Open Loop Scrubbers – An Overview

Open Loop Scrubbers – An Overview

With the IMO’s 2020 global sulphur cap deadline getting closer, owners and operators are faced with having to make a significant operational and commercial decision – whether to continue operating on low sulphur fuel, post 2020, or whether to finance the retrofitting of a scrubber system to the vessel.

Depending on the type of scrubber selected, retrofitting the system to an existing vessel can be a complex and expensive process. Setting aside the significant capital cost of the technology itself, the engineering required to integrate the scrubber system, supporting structure, pipework and all required modifications to the vessel can be vast.

However, perhaps the most critical decision to be made by owners and operators is the type of scrubber system itself. With three different types of scrubbers available, namely open loop, closed loop and hybrid systems, each with their own inherent benefits and drawbacks, the selection of the most appropriate system for the vessel’s trading patterns is highly important.

Sticking to our commitment to being owners and operators trusted advocate in achieving environmental compliance, Cleanship Solutions has developed a three part deep-dive series on each of these scrubber types, highlighting critical considerations of each option.

Part one is the open loop scrubber.

Please note that this article does not discuss the physical retrofit practicalities of the open loop scrubber option – as this will be covered in detail in future scrubber related articles. Rather, this article has been developed to identify critical commercial considerations of the open loop scrubber option, and their potential impact on the vessel’s performance.



Open loop wet scrubber systems generally use sea water as the scrubbing liquid, which is typically pumped directly from a dedicated sea chest (which is often a new sea chest due to the significant volumes of water required). Once the scrubbing process has been completed, the wash water is filtered and separated accordingly and discharged overboard.

Sludge is collected separately for later discharge to shore. The wash water is generally closely monitored by an effluent monitor to ensure the discharging water meets the legislative requirements.


Primary Benefits of Open Loop Scrubbers

Open loop scrubbers offer a number of immediately identifiable benefits over both closed loop and hybrid scrubber systems.

The main benefit is their simplicity. Sea water is simply pumped through the system, filtered, and discharged back overboard – a process that most owners, operators and ship based crew are familiar with and comfortable with. With such simplicity, in our experience, the perception from owners and operators is that there are fewer things to go wrong, and as such reliability is significantly higher than other scrubber options. Such simplicity also impacts the retrofit engineering and installation of the scrubber – offering a much more cost effective solution.

Another significant benefit of open loop scrubbers is their lack of reliability on additional chemicals to ensure their efficacy. Both hybrid and closed loop systems can be reliant on additional chemicals to condition the scrubbing liquid to a sufficient level to guarantee efficacy. Without a requirement for chemical storage or the treatment and re-circulation of scrubbing liquids, open loop scrubbers again offer both CAPEX and OPEX savings against both closed loop and hybrid scrubber options.


Critical Considerations

Whilst open loop scrubbers have some attractive benefits, there are a number of critical considerations owners and operators are well advised to examine when determining the suitability of an open loop scrubber option. We take great care to analyse these aspects when conducting scrubber feasibility studies on behalf of our ship owner and operator clients, and are excited to share such important insights with our readers.


Vessel Trading Patterns & Water Characteristics

The success of open loop scrubbers is highly dependent on the alkalinity of the water being used as the scrubbing liquid – in marine cases this intuitively being the water in which the vessel is trading. The alkalinity of the scrubbing liquid neutralises the acidity of the SOx in the exhaust gas, keeping the pH suitably high. Without this alkalinity, the efficacy of the scrubbing process is seriously and detrimentally affected.

Whilst most sea water locations have high alkalinity, there are certain key geographical areas which do not offer the required alkalinity, including the Great Lakes and the Mississippi, potentially rendering the open loop scrubber an unviable solution, so owners and operators are well advised to examine the historic and planned (where known of course) trading patterns of the vessel, and determine the range of water alkalinities across these regions.

System Wear and Fouling

The water conditions have another critical impact on the performance and maintenance of open loop scrubbers – primarily in the form of system wear and long term fouling. Even with smaller filter meshes installed in sea-strainers, it is likely that the scrubbing liquid will enter the scrubber unit inclusive of small particles such as sand, as well as small organic species such as jellyfish and algae. The former will, over time, cause wear on the scrubber components, particularly the injection nozzles, and this additional maintenance and spares should be factored in by the owners and operators. In the case of the latter, it is likely that the organic species will build up in way of the nozzles of the scrubber, and cause unwanted fouling – decreasing performance and requiring frequent maintenance. Again, owners and operators should remain conscious of these limitations and potentially examine the use of self-backflushing filters as an additional step to minimise these impacts.

Power Availability & Operating Costs

With open loop scrubber systems requiring such vast quantities of water being pumped continuously from the sea-chests up to near the highest points in the funnel, the additional power consumption for these pumps alone is likely to be significant. Indeed, owners and operators may be faced with upwards of 150kW of new sea water pumps, due to not only significant volume demand from the scrubber, but also the discharge head necessary to reach the top of the funnel.

Setting aside the obvious impact on fuel consumption onboard, which is not insignificant considering the constant requirement to run these pumps, such significant power requirements may also cause issues with some existing vessels – particularly those that are already underpowered. Whilst this is most likely to manifest itself during power hungry operational modes onboard – such as cargo operations of thruster operations, it is, nonetheless, something owners and operators should pay close attention to during the determination of the feasibility of an open loop scrubbers system.

Impact on Waste Heat Recovery and SCR Systems

One of the most detrimental effects of open loop scrubber systems is their impact on the temperature of the exhaust gas. Open loop scrubber systems, due to using a constant supply of sea-water, can significantly lower the temperature of the exhaust gas to the extent that it has a negative impact on any existing exhaust based systems – such as waste heat recovery systems or selective catalytic reduction systems (SCR). Whilst one may intuitively relocate such existing technologies upstream of the scrubber, this too can have negative impacts on the temperature of the exhaust gas, and cause detrimental performance issues on the scrubber. Therefore this process is somewhat iterative and requires close discussions between the scrubber manufacturer and the manufacturer of the waste heat recovery or SCR systems, to ensure both are operating optimally, or, at the very least, within an acceptable compromise.

Acceptability of Open Loop Scrubbers in Port

Whilst open loop scrubbers do offer a viable compliance solution to meet the requirements of Annex VI and other sulphur reduction requirements, their acceptance for use in port is a subject that is, at the time of writing, continuing to be hotly debated within the industry.

Many stakeholders on the environmental side of the divide believe that removing the sulphur content and relative acidity from the exhaust gas and discharging it (loosely speaking) into the sea is potentially hazardous to marine life, particularly in shallow waters, and are calling for such activities to be banned within port areas. Indeed, many ports around the world have mooted that they may consider banning such discharges in the near future.

Whilst this is yet to be officially legislated by any industry body or IMO, it is a topic that Port States have jurisdiction over in the meantime, and presents a real and significant risk to owners and operators considering open loop systems. Owners and operators are well advised to seek the advice of key Ports within their trading patterns.


Whilst the open loop scrubber system is an attractive and cost effective solution for complying with forthcoming legislation, owners and operators are advised to ensure a thorough review of the impact of the key commercial implications discussed within this article prior to confirming suitability.

Cleanship Solutions offer comprehensive scrubber feasibility studies for owners, analyzing the various options available and objectively determining the most suitable system for any given vessel. We have no commercial ties to any particular manufacturer, meaning we can offer vessel specific recommendations free of any subjective preference…