The Impact of Regulation on Cylinder Oil Lubricant Selection (Part III)

In this final part, we would like to give you a detailed look into the effect of cylinder oil on vessels’ operation

IV. IMPACT ON CYLINDER LUBRICATION
Not utilising the correct cylinder oil lubricant in two-stroke engines with an optimised feed rate has serious financial implications for ship operators:

1. Cylinder oil lubricants are a single use product. If feed rates are not optimised, then there can be costly, unnecessary oil consumption.
Many vessels have the feed rate set higher than is necessary by up to 50% or more which means that a significant volume of cylinder oil is wasted every year through over lubrication. On a single large vessel this over lubrication can cost as much as an additional $100,000 in a year.

2. Incorrect use of lubricant and mis-match to the type of fuel used can have serious mechanical consequences including excessive wear of engine components.
This can cause shortened life spans of engine components and costly replacements through to potentially catastrophic engine failure.

In order to stay competitive in a challenging market it is critical ship operators utilise the correct lubricants as well optimising the quantities utilised for each vessel.

4.1 Impact of SOx Regulations on Lubrication
4.1.1 Operation with Traditional Marine Fuels
During combustion the sulphur in the fuel is subject to oxidation and SOx (sulphur dioxide SO2 and sulphur trioxide SO3 ) formation. The SOx compounds react with the water created from combustion to sulphurous and sulphuric acids.

These corrosive compounds, if not neutralised, will corrode the cylinder liner wall. One of the principal functions of the cylinder oil lubricant is to act as a neutralising agent.

The BN (Base Number) of a lubricant represents the ‘neutralising power’ of a lubricant.

What is the base number (BN) number?
The BN is what is often referred to as the ‘alkalinity’ or ‘base’ of a lubricant, however it is actually the quantity of acid, expressed in terms of the equivalent number of milligrams of alkaline potassium hydroxide, that is required to neutralise all alkaline constituents in one gram of sample.

The higher the sulphur content of the fuel, the more acidic compounds form and the higher the BN of the lubricant that is required for effective neutralisation.

Therefore, a careful balance is required.

The BN of the lubricant must be matched to the sulphur content of the fuel being burnt.

What Are the Risks?
A BN too low? The corrosive sulphuric by-products of combustion remain un-neutralised. These create excessive corrosion of the cylinder liner leading to metal to metal contact and scuffing as oil control is lost.

A BN too high? There is more base (calcium carbonate, CaCO3 ) than is required. This starts to form hard deposits. These deposits can cause fouling of the piston crown which in turn can lead to bore polishing as well as deposition of ash in the combustion chamber, exhaust vales and turbocharger. Again, loss of oil control results in metal to metal contact and adhesive wear.

Heavy Fuel Oil (with a maximum of 3.5% sulphur content)
Recommendation:

• For newer-generation NOX Tier II compliant engines and earlier engines with modifications to operate under low load conditions, cylinder oils of up to 100BN are recommended by OEMs.

• For older engines and those not operating under conditions where cold corrosion is likely to be a problem a 70BN remains the standard.

Distillate/HFO Blends (with a maximum of 0.5% sulphur content)
Recommendation: 25 – 40BN cylinder oils for two stroke engines. Extra care with regard to monitoring and control of feed rates is advised.

Distillate Fuel and Marine Gas Oil (with a maximum of 0.1% sulphur content)
Recommendation: Low BN cylinder oils (~25BN) for two stroke engines.

4.1.2 Fuel Switching
As the BN of a lubricant must be carefully matched to the sulphur content of the fuel and the operating conditions of the engine, switching between fuels of differing sulphur contents causes a mismatch.

What Are the Risks?
It will depend on the length of time a vessel operates with a lubricant that is not matched to fuel sulphur content.

Recommendation: Since the introduction of switching between fuels with varying sulphur content there has been no ‘safe period’ that has been established by OEMs. Vessels have been found to be particularly at risk when leaving ECAs.

Therefore, the advice is to switch lubricants when switching fuels, always taking care to align the timing of fuel change with lubricant change.

4.1.3 Operation with Alternative Fuels (such as LNG, methanol, ethane and LPG)
For dual-fuel operations, two very different types of fuel are utilised interchangeably.

Recommendation: Operators can either switch between two types of cylinder oil lubricant- one for gas and one for liquid fuel operation.

Alternatively, depending on the predominant operating profile of the vessel, it is possible to utilise a single lubricant for their dual-fuel engines designed to match main operating condition.

4.2 Impact of NOx Regulations & EEDI on Lubrication
4.2.1 Tier II NOx Reductions & EDDI
Tier II NOx regulations (combined with Energy Efficiency Design Index (EEDI) guidelines) have led to the development of new engine types to meet the requirements.

By utilising longer piston strokes these newer engine designs achieve improved fuel oil consumption.

What Are the Risks?
To achieve this the cylinders are operating under increased peak pressures and reduced operating temperatures.

This creates conditions below the dew point meaning that water may condense on the cylinder liner walls leading to significant sulphuric acid formation as compared to traditional engine designs.

If the excessive sulphuric acid is not neutralised by the cylinder lubricant it accelerates cylinder liner wear.

The phenomenon is known as cold corrosion.

Recommendation: Firstly, if cold corrosion is suspected it is important to identify how serious the problem is. OEMs recommended that cylinder scrape down monitoring is done to understand what is happening in the engine.

MAN Diesel recommends the use of a ‘Sweep Test’ and Wärtsilä recommends its ‘Quick Test’.

These tests should be done as part of an established condition monitoring programme. They ensure that the performance of the engine is known so that the feed rate can be set appropriately, and the tests should be repeated whenever parameters change, such as a new fuel in use.

Not only will it enable the correct cylinder oil to be selected but it will enable the feed rate to be optimised to suit the engine, operating conditions and cylinder oil in use.

4.2.2 Tier III NOx Reductions
The impact on cylinder oil lubrication will depend on the NOx abatement technologies fitted:

Selective Catalytic Reduction (SCR) Technology
SCR is post combustion technique.

Cylinder oil lubricant selection therefore should be based upon matching the BN to the fuel sulphur content of the marine fuel used. The possible effect of cold corrosion must also be considered.

Specific considerations for SCR technology:

• Cylinder oil lubricant additives can have an impact on the long term performance of SCR catalysts, therefore the suitability of the cylinder oil lubrication match with the technology is a key factor to consider.

• A cylinder oil lubricant that has low sulphated ash is recommended to increase both the lifespan of SCR systems and the associated soot filters.

Exhaust Gas Recirculation Technology
Newer engines may now integrate EGR technology, which works by filtering, cooling and recycling a fraction of the exhaust gas back into the combustion chamber. This lowers the oxygen content of the scavenged air, resulting in a reduction of peak combustion temperature to reduce the formation of NOx.

4.3 Impact of New Generation Engines and Changing Operational Profiles on Lubrication
In order to comply with NOx Tier II regulations, there is a new generation of engines. Engine operating profiles are also changing to increase low-load and part-load operation.

What Are the Risks?
The new generation of engines are particularly susceptible to the issue of cold corrosion (see section 4.2).

Cold corrosion can also be an issue or further exacerbated when the ship operates at low load for long periods of time as the engine is operating at lower temperatures in this state.

Recommendation: OEMs’ recommendations will vary and the OEMs must be consulted for each specific engine type for optimal lubricant selection under different load conditions but a cylinder oil scrape down monitoring programme is always recommended.

In newer engines a number of design changes are being implemented including changes to the liner cooling systems and cylinder oil injection points to address cold corrosion.

Engine model, engine modification, engine load and fuel sulphur content must be taken into account when selecting a cylinder oil lubricant.

V. THE FUTURE
As legislation around the fuel-related emissions from shipping operations continues to evolve, coupled with the drive for efficiency, the role of cylinder oil lubricants and their requirement to perform under varying engine conditions will become progressively more challenging.

The anticipated drop in the global sulphur cap in 2020 or 2025 will impact marine fuel use across the global fleet and this in turn will mean that ship operators will need to reconsider the cylinder oil lubricants used on a vessel by vessel basis.

In addition, the need for a monitoring programme to ensure the safe and efficient operation of the engine will become increasingly more important.

It will therefore be vital for ship owners and operators to not only select the lubricant that best fits their needs, but also to choose a supplier that compliments their operational requirements and can provide the right level of global support for their fleet.

This report was delivered by Fathom Maritime Intelligence in collaboration with ExxonMobil