Figure 12-5. A typica. residua. fue. treatment system.
Figure 12-6. .ue. b.ending for specifi. gra.ity reduction. Specific gra.ity hea.y oi. = 1.0. Specific gra.ity .ight oi. = .88.
which is the average of the constituents. However， viscosity blending is a logarithmic relation as shown in Figure 12-7. To reduce viscosity from10，000 to 3000SSU， a 3:1 reduction requires a dilution of only 1:10. An added advantage of the centrifugal process is that the sludge and particulates that can cause fuel system fouling are removed.
Electrostatic separators operate on a principle similar to centrifugalseparation. The salt is first dissolved in thewater， and the water is then separated. Electrostatic separators utilize an electric field to coalesce dro-plets of water for an increase in diameter and an associated increased settlingrate. The DC separators are most efficient with light fuels of low conductivity，and AC separators are used with heavier， highly conductive fuels. Electro-static separators are attractive because of safety considerations (no rotatingmachinery) and maintenance (few overhauls). However， sludge removal is more difficult. Water washing systems are summarized in Table 12-6.
Vanadium originates as a metallic compound in crude oil and is concen-trated by the distillation process into the heavy-oil fractions. Blade oxidation occurs when liquid vanadium is deposited onto a blade and acts as a catalyst. Vanadium compounds are oil-soluble and are thus unaffected by fuel washing. Withoutadditives， vanadium forms low-melting-temperature
Figure 12-7. .ue. .iscosity b.ending chart. High-.iscosity oi. = 10.000 SS.. .ow-.iscosity oi. = 40 SS..
Table 12-6 Selection of Fuel Washing Systems
Fuel Washing System
Distillate Centrifugal or DC electrostatic desalter Heavy distillates Centrifugal or AC electrostatic desalter Light-medium crudes Centrifugal or AC electrostatic desalter Light residual Centrifugal or AC electrostatic desalter Heavy crudes Centrifugal desalter and hybrid systems Heavy residuals Centrifugal desalter and hybrid systems
compounds， which deposit on a blade in a molten slag state that causes rapidcorrosion. However， by the addition of a suitable compound (magnesium，forexample)， the melting point of the vanadates is increased sufficiently toprevent them from being in the liquid state under service conditions.Thus， slag deposition on the blades is avoided. Calcium was initially selected as theinhibiting agent， as tests indicated it was more effective at 1750 0F (954 0C). Subsequent tests showed magnesium gave better protection at 1650 0F (899 0C) and below. However， at temperatures of 1750 0F (954 0C) and over， magnesium no longer inhibits but rather accelerates corrosion. Magnesium also provides more friable deposits than calcium inhibitors. A magnesium/ vanadium ratio of 3:1 reduces corrosion by a factor of six between tempera-tures of 1550 0F (843 0C) and 1400 0F (760 0C).
The particular magnesium compound selected for inhibition is dependentupon fuel characteristics. For low-vanadium concentrations (below 50 ppm)， an oil-soluble compound such as magnesium sulfonate is added in the correct proportion to the vanadium present. The cost of oil-soluble inhib-itors becomes prohibitive above concentrations of 50 ppm.
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