Page 246 - Boiler_Operators_Handbook,_Second_Edition

Consumables 231

contains about the same amount of sulfur as low sulfur just heavier than lighter oils. One other confusing factor
heavy oil. is the use of “gravity” to define an oil. The API gravity
To reduce sulfur oxide emissions, transportation of a fuel oil increases as the fuel gets lighter. API gravity
and off road equipment is now required to burn ultra is the ratio of a weight of oil of a specified volume com-
low sulfur diesel oil (which doesn’t materially differ pared to the weight of the same volume of water at the
from No. 2 oil) with a sulfur content of less than 15 ppm. same temperature. To determine the specific gravity of
It stands to reason that restriction will shortly apply to an oil add 131.5 to the API gravity and divide the result
heating fuel as well. into 141.5. Multiply that result by 62.4 to determine the
6
Grade 3 was dropped from consideration in 1948. pounds per cubic foot. An oil with an API gravity of 10
That’s why nobody knows about it unless they’re over 60. will have the same weight as water. Higher numbers are
Numbers 4 through 6 are referred to as “heavy fuel lighter than water.
oil,” they are dark in color, require some heating before Number 4 oil has a typical heating value of 146,000
they will burn and exhibit varying degrees of soot for- Btu per gallon, weighs about 7.7 pounds per gallon and
mation and other problems with burning. Numbers 5 has an air-fuel ratio requirement of 14.01 pounds of air
and 6 require heating to reduce the viscosity of the fuel per pound of fuel. That is approximately equal to 108.2
so it can be pumped. Number 6 fuel oil has to be heated cubic feet of air per gallon, 0.74 cubic feet per million Btu.
so it will flow. I have a sample of it that I carry for semi- Number 6 oil has a typical heating value of 150,000
nars. It looks like a puddle of oil when it’s resting on a Btu per gallon, weighs about 8.21 pounds per gallon and
table but I can pick it up and tap out a tune with it, it’s has an air-fuel ratio requirement of 13.95 pounds of air
that hard at room temperature. I then explain that it will per pound of fuel that is approximately equal to 114.6
flow like water if it is heated to about 200°F. cubic feet of air per gallon, 0.76 cubic feet per million Btu.
The viscosity (resistance to flowing) of these fuels Pour point is one of the important values the
varies considerably with temperature. The viscosity, operator should monitor when firing heavy fuel oils,
not the temperature, has to be maintained at the value especially Number 6. Before acid rain was recognized
prescribed by the burner manufacturer and the operator as a problem the pour point of fuel oils was fairly stable.
has to set the oil temperature to achieve the required vis- When it became necessary to remove the 3 to 5% sulfur
cosity for proper atomization. The analysis of the fuel, in the oil to reduce emissions the process changed the
provided by the fuel supplier, will indicate a viscosity at characteristics of the oils introducing a problem with
a standard temperature and charts or graphs furnished elevated pour points. The Pour Point is the temperature
by the fuel supplier or the burner manufacturer must be at which the oil will start to flow. Oil in a storage tank
used to determine the required temperature for burning. that is allowed to cool below its pour point will not flow
If you’re burning a heavy fuel your fuel supplier should to the heater to be heated and pumped out of the tank.
furnish you with temperature—viscosity charts and Heating the oil to a higher temperature ensures the oil
guidance in maintaining the proper viscosity. will flow.
That will give you a starting point. An oil burner is Desulferized fuels have a tendency to develop
designed to atomize the oil at a specific viscosity, most elevated pour points. Once the oil cools below its pour
of them at 200 SSU (Seconds Saybolt Universal). That point and sets up it must be heated to a much higher
simply means it takes 200 seconds for a 60 milliliter oil temperature before it will flow again. Repeat the cooling
sample at 100°F to flow through an orifice in the Say- and heating process enough times and the oil becomes a
bolt Viscometer. I like to vary the viscosity, by varying solid mass that will not flow and can’t be pumped. The
the temperature, a little each side of the specified value only solution to a gelled oil tank is to add chemicals and
and see what it does for the boiler performance. It the oil to dissolve the mass. Regrettably it can’t be chopped
performance improves or I seem to be getting cleaner up and burned as coal because once it gets in the furnace
combustion at that viscosity I’ll change it a little more. it will melt, becoming a liquid again at the high furnace
Eventually I’ll find the best viscosity for my burner and temperatures.
that’s what I’ll heat the oil to get. The result of that activ- Flash point is another property of fuel oils that
ity should be recorded in the maintenance log for that should be watched. Those Pacific Specifications required
particular burner. Number 2 fuel oil have a flash point higher than 100°F.
When we hear the term “heavy” applied to oil Heavier oils were listed for higher flash points, above
it can conjure up thoughts of extreme weights but the 150°F. There are two methods for determining flash
truth is that all oil is lighter than water. Heavy oils are point, the common one being the open cup method