Exhaust gas cleaning systems are an important part of maritime emission compliance, but not all scrubber configurations behave the same in operation.
For shipowners, the choice between open loop, closed loop or hybrid scrubber operation is not only a regulatory decision. It is an engineering decision that affects water chemistry, pump load, energy consumption, maintenance and long-term operational flexibility.
Open Loop and Closed Loop: The Core Difference
In an open loop scrubber, seawater is used as the scrubbing medium. The natural alkalinity of seawater neutralizes the acidic components created when sulphur oxides are removed from the exhaust gas.
In a closed loop scrubber, the washwater is recirculated inside the system. Instead of relying mainly on seawater alkalinity, the system uses chemical dosing to control pH and maintain the scrubbing process.
This creates two very different engineering profiles.
Open loop systems are typically more dependent on high seawater flow and local water conditions. Closed loop systems offer greater chemical control, but require more process equipment, monitoring and operational discipline.
Water Chemistry: Natural Alkalinity vs Controlled Process
Water chemistry is one of the biggest differences between open loop and closed loop operation.
Open loop systems rely on the natural alkalinity of seawater. This can be effective, but performance may vary depending on the water area in which the vessel operates. Lower alkalinity can require higher washwater flow to maintain scrubbing performance.
Closed loop systems are less dependent on seawater chemistry because alkalinity is managed through dosing. This gives more control, but also adds more parameters that must be monitored and maintained.
A closed loop system requires control of:
- pH levels
- Chemical dosing
- Bleed-off water
- Sludge formation
- Tank levels
- Sensor performance
- Water treatment efficiency
The trade-off is clear: open loop is simpler in chemistry, while closed loop gives more control but adds system complexity.
Pump Load and Energy Consumption
Pump load is a key operational factor in scrubber design.
Open loop scrubbers usually require large volumes of seawater to be pumped through the system. This places higher demand on seawater pumps, piping, valves, strainers and discharge systems.
As a result, open loop operation can increase auxiliary power consumption, especially when the system must handle high engine load, high sulphur fuel and changing seawater conditions.
Closed loop systems normally circulate a smaller water volume, but they introduce other energy consumers, including:
- Circulation pumps
- Dosing units
- Water treatment equipment
- Sludge handling systems
- Monitoring and control systems
So the difference is not simply that one system uses more energy than the other. Open loop systems are often flow-intensive. Closed loop systems are more process-intensive.
For shipowners, the important question is which configuration delivers the best total operating profile for the vessel's route, port calls and compliance exposure.
Maintenance Implications
Open loop and closed loop scrubbers also create different maintenance needs.
In open loop operation, maintenance is often focused on the seawater side of the system. Pumps, valves, filters, strainers, overboard lines and emission washwater monitoring equipment must be kept in reliable condition.
Closed loop systems require a broader process maintenance approach. In addition to pumps and scrubber components, the crew must manage dosing equipment, process tanks, water treatment units, sensors and sludge handling, also to include often a more complex control system.
Typical maintenance focus areas include:
- Pump condition
- Sensor calibration
- Chemical dosing accuracy
- Water treatment performance
- Sludge management
- Software and control settings
- Spare parts availability
This is why preventive service, crew training and reliable spare parts logistics are important parts of long-term scrubber performance.
Compliance and Operational Flexibility
Open loop scrubbers can be effective where washwater discharge is permitted. However, restrictions in certain ports and coastal areas can reduce operational flexibility for vessels that rely only on open loop operation.
Closed loop operation allows vessels to continue scrubbing in areas where open loop discharge is restricted, as the washwater can be recirculated and managed onboard.
For many shipowners, a hybrid scrubber configuration offers the most flexible solution. It allows the vessel to operate in open loop mode where permitted and switch to closed loop mode where required.
This is also why open loop to hybrid scrubber conversion has become relevant for vessels that need to protect port access, trading flexibility and long-term compliance.
When Conversion Becomes Relevant
For vessels already equipped with an open loop scrubber — regardless of the original system manufacturer — conversion to hybrid configuration can be a practical way to extend the value of the existing installation.
ME Production carries out conversion projects on existing scrubber installations across brands. The scope of work is the same whether the vessel runs an ME Production system or another manufacturer's installation, since the engineering challenge is defined by the vessel's existing setup, not by who originally supplied it.
A conversion project may involve:
- Chemical dosing systems
- Water treatment equipment
- Process tanks
- Piping modifications
- Control system automation upgrades
- Sensor integration
- Sludge handling setup
- Crew training and service planning
The goal is not only to meet current rules, but to prepare the vessel for a more complex regulatory environment — no matter which scrubber brand is already on board.
A Note on US Waters
Shipowners trading in US waters face an additional layer of complexity under the Vessel General Permit (VGP), where discharge pH limits shape how scrubber washwater can be managed. NaOH dosing is one alternative some operators consider as an option to closed loop conversion. This is a topic in its own right — we'll cover it in a dedicated post soon.
ME Production's Engineering Approach
ME Production supports shipowners with exhaust gas cleaning systems for both newbuildings and retrofits, including open loop, closed loop and hybrid configurations.
This support is not limited to ME Production's own systems. With dedicated project competencies for converting existing scrubber installations, ME Production works across scrubber brands — helping shipowners upgrade whatever system is already installed on board, based on the vessel's actual operating profile.
With in-house engineering capabilities, global service support and experience in scrubber conversions across manufacturers, ME Production helps evaluate the right solution regardless of the original supplier.
The right scrubber configuration depends on:
- Vessel type
- Trading route
- Port restrictions
- Fuel strategy
- Available space
- Pump load
- Energy consumption
- Maintenance capacity
- Long-term compliance needs
The best solution is not always the most advanced system. It is the system that fits the vessel, the crew and the operating profile — whichever brand it started with.
Getting the Configuration Right
Open loop and closed loop scrubbers both support SOx compliance, but they do so through different engineering principles.
Open loop systems rely on seawater chemistry and high washwater flow. Closed loop systems rely on controlled water chemistry, dosing and onboard process management. Hybrid systems combine both and give shipowners greater operational flexibility.
For shipowners, understanding these trade-offs is essential. Water chemistry, pump load, energy consumption and maintenance all influence the long-term value of the system.
ME Production helps shipowners evaluate, convert and service scrubber systems based on real operational needs — from compliance requirements to long-term performance.