Why Heavy Fuel Oil Ends Up in the Sludge Tank

When discussing fuel efficiency onboard ships, most attention goes to SFOC figures and engine performance.

But there is another part of the fuel story that is often overlooked:

How much of the fuel you purchased actually reaches the engine?

In practice, around 1–2% of bunkered fuel may end up in the sludge tank. In some situations — especially with unstable fuels — this percentage can be significantly higher.

This part is usually not included in SFOC calculations.

Fuel Purchased Does Not Always Equal Fuel Burned

Modern marine fuels are becoming increasingly complex and unstable.

Operators regularly face challenges such as:

• Increased sludge formation
• Fuel incompatibility
• Asphaltene instability
• Difficulty keeping fuels within specification after separation
• Variable fuel quality between bunkerings
• Biofuel stability concerns

As a result, separators and purifiers are under increasing pressure to keep the fuel within specification while protecting engines and fuel systems.

Separators and purifiers remain essential onboard systems. Their job is to remove water, cat fines, and unstable fractions from the fuel before it reaches the engine.

However, under difficult fuel conditions, a larger amount of potentially usable fuel may also end up in the sludge tank.

What Is Sludge?

Sludge is a composition of solids, oil, and water removed from the fuel during separation and purification.

A typical sludge composition consists of:

• 2% solids (inorganic particles)
• 28% oil
• 70% water

According to the traditional "German Rule," sludge disposal below approximately 1.5% of the daily fuel consumption was historically often not accepted or considered realistic.

However, with today’s increasingly unstable fuels, sludge volumes can often become significantly higher.

The Hidden Cost of Sludge

Sludge is not only a waste product.

It also represents:

• Purchased fuel that never reaches the engine
• Additional sludge disposal costs
• More separator and purifier load
• Increased maintenance and operational attention
• Fuel handling inefficiencies that are usually not visible in SFOC calculations

Especially the oil fraction inside the sludge is important.

A considerable part of the sludge tank content may still consist of usable fuel.

Improving Separator and Purifier Performance

The IPCO Power FID Reducer is installed before the separator or purifier.

The system mechanically conditions the fuel before separation by homogenizing the fuel and improving consistency.

This helps:

• Reduce sludge formation
• Improve fuel stability
• Support separator and purifier performance
• Increase the usable portion of the bunkered fuel

Rather than replacing separators or purifiers, the FID Reducer works together with these systems.

Practical Results and Separator Optimization

In practical operation, we have seen sludge volumes reduced so significantly after installation of the FID Reducer that authorities initially questioned whether the reported sludge disposal volumes were correct.

This was mainly achieved by extending separator discharge intervals (shoot time), resulting in less usable fuel being discharged to the sludge tank.

In one practical example, separator desludging frequency was reduced from 24 discharges per day to only 6 discharges per day after installation of the FID Reducer.

The reason was simple: less usable fuel ended up in the sludge tank.

This directly affects return on investment.

Lower sludge volumes mean:

• Lower disposal costs
• More usable fuel available for combustion
• Reduced fuel losses
• More stable fuel system operation

Improving Separator Performance Before the Separator

The IPCO Power FID Reducer is installed before the separator.

The system mechanically conditions the fuel before separation by homogenizing the fuel and improving consistency.

This helps:

• Reduce sludge formation
• Improve fuel stability
• Support separator and purifier performance
• Increase the usable portion of the bunkered fuel

Rather than replacing separators or purifiers, the FID Reducer works together with these systems.

The goal is simple:

First maximize the amount of usable fuel from the bunker fuel purchased.

Then ensure the remaining fuel is burned as efficiently as possible.

Fuel Efficiency Beyond Traditional SFOC Calculations

A vessel may report good SFOC figures while still losing a noticeable amount of purchased fuel through sludge discharge.

That is why fuel efficiency should not only focus on combustion efficiency inside the engine.

It should also consider:

• Fuel losses before combustion
• Separator and purifier performance
• Fuel stability
• Sludge generation
• Overall fuel handling onboard

Especially with increasing fuel prices and more unstable fuel blends entering the market, these factors become increasingly important.

A Practical Approach

At IPCO Power, we believe fuel treatment should remain practical.

Not by adding unnecessary complexity, but by improving fuel handling where it matters most.

Reducing sludge means:

• More usable fuel
• Lower disposal costs
• Better fuel system stability
• Improved operational efficiency

And most importantly:

Making better use of the fuel you already purchased.

Interested in discussing sludge reduction, separator performance, or fuel conditioning onboard your vessels?

Feel free to contact IPCO Power for more information.