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November 2002 | |
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0251 1318-SDTDC | ||
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Lois Sicking, Mechanical Engineer
Introduction
Field personnel have
requested information regarding the safest and most efficient method to counter
the hose nozzle reactive force generated during annual fire engine service testing.
The San Dimas Technology and Development Center recommends that fiberglass sandbags
be positioned across and down the entire length of the hose, with more weight
added at the nozzle end to secure it in place. To ensure safe practices, no
one should hold the nozzle. Follow all safety precautions discussed in the "Wildland
Fire Hose Guide," PMS 466, NFES 1308, Febbruary 1997, "Safety in Testing"
section, and NFPA 1911, "Standard for Service Tests of Fire Pump Systems
on Fire Apparatus," 1997.
It is important to use the proper weight in sandbags to effectively counter the amount of hose thrust or nozzle reactive force. The counterweight can be determined by calculating the nozzle pressure and then the nozzle reactive force. Use the bore diameter of the nozzle and the flow in gallons per minute to calculate the nozzle pressure. Use that to determine the nozzle reactive force (See figure 1).
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| Figure 1—Nozzle reactive force generated during service testing. |
Example: How many pounds of sandbags should be used to counter the reactive force generated by a 1.25 inch-or a 1-inch-bore-diameter nozzle at 400 gallons per minute?
Calculating
the Nozzle Reaction for a 1.25-inch-Bore-Diameter Nozzle
The solid bore discharge
formula is gallons per minute (gpm) = 29.71 D² NP¹/². The formula
can be rewritten to solve for the unknown, nozzle pressure:
Nozzle Pressure = NP psi
= [(gpm)/(29.71 x D²)]²
Where:
gpm = 400 gpm
bore diameter (D) = 1.25 in
therefore:
NP = [(400gpm)/(29.71
x 1.25²)]²
NP = 74.25 psi
The formula for nozzle reaction
(NR) for sloid bore nozzles is NR = 1.57 D²NP
therefore:
NR = 1.57 x 1.25²
x 74.25
NR = 182 lb reactive force
A Factor of Safety of 2 to 3 is recommended. Nozzle reaction x Factor Of Safety of 2 to 3
182 lb x 2 = 364 lb
182 lb x 3 = 546 lb
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| Figure 2—Sandbags are used to counter the reactive force during service testing. |
Calculating the Nozzle
Reaction for a 1-inch-Bore-Diameter Nozzle
If the bore diameter
is reduced to 1 inch, the nozzle reaction at 400 gallons per minute is significantly
increased to 285 pounds reactive force. With a Factor of Safety of 2 to 3, it
is recommended that 600 to 850 pounds of sandbags be used to counter the nozzle
reactive force.
Use 400 to 600 pounds of sandbags for the 1.25-inch-bore-diameter nozzle and 600 to 850 pounds for the 1-inch-bore-diameter nozzle at 400 gallons per minute.
How To
Use The Fiberglass Sandbags
For added safety and
strength, use double-bagged fiberglass sandbags. Double-bag by filling a bag
with sand, tightening up the fastener, turning the bag upside down into a second
bag, and tightening up the second bag's fastener (See figure 3). Some hardware
stores sell fiberglass sandbags with an attached metal ring designed to tie
two sandbags together. Using a scale, fill all sandbags to the same weight.
Always doublecheck the weight. A bathroom scale is adequate.
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| Figure 3—Double-bag for increased strength. |
General-purpose sand that is packaged in either a plastic bag or a paper wrapper should be double-bagged with the fiberglass bags for added safety. Bags wrapped in paper should be split open inside the double sandbag, allowing the sandbag to be pliable and drape over the hose.
Cost
Fiberglass sandbags
at a home improvement store cost approximately 25 cents each. General-purpose
sand, also available at home improvement stores, costs approximately $1.25 per
38 pounds (0.5 ft³). The total cost of one 38-pound sandbag with double-bagging
is $1.75. If 600 pounds of sandbags are used, the cost will be $28. Labor required
to double-bag is minimal.
For further information, contact fire program leader by phone at (909) 599-1267, or by fax at (909) 592-2309.
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 E-mail: mailroom_wo_sdtdc@fs.fed.us 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. The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, sex, religion, age, disability, political beliefs, sexual orientation, or marital or family status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA's TARGET Center at (202) 720-2600 (voice and TDD). To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326-W, Whitten Building, 1400 Independence Avenue, SW, Washington, D.C. 20250-9410 or call (202) 720-5964 (voice and TDD). USDA is an equal opportunity provider and employer. |