provides. This sets it apart from a closed loop scrubber, which recirculates a caustic-dosed fresh water stream and only bleeds off a small treated portion, and from a hybrid scrubber, which can switch between the two modes. Open loop needs seawater with enough natural alkalinity to neutralise the sulphuric acid formed when SO2 dissolves, which is why it performs less consistently in low-alkalinity waters such as parts of the Baltic.
Main components
Scrubber tower
Vertical vessel where seawater is sprayed countercurrent to the exhaust gas across spray nozzles or a packed bed, stripping SO2 and much of the particulate matter.
Seawater supply and lift pumps
High-capacity pumps drawing large volumes of seawater, since open loop operation depends on flow rate rather than chemical dosing to achieve the required wash ratio.
Wash water treatment unit
Settling tank or hydrocyclone removing particulates and free oil from the discharge stream before it goes overboard, feeding sludge to a residue tank.
Monitoring skid
Continuously measures wash water pH, PAH (polycyclic aromatic hydrocarbon) content, turbidity and flow, alongside SO2/CO2 ratio in the cleaned exhaust, to prove compliance.
Bypass and changeover arrangement
Allows the engine to run on the scrubber or bypass it if fitted with dual-fuel capability to run on compliant low-sulphur fuel instead.
Selection and sizing
Sizing follows engine exhaust gas flow at full load, not ship size directly. The scrubber and its exhaust gas system must handle the combined main engine and auxiliary engine exhaust volumes it is designed to serve, at the required SO2 reduction to meet a 0.10% or 0.50% sulphur-equivalent limit depending on the operating area. Wash water flow rate, usually expressed in m3 per MWh of engine power scrubbed, is the other key sizing figure, since it drives pump capacity, piping diameter and the footprint of the treatment skid.
Regulations and class
- MARPOL Annex VI Regulation 4 permits exhaust gas cleaning as an equivalent means of meeting the fuel sulphur limits, provided the system meets the 2015 IMO EGCS Guidelines (MEPC.259(68)).
- Wash water discharge criteria cover pH at the overboard point, PAH concentration relative to a washwater/seawater ratio, turbidity above ambient, and nitrate addition rate.
- A growing list of ports, coastal states and entire sea areas, including several in the Baltic, North Sea and various national waters, restrict or prohibit open loop discharge; the ship's trading pattern determines whether open loop alone is workable or a hybrid unit is needed.
- Class requires continuous monitoring records to be retained and available to port state control.
Typical faults
| Fault | Consequence |
|---|---|
| Spray nozzle scaling or blockage | Uneven gas-liquid contact, SO2 reduction falls below the required ratio |
| Wash water pump cavitation at low sea chest level or in heavy weather | Loss of wash flow, automatic changeover to low-sulphur fuel or engine power reduction |
| pH monitor fouling or drift | Discharge criteria breach going undetected, or false compliance failures forcing unnecessary bypass |
| Tower internals corrosion from continuous acidic wash | Structural thinning over time, requiring coating renewal or plate replacement at dry dock |
What to look for in a supplier
- Demonstrated SO2 reduction and wash water compliance data from ships in comparable trades, not just bench test figures.
- Materials of construction for the tower and piping, since acidic wash water is aggressive to ordinary steel.
- Whether the unit is offered as open loop only or hybrid-capable, given the expanding list of open loop discharge bans.
- Footprint and weight impact on the ship, since retrofits often compete with cargo space or stability margins.
Check the port and coastal state discharge ban list before the voyage is planned, not after the ship arrives outside the limit; an open loop scrubber that cannot discharge has to switch to compliant fuel, and that fuel has to already be aboard.
2 manufacturers · 2 models
Alfa Laval
1
PureSOx Open Loop
variable
· Exhaust Gas Treatment
· Open‑loop exhaust gas scrubber
Type
open_loop
Marpol
ja
Technical Specifications
- Sox-abscheidung
- >98% Removal mit Partikelfiltration
- Betriebsweisen
- Open-Loop, Closed-Loop, Hybrid, mit optionalem Energy Reduction Mode
- Wasser-reinigungseinheit
- Centrifugal-Separator (PureSOx H2O), 6 m² Grundfläche, modulare 3-Skid-Konstruktion
- Installationen
- 100+ Schiffe weltweit, 200+ Retrofit-Projekte abgeschlossen
- Service-Verfügbarkeit
- 24/7 global support, 24-hour spare parts dispatch, dry dock services, crew training
Product Lines
- PureSOx U-Design
- PureSOx I-Design (Inline)
- PureSOx Express
- PureSOx Hybrid
- PureSOx Closed-Loop
- PureSOx Open-Loop
Common Failures & Inspection Points
- Area: Nozzle and spray system wear: nozzles can become clogged/damaged through erosion, sludging or mineral deposit, affecting spray pattern andCheck: Visual inspection of spray nozzles for wear, cracks, deposits; functionality test of spray pattern; use genuine replacement nozzles via Alfa Laval sprayer replacement programme with manufacturer calibration
- Area: pH and PAH sensor calibration: sensors (PAH, turbidity) must be calibrated by manufacturer every 2 years (MEPC standard). Inaccurate measurements lead to compliance-Check: Check calibration date (should not be older than 2 years); sensor replacement via Alfa Laval exchange programme; documentation of calibration certificates and measurement logs (current standard: logging every ~90 seconds)
- Area: Absorber corrosion: The absorber module (separated from the jet module by natural water trap) is exposed to saltwater/acid environment and can corrode; corrosCheck: Dry dock condition assessment to be carried out (ideally planned 6 months before docking); visual inspection for corrosion spots, material loss on walls/floors; wall thickness measurement (critical areas); documentation and repair recommendations
- Area: Wash water monitoring and leak detection: In closed-loop/hybrid operation, dirty water with soot, minerals and oil is generated; insufficient monitoring or leak iCheck: Testing of the water treatment unit (PureSOx H2O centrifugal separator) for wear/sludging; verification of pH/PAH/turbidity measurements and records (continuous logging required); visual inspection for leaks in piping and fittings; validation that turbidity remains <25 FTU
- Area: Jet and absorber module piping and damper valves: U-design or inline design utilize complex internal pipe systems; deposits, corrosion or valve clogCheck: Dry dock inspection: Visual inspection of internal pipes (using endoscope if necessary) for deposits/corrosion; functional testing of damper valves and pumps; inspection of critical components per Alfa Laval dry dock service kit (damper valves, pumps, sensors); replacement of worn parts with original spare parts
- Area: Discharge water testing and compliance documentation: IMO MEPC requires laboratory tests of discharge water after commissioning and before each class survey; testsCheck: Verification of discharge water test certificates (laboratory-confirmed) and their date; comparison of measured values against limit values (pH ≥6.5, PAH ≤50 µg/L, turbidity ≤25 FNU, temperature ≤60°C); in US waters: at least 2 tests in year 1, then annually 1x; in other regions possibly more frequently; documentation in ship's compliance logbook
Type-universal inspection/maintenance points for exhaust gas cleaning (Alfa Laval, Mega-Schwarm 2026-06). Per-model specs not auto-filled.
Service: PureSOx brand. Open-loop mode. Washwater treatment required.
Spare Parts: Alfa Laval: Spezialteile per Hersteller. Lead time: 4-8 Wochen. Gasdetektions-Sensoren an Bord.
Strengths
- Proven SOx removal efficiency (>98 %) meeting IMO Annex VI limits
- Modular 3‑skid design (≈6 m² footprint) simplifies installation and retrofits
- Global 24/7 spare‑part support, dry‑dock services and crew training from Alfa Laval
- Flexibility to operate in Open‑Loop, Closed‑Loop or Hybrid modes with an Energy‑Reduction option
- More than 100 vessels already equipped, providing a large operational knowledge base
Weaknesses
- Requires continuous discharge of wash water; not permitted in many coastal and US waters without additional treatment
- Nozzle erosion and spray‑system fouling demand regular visual inspection and possible replacement
- Absorber module is exposed to acidic seawater and can suffer corrosion if not inspected at dry dock
- Higher initial capital cost compared with some compact closed‑loop units
- Complex piping and damper valves increase dry‑dock inspection time
Typical Vessels:
Crude oil tankersProduct tankersBulk carriersContainer shipsGeneral cargo vessels
Certifications:
IMO Type Approval (Annex VI)DNV Recommended Practice RP OGS approvalABS Approved Equipment
Decision Guide: Choose if the vessel operates primarily in international waters where open‑loop wash‑water discharge is allowed, you need a high‑efficiency (>98 %) SOx solution, and you value extensive aftermarket support for retrofits. Avoid if the ship will spend significant time in emission‑control areas (ECAs) that ban wash‑water discharge, or if space/weight constraints make a smaller closed‑loop system more suitable.
Use Cases: The PureSOx Open‑Loop is typically installed on existing tankers and bulk carriers during dry‑dock periods to meet the 2020 IMO SOx cap. It is also selected for new builds where designers prefer an open‑loop layout to minimise auxiliary power consumption while retaining the option to switch to closed‑loop mode when operating in restricted waters.
Wärtsilä
1
Wartsila Open Loop Scrubber
sized per engine power
· Exhaust Gas Treatment
· Open-loop marine SOx scrubber
Type
open_loop
Marpol
ja
Technical Specifications
- Sox-reduktion
- 98% Schwefeldioxid-Entfernung
- Operationsmodi
- Open Loop, Closed Loop, Hybrid
- Seewasser-Alkalinität erforderlich
- 2300 µmol/l zur Reduktion von 3,5% auf 0,1% Schwefelgehalt
- Abwasser-pH Entladung
- Minimum 6,5, Maximum Temperatur 60°C, max. 2 pH-Einheiten Differenz Inlet-Overboard
- Überwachungsparameter
- pH, PAH (Polyzyklische aromatische Kohlenwasserstoffe), Trübung, Temperatur
- Entschwefeleinsätze
- Venturi-Absorber mit 3-stufiger Wasserzerstäubung, Hochleistungs-Edelstahl
Product Lines
- Conventional Quench and Venturi Design
- Inline Scrubber Design
- IQ Series Scrubber (compact, 25% weniger Platz, 30% leichter)
- Hybrid Scrubber (Open/Closed Loop kombinierbar)
Common Failures & Inspection Points
- Area: Absorber/venturi erosion and wearCheck: Six-monthly visual inspection of absorber for signs of erosion and corrosion; pressure test of spray headers (low pressure = erosion); inspection for wear on venturi walls and deflector plates
- Area: Spray nozzle plugging and erosionCheck: Weekly inspection of spray headers and header on plugging or erosion; inspection of liquid pressure gauges to detect abnormally low pressures (indicator of erosion); six-monthly internal inspections for plugged or eroded nozzles
- Area: Effluent sensor and pH probe calibrationCheck: Annual full service and calibration of all sensors by certified technicians; regular inspection for probe fouling and sensor sensitivity; calibrate CEMS and water monitoring systems per manufacturer specifications
- Area: Abwasserqualität: pH, PAH, Trübung, Temperatur MonitoringCheck: Continuous monitoring of inlet and outlet water quality; documentation of pH (min. 6.5), PAH, turbidity, temperature (max. 60°C); annual sampling to be performed no later than after first year of operation; 3-year retention of all test results
- Area: Corrosion on effluent pipes and discharge linesCheck: Wall thickness measurements of all effluent pipes and overboard lines at every annual class survey (DNV requirement); visual inspections for signs of corrosion and erosion; inspection of integrity of pipe supports and welds; particular attention to acid corrosion prior to seawater dilution
- Area: Mist eliminator fouling and blockageCheck: Six-monthly internal inspections of mist eliminator for fouling, plugging and corrosion; inspection for solids accumulation in eliminator; functional test of eliminator washdown systems; cleaning or replacement when damaged
Type-universal inspection/maintenance points for exhaust gas cleaning (Wärtsilä, Mega-Schwarm 2026-06). Per-model specs not auto-filled.
Service: Open-loop uses seawater. Restricted in some ports/ECAs. Washwater monitoring mandatory.
Spare Parts: Wärtsilä: Spezialteile per Hersteller. Lead time: 4-8 Wochen. Gasdetektions-Sensoren an Bord.
Strengths
- Very high SOx removal efficiency (≈98%) meeting Annex VI limits
- Multiple operating modes – open, closed and hybrid – for regulatory flexibility
- Compact IQ series reduces installation space by up to 25% and weight by ~30%
- Integrated real‑time monitoring of pH, PAH, temperature and turbidity for compliance reporting
- Wärtsilä lifecycle support with documented inspection intervals
Weaknesses
- Requires seawater with alkalinity ≥2300 µmol/l; not always available in low‑alkalinity regions
- Open‑loop discharge is prohibited in many ECAs and several ports, limiting operational zones
- Higher water consumption and potential for corrosion of discharge piping
- Maintenance intensive – regular inspection of venturi, spray nozzles and pH sensors
- Discharge monitoring and record‑keeping add administrative workload
Typical Vessels:
Container ships (≥8,000 TEU)Bulk carriersCrude and product tankersGeneral cargo vesselsCruise ships operating on low‑sulfur fuel regimes
Certifications:
IMO MARPOL Annex VI complianceDNV GL SCRUBBER notation
Decision Guide: Choose if the vessel operates primarily in open waters or ports where open‑loop washwater discharge is permitted, needs a proven high‑efficiency SOx solution and values flexibility between open, closed and hybrid modes. Avoid if the ship will spend most of its time in strict ECAs or ports that ban open‑loop scrubbers, or if water‑quality constraints make seawater alkalinity unreliable.
Use Cases: Deployed on newbuilds and retrofits of large container, bulk carrier and tanker fleets to meet global sulfur caps while retaining the option to run on high‑sulfur fuel in regions where open‑loop discharge is allowed. Frequently installed on vessels that require rapid compliance with Annex VI without committing to low‑sulfur fuel logistics.