Kohler 20--2800 kW manual LP Gas Fuel Characteristics, Vapor Withdrawal 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 m3) of butane gas in each gallon (3.78 L) of liquid, and 36.39 cubic feet (1.03 m3) 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 Fuel can be withdrawn in liquid form and vaporized in an electrically heated, engine water jacket-heated, or LP gas-heated vaporizer.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). Even at +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 Figure 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.

6.14Dual 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.

TP-5700 7/93 Section 6 Fuel Systems 33