Crude Oil Settling Tanks
A great deal of sludge can be removed from crude oil by proper settling. A recommended settling system consists of two conebottomed tanks, Figure 11, each holding a little more than four days usable supply of fuel.
Sludge in the bottom third is discarded before refilling. The tanks must be housed in a heated building, and each fitted with heating coils.
Immediately after filling, hot water is circulated through the heating coil until the tank is heated to 100°F (38°C). The heat is then shut off and the fuel allowed to settle undisturbed for four days.
During this time, fuel is being used from a second tank. Temperature inside the settling tank building should be maintained above 70°F (21°C), and the tanks must be vented outside the building.
A two-day supply of diesel fuel should be maintained for emergency use. This supply can also be used to start and stop engine when the crude oil fuel is highly viscous or heavy with paraffins.
Crude Oil Maintenance Intervals
Engine inspection intervals should be reduced by 50% when using crude oil as fuel, and maintenance routines should be modified based on the results of these increased inspections.
Pretreatment of Crude Oils
- 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.
Crude Oil Fuel
Note: Crude oils are not suitable for use as fuel in all engine applications.
The suitability of these fuels for use is determined on a case-by-case basis. A complete fuel analysis is required.
NOTICE: Use of permissible crude oil fuels can result in higher maintenance costs and in reduced engine service life.
NOTICE: Caterpillar does not recommend using any of the heavier fractions such as residuals or bottoms in engines that are configured to use distillate diesel fuel. Failure to follow this recommendation will result in severe wear of components and engine failure.
Residual fuels or blended fuels with residuals are unsuitable because they have a high viscosity range, low ignition quality and high vanadium and sodium contents that shorten engine life. Such fuels may cause high wear rates in the fuel system, on the piston rings, cylinder liners, and exhaust valves. Also, filter problems and deposits in the piston ring belt may be evidenced.
Special crude oil fuel pretreatment equipment may be required and is available from suppliers of fuel treatment equipment. Also, it may be essential to start and stop the engine on a better quality, ASTM No. 2-D type fuel to prevent plugging and sticking fuel system components and to permit satisfactory starting capability.
The same diesel power ratings may not always apply for Cat engines burning crude oil. Reasonable engine service life can be achieved when proper procedures are followed. However, the greater risks involved make it good practice to include slightly higher than normal maintenance costs when figuring the overall economics to be gained.
A fuel analysis should be performed. Include a distillation curve. Operation at light load is not recommended. On occasion, operation at 50% load has reportedly caused smoking.
Engines for crude oil fuel operation should be equipped with higher temperature thermostats, bypass centrifugal oil filter, and fuel injector pushrod keepers.
Day Tank Calculations
The following information is required to perform the calculations:
- Engine model
- Engine developed power (MCR or CSR)
- Engine speed
- Brake specific fuel consumption (bsfc)
- Initial day tank fuel temperature
- Storage tank fuel temperature (Make-up)
- Ambient air temperature
- Day tank length, width, and height
- Typical full day tank fuel height (assume 95% of tank capacity)
- Engine fuel transfer pump flow rate (see TMI)
- Fuel heat rejection from the engine (see TMI)
- Incremental time element
Day Tank Thermal Capacity Calculation
Example:
- Application: Single main engine
- Engine Model: 3612
- Rated Power: 4640 bhp (CSR)
- Rated Speed: 900 rpm
- bsfc: 0.326 lb/bhp-hr
- Initial Day Tank Fuel
Temperature = 85°F - Storage Tank Temperature = 85°F
- Ambient Air Temperature = 95°F
- Day Tank Dimensions:
o Length (L) = 12 ft.
o Width (W) = 8 ft.
o Height (H) = 8.42 ft. - Fuel Height (@ 95% of total Capacity) (H) = 8 ft.
- Engine Fuel Oil Transfer Pump Flow Rate: qxfer = 19.0 gpm
- Heat rejection from engine to fuel oil: Q = 1252 Btu/min
- Incremental time element: t = 60 min.
Assume that the day tank will be replenished from the fuel storage tanks when the day tank level falls to approximately 50-55% of normal operating capacity.
Some of the data above must be converted to other units prior to beginning calculations. The following formulas can be used:
- Engine Driven Transfer Pump Mass Flow Rate = Mxfer (lb/min)
Assume: #2 DO with an API gravity of 35 (7.1 lb/gal)
Mxfer = qxfer x 7.1 lb/gal = 19.0 gpm x 7.1 lb/gal = 134.9 lb/min - Engine burn rate under full load conditions:
3. Engine fuel return rate under full load conditions:
Day Tank Sizing (When Day Tank Serves as a Heat Sink)
The fuel supply temperature must be within specified limits for optimum injector life and maximum power capability.
Fuel systems without fuel coolers rely on the day tank to dissipate the heat of fuel returning from the engine. Day tank temperatures are affected by the following conditions.
- Day tank wetted surface area (including tank bottom)
- Engine(s) fuel consumption rate
- Day tank replenishing level
- Storage tank fuel temperature
- Ambient temperature
- Spaces contiguous to the day tank (void tanks, cofferdams, vessel shell plating, etc.)
- Return fuel temperature
Tank temperature calculation are performed in five [5] steps. The first determines the fuel mass in the tank at each time interval. The second step is based on a fuel mix temperature resulting from the engine driven transfer pump flow
rate to the engine and the return flow rate to the day tank. The third step determines the day tank fuel height for each incremental time element. Typically, the calculations will be based upon a 30-60 minute iterative time function. The end point for the calculation is assumed to be when the day tank is refilled. The fourth step approximates the heat transfer from the tank to the surrounding environment due to the temperature difference between the fuel mix temperature and the ambient temperature. This convective heat transfer then determines the resultant tank temperature. The fifth step evaluates the impact of the final fuel supply temperature on the engine’s maximum power capability.
The included example calculations should only be used to provide general guidance. If the day tank size is marginal, use a fuel cooler.
To simplify the following calculations, it is assumed the day tank walls are surrounded by free moving air. If the tank walls are contiguous to the shell plating, heat transfer from the day tank will be enhanced. Conversely, if the day tank is bounded by void spaces and cofferdams, heat rejection from the day tank will be retarded. Typically, most day tanks are located with various combinations of the preceding boundary elements. The individual performing the evaluation
must be familiar with the installation as well as the fundamental engineering concepts of the formulas used in the calculations.
Fuel Conservation Practices
Fuel costs typically represent the single highest operating cost associated with any diesel engine application. This has promoted various fuel conservation practices that can usually be applied to all applications.
- Avoid fuel spillage. Do not overfill the fuel tank. Fuel expands when warm and may overflow, especially when
tank is not designed correctly. - Operate the engine with a good electrical system. One bad cell in a battery will overwork the alternator, consuming more engine horsepower and fuel. A poor electrical system can also lead to hard starting, which
encourages excessive idling. - Size the engine or generator set to the job. Engines operate more efficiently at relatively high load factors.
- Do not increase fuel settings to obtain more power.
- Make sure all air hoses and connections do not leak.
Leaks keep the compressor working unnecessarily. - Make sure the turbocharger is turning freely so that proper air-fuel ratio is maintained.
A clean burning exhaust should indicate these items are functioning correctly. - Operate the engine with a thermostat all year; cold engines consume more fuel and wear out more quickly.
- Keep air cleaners clean. Use an air cleaner restriction indicator to avoid guessing at air cleaner condition.
Continuous Blending
If the installation warrants, used lubricating oil can be blended and used in the engine in a continuous manner. The normal method uses a centrifuge module similar to Figure 9. The following information describes this system.
Centrifuge No. 1
Engine crankcase oil is continuously centrifuged except when the clean waste oil tank is low, at which time the dirty waste oil is centrifuged and directed to the clean waste oil tank.
Centrifuge No. 2
Distillate fuel/oil mixture daytank is continually centrifuged.
Metering Pump
Adds up to 5% clean waste oil to the distillate fuel (from the main supply tank) when the daytank low level switch calls for more fuel.
Static Mixer
Runs when the metering pump is on to insure a proper homogeneous mixture of the fuel and clean waste oil.
The centrifuge module is electronically controlled and includes the components within the dotted line as shown in Figure 10. Size the system for appropriate fuel delivery.
Burning Used Crankcase Oil
With legislation and ecological pressures, it is becoming increasingly difficult to dispose of used oil. The burning of used crankcase oil in 3600/C280 engines is not recommended due to the detrimental effects on exhaust emissions. However, if ancillary methods of reducing exhaust emissions to acceptable limits are used, or if emissions are not a
problem, burning crankcase oil in 3600/C280 engines is possible with the following guidelines.
- It is necessary to collect, store, and dispose of used crankcase oil from engines correctly. It is not acceptable
to dump used crankcase oil into the oceans, rivers, and harbors from vessels or offshore drilling and production platform installations. It may be necessary for engine operators to consider burning crankcase oil in their Cat
engines. This can be done, providing the precautions below are carefully followed: - Only diesel engine crankcase oils can be mixed with the diesel engine fuel supply. The ratio of used oil to fuel must not exceed 5%. Premature filter plugging will occur at higher ratios. Under no circumstances should gasoline engine crankcase oil, transmission oils, special hydraulic oils not covered by Caterpillar recommendations, grease, cleaning solvents, etc., be mixed with the diesel fuel. Do not use crankcase oils containing water or antifreeze from engine coolant leaks or poor storage practices.
- Adequate mixing is essential. Lube oil and fuel oil, once mixed, will combine and not separate.
Mix used filtered crankcase oil with an equal amount of fuel, then add the 50-50 blend to the supply tank before new fuel is added (maintaining the 5% used oil-to-fuel ratio). This procedure should normally provide sufficient mixing. Failure to achieve adequate mixing will result in premature filter plugging by slugs of undiluted oil. - Filter or centrifuge used oil before putting it in the fuel tanks to prevent premature fuel filter plugging, accelerated wear, or plugging of fuel system parts. Soot, dirt, metal, and residue particles larger than 5 microns
(0.000197 in.) should be removed by this process. If filtering or centrifuging is not used before adding the oil to
the fuel, primary filters with 5 microns (0.000197 in.) capability must be located between the fuel supply and
engine. These will require frequent servicing. - Clean handling techniques of the used crankcase oils are essential to prevent introducing contaminants from
outside sources into the diesel fuel supply. Care must be taken in collecting, storing and transporting the used
crankcase oil to the diesel fuel tanks. Diesel fuel day tank sight glasses may become blackened in time due to the carbon content in the crankcase oil. Ash content of the lube oil added to the fuel may also cause accumulation of turbocharger and valve deposits more rapidly than normal.