Questions and Answers
NOTE—Readers are invited to send in questions, which will be answered in the order received. Names are omitted from questions unless otherwise specified
Flow From Small Nozzle
To the Editor :
Will you kindly inform me it my method of determining g.p.m. flow from a 1-inch diameter nozzle is correct, as follows:
Pitot tube blade placed in stream flowing from nozzle registers 50 pounds pressure on gauge attached: 30 x 1″ x 1″ x √50 = 30 x 1″ x 1″ x 7.07 = 212.10 U. S. g.p.m. flowing.
H. F. J.
Answer: Your calculations are correct. Slightly more accurate results may be secured by using 29.7 in place of 30 in your formula, although the latter figure is the more widely used one because of its simplicity.
Table values show the discharges from a 1-inch nozzle at 50 pounds pressure to be 209 gallons per minute.
Using 29.7 in the formula, the discharge solves out to be 209.979 or 210 gallons per minute. However, your solution, as noted above, is substantially correct.
Testing Fire Hose
To the Editor:
Will you please tell me the proper method of procedure in testing new fire hose. I would like to know in detail the maximum elongation and also what right and left turns are allowed.
Answer: On the testing of fire hose the following may be said:
In purchasing fire hose many cities, including New York, set a limit on the amount of elongation and twist permitted in hose under test pressure. For instance, in one city, specifications allow 54 inches elongation and not more than two revolutions of twist for a 50-foot section of 2 1/2-inch hose submitted to a pressure of 400 pounds per square inch.
Some authorities contend that it is bad practice to limit elongation to 54 inches, as the hose constructed to meet such requirements must necessarily be made of larger bulk in the fabric by using middling grade of yarn rather than sea island cotton or grades approximately of this standard, because the best cotton is the most elastic and of course the fabric is woven up with the highest grade yarn and the tendency to stretch will be greater.
A hose that would stretch 108 inches without twisting more than two turns must be well constructed mechanically and the yarn must be of the very best quality to permit such a stretch under 400 pounds pressure, and come back nearly to its normal length of 50 feet in a test of a single section of hose. This, according to the above authorities, is a remarkable performance and any one using such a hose should have no anxiety whatever. But it is questioned whether such a high quality hose could be produced with yarn of the present day product. The principal objection to socalled extensive elongation is on account of the tendency of such a hose to twist four, five or even six turns, but if this tendency to twist can be resisted, it is decidedly a very great advantage rather than being objectionable. Such a hose would only be lined, of course, with the very highest grade of rubber tubing.
On the other hand, other authorities are of the opinion that double-jacket fire hose which, under 400 pounds pressure will elongate more than 54 inches, shows very clearly a weakness in manufacture. Other arguments they advance follow: In addition to the first defect, it is rather cumbersome to handle.
To illustrate this statement, it would be hazardous, when operating a three or four hundred-foot line of hose on a ladder, to have the pressure released and the hose in turn contract more than 54 inches. Again, it would be almost impossible to manufacture a hose which would show no elongation under 400 pounds pressure. Another manufacturing weakness would therefore be shown if the hose in question showed no twist or elongation. It is for this reason that certain limits should be prescribed.
Regarding the subject of twist, all fire hose couplings are designed with a right hand thread. It is therefore necessary that any twist in the hose take place toward the right, otherwise the hose might uncouple itself. Too great a twist makes handling of the hose rather troublesome, as in the case of elongation. A double-jacket fire hose which, under 400 pounds pressure, shows no expansion has no practical value, inasmuch as the slightest overload in pressure will disrupt the hose. It is therefore necessary that a slight degree of expansion be present. Too great an expansion shows a manufacturing weakness, in that the hose would be short-lived. For this reason certain limits of expansion under 400 pounds pressure should be prescribed.
A Deck Gun Problem
To the Editor:
Your solution of the following problem will be greatly appreciated. A 1,000-gal. type pump is pumping through a single-line of 2 1/2–inch hose 350 feet in length, which is connected to a deck pipe, equipped with a 1 3/4-inch tip. A 750-gal. type pump is also pumping into this deck pipe, through the same size hose and same length of hose. What engine pressures are necessary to maintain 75 pounds nozzle pressure. What is the discharge from each pump.
Answer: The simplest way to solve this problem is to assume the same pressure at each engine and then calculate the nozzle pressure. The variations possible between the two engine pressures might be infinite, that is, the same nozzle pressure might be secured where any number of different pressures at the two engines were employed, but the most sensible way of handling the problem is to assume that each engine is operated at the same pressure and then solve for this common engine pressure.
The elevation of the nozzle on the deck gun is about 9 feet above the street so that we will assume that this back pressure must be overcome in addition to the friction loss in the hose.
The value of “K” for two parallel lines of 2p2-inch hose feeding 1 3/4-inch nozzle is .242.
E. P. = N. P. X (1.1 + K. L.)
“L” in this case is the number of 50 foot lengths in either one of the parallel lines, or 7.
Then E. P. = 75 x (1.1. + .242 x 7)
= 75 x (1.1. + 1.694)
= 75 x 2.794, or 209.55 pounds.
To this must be added the back pressure due to the elevation of the nozzle, or 9 X .434 or 3.9 pounds.
The correct engine pressure is then 213.45 pounds which, incidentally, is the pressure at each of the engines.
To the Editor:
Will you please answer the following question?
A 1,000 gallon pumper is taking water from a hydrant which has 75-lbs pressure and is fed from an 8″ main with a 2 1/2″ outlet from hydrant, and 4 1/2″ suction to pumper. Three 2 1/2″ lines are taken from pumper, each of 550 feet length.
What size nozzle tips should be used to obtain an effective fire stream? What should engine pressure be? Also nozzle pressure?
B. B. C.
Answer: In the layout such as you give, one and oneeighth inch nozzle would be most suitable. With this size tip, 50 pounds is about as much pressure as you would ever require. Such pressure gives satisfactory stream without excessive spraying, and enables the men to use the line without difficulty.
Assuming, therefore, that 1 1/8 inch nozzles are used and the nozzle pressure is to be 50 pounds, the engine pressure will be equal to the nozzle pressure plus the friction loss in the hose. The discharge from a 1 1/8 inch nozzle at 50 pounds pressure is 265 gallons per minute. With this flow, the friction loss in 100 feet of 2 1/2 inch hose is approximately 17 pounds.
The engine pressure would therefore have to be 17 x 5.5 + 50, or 143 1/2 pounds. In the preceding, the length of the hose line is 550 feet and the friction loss is 17 pounds per 100 feet. The friction loss is therefore 17 x 5.5.
This same engine pressure would apply to all three lines. The total discharge with this arrangement, from the three nozzles, would be 3 x 265 gallons per minute or 795 gallons per minute. The 1,000 gallon pumper could easily handle this discharge, being helped as it is by 75 pounds hydrant pressure.