Standby Generator Sets
Fuel Systems
FUEL SYSTEM
FUEL REQUIREMENTS
The standby generator may be equipped with one of the following fuel systems:
•Natural gas fuel system
•Propane vapor (PV) fuel system
The Manual Drawing Listing that is affixed to the unit includes the “Identification Code,” which may be used to identify the type of fuel system installed on the unit.
Recommended fuels should have a Btu content of at least 1,000 Btus per cubic foot for natural gas; or at least 2,520 Btus per cubic foot for LP gas. Ask the fuel supplier for the Btu content of the fuel.
Required fuel pressure for natural gas is 5 inches to 14 inches water column (0.18 to 0.5 psi); and for liquid propane, 5 inches to 14 inches of water column (0.18 to 0.5 psi).
NOTE:
Any piping used to connect the generator to the fuel supply should be of adequate size to ensure the fuel pressure NEVER drops below five inches water column for natural gas or 5 inches water column for propane vapor for all load ranges.
NOTE:
It is the responsibility of the installer to make sure that only the correct recommended fuel is sup- plied to the generator fuel system. Thereafter, the owner/operator must make certain that only the proper fuel is supplied.
NATURAL GAS FUEL SYSTEM
Natural gas is supplied in its vapor state. In most cases, the gas distribution company provides piping from the main gas distribution line to the standby generator site. The following information applies to natural gas fuel systems.
•Gas pressure in a building is usually regulated by national, state and local codes.
•To reduce gas pressure to a safe level before the gas enters a building, a primary regulator is needed. The natural gas supplier may or may not supply such a regulator.
•It is the responsibility of the gas supplier to make sure sufficient gas pressure is available to operate the primary regulator.
•Gas pressure at the inlet to the fuel shutoff sole- noid should not exceed approximately 14 inches water column (0.5 psi). Optimum pressure at the fuel shutoff solenoid is 11 inches water column (0.4 psi).
PROPANE VAPOR WITHDRAWAL FUEL SYSTEM
This type of system utilizes the vapors formed above the liquid fuel in the supply tank. Approximately 10 to 20 percent of the tank capacity is needed for fuel expansion from the liquid to the vapor state. The vapor withdrawal system is generally best suited for smaller engines that require less fuel. The installer should be aware of the following:
•The natural gas and LP gas systems are similar. However, the natural gas system delivers gas at a pressure of approximately five inches water col- umn to the carburetor.
•When ambient temperatures are low and engine fuel consumption is high, the vapor withdrawal system may not function efficiently.
•Ambient temperatures around the supply tank must be high enough to sustain adequate vaporiza- tion, or the system will not deliver the needed fuel volume.
•In addition to the cooling effects of ambient air, the vaporization process itself provides an additional cooling effect.
LP FUEL SYSTEM
LP is supplied as a liquid in pressure tanks. It is usually made up of propane, butane, or a mixture of the two gases. Propane tends to vaporize readily even at temperatures as low as
LP in a liquid withdrawal system must be converted to its gaseous state before it is introduced into the engine carburetor. A
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