SECTION 7 - Exhaust System

7-1. General (Exhaust System)

Generator engines give off deadly carbon monoxide gas through their exhaust systems.

Carbon monoxide gas, if breathed in sufficient concentrations, can cause unconsciousness or death. Exhaust gases must be piped safely away from any room or enclosure that houses a generator and to a well ventilated area where people will not be endangered.

Besides the possibility of carbon monoxide poisoning, exhaust piping becomes extremely hot during operation and remains hot for a long time after shutdown. For that reason, the following precautions are necessary:

• Avoid contact with hot engines, exhaust manifolds, exhaust piping and mufflers. Any of these can cause severe burns.

• Where piping must pass through combustible walls or ceilings, special precautions must be taken to prevent fire or heat damage such as using heat thimbles through walls and ceilings.
  

7-2. General Rules for Exhaust Systems

When installing an exhaust system for a generator, the following rules should be considered:

• Exhaust piping should be of wrought iron or steel having adequate strength and durability.

• Exhaust fittings may be of cast iron. A 9 inch spacing (10 inches (250mm) recommended) from the exhaust pipe and walls is also required by most local codes.

• Low points in horizontal runs of piping should be provided with condensation traps, as well as condensation drains.

• Piping and mufflers must be properly supported and connected.

• A flexible length of exhaust pipe is required between the engine exhaust manifold and rigid exhaust piping.

• Exhaust piping must be terminated safely outside a structure that houses a generator, in such a way that hot gases and sparks will be discharged harmlessly and will not blow against any combustible surface or material.

• Exhaust piping must not terminate under loading platforms, structures, or near any opening in a building.

• Where necessary, exhaust piping must be guarded and/or insulated to prevent burns.

• Provide a clearance of at least 9 inches (229mm)(10 inches (250mm) recommended) between exhaust piping and any combustible material.

• Keep exhaust piping well clear of fuel tanks, fuel lines, etc.
  

7-3. Routing Exhaust Piping Through Combustible Walls

• Exhaust piping that passes through any combustible wall or partition must be guarded at the point of passage by:

• A ventilated metal thimble that is at least 12 inches in diameter larger than the piping, or

• Metal or burned fire clay thimbles built in brickwork that provides not less than 8 inches of insulation between the clay thimble and any combustible material.

• Thermal insulation or protective guards are typically required for exhaust pipes/system by national and local codes to protect users from burns.

Figure 10.  Wall Thimble

7-4. Routing Exhaust Piping Through Combustible Roofs

Exhaust piping that passes through any combustible roof must be separated from the roof by a ventilated metal thimble that is at least 6 inches in diameter larger than the piping. The thimble must extend at least 9 inches (229mm)(10 inches (250mm) recommended) above and below roof construction.

Figure 11.  Roof Thimble

7-5. Rain Cap

A rain cap is recommended on the end of the exhaust pipe. The rain cap is attached to the end of the pipe and opens due to the pressure from the exhaust discharge force. The rain cap protects the exhaust system from the environment when the system is not running.

7-6. Spark Arrestor

Use of a spark arrestor is required by the U.S. Department of Forestry if located on lands under their jurisdiction. The spark arrestor is recommended in areas where combustible materials may ignite such as dry grass, leaves, or other combustible materials.

7-7. Exhaust Back Pressure

The exhaust back pressure of the generator when measured at full load must not exceed the manufacturer's recommendations. The size of exhaust pipe, number and type of ends and fittings together with the selection and location of muffler determine exhaust back pressure.

A typical 90 degree bend in an exhaust system is equal to adding 8 feet (2.67 meters) of pipe.

SECTION 8 - Gaseous Fuel Systems

8-1. General (Gaseous Fuel Systems)

Some generators are equipped with fuel systems that utilize Liquefied Petroleum (LP) or Natural Gas as a fuel.

Local fuel gas codes may vary widely. For that reason, it is recommended that a local gas distributor or installer be consulted when installing a gaseous fuel supply system. In the absence of local fuel gas codes and regulation, booklets published by the National Fire Protection Association (NFPA) may be used as sources of information.

The installer must ensure that the correct fuel delivery system is installed, and that applicable standards and codes are strictly complied with.

8-2.  Advantages of Gaseous Fuels

Use of Natural and LP gas as a fuel may result in a slight power loss. However, that disadvantage is usually compensated for by the many advantages of gaseous fuels. Some of the advantages of gaseous fuels are:

Low residue content, resulting in minimum carbon formation.

• Reduced sludge build-up in engine oil.

• Reduced valve burning, as compared to gasoline.

• No "wash down" of engine cylinder walls during start up.

• No tetra-ethyl lead to foul spark plugs and other engine parts.

• Excellent anti-knock qualities.

• Reduced amounts of contaminated residues.

• A nearly homogeneous mixture in engine cylinders.

• Fuel can be stored for long periods without breakdown.

8-3. Gaseous Fuel System Variations

Any one of four different types of gaseous fuel systems may typically be installed by the factory on your generator system, dependent upon the model. These are:

• Liquefied Petroleum (LP) gas vapor withdrawal.

• Liquefied Petroleum (LP) gas liquid withdrawal.

• Natural Gas.

• Dual Natural and LP gas (Additional regulator and hardware are not standard on most generators Contact the generator's manufacturer if you need this option).

• Combination Gas-Gasoline systems (Additional regulator and hardware are not standard on the generator systems. (Contact the manufacturer if you need this option).

8-4. Properties of Gaseous Fuels

Natural Gas: Is lighter than air and tends to settle in high places. Natural gas is found in the gaseous state only at normal ambient conditions. Natural gas is highly explosive and accumulations of the gas can be ignited at the slightest spark. For that reason, adequate ventilation is absolutely essential and fuel lines must be free of leaks. Local fuel/gas codes usually dictate the maximum pressure at which natural gas can enter a structure. A primary regulator is required, to reduce the pressure of the delivered gas to the reduced pressure required by code.

LP Gas: Is heavier than air, tends to settle in low places. The gas is highly explosive and the slightest spark can cause an explosion. LP gas is usually supplied in pressure tanks as a liquid, but is found in gaseous form at normal atmospheric temperature and pressure. It may consist of (1) butane, or (2) propane, or (3) a mixture of these two gases. Fuel suppliers may fill the supply tank with a mixture made up primarily of butane in warm weather. Butane may not provide sufficient vapor pressure in colder weather and more propane may have to be added to the mixture. The ratio of butane to propane is especially important when a large outdoor supply tank is used. LP gas must be converted to its vapor state before it enters the engine carburetor.

8-5. Natural Gas Fuel System

The maximum pressure at which natural gas can enter the building is established by code and may vary from area to area. The gas distribution company will usually provide piping from the main distribution line to the standby generator site. A primary regulator is needed to reduce gas supply pressures to the required safe level before the gas enters a building. Such a regulator may or may not be provided by the gas supplier. It is the responsibility of the gas supplier to ensure that sufficient gas pressure is available to operate the primary regulator.

Gas outlet pressure from the primary regulator to the standby generator's shutoff valve should typically not exceed approximately 0.50 pounds per square inch (psi), or 14 inches of water column. Optimum supply pressure to most small generator's shutoff valve is 11 inches of water column. Depending on the characteristics of the specific shutoff valve in use, the valve may or may not open at supply pressures greater than 0.50 psi (14 inches water column).

Install a flexible length of fuel line between rigid piping and the Generator engine's natural gas connection point.

Natural gas is delivered to the primary regulator. From the primary regulator, in most installations, the gas flows through a solenoid operated fuel shutoff valve, a pressure reducing valve and the engine's natural gas carburetor. The shutoff valve is electrically energized open during startup and running, is de-energized closed on shutdown. The carburetor measures engine air flow and meters gas to the engine based on throttle setting and load. The carburetor also provides a positive gas shutoff.

8-6. LP Gas Vapor Withdrawal System

This type of system utilizes the vapors formed above the liquid fuel in the supply tank. Approximately 10-20 percent of the tank capacity is needed for fuel expansion from the liquid to the vapor state.

Ambient temperatures around the supply tank must be high enough to sustain adequate vaporization or the system will not function properly. In addition to the cooling effects of ambient air, the vaporization process itself provides an additional cooling effect. Vapor withdrawal systems are generally more suited for smaller engines that need less fuel.

When ambient temperatures are low and fuel consumption is high, the vapor withdrawal system may not function efficiently. This is particularly true with larger engine machines.

Many LP gas and Natural Gas vaporous fuel systems are identical as a demand regulator is used to provide fuel to the engine.

8-7. LP Liquid Withdrawal System

This type of system delivers gas in liquid form to a generator. The liquid fuel must then be vaporized before it is delivered to the engine carburetor.

Liquid withdrawal (LP) gas systems usually employ a "vaporizer-regulator" to convert the liquid to its vapor state. A "vaporizer-regulator" is mounted in the air flow of the engine to provide heat to the regulator for fuel vaporization.

LP liquid withdrawal is typically used for equipment used in remote locations where size and availability of refilling the tank is limited. Liquid withdrawal is also used for trailered and construction site equipment.

8-8. Dual Natural/LP Gas Fuel System

In some areas, the cost of Natural gas may be reduced considerably by procuring the gas on "interrupted service" rates. Such "interrupted service" can be obtained by using LP gas as an emergency fuel whenever Natural gas is not available.. Automatic changeover is accomplished by using two regulators - a line pressure regulator for natural gas and a vacuum operated regulator for LP gas. The differences in pressures compensates for the greater BTU value of LP gas.

During operation on Natural gas, a 5 inch (water column) (typical) pressure exists in the common line to the carburetor. This pressure closes the LP gas regulator. Loss of Natural gas pressure causes loss of pressure in the line; the LP gas regulator then opens to admit LP gas into the system. A separate power mixture adjustment in the LP gas line provides precise setting of air/fuel ratios for each of the two fuels. Changeover is automatic with the engine operating.

8-9. Gaseous Fuel System Piping

The following general rules apply to piping used in gaseous fuel systems:

• Piping should be of black iron.
  

• Piping should be rigidly mounted and protected against vibration.
  

• Install an approved length of flexible hose between the generator fuel line connection point and rigid piping.
  

• Piping must be of the correct size to maintain required supply pressure under varying conditions, especially when fuel in gaseous form is being supplied (Natural gas and LP gas vapor withdrawal).
  

• Installed piping must be properly purged and leak tested, in accordance with applicable standards.
  

NOTE: In the absence of local purging and leak test standards, NFPA No. 54 may be used as a guide.

8-10. Gaseous Fuel Pipe Sizes

A "Gas Flow Pipe Sizing Chart" (Table 8-1) is provided below. Use the chart to determine the correct piping diameter in gaseous fuel systems (such as Natural gas and LP gas vapor withdrawal type).

To find the proper pipe diameter, the installer must know (a) the length of the gas piping run, and (b) the cubic feet of gas needed by the generator when under full load.

First, find the length of the piping run on the chart.

From the pipe length figure on the chart, move horizontally across the chart until you reach a number that is just higher than the cubic feet of gas needed under full load. From the cubic feet of gas figure, move straight up vertically in the chart to the pipe diameter given in that vertical column. This is the pipe size required.

LP Gaseous Fuel Example: A small 16 horse power generator when operating at full load requires a supply of 51 cubic feet per hour of LP gas. Length of the piping run from the supply tank is 60 feet. Propane gas having a specific gravity of 1.5 and with a multiplier of 0.633 is used (Table 8-2). From the pipe length in the chart, trace horizontally across to "86" (the first number larger than 51 cubic feet). Moving vertically upward in the chart, a 3/4 inch pipe is needed. Applying the chart conversion factor (86 x 0.633=54.44) and the 3/4 inch pipe is still adequate.

Natural Gas Example: A small 16 horse power generator operating at full load requires 115 cubic feet per hour of Natural gas. Length of the piping run from the supply tank is 75 feet. Natural gas having a specific gravity of 0.65 and with a multiplier of 0.962 (Table 8-2) is to be used. From the pipe length in the chart, trace horizontally across to "155" (the first number larger than 115 cubic feet). Moving vertically upward in the chart, a 1 inch pipe is required. Apply the conversion factor (155 x 0.962=149.1) and the 1 inch pipe is still adequate.

Back to Table of Contents

Go to APPENDIX A - Applicable Codes

Tips on Hooking up a Generator (genset) to a Uninterruptible Power System (UPS)

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