I imagine this stuff to be a dark tarry stuff that has almost the consitatcy of chewing gum.
If you’ve ever seen pitch or asphalt tar, you have the idea of the kind of ‘residual oil’ that the Navy used to call Bunker C. I think that in practice the straight residual is admixed with a little of some lighter fraction, like #2 or ‘gas oil’, to produce actual Bunker C/#6. Here is a nice introduction to fractional distillation that shows what the stuff resembles.
Keep in mind that steam locomotives (and the UP turbines) didn’t burn “Bunker C”, they used Bunker B or #5, which is not quite so bad (but still needs to be heated to flow effectively in lines and burners, and has a high enough volatile content at not too much higher temperatures to give you problems with gassing).
To my knowledge this stuff is viscous, not rubbery or thixotropic, so more like something like molasses than chewing gum.
I expect someone like Midland Mike can tell you everything you want to know from production to 95 SUS.
The ship I was on in the 1970s formerly ran on bunker C but was converted to ND (Navy Distillate) fuel which was the same as diesel fuel. The flight deck trucks and the ship’s boats all ran on it, maybe the jets, too. After the Navy I worked in a boiler room where we would burn number 6 fuel. Nasty stuff, had to be heated to make it flow and also to make it burn. Yep, just like tar. We would have change over to it from nat gas whenever there was a cold spell so the local hospitals were ensured that they would have an adequate supply. Every single time we were told to go over to number 6, the guy on night shift (I was on 4-12) would call in sick.
SP set up a bunch of SD9s with partitioned fuel tanks to burn Bunker oil in the late 1950s. The “mudburner” feature was not popular with engine crews, and didn’t last long in service. I guess they started on Diesel and mixed the Bunker in. The engines so equipped tended to stay in Tehachapi service, since they had no way of heating the oil even though at least some of the units were S/G equipped.
Don’t forget the UP doing the same thing on GP9s and SD24s. Here is Don Strack’s account:
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In a move to reduce operating costs, many of UP’s 300 class GP9s were equipped to burn low grade heavy fuel. This fuel was similar to the fuel that UP was using in its Gas Turbine locomotives [i.e. the #5 that replaced the GE-modified spec Bunker C]. Those GP9 units that were modified to burn heavy fuel received a large 2,400 gallon fuel tank which contained the electrical heating coils needed to heat the fuel and keep it flowing.
The use of heated heavy fuel as a fuel meant that heat built up under the walkways, so the modified GP9 units were equipped with open metal grating applied as walkways to dissipate excess heat. Also included was a two-stage fuel filter, located between the air compressor and the equipment rack in the rear of the carbody interior. The larger fuel tank forced one of the twin, GP9 air reservoirs to be mounted cross-wise in the area just ahead of the battery boxes at the front of the locomotive.
The larger fuel tank forced the air cooling coils from the normal GP9 location under the walkway, out to the right side of the modified locomotive. The large fuel return line was located on the left side of the new fuel tank. Later modifications to the 300 class included the same 2400-gallon fuel tank as a simple increase in fuel tank size. This modification used the same design as the heavy fuel tank, but lacked the heating coils and large return line. All of the units originally equipped to burn heavy fuel were later changed to burn diesel fuel, but retained the larger fuel tank.
The SD24s were purchased as heavy fuel locomotives in 1959, to also make use of this inexpensive fuel. The SD24s were delivered with the same electric heaters and two-stage fuel filters, with these features either being removed, or retire
My experience is in the drilling & production end. Michigan mostly produces light crudes and condensate. I suspect that residual was not a big output from the state’s refineries (there is only one left now), up until maybe the last few years when they started importing oil sand dilbit.
" Would be interesting to see the thermostatic arrangements to keep the elements from overheating and/or gassing the heavy fuel!"
Assuming that “gassing” means boiling then it would not be much of a problem. Less than 1 % of bunker C typically boils below 160 degrees C.
http://www.etc-cte.ec.gc.ca/databases/oilproperties/pdf/web_bunker_c_fuel_oil.pdf
160C is way above what is required for flow through pipes, pumps and such. As long as the oil bunker is vented to atmosphere there is no problem.
According to the “1973 Southern Pacific Motive Power Annual” by Joseph A. Strapac (Chatham Publishing, Burlingame CA 1974):
1956-built DF-124 5449-5463 had boilers and dual fuel when new, so they were equipped with triple tanks; 800 gallons of water, 400 gallons of diesel fuel and 1200 gallons of heavy oil. When the steam generators were pulled in 1958-1960 the tank cpacity was changed to 1800 gallons of heavy oil and 600 gallons of diesel fuel. The mudburner feature is gone now[1973]; the stable is all pegged at 2,400 gallons capacity.
No mention is made of heating coils, though even by 1956 the steam generators would not have seen much use. They were probably equipped with electric coils. In any event they spent their time on Tehachapi before the “mudburner” feature was removed, so the fuel wouldn’t have required as much heating as in other SP (or UP) territories.
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Very-O.T: Navy aircraft have used a specially formulated jet fuel known as JP-5 since the early 1950’s. I suspect that this is not the ND fuel you mention…
The usual mission creep that happens when you look something up…
SP also equipped 25 GP9s (Class DF-605, numbers not listed) with dual fuel and ATS for Roseville pool service (which includes Donner Pass). Freight units, they did not have steam generators, so fuel must have been electrically heated.