Fuel filter blocking: Get ready for winter – Guest blog – 21 October 2020

As we move from autumn into winter, and the prospect of further problems with fuel and filters, it is worth looking at the seasonality factors that affect diesel and gasoil fuels writes Nigel Elliot, chair of the BSI Agricultural Filter Blocking Taskforce in this guest blog for NFU Scotland.

(For a full biography on Nigel Elliot click here)

The take home messages are:

  • Winter grade fuel will operate all year round, whilst summer grade will not perform in winter due to waxing. Therefore, Farmers and crofters should aim to use all their summer fuel before the winter months.
  • The application of cold flow additives is specific to the fuel blends and best done in warm conditions, before waxing occurs. Nigel notes “Aftermarket cold flow additives can disrupt the careful matching process and deteriorate cold flow performance and are generally not recommended.”
  • Farmers and crofters should aim to have used as much fuel as possible to prepare to receive winter grade fuel from 16 November.

Diesel and gasoil fuels are a complex mixture of hydrocarbons and, more often than not these days, are required by legislation to include renewable biodiesel such as FAME (Fatty Acid Methyl Ester) and HVO (Hydrotreated Vegetable Oil).

Depending on the source of the petroleum crude and on the level of refinery processing, some 15 to 30 percent of the fuel contains heavy paraffinic hydrocarbons, often referred to as normal paraffins or n-alkanes. They have limited solubility in the fuel and if cooled sufficiently, will drop out of solution as wax.

The introduction of renewable biodiesel blend components has brought an additional complication and is a source of saturated material that can further deteriorate the cold flow performance of the diesel fuel blend and promote wax crystal growth if not carefully specified and managed.

So why not eliminate these waxy molecules from the fuel blend? Normal paraffins are an essential part of the diesel and gasoil fuels as they provide good ignition quality (high Cetane) which is particularly important under cold start conditions and also helps provide low exhaust emissions.  

In practice, when the temperature of the fuel is lowered below the cloud point, the wax crystals become visible and in untreated fuels appear as thin flat rhomboid shapes. These crystals can quickly block vehicle fuel system filters preventing the flow of fuel leading to loss of power, driveability issues and eventually, in the worst cases, engines stopping due to lack of fuel.

A test method known as the Cold Filter Plugging Point (CFPP) was developed in the 1970s to simulate this effect and to predict the temperature at which blockage was likely to occur. The CFPP is now used as the basis for setting cold operability specifications for diesel and gasoil fuels across Europe and elsewhere in the world.

To maintain vehicle and machinery operation during low temperatures, cold flow additives are added to the diesel and gasoil fuels during manufacture to modify the size and shape of the wax crystals that are formed as the fuel cools. The cold flow additives modify the wax crystals to make them smaller and needle like in shape. These needle-like crystals form a matrix on the filter that still allows the remainder of the fuel to pass through the filter and keep the engine running. As the engine warms up the excess recycled fuel from the injection system is routed back to the fuel filter and the vehicle/machinery fuel tank. The fuel recycled from the injection system is warm and soon dissolves any accumulated wax on the filter allowing the engine to remain running.

This is a delicate balance, but cold flow additives have been used for many years successfully around the world to deliver cold vehicle and machinery operability.

Addition of lower wax content fuels such as kerosene into the diesel/gasoil blend will improve cold flow performance but degrade ignition quality. Kerosene is also in great demand as aviation fuel, so the use of cold flow additives allows both diesel/gasoil and aviation fuel demand to be met by oil refineries.

Cold flow additives are specifically selected to match the fuel blend by the manufacturing refinery, depending on the amount and distribution of the wax across the fuel boiling range. Aftermarket cold flow additives can disrupt the careful matching process and deteriorate cold flow performance and are generally not recommended.

Cold flow additives also need to be thoroughly blended into the fuel and this is ideally done under warm conditions with good mixing energy, such as during production at the refinery.  It is therefore a waste of time adding additional cold flow additives at low temperatures as they will not be mixed properly and if wax precipitation has already started, will not have any effect and can make things worse by increasing filter blocking.

So why do we have different fuel quality in the summer and winter? The addition of cold flow additives and modification to the fuel blend to reduce wax content in the wintertime is an expensive process and not required all year round to ensure vehicle/machinery operability.

Seasonal cold flow specifications have been developed over many years based on individual country climatic data and experience, to ensure vehicle/machinery operability under typical seasonal ambient temperature conditions.

In recent years with the move to sulphur free (< 10mg/kg Sulphur) diesel and gasoil fuels, BS EN590 diesel fuel is often dual graded and also sold as BS 2869 A2 and D gasoil as it fully complies with the specification.

For this reason, the seasonal changeover dates have been aligned between the two standards, although the actual CFPP specifications are slightly different with BS EN590 diesel affording superior cold flow performance.

The various implementation dates for refineries, imports, terminals and delivery to end users are designed to ensure that summer quality fuel is changed over throughout the distribution system in a timely manner.

Winter specification fuel will operate satisfactorily all year round whereas summer specification fuel will likely result in vehicle/machinery operability problems in the wintertime due to waxing.

It is therefore recommended that farmers schedule fuel purchases such that their storage tanks are run down as low as possible prior to winter quality deliveries made from the 16th November. This should ensure the maximum protection through the winter period.

Nigel Elliott is a fuel quality expert with 40 years of experience in the oil industry. He worked for ExxonMobil (Esso) for 35 years in fuels and lubricant testing and fuels development. Prior to retirement in 2012, he was the Senior Fuels Technical Advisor for ExxonMobil Research and Engineering in Europe, Africa and Middle East. He chaired the European CEN TC19/WG24 fuels specification standardisation group responsible for the European EN590 diesel and EN14214 FAME specifications for 10 years from 2005 to his retirement in 2015. He has been a long-standing member of BSI PTI/2 group responsible for the UK versions of EN590 and EN14214 as well as the BS 2869 gasoil specification. More recently he chaired the CEN TC19/WG24/Abrasive particles task force, investigating abrasive damage to fuel injection equipment in Northern Germany and the UK due to contaminated diesel fuel imports and is now leader of the PTI/2/-/3 Agricultural panel investigating BS2869 gasoil filter blocking issues.

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