The crude may contain excessive amounts of sediment and water that will require removal before they get to the engine. This can usually be accomplished with a settling tank, Figure 11, a centrifuge or special filtering equipment or a combination of these methods. The crude may also contain solid particles of wax at ambient temperature that would plug the filters rapidly. It is impractical to try to remove the wax, but the crude can be heated sufficiently to dissolve it. The amount of heat needed will vary from one crude to another and each situation requires an assessment. Jacket water heated fuel filters, available from fuel equipment suppliers, are often suitable for heating the crude.
If this is not appropriate for you application, an external heating system will be necessary.
The crude oil must not have too high a viscosity. For maximum life and minimum maintenance of the fuel pumping and injection systems, the viscosity of the crude oil in these systems should be within 1.4 to 20 cSt at 104°F (40°C). If the crude’s natural viscosity is higher than this, it may be heated or diluted to reduce it.
The degree of heating required will vary from one crude oil to another and will have to be established in each case. Another method of reducing viscosity is to blend the original crude with a sufficient amount of lighter distillate material. Again, the blending proportions would have to be determined for each crude oil.
The crude must have a cetane number of at least 40. This brings its distillation characteristics into the picture. The cetane number should be determined by actual engine test because calculated numbers of crude oils are unreliable. The cetane number of a crude oil is a function of its composition. Crude is generally subdivided into fractions by boiling temperatures. The combination of the gasoline and naphtha fractions, which have low cetane numbers, should not exceed 30% of the total crude. The kerosene, distillate and gas oil fractions combined should make up at least 30% of the total because they have high cetane numbers.
Another problem created by highly volatile crude oils (low initial boiling points) is vapor locking of the fuel system. This situation can be handled by an “air eliminator.” This, in some cases, can be an ordinary float-type steam trap inverted, but it should be made of corrosion-resistant materials. It should be located after the auxiliary filters. If the engine is stopped occasionally and allowed to cool, coagulation may build up in this vapor trap and cause it to be inoperative.
The proper oil change recommendation must be made in each case. Many crude oils contain large amounts of material that accelerate lube oil deterioration. For this reason, the standard change period with recommended oils should be reduced by one-half. From this point, the length of change period with crude is determined by sulfur content the same as with distillate fuels. With 0.4-1.0% sulfur, the change period should again be reduced by one-half. When sulfur content exceeds 1.0%, still further reduction is recommended. In many cases, it may be desirable to install a larger capacity lube oil system to avoid short oil changes. The use of Cat S•O•S SM, is
strongly recommended.