Ralph H. Gonzales, Mechanical Engineer
Equipment requiring the use of a general-purpose spark arrester can be found in all types of wildland settings. This equipment ranges from huge tractors involved in road construction to rototillers and lawn mowers used by homeowners.
PARTICLES—AN EXHAUST BY PRODUCT
A fallout arrester uses a long
chamber that allows the heavier carbon particles to lose velocity and
drop to the bottom of the chamber. The lighter particles with a diameter
less than 0.023 in are carried through the chamber by the exhaust gases.
This design is seldom used because it is too large for most applications.
The trap arrester is the most common general-purpose spark arrester. The arrester works in two different ways: either by trapping carbon particles through centrifugal force, or by screening the particles out of the exhaust using a screen with openings smaller than 0.023 in. Through centrifugal force, the heavier carbon particles are thrown against the inside walls of the arrester and are directed into a trap. This is achieved by a series of stationary fins or fan-like louvers that set the exhaust gas into circular motion as it enters the arrester. Gases are allowed to escape into the atmosphere through the outlet, but the heavier carbon particles are trapped within the arrester. Trap spark arresters require periodic removal of the trapped carbon particles.
Through the use of measuring devices and instruments, the arresters are tested for their ability to remove pieces of carbon greater than 0.023 in. The arrester must also collect at least 80 percent of the particles. Additionally, the arrester must pass a series of other tests to become qualified for use. When the arrester is qualified, SDTDC publishes the information in the next edition of the Spark Arrester Guide.
ARRESTERS MUST BE INSTALLED PROPERLY
The exception for passenger and transportation vehicles is granted because they generally are not driven in close proximity to flammable vegetation. Although mufflers and catalytic converters can help reduce the problem of spark emissions, they are not spark arresters. Catalytic converters also pose a special fire threat as they generate temperatures in excess of 2,000 °F that can ignite wildland vegetation.
ARRESTER GUIDE PROVIDES INFORMATION
After determining the name
and model number on the body of the arrester, refer to the greensheet
list in the guide for a listing of qualified spark arresters. The arresters
are listed by the manufacturer name and model number. If the manufacturer’s
name and model number is not found, the unit is not qualified.
ARRESTER INSPECTION PROCEDURES
The next column in the guide lists the page number where a line drawing is provided for a complete identification. The engine should have a stamped plate indicating the number of horsepower and cycles. There is also a maximum horsepower limitation requirement for the engine application. This limitation is noted in the remarks section of the greensheet list.
The spark arrester must be checked to verify that it has been properly maintained and cleaned out. There should be no heavy carbon deposits or heavy corrosion on the screen or in the trap. The spark arrester must be free of modifications or any deterioration and must appear to have had regular maintenance.
No cracks or holes should be visible in the exhaust pipe leading to the spark arrester, and there must be a tight fit where the spark arrester connects to the exhaust pipe. Once the spark arrester is determined to be qualified, look for a cleanout plug or band. The pulverizing arrester does not require a cleanout mechanism. All other arresters require periodic cleaning of trapped carbon particles.
To clean the arrester, locate the cleanout device. It is usually a metal plug or a metal band around the arrester. Have the operator remove the plug or band. If the cleanout plug or band is rusted, it is doubtful that proper maintenance has been performed. Sometimes the carbon has hardened or crusted over. In this case, it is helpful to have the operator insert a small wire or stick to break up the carbon crust. Start the engine in a cleared area; the exhaust gases will force the built-up carbon from the arrester cleanout hole.
Sometimes the exhaust outlet needs to be covered to force out the collected carbon. Make sure that the cleanout plug or band is properly replaced and tightly closed. Check the arrester for dents that may cause the arrester to operate less efficiently, or not at all.
When inspecting centrifugal-type spark arresters, check the internal parts to ensure that they have not been removed or damaged. Internal parts have been removed or damaged if a 8-in probe can be passed the full length of the spark arrester.
Manifolds are part of the exhaust
system and are located on the outlet side of the engine block. Manifolds
are joined to the engine block by bolts and are sealed with gaskets. Manifold
surface temperatures may exceed 1,000 °F.
Exhaust pipes lead away from
the manifold. Inspection must include a check for cracks, loose connections,
or exhaust leaks. Leaks are often minute and result from rusting or metal
There are two volumes to the guide: General Purpose and Locomotive, Volume 1, and Multiposition Small Engine, Volume 2. A revision of the guide is published every year. Therefore, each volume is published every 2 years. An online guide, updated every quarter, is available on the USDA Forest Service Intranet at http://www.fsweb.sdtdc.wo.fs.fed.us. It is a searchable database that allows the user to make powerful searches.
Attn: Spark Arrester Program Leader or http://www.fsweb.sdtdc.wo.fs.fed.us
Project Leader, Fire Management
San Dimas Technology & Development Center
444 East Bonita Avenue, San Dimas CA 91773-3198
Phone 909-599-1267; TDD: 909-599-2357; FAX: 909-592-2309
Information contained in this document has been developed for the guidance of employees of the Forest Service, United States Department of Agriculture (USDA), its contractors, and cooperating Federal and State agencies. The USDA assumes no responsibility for the interpretation or use of this information by other than its own employees. The use of trade, firm, or corporation names is for the information and convenience of the reader. Such use does not constitute an official evaluation, conclusion, recommendation, endorsement, or approval of any product or service to the exclusion of others that may be suitable.
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