Doosan G20P-5, Руководство по обслуживанию

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G424F(FE) Service Manual 299 Appendix
LPG And LPG Fuel Tanks
LPG Fuel Supply
Liquefied petroleum gas (LPG) consists mainly of
propane, propylene, butane, and butylenes in
various mixtures. LPG is produced as a by-product
of natural gas processing or it can be obtained from
crude oil as part of the oil refining process. LPG, like
gasoline, is a compound of hydrogen and carbon,
commonly called hydrocarbons.
In its natural state, propane is colorless and
odorless; an odorant (ethyl mercaptan) is added to
the fuel so its presence can be detected. There are
currently three grades of propane available in the
United States. A propane grade designation of HD5
(not exceeding 5% propylene), is used for internal
combustion engines while much higher levels of
propylene (HD10) are used as commercial grade
propane along with a commercial propane /butane
mixture.
APPROXIMATE COMPOSITION OF HD5
PROPANE BY VOLUME
Propane
(C3H8)
Propy
lene
Butane
(C4H10)
Iso-
Butane
Methane
(CH4)
TOTAL
90.0%
min.
5%
max.
2.0% 1.5% 1.5% 100%
An advantage of LPG is the ability to safely store
and transport the product in the liquid state. In the
liquid state propane is approximately 270 times as
dense as it is in a gaseous form. By pressurizing a
container of LPG we can effectively raise the boiling
point above –44 deg. C / -42 deg. C, keeping the
propane in liquid form. The point at which the liquid
becomes a gas (boiling point) depends on the
amount of pressure applied to the container.
This process operates similarly to an engine coolant
system where water is kept from boiling by
pressurizing the system and adding a mixture of
glycol. For example water at normal atmospheric
pressure will boil at 212 deg. F / 100 deg. C. If an
engines operating temperature is approximately 230
deg. F / 110 deg. C, then the water in an open un-
pressurized cooling system would simply boil off into
steam, eventually leaving the cooling system empty
and over heating the engine. If we install a 10 PSIG
cap on the radiator, pressurizing the cooling system
to 10 PSIG, the boiling point of the water increases
to 242 deg. F / 117 deg. C, which will cause the
water to remain in liquid state at the engines
operating temperature.
The same principle is applied to LPG in a container,
commonly referred to as an LPG tank or cylinder.
Typically an LPG tank is not filled over 80% capacity
allowing for a 20% vapor expansion space. Outside
air temperature effect’s an LPG tank and must be
considered when using an LPG system. (Figure 2)
shows the relationship between pressure and
temperature in a LPG tank at a steady state
condition.
With 128 PSIG vapor pressure acting against the
liquid propane the boiling point has been raised to
slightly more than 80 deg. F / 27 deg. C.
NOTE:
Vapor pressure inside an LPG tank depends
on the ambient air temperature outside the tank, not
the amount of liquid inside the tank. A tank that is ¾
full of liquid propane at 80 deg. F will contain the
same vapor pressure as a tank that is only ¼ full of
liquid propane.
LPG’s relative ease of vaporization makes it an
excellent fuel for low-rpm engines on start-and-stop
operations. The more readily a fuel vaporizes the
more complete combustion will be.
Because propane has a low boiling point (-44F), and
is a low carbon fuel, engine life can be extended due
to less cylinder wall wash down and little, if any,
carbon build up.
0
50
100
150
200
250
300
-20 0 20 40 60 80 100 120 140
Temperature, deg F
P
res
su
re,
p
si
g
LPG Tank Pressure VS Temperature
Figure 2
Compressed
Vapor
128 PSIG
Liquid
Propane
LPG Tank
Figure 3