QUESTIONS AND ANSWERS
B.Sc., M.E., Technical Editor
“He that questioneth much shall learn much”-BACON
NOTE—Readers of FIRE AND WATER ENGINEERING are invited to send in questions, which will be answered in the order in which they are received. Names are omitted from questions unless otherwise specified.
Capacity of Hydrants and Mains
To the Editor:
I have been advised that you are an authority on hydraulics, with the result I take the liberty of writing to you for the following information: How many lines of two and a half-inch hose with one and one eighth-inch nozzles could be taken from three six-inch mains of 1,000-feet lengths and fed at each end with twelve and ten-inch mains respectively, the hydrants placed half-way between each 1,000-feet of said six-inch mains. Pressure at hydrants 85 lbs.; and assuming that each line of hose would be 400-feet in length, what would be the total discharge in gallons from said lines. I herewith inclose a sketch of system showing size of mains and location of hydrants. Trusting I am not encroaching on your time for the above information, and
Thanking you in advance for same,
Brandon, Manitoba, Mar. 29, 1920.
Answer: If each hose line were 400-feet long, the 85 pounds pressure would not give effective streams. Nozzle pressures of 40 to 60 pounds per square inch with nozzles ranging from one-eighth to one and one quarter-inch will give streams which may be classed as good; pressures less than these cannot be considered as satisfactory. With the full 85 pounds at the hydrant and hose line made up of 400-feet of new hose equipped with one and one-eighth-inch nozzle, the nozzle pressure would be 35 pounds. It is found as follows:
N.P. = Engine Pressure / 1.1 + KL = 85 / 1.1 + 0.167 x 8 = 35 lbs.
L = No. of 50 ft. lengths of hose in line.
“K” is a constant for 2 1/2-inch hose and 1 1/8-inch nozzle.
With this pressure the discharge would be approximately 222 gallons per minute. Discharge is found from formula:
Q = 29.7 d2 p where d = diameter of nozzle in inches p = nozzle pressure in pounds.
This figure for discharge per stream will not be taken as a basis for the following calculations for it represents a deficient nozzle pressure, but instead, a discharge of 250 gallons per stream will be taken as an average. This discharge represents a nozzle pressure of 46 pounds, approximately. It is assumed that the ten and twelve-inch mains feeding the six-inch lines are not excessively long, and that they have sufficient supply so that the six-inch mains in turn may be considered as connected at either end to sources which will meet whatever demand they may require. The distance from any one of the three hydrants in question to the feeding mains is 500 feet. In other words, whatever water the hydrant delivers must be taken through two 500-foot lines of six-inch pipe, one on either side. The problem then resolves itself into one in which the friction in the sixinch pipe is the determining factor. The follownig figures give the pressure drop in 500-feet of six-inch pipe for various flows:
Now, for a flow of 250 gallons per minute, the friction loss in two and a half-inch fire hose is approximately 15 pounds per 100 feet, and a nozzle pressure of at least 40 pounds being required where a one and one-eighth-inch tip is employed, the hydrant pressure for a 100-foot line would have to be 40 plus 15, or 55 pounds; for a 200-foot line, 40 plus 30, or 70 pounds; for a 300-foot line, 40 plus 45, or 85 pounds; and for a 400-foot line, 40 plus 60, or 100 pounds. On the assumption that the pressure in the supply mains (10 and 12-inch) remains practically constant, it is then only a matter of finding what flow can be carried in the six-inch pipes and which will not cause a drop in pressure sufficient to make the streams ineffective. As an example, assume that 350-foot lines of two and a half-inch hose are used. Hydrant pressure must be 40 plus fifty-two and a half pounds, or ninety-two and a half pounds. This permits a drop of seven and a half pounds in the 500 feet of six-inch main to give 85 pounds at the hydrant. From the table of friction losses given above it will be seen that this represents approximately 400 gallons per minute. But the drop is the same from either supply main, indicating that the flow in each line is 400 gallons per minute, or a total of 800 gallons per minute is being supplied to the hydrant. One stream represents a discharge of 250 gallons per minute, so that three streams can easily be accommodated with satisfactory hydrant and nozzle pressures. Other layouts can readily be worked out from the above data by similar calculations.
Correcting an Error
To the Editor:
Will you kindly straighten me out in regards to example No. 1, on page 94. “Practical Hydraulics for Firemen.” I am a subscriber of the FIRE AND WATER ENGINEERING and would like you to answer this problem (where I am confused) at your leisure.
Example—Four lines of three-inch hose, 250 feet in length, soamesed into 200 feet of three and a half-inch hose, nozzle one and a half-inch Engine Pressure 150 lbs. Factor for four, three-inch lines siamesed 32. I fail to see how one can arrive at the quotient of 78.1 by dividing 250 by 32. If 32 is a typographical error, and should read 3.2 I can readily understand.
Thanking you in advance, I am,
Philadelphia, Pa., April 12, 1912. S. R.
Answer: The error is in the position of the decimal point in the quotient, i. e. 250 / 32 = 7.81 instead of 78.1. This error throws the balance of the problem off.
Report of Accident in Responding to an Alarm
To the Editor:
Please give me an answer in your journal to the last question on the following: “Examination for Promotion in the San Francisco Fire Department from the rank of lieutenant to the rank of captain, April 16, 1919. Subject: General Knowledge of Duties.
“Q. 1. What protection is required by ordinance on the inside of acid rooms, constructed for the storage of more than one carboy of nitric or sulphuric acid?
“Q. 2. State briefly the provisions of the ordinance as to fire drills for persons employed in factories, etc.
“Q. 3. Subject—Writing of Report. Assume that you are a captain of a hook and ladder company and that while going to a fire an automobile collides with your apparatus killing the driver of the automobile and disabling one of your men, but failing in injure your apparatus. Write a report containing at least one hundred (100) words giving the details of the accident and of what you did thereafter in the performance of your duty as Captain of the company. Address your report to William Roe, Battalion Chief, and sign your report John Doe, Captain, Truck Company No. 2. NOTICE—Avoid indentification marks as they will exclude your entire examination papers.”
I am sending herewith a copy of regulations for your information.
San Francisco, Cal., April 20, 1920.
Answer: William Roe, Battalion Chief, Fire Department, San Francisco. Sir: I have the honor to report an accident which occurred while the truck of this company was responding to an alarm of fire from Box No. 100, and in which accident one civilian was killed and a member of this company injured. The accident occurred at the corner of Black and White streets. This truck was passing south along White street, holding well to the right of the street. Siren and bell were sounded as the truck approached Black street. At a point about five feet before the street line of Black street was reached, a medium weight pleasure car (automobile) bearing license number 145,000, California, swung around the corner from the left on Black street, and was struck a glancing blow by the truck, but sufficient to overturn the car. From witnesses interviewed, it appears the driver of the automobile became confused on hearing the fire apparatus and turned directly into the street on which the apparatus was approaching. Following are names and addresses of witnesses: John Smith, 54 White street; Thomas Brown, 145 South 12th Street; James Jones, 442 Black street. The driver of the automobile was killed instantly and one member of this company, James Doe, slightly injured. Windshield, radiator, front left fender and one wheel of automobile were crushed. A member of the company was ordered to call an ambulance while the remaining members carried the injured to a nearby drug store. First aid administered to fireman until arrival of ambulance, which took him to Emergency Hospital, where he is now confined. The body of the automobile driver was left in charge of the police department. Company ordered back to station and reported in service, having been delayed twenty minutes at point of accident. Truck was uninjured except for minor scratches. Details of accident entered in Company Journal.
JOHN DOE, Captain,
Truck Company No. 2.
Pumping Engine Capacity in Fire Departments
To the Editor:
Is there any way of estimating the total fire engine capacity of a fire department when the water pressure in the mains is poor, and also when it is good? That is, how would you figure what the total capacity of fire engines should be under these conditions? Awaiting with interest your advice on the above. I am,
White Plains, N. Y., April 9, 1920.
Answer: The National Board of Fire Underwriters has worked out what seems to be the simplest method of arriving at the answer of your question. In the first place, assuming that the population of White Plains is in the neighborhood of 40,000, then the required flow at a point in the high value district should be around 6,000 gallons per minute. Having this figure, the fire engine capacity is determined directly from a table prepared by the Board, and which is as follows:
When fire flow, (allowing for domestic flow) is obtainable at pressure of:
No engine capacity required where full fire flow is available at 150 pounds and no building exceeds 10-stories in height; for pressures intermediate between 90 and 150 pounds, proportional engine capacity is required.
Akron city firemen have been granted an increase in pay. It has been a long drawn out fight on the part of Akron firemen to secure recognition of their demands for increased pay. Under the new schedule fourth year men will receive $1,960; third year men, $1,920; second year men, $1,860; and first year men, $1,800. The pay of Chief John T. Mertz will continued at $4,500 a year, adopted by council early last year.