Kohler Generator Sets 20--2800 kW User Manual download pdf (Page 45)

TP-5700 7/93 33Section 6 Fuel Systems

6.11 LP Gas Fuel Characteristics

LP gas is supplied as a liquid in pressure tanks. It is

easily adaptable to stationary applications where

complete independence of an outside fuel supply is

required. Since LP gas does not deteriorate in long

periods of storage as gasoline is known to do, a large

supply of fuel can be kept on hand indefinitely for

operation during emergency conditions.

LP gas is propane, butane, or a mixture of the two gases.

The ratio of butane to propane is especially important

when a large outdoor tank is used—a fuel supplier may

fill the tank in the warm summer months with a mixture

composed mainly of butane; however, this mixture may

not provide sufficient vaporized pressure at extremely

cold temperatures to start and operate the engine. A

local fuel supplier is likely to be the best source of

information on what size tank will be necessary to

provide adequate fuel vapor.

Since LP gas is supplied in pressurized tanks in liquid

form, it must be converted to a vapor state before being

introduced into the carburetor. There are 31.26 cubic

feet (0.88 m

) of butane gas in each gallon (3.78 L) of

liquid, and 36.39 cubic feet (1.03 m

) of propane in each

gallon of liquid. See the individual generator spec

sheets for fuel consumption at different loads, and

contact your fuel supplier for information regarding tank

sizes and fuel mixtures.

6.12 Vapor Withdrawal Systems

The liquid level in LP gas tanks must not exceed 90% of

the tank capacity. Generally, 10 to 20 percent of capacity

is allowed for expansion of the gas from a liquid to a

vapor state. A vapor withdrawal system utilizes vapor

forming in the space above the liquid. Temperature of

the air surrounding the tank must be high enough to

sustain adequate vaporization of the liquid fuel. In the

colder climates, an independent heat source may be

necessary to supplement natural vaporization within the

tank. Fuel can be withdrawn in liquid form and vaporized

in an electrically heated, engine water jacket-heated, or

LP gas-heated vaporizer. Straight butane gas has little

or no vaporization pressure in temperatures below

+40°F(4°C).Evenat+70°F(21°C) the pressure is only

approximately 18 psi (124 kPa). Some primary

regulators will not operate if tank pressure drops below

+30 psi (207 kPa) while others operate at incoming

pressures as low as to 3-5 psi (20.7-34.5 kPa). The fuel

mixture and its vaporization pressure at the anticipated

temperatures influence the selection of regulatory

equipment. The components of the vapor withdrawal

system used in a typical stationary application are

shown in F igure 6-5 and Figure 6-6.

6.13 Liquid Withdrawal Systems

Liquid withdrawal fuel systems can be supplied for

generator sets but are not recommended for automatic

standby service. With these systems, high-pressure LP

at 150-200 psi (1034-1379 kPa) is piped to the engine in

liquid form. A combination of converters (vaporizers)

and regulators can then reduce the gas to acceptable

pressures. In Figure 6-7, a converter (combination of

vaporizer, primary, and secondary regulators) changes

the liquid to vapor using heat from the engine cooling

system. In such a system, for a short period after

start-up, there may be problems vaporizing enough fuel

for an engine running under load. The engine, which

supplies heat to the converter (vaporizer), needs time to

warm sufficiently to allow the converter to vaporize

enough fuel to supply the engine.

Many areas have codes prohibiting gas fuel at more

than 5 psi (34.5 kPa) inside of buildings. This might

preclude the use of a liquid withdrawal system. In order

to meet codes, converters are sometimes located

outside of the building that houses the generator set.

This can cause start-up problems because the great

length of pipe between the converter and the carburetor

does not allow sufficient heat buildup and heat retention.

6.14 Dual Systems

(Natural and LP Gas)

In many applications, natural gas is the main fuel and LP

gas is used as the emergency fuel when natural gas is

not available.

The dual fuel system in common use offers automatic

changeover from one fuel to the other. This is

accomplished by the use of two separate regulators and

solenoid valves. A pressure switch placed on the

primary source of fuel closes with a drop in pressure and

energizes a relay which closes the primary fuel solenoid

and opens the secondary or emergency fuel solenoid.

To ensure proper carburetion upon changeover to LP

gas, a separate LP gas load adjustment is located in-line

between the secondary regulator and the carburetor.

See Figure 6-8 and Figure 6-9.

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Kohler Generator Sets 20--2800 kW User Manual Page 45

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