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Article: Exhaust Gas Cleaning Systems (Scrubber) Guidance

News & Insights 11 December 2019

If members elect to comply with the IMO global sulphur cap by fitting scrubbers, they will need to consider a few factors.


In a few weeks, IMO’s ‘global sulphur cap’ will take effect. The permitted levels of SOx in ship emissions will be restricted in association with the revised MARPOL Annex VI, ie the current sulphur limit of 3.5% of any fuel oil used onboard ships will be reduced to 0.5% from 1 January 2020, except for ships using ‘equivalent’ compliance mechanisms.

It is yet to be proven how reliable the quality and supply of low sulphur compliant fuel (LSFO) is. There is a possibility that the price of high sulphur fuel oil (HSFO) could see a sharp decline in 2020. 

The ‘equivalent’ compliance mechanism includes fitting ships with an Exhaust Gas Cleaning System (EGCS), or scrubber that cleans exhaust gasses from the engines. Ships with scrubber installations are allowed to run on HFSO under the new regulations. Considering fuel price differentials, it is possible for shipowners to benefit from the lower prices of HSFO by choosing this method of compliance with the regulations.

Should members elect to continue operating on HSFO with scrubbers, they will need to consider a few factors which are outlined below.

Exhaust Gas Cleaning Systems

The available SOx scrubbing technologies fall into two main categories: dry and wet scrubbing systems.

Dry scrubbers, as the name implies, do not use water or any liquid substances to carry out the scrubbing process. Instead, the system exposes hydrated lime-treated granulates to the exhaust gas to create a chemical reaction that removes the SOx emission compounds. They are more commonly used on land-based installations and not so common for marine use.

Wet scrubbers are more common in the marine industry. These fall into three design variations:

  1. Open loop system, where seawater with naturally existing alkali is used for scrubbing and discharged back into the sea. These are only effective in removing SOx compounds from the exhaust gas when the source water is alkaline, therefore their effectiveness depends on the alkalinity of the water the vessel is operating in. Open loop scrubbers are relatively simple and comparatively cheaper than other wet scrubbing systems. Among the limitations of this system is the need to position the overboard discharge well away from the sea chests, as well as strict domestic wash water discharge regulations in some parts of the world (see table below).
  2. Closed loop system, where seawater or fresh water is typically treated by adding sodium hydroxide (caustic soda) to achieve the required alkalinity for scrubbing and neutralization. This system requires a separate tank for collecting any residual waste. This could incur additional expense, and logistical coordination in obtaining supplies and arranging for the discharge of waste to shore reception facilities.
  3. Hybrid system, which offers maximum flexibility, is a combination of the open and closed loop; but it is more complex and expensive.

Overview of scrubber discharge bans 

Some port states have concerns regarding water borne sulphates resulting from the neutralizing process being discharged into the sea, as they have been known to significantly change the pH of sea water. The changes may have an impact on marine biodiversity, especially among fish species. Therefore, discharge limitations set by the IMO and various region regulations stipulate that the pH of the wash water discharge must be measured prior to overboard discharge. 

Wash water from open loop scrubbers must be treated and monitored at the inlet and outlet to ensure that it conforms with the MEPC 259(68) discharge criteria, with no risk of harm to the environment. However, some coastal states and ports have implemented local regulations with more stringent requirements for the quality of wash water being discharged overboard. Details of these ports are given below:

China Discharge prohibited in inland & coastal ECAs and Bohai Bay waters.
**Ban in the entire coastal domestic ECA may be announced soon.
Hong Kong Discharge may be allowed but vessels are to request permission to use non-compliant fuel 14 days before arrival.
Singapore Use of open loop scrubbers prohibited within Singapore port limit.
Malaysia Use of open loop scrubbers prohibited throughout Malaysian territorial waters & ports.
India Generally permitted but some ports may have more stringent requirements in place.
UAE Abu Dhabi may allow discharge overboard if free from pollutants, whilst scrubber sludge should be discharged to a licensed waste disposal contractor. 
Fujairah has banned the use of open loop scrubbers in territorial waters.
Belgium Discharge prohibited in ports and inland waterways.
Germany Discharge prohibited in inland waterways and on the Rhine.
Lithuania Discharge prohibited when in port.
Latvia Discharge prohibited in territorial and port waters.
Ireland Discharge prohibited in ports of Dublin, Waterford and Cork.
Norway Discharge prohibited in Norwegian fjords (visible emissions reduction also required).
Panama Use of open loop scrubbers prohibited in Panama Canal waters.
Hawaii Currently permitted subject to special conditions.
Conneticut Discharge prohibited.
California Use of scrubbers is banned.

Members with an open-loop (or hybrid) EGCS system installed on their ships would need to be mindful when their ships operate in regions where scrubber discharge is banned. The ships will need to switch to either closed-loop mode or to compliant fuel well in advance of the ship’s arrival at such ports.

Scrubber type considerations

Vessels' operational profiles will differ vastly, and each ship will have its own unique solution. Some of the factors for deciding which type of scrubber to install, include: 

  • Vessel’s trading pattern - alkalinity of water on deep sea passage and length of periods in/out of restricted areas where scrubber wash-water is banned.
  • Commercial aspects - charterparty agreement, current fuel oil consumption, fuel oil bunker capacity, etc.
  • Initial investment, operational and maintenance costs - is the scrubber system type approved and inclusive of after-sales service provided by the manufacturer.
  • Installation time - installation times may vary depending on the ease of access to the funnel space and on the complexity/size of scrubber to be fitted. 
  • Vessel configuration - space availability onboard and complexity of the system (piping, tanks, etc).
  • Power requirement - additional load availability to operate the scrubber support systems.
  • Stability and lightship weight - these need to be evaluated due to the additional weight of the scrubber system.
  • Recalculation of the ship’s Gross Tonnage (GT) - due to enclosed superstructure being enlarged, causing volumetric change. 
  • Strengthening and Structural Enforcement - scrubber that is installed at the stern section of the ship may increase the ship’s hogging moment.
  • Increase of exhaust backpressure - may result in additional fuel consumption, increased component temperatures and increased wear.
  • Crew training - this will include operating the scrubber unit, monitoring control systems and carrying out maintenance, but also safe handling of the chemicals used and scrubber waste. The maintenance checklist and critical spares list will need to be prepared in consultation with the manufacturer.

Scrubber breakdown

Scrubbers, like any other piece of mechanical equipment, can break down from time to time. In the case of an exhaust gas scrubber installation, it is likely this will involve the external support equipment, pumps, pipes, valves, etc. Apart from the nozzles, there is very little that is likely to fail within the scrubber itself.


The wash water from scrubbers is acidic and highly corrosive. The effects are aggravated by high temperatures of exhaust gases. Therefore, the structural integrity of the scrubber and associated pipework will need to be regularly checked for signs of leakage or corrosion.

There are recent reports of scrubber pipework failing very early in the life of the installation, sometimes within six months of commissioning. These failures could be traced back to quality issues during its installation. Corrosion has been discovered in pipes leading from the scrubber and in way of the residue treatment systems which handle acidic substances. 

Pipes in these systems should ideally be manufactured from a corrosion resistant material, but as the demand to install scrubbers moves at an increasing pace, these materials are short in supply.

As an alternative, steel pipes are applied with a corrosive resistant protective coating and, like any other protective coating, for example ship shell coatings, the preparation of the substrate is very important. If the preparations are not carried out correctly, for example correct cleaning procedures with etching or pickling to remove grease and acid deposits from the internal surfaces of the pipe and for all corrosion products (for example surface rusting) to have been removed, the coating will not bond with the substrate correctly leading to an early failure of the coating. Similarly, it is essential the complete recommended coating system is correctly applied to the required dry film thickness (DFT) as set out by the coating manufacturer.

When overseeing a scrubber installation, members need to pay extra attention to the installation and coating of the system pipework, remembering that welding onto the surface of a pipe may well destroy any internal coating and could lead to an unexpected early failure of the pipe.

Failure of the above equipment will render the scrubber non-operational, making it mandatory for the ship to report it to the flag and relevant port state authority immediately.

Contingency planning

It is recommended that the Ship’s Implementation Plan (SIP) include a section on ‘contingency planning’ as there could be a potential non-compliance and off-hires in circumstances where the scrubber system breaks down or is defective. 

In case of a breakdown or reduced performance, members would need to notify both the flag and port states immediately. This should be viewed as a temporary non-compliance, but the vessel would not be in breach of the regulations if the vessel immediately took steps to change over to a compliant fuel. If a compliant fuel is not available, it must be bunkered at the next port alongside the necessary repairs.

Difficulty in repairing scrubber units while the vessel is in remote regions should be anticipated. Due diligence and preparation would require members to revisit their critical spares list, and in consultation with the manufacturer, add items which are most likely to break down, including the continuous emissions monitoring system. 

The Exhaust Gas Cleaning Systems Association (EGCSA) has produced a logic flow diagram which examines this scenario and provides guidance on appropriate steps to rectify the problem and remain compliant with MARPOL Annex VI, this is shown here. The Association also published a guide which can be found here.


The case for scrubbers largely depends on the ship’s trading pattern, fuel availability and price differential between the high and low sulphur (compliant) fuel oil.

While this compliance option may be lucrative for large vessels with high fuel consumption, in order to gain the competitive edge, the shipowner/operator would also need to consider the right technology and utilize reliable suppliers as any breakdown or malfunction of the scrubber could lead to significant fines.

The contractual agreements would also need to consider the down time for scrubber installation and other aspects which are covered in the club's article discussing charterparty FAQs

With thanks to Charles Taylor Adjusting / Marine for their contribution to the technical aspects of this article

类别: Alternative Fuels, Pollution

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