MIL-HDBK-1003/6
c) For rules and regulations, refer to NFPA no.31, Standard for
the Installation of Oil Burning Equipment.
5.4.2.2
Oil Handling in Plant
a) Preheating and Recirculation. When a boiler plant is shut
down, heavy oil will congeal in an oil burning system. Where cold start-up of
the plant is required, provide a thermoelectric method to trace heat the
system oil and piping. Provide pumps to circulate oil through electric
heaters, piping, storage and return the heated oil to tank for mixing with
tank oil at suction. For economic reasons, a steam or hot water oil heater
should be installed in parallel with the electric heater for use at all times
other than start-up. Fuel oil may be heated to pumping viscosity by a tank
heater, tank suction heater in the suction line to the pump, or by a return
line heater. Recirculation of oil back to the tank should be continuous to
keep the formation of sludge in the oil tanks to a minimum.
b) Temperature. Oil in the suction line to the pump must be
heated to the temperature corresponding to optimum pumping viscosity. Oil
from the pump discharge must be heated to the temperature necessary for
optimum atomizing viscosity which is defined by the burner manufacturer. If
the pump suction is under negative pressure, avoid vapor locking by keeping
the pump temperature below the oil flash point. Avoid temperature buildup of
oil above the flash point in the oil tank. This buildup is due to oil at
atomizing temperature (which is above flash point) being returned from the oil
burners. Provide a high temperature bypass around the heater to the oil
return to mix with hot oil returned from burners to limit oil temperatures in
the tank.
Pumping. Base the pumping rate on the amount of return oil
5.4.2.3
necessary to mix with tank oil to form a suction mixture at optimum pumping
viscosity, plus the burning rate of all burners. Rate of oil return must be
controlled to preclude overheating of the oil with its resultant pumping
problems.
Blending. Blending (mixing) of fuel oils should not be attempted
5.4.2.4
without establishing the compatibility of oils in question. Make sure proper
research and testing is done before mixing, because oils of a different
character, such as distillates or residuals, straight or cracked (thermally or
catalytically) may react to cause heavy sludging and handling problems.
Oil Tank Heating. To avoid coking of oil in contact with the coils
5.4.2.5
in the oil tank, the tank heating coils should not exceed 240 degrees F (115
degrees C). Circulation of HTW through coils is preferable to steam because
of better control. If the oil pressure exceeds the steam or HTW pressure in
the heating coils, use double-walled tubing in the coils to prevent
contamination of steam or HTW with oil. Use steam or HTW in the inside tube,
a thermal liquid between the inner and outer tubes, and oil on the outside of
the outer tube. The thermal liquid should be at very low pressure and
pressure sensitive alarm with sight glass provided to indicate leak of oil or
steam into the thermal liquid.
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