Centrifugal Separators & Operational Efficiency – An Important Insight


ISO 8217 compliant fuels are used on board ships for propulsion and power generation. Environmental regulations have forced the maritime industry to use different types of fuels. Uni-fuel ships have become multi-fuel ships with complex operational procedures and new engineering challenges. Most engine makers have moved towards electronic engines and demand a better fuel quality. However, ISO 8217 still caps the catfines at 60 ppm in bunker fuels. Most engine makers recommend not more than 10 ppm of catfines at engine inlet. Can you see the GAP?

There is no regulation or requirement to fulfil this GAP and this has been vested on ships crews and managers. This technical write-up will reinforce and enlighten on performance and efficiency of centrifugal separators.

It is evident that removing catfines alone is not important but the size of remaining catfines in fuel matters a lot. A purifier can remove water, sludge, and abrasives from fuel oil. If properly maintained and operated, a separator can remove particles down to approximately 5 µm. Smaller particles are difficult to remove, hence good maintenance and operating procedures gets significant.

The efficiency is important and should be checked every third month as a minimum. Check the efficiency by drawing samples before and after the separators and send these to an MAN PrimeServ Lab or similar for analysis. Samples should be taken more often when operating on fuel oil bunkered with more than 25 ppm cat fines. (Source: MAN Diesel & Turbo)

MAN Diesel & Turbo’s requirements – Quality of Fuel Oil after Separation.

The good old theory for better separator efficiency is dependent on two important parameters:

  1. Flow
  2. Temperature

There are other factors like back pressure, etc. which are intrinsic operational factors which can be neglected considering the scope of this write-up. A high temperature and low flow removes a high amount of particles – good efficiency, while a low temperature combined with a high flow barely removes any particles at all – poor efficiency.

It is evident from the above graph that the particle removal is better when the flow or feed into the purifier is kept minimal and the temperature as high as possible (close to 98 degree Celsius).

Why is it essential to keep an eye on flow rate?

The main propulsion and auxiliary engines are designed to carry a specific amount of cargo and at a particular speed. Thus, the fuel consumption per unit time will need to be considered when selecting a purifier. A ship which consumes 50 MT/day usually houses a purifier of at least 4 cubic meters per hour. This figure may vary depending upon various factors (tank size, etc). An important point to note is that the purifier capacity is much higher than the required fuel consumption. Ships crews allow the purifier to operate at maximum flow rate and thus overflowing service tank back into settling tank.

Higher flow rate may not be effective in removing contaminants and thus, it is essential to keep flow rate as low as possible and meeting the required fuel consumption per day (plus some buffer).

Another issue to address is that good maintenance is important, often overlooked, and difficult to quantify. However, if the bowl is not cleaned in time, deposits will form on the bowl discs, the flow velocity will increase, and the separation efficiency will drop.

It is well known that settling and draining the tanks (both settling and service) may remove some high density particles and some catfines. It is estimated that upto 5% of catfines can be removed in settling tanks by settling and draining the tanks effectively. Holding down the drain cocks open for just a minute may not help and it is essential to train the ship’s crews stating the importance of draining fuel tanks.

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