Natural gas is commonly used as a standby generator fuel. There are two reasons. It is cheap and convenient . Aside from that, Natural gas is a poor choice.
1. It may be interrupted.
A generator is intended to be used during emergencies like hurricanes or tornadoes, but when a devastating storm hits, the utility companies often turn the gas off to large areas in order to prevent fires. When the tornado hit Joplin, MO in May of 2011, the gas company had so many gas leaks due to houses and building being knocked down they turned the gas off to most of the city. This rendered the natural gas generators useless.
Because of this problem the building code that governs generators as well as the National Electric Code require an automatic switch-over to an onsite fuel source for most installations. The onsite source is usually limited to only 2 hours of supply.
2. It can be contaminated.
Air or inert gas may enter the line when the gas company works on the gas line. If they do not carefully purge the system the fuel you get may be so dilute that the engine will not run on it.
Water can also be a contaminant. We service a generator where the natural gas is so wet we have to wrap the fuel delivery system with heat tape to keep regulators and valves from freezing up in the winter.
3. It has very low heat value.
An engine only converts the heat value, or energy content, of a fuel into torque you can use to turn a generator. The more power you need to take out of the generator the more BTUs you need to put into the engine.
Natural gas has a heat value of 910 BTUs per cubic foot. Vaporized LPG has a heat value of about 2500 BTUs per cubic foot of vapor. Gasoline vaporized has a heat value of about 3500 BTUs per cubic foot.
At 910 BTUs the engine is hard to start, low powered, and temperamental. It will work, but when you start having trouble it can get expensive. LPG and gasoline are more forgiving and easier to use.
(Did you ever wonder why we don’t drive hydrogen cars? That heat value is only 290 BTUs per cubic foot. You just cannot satisfactorily run an automobile engine on the stuff unless you increase the compression ratio. Then you can’t run the car on anything else!)
4. Engine choice is limited
Nobody builds a generator engine anymore. The standby generator industry follows the engine industry in that they select readily available mass produced engines to adapt to generators. There are two types of engines mass produced; spark ignited and diesel.
Natural gas engines are spark ignited engines. These are generally selected from available automotive (car) engines. As a result, compression ratios are usually restricted to 8.5:1 which is not ideal for natural gas but works well for gasoline and LPG. Also, generator size is often limited to 125kW and smaller since the largest readily available car engine is the ford 10 cylinder.
When standby generator manufacturers want to build a bigger gas engine they use a large industrial gas engine or a converted diesel. Both add a lot of cost to the system.
5. Correct gas plumbing is complicated.
Gas pressure for use inside of a building is limited to about 4ounces. This is very low. An engine uses a lot of fuel, so in order to provide enough fuel the installing contractor and consulting engineer must pay close attention to the fuel system layout. Every 900 elbow is equal to a 10 ft. joint of straight pipe. At 4 ounces of pressure, every 10 ft. of pipe reduces the delivery pressure. If that pressure drops below 2 ounces at full load the engine starves and performance suffers.
No equipment other that the standby generator may share the generator’s fuel line. The generator must have a separate fuel shutoff before the building’s main shutoff to allow the fire department to shut the main fuel supply off while allowing the generator to run.
Just use diesel.
Generators and automatic transfer switches as well as their appurtenant devices employ high voltages that can hurt you. Do not attempt to work with this equipment unless you are qualified. Observe all rules and cautions found in the manufacturer’s manuals as well as NFPA 70E, Standard for Electrical Safety in the Workplace.
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