Loss of Power (Blackout) Due To Filter Choking

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filter

Incident:

A fully loaded tanker en route to the port of Hamburg changed over to LSMGO prior entering the ECA zone.  The vessel did not observe any problem until 12 hours after the changeover.

As a precautionary measure, the engine room remained manned for the ECA zone anticipating machinery problems.   At 13:00 hours, the third engineer observed fluctuation of fuel oil pressure on the diesel generator engine.  He changed over the fuel oil filter and started cleaning the isolated filter.  He found white, waxy substance on the filter and neglected it, thinking that it could be some normal debris from the tank.

Two hours later, the fuel oil pressure once again fluctuated but this time the engine came to halt resulting in a blackout.  The engineers attempted to start other diesel generator engines to restore power and the propulsion.  However, their attempt was not fruitful.

Upon investigation, it was noted that the filters were clogged with white waxy substance.  The fuel circulating pumps were not building up the pressure as the suction filters were found clogged.  Further investigation by the engineers revealed that the entire LSMGO tank was found to have solid wax like paraffin which was not flowing.

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Phenomena:

As a distillate fuel cools, the first phenomenon to occur is the clouding of the sample. Therefore, the cloud point is the first value.  As the fuel is cooled further, the Cold Filter Plugging Point (CFPP) occurs when small wax formation will tend to block the filters.  When the MGO is cooled still further, at the lowest point, the pour point takes place when the entire MGO sample solidifies.

The important point to note is that the cloud point and CFPP will depend on the wax content in the fuel.  Greater the wax, higher the temperature at which CP and CFPP will take place.  Suppliers are required to treat the fuel with wax modifiers which would result in a greater difference between CP and CFPP.

Additive manufacturers have been developing products to improve cold flow properties of Marine Gas Oil (MGO), and specifically for DMA grade.  As per one major manufacturer, for fuels which have been pre-additized, typical responses seen are between 3 and 13°C reduction in CFPP (average 4.6°C).

There is also a grade of fuel available in Europe called Arctic Diesel.  This grade is specially formulated for use in cold weather regions with a UPP below -20 degC and low CP and CFPP values.

The CFPP and CP tests are not included in the ISO 8217 standards.

Information You Need To Have On Cold Flow Properties:  

The cold flow properties of a fuel oil are temperature dependent and can have an adverse effect on low-temperature operability.  For example whilst one fuel at very low temperatures may remain fluid another of a similar grade may either stop flowing or result in the deposition of wax crystals at the filters causing fuel starvation and possible engine shut down.  

ISO8217 limits the cold flow properties of a fuel through setting a limit on the pour point (PP).  However, given that wax crystals form at temperatures above the PP, fuels that meet the specification in terms of PP can still be challenging to operations in colder operating zones, as the wax particles can lead to rapidly blocking filters, potentially blocking them completely

The cold flow properties of a fuel can be evaluated through one of, or a combination of, the three most commonly used test methods, these being: cloud point, cold filter plugging point and pour point.  

Cloud Point (CP)

The cloud point is defined as the temperature at which wax crystals start to visibly form in the fuel and a transparent fuel becomes cloudy (thereof the name) (ISO3015).

Cold Filter Plugging Point (CFPP)

The cold filter plugging point is the highest temperature, expressed in multiples of 1°C, at which a given volume of fuel fails to pass through a standardized filtration device (45 microns) in a specified time (60 sec) when cooled under the conditions prescribed in this test method (ASTM D6371).

This test gives an estimate for the temperature at which a fuel will give trouble-free flow in certain fuel systems.  

Pour Point (PP)

The pour point is defined as the temperature at which the fuel ceases to flow, experienced as the fuel being perceived to have turned solid (ISO3016).

Typically, the difference between the above temperatures will be about 5‐10 degrees for untreated fuels although higher differences are recorded, with the CP having the highest temperature and the PP the lowest.  

The CP identifies the temperature at which crystals will start forming in the fuel, whereas CFPP provides a relative indication of the point where filterability problems may start to occur, leading to possible restrictions of fuel flow to the engine.  Statistically, it is not possible to determine the CP and CFPP from just knowing the PP figure as the correlation is very broad.

When it comes to the CFPP, recently there have been a number of cases where the differences between the CFPP and PP have been as much as over 24 deg C; The PP was meeting specification being well below zero degrees Celsius. Ships were experiencing heavy wax deposits in the separators, restricted filters along with heavy wax deposits in the storage tanks.

Cold flow improvers (CFI) ‐ additives

Improving and managing the cold flow properties of a fuel by use of additives is a complex task requiring several considerations:

  • Additives can be applied to depress the CFPP and PP; however, additives cannot influence the CP
  • The CFPP reducing additives work by changing the shape of the wax crystals to facilitate the passage through the filters
  • Some additives only impact the CFPP or PP whereas other will have an influence on both CFPP and PP

Onboard Test for determining the point at which the fuel starts to deposit waxes

  • Take a sample about 50‐100 ml of the fuel  in a clear glass jar  
  • Clearly mark the glass jar: “Not for consumption”
  • Place the fuel filled glass jar in a fridge at 3‐6 deg C for 2‐3 hours
  • Confirm that the temperature of the fuel is at fridge temperature
  • Note if the fuel has become cloudy or even deposited wax on the glass sides If so, the CP (and possibly the CFPP) exceeds the fridge temperature

Sources : CIMAC, INTERTANKO and others.

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