QUESTIONS and ANSWERS
Wherein are answered questions relating to current problems in Fire Protection.
To the Editor:
An engine is pumping through two lines of 2 1/2-inch hose, one 300 feet long and equipped with 1 1/8-inch tip operated from the street, and the other 400 feet long equipped with 1 1/4-inch tip, stretched up a ladder to the fourth floor of a building. Nozzle pressure on the second line is 50 pounds. What will be the engine pressure?
V. O. B.
Answer: Assume 12 feet per floor as the height of the floors in the building.
The nozzle is then 36 feet above the engine outlet.
The back pressure is 36 X .434, or 15.624 pounds.
The nozzle pressure is 50 pounds.
Discharge = 29.7 X 1 1/4 X 1 1/4 X √50
= 328 gallons per minute approximately.
Friction loss in 100 feet of 2 1/2-inch hose carrying 328 gallons per minute = 2 X 3.28 X 3.28 + 3.28, or 25 pounds approximately.
The friction loss in 400 feet is 4 X 25, or 100 pounds.
Then the engine pressure = nozzle pressure + friction loss + back pressure = 50 + 100 + 15.6 = 165.6, or 166 pounds approximately.
With the engine pressure known, the pressure at the 1 1/8-inch tip on the street line can be readily solved, although it’s not asked for in the problem.
Engine Pressure on Branched Layout
To the Editor:
A pumper is pumping through 500 feet of 3-itich hose to a second pumper from which two lines of 2 1/2inch hose are stretched; one line is 200 feet long and branched into two 100-foot lines of 1 1/2-inch hose each equipped with 1/2-inch nozzle; the other line is 300 feet long and equipped with a 1-inch tip. Nozzle pressure at each of the three tips is 50 pounds.
What pressure is required at each pump? Should largest capacity pump be at source of supply?
F. H. S.
Answer: The discharge from the 1/2inch nozzle at 50 pounds pressure is 52.5 gallons per minute (29.7 X 1/2 X 1/2 X √50).
Friction loss in 100 feet of 1 1/2-inch hose carrying 52.5 gallons per minute = 40 X (2Q2 + 1/6 Q) X C, where Q = 52.5 / 100 — .525, and C, a factor, to be used with 1/2-inch nozzle, is .4.
Then the friction loss = 10.22 pounds per 100 feet. Thus the pressure at the wye will be 50 + 10.22, or 60.22 pounds.
The flow in the 2 1/2-inch branched line will be 2 X 52.5, or 105 gallons per minute.
The friction loss in 200 feet will be 2 X 3.255 = 6.51 pounds.
The pressure at Engine No. 2 will then be 60.22 + 6.51 = 66.73 pounds.
The discharge from a 1-inch nozzle at 50 pounds pressure = 209.979, or 210 gallons per minute.
The friction loss in 100 feet of 2 1/2inch hose carrying 210 gallons per minute = 2 X 2.1 X 2.1 + 2.1 = 10.92, or 11 pounds.
The friction loss in 300 feet = 3 X 11, or 33 pounds.
Engine pressure at Engine No. 2 would therefore be 50 pounds + 33 pounds, or 83 pounds.
This does not check with the engine pressure on branched line. Therefore the discharge valve on the 200-foot branched line must be throttled down to give 66.73 pounds, as the pressure at the engine for taking care of the single 300foot line of 2 1/2-inch hose would have to be 83 pounds.
The total flow through the 3-inch hose = 105 + 210 = 315 gallons per minute.
The friction loss in 100 feet of 2 1/2inch hose carrying 315 gallons per minute = 2 X 3.15 X 3.15 + 3.15 = 22.995.
The friction loss in 100 feet of 3-inch hose carrying the same quantity would be 22.996 / 2.6 = 8.84, or 9 pounds approximately.
The friction loss in 500 feet of 3-inch hose would then be 5 X 9, or 45 pounds.
The pressure at Engine_ No. 1 will thus be equal to 83 + 45 + 5 (inlet pressure to Engine No. 2) = 133 pounds.
Open Butt Discharge
To the Editor:
Why isn’t the interlocking type of coupling used in place of our threaded type?
An engine is pumping at 42 pounds engine pressure through three 800-foot lines of 2 1/2-inch hose open butt. What will be the discharge?
G. V. C.
Answer: The interlocking type of coupling is used in a few fire departments in the United States, but very few. The National Standard Screw Thread is standard with possible 85 per cent of the fire departments in the United States. The National Board of Fire Underwriters have strongly endorsed the use of the National Standard as has also the International Association of Fire Chiefs. We are familiar with quite a number of different types of snap couplings, including the cumbersome one used in Great Britain, and which has two handles for disengaging the lugs. None of them possesses material advantage over the screw coupling, and as the latter is much less expensive and at the same time simpler, it is the type that is generally favored .by fire departments.
The trend toward National Standard coupling started about 1885, and has been maintained since that time. We cannot hope that some of our large cities, such as New York and Philadelphia, will soon change to the National Standard, for it represents quite an item of cost to change over all hose and hydrant connections. Some of their suburban communities may also be slow to change. But this much may be said: A change from National Standard to the other type of coupling has been unknown in the fire service of this country during the past forty years.
With regard to the question you submit, we assume that the total hose in each line is 800 feet. You have three lines each 800 feet long, and each equipped with 2 1/2-inch open butt. The formula for finding engine pressure on a line of 2 1/2-inch hose, employing open butt, is as follows:
Butt pressure = E.P. / (1.1 + KL), where K is a factor depending upon the hose and nozzle diameters, and is, for 2 1/2-inch hose and 2 1/2-inch open butt, 3.35; L, the number of 50-foot lengths of hose in the line, is 800 / 50, or 16.
Then Butt Pressure
= 42 / (1.1 + 3.35 X 16)
= 42 / 54.7
= .76 pounds.
The discharge = 29.7 X d2 X √P X .90
= 29.7 X 2.5 X 2.5 X √.76 X .90
= 29.7 X 6.25 X .87 X .90 = 145.8 gallons per minute from each line.
There are three lines, and the total discharge will be three times this amount, or 436.4 gallons per minute.