Winter Waxing of Diesel Fuels
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THE marketing of summer-grade and winter-grade diesel fuels by the leading oil companies accords with a policy of reducing overall refining costs to a minimum whilst matching the characteristics of the fuel to prevailing temperature conditions.
Ensuring that a fuel will operate satisfactorily in low ambient temperatures requires extra processing to remove the wax, to an extent depending upon the properties of the batch of crude oil being processed as well as the range of operating temperatures, and this increases production costs. Adding lighter fractions also tends to increase fuel consumption measured in volume (m.p.g.) because it reduces the specific gravity of the fuel. Special tighter fraction fuels of relatively low specific gravity are essential for arctic operations, whilst fuels can be used in tropical conditions that would be unsuitable for temperate climates.
It can be assumed that the winter-grade fuels offered by the larger oil companies in this country provide sufficient latitude for operating in the lowest temperatures normally recorded, given that the vehicle designer plans the fuel system to the best advantage and that the user takes reasonable precautions. To produce a "universal fuel" suitable for use in all types of fuel system (even in the "worst possible" layout) operating at low temperatures is unnecessary and would be unrealistic and uneconomic.
These comments give some indication of the problems facing the oil companies in catering for changes in ambient temperature. The suggestion that the low-temperature characteristics of fuels tend to vary is generally valid to the extent that the super-normal properties of some batches of crude oil raise the standard of the refined fuel above the minimum requirement. Artificial de-rating would be pointless as well as costly. The effect of reduced temperatures on diesel fuel is mainly a function of "cloud point" and "pour point ", and it is therefore initially appropriate to give a definition of these terms according to the Institute of Petroleum "Standards for Petroleum and its Products ".
Cloud point is defined as the temperature, measured in multiples of 2°F (1°C), at which a cloud or haze appears when the oil is cooled under prescribed conditions. Visual clouding of the fuel marks the initial onset of crystal precipitation or waxing and, depending on their size, the crystals may be trapped in filter elements or at other points of restriction in the fuel system.
Pour point is the temperature, measured in multiples of 5°F (3°C), that is 5°F above the temperature at which the oil just fails to flow (from a test tube) when cooled under prescribed conditions. Although the fuel does not flow freely, it may be possible to pump it by vacuum or pressure along a pipe. Many systems can handle fuel at a temperature considerably below the cloud point, and some simple types of farm tractor system can cope with semi-solid fuels at and below the pour point, which is usually some 10-15°F (5.4-8.1 °C) below the cloud point. A particularly fine filter, or one which is partially blocked by dirt, may become completely blocked by wax at a few degrees below the cloud point. Agitation of the fuel in the tank tends to prevent clouding. Normally, fuel starvation from waxing is less likely to be experienced during continuous operation of the vehicle, although waxing may occur in severe running conditions if the filter is cooled by fan draught or is located in a very exposed position and the vehicle is running light. Starvation normally becomes manifest when the vehicle has been standing for a long period in a low ambient temperature and after the engine has been running for a short time, the fuel under pressure in the injection pump and between the pump and injectors being generally usable, even if the temperature has fallen below the pour point.
Apart from location of the main fuel filter in the fan draught or in an exposed position, features of a system that promote waxing, or increase susceptibility to waxing, include the use of a gauze filter in the fuel tank and fuelfeed lines with sharp bends, both of which tend to trap the wax. Because waxing is partly a function of the rate of cooling in relation to fuel bulk for a given temperature, the more nearly the tank is filled to capacity, the longer will be the interval before the onset of waxing. A fuel tank also reduces the risk, of condensation and, consequently, of pipe or filter blockage, due to frozen water droplets.
In a. paper on "The Influence of Tractor Service Conditions on the Quality of Fuels, Lubricants and Protective Materials ", presented at meetings of the Automobile Division of the Institution of Mechanical Engineers in 1961 by Mr. E. S. Bates and Mr. R. P. Strettell, of the British Petroleum Co. Ltd., tests were described of six pairs of farm tractors operating in arctic conditions in Sweden, which were also applied to various commercial vehicles. The tests were performed to investigate the influence of vehicle and fuel-system design on blockage and to establish whether any relation existed between known fuel criteria, such as cloud point and pour point, and the minimum operating temperature of the vehicles. It was emphasized by the authors that diesel fuels contain a much higher proportion of wax than either vaporizing oil or petrol, and that the removal of wax during the refining process was an expensive operation which materially reduces the yield of gas oil from crude. It was, therefore, desirable to obtain factual data on the relative importance of the features that contribute to fuel-system failure in low ambient temperatures.
Tests were conducted over a temperature range from —10°C (14°F) to —35°C (-31°F) with fuels having a cloud point varying between —5°C (23°F) to —35°C. After being parked overnight in the open, the vehicles were, if necessary, started with ether or by towing and driven round a test track for a maximum of 30 min. This was the limiting period above which further running invariably raised the fuel temperature above the critical range. Temperatures and pressures at multiple points in the fuel system were continuously recorded. .
The tests underlined the importance of the type and location of the filters. For example, they showed that a greater blocking tendency can be expected if .a given filter is placed in the line between the tank and the feed pump rather than in the line between the feed pump and the injection pump. The practice of fitting a suction-side filter is less popular today than it was, but when suction-side filters are used they are often placed on the outside of the thissis, where they are quite unprotected from cold winds ind the vehicle's slip-stream. Obviously, the suction line itself should be as short as possible with a reasonably large liameter to promote easy flow, and so far as possible it thould be free from bends and junctions. It is often the kccumulative effect of these factors that makes a particular vehicle susceptible to fuel system failure due to wax.
Another example of a minor design feature that >ignificantly influenced performance was the absence of a radiator blind on a vehicle with its main filter mounted rnmediately behind the fan. An alternative to fitting a And would have been to re-position the filter on the 'ulkhead, where it would have received some warmth from he engine.
Despite the fact that the tank filters were close-mesh ;creens rather than filters, they were frequently a source of 'lockage. It was noted in the paper that the flow rate B3s! through the tank filter is normally much greater than the maximum fuel rate demanded by the engine (a substantial proportion of the fuel being returned from relief valves and air vents, and by virtue of back leakage and so On, to the tank), which increases the throughput of wax crystals in the tank filter.
It was also found that the surface area of the filtering element was critical in relation to •the rate of fuel flow. Even in the case of a screen upstream of the lift pump, the element was blocked after a short period unless it had a large surface area.
On one of the trucks, the fine-felt filter was positioned in front of the engine block immediately behind the radiator fan and was found to be much more prone to blockage than similar or even finer elements that were better sited on other vehicles. It was observed that the finer the filter mesh or the smaller its pore size, the greater was the blockage tendency, but that the fineness of the filter was generally less important than the location.
The conclusions drawn from those tests are that vehicles will run as long as the ambient temperature is above the cloud point of the fuel and that failure is likely somewhere between the cloud point and the pour point. Although a simple fuel system may operate at temperatures below the pour point, this is unusual. Small design changes in the fuel system. can lower the minimu.m operating temperatures of the vehicle by several degrees.
The importance of filter features when operating in low ambient temperatures is also emphasized in a book published by the Esso company in 1960 entitled "Diesel Fuels and Lubricants ", in which it is stated that the cloud point of fuels is becoming increasingly important because of the tendency to fit finer filters, a point borne out by the BP work and supported by the fact that oil companies appear to be favouring cloud point rather than pour point as a low-temperature performance criterion.
After emphasizing that the finer the filter the more readily it will become clogged, the authors of the Esse publication observe that increasing the bore of the fuel lines and the size of the filters reduces the effect of fuel deposits and therefore widens the range of fuels that can be used. Mention is also made of the fact that the pour point of a fuel is only an indication of the lowest temperature at which it can be pumped, and it is confirmed that oils can in some circumstances be pumped at temperatures below their tested pour point.
According to another authority, random samples of dery obtained at service stations in different parts of the United Kingdom between May and October showed that cloud points varied between less than 0°F (-17.7°C) and 18°F (-7.8°C), one sample taken in October having a cloud point of 16°F (8.9°C). •The smallest difference between cloud point and pour point was 2°F (1.1°C), whilst the greatest was 14°F (7.6°C). It is notable that the difference of 2°F applied to a sample taken in August having a cloud point of 2°F (-16.6°C) and a pour point of 0°F (-17.7°C). On average the difference was around 10°F (5.4°C), the larger differences applying to fuels with a relatively high cloud point. For example, the fuel with a cloud point of 18°F had a pour point of 5°F (-15°C).
It should be noted that all the fuels were being sold in the normal " summer " period, and it is re-emphasized that the major oil companies market special winter grades with improved low-temperature characteristics from autumn to spring. Operators should, therefore, make sure that summer-grade fuel is not used in wintry weather.
These observations and reports by operators confirm that a fuel system designed to reduce the effects of waxing to a minimum should normally provide immunity from fuel starvation throughout the year, except when the ambient temperature falls to an abnormally low level.