What Friction Loss in Hose Really Means
Tests of Ellis and Freeman—Effect of the Diameter of Fire Hose on Friction Loss—Some Advantages of Two and Five-Eighths-lnch Hose
THE matter of the most advantageous size of fire hose for exterior work in large fires is one which engages the attention of every fire chief. The necessity for the adoption of a standard for the large size of hose to be used by the fire defiartment is advocated in the following paper:
From the earliest days before Christ when clay pipes were used for conveying water, keen students of the laws of nature observed that there was a loss in pressure when water flowed through pipes and the longer the pipe the greater the drop in pressure. This phenomenon had such a great effect that it was noted that with very long lines of pipe, even of comparatively large size, the quantity of liquid discharged was very small, even though the head or pressure at the inlet was large.
What Friction Loss Is
These rough observations were made in very early days, but it was not until comparatively modern times that the true nature of loss in pressure, or the friction loss as it is now generally called, was discovered. Friction loss in liquids resembles in some respects that in solids, if a body free to move is given a push or movement on a fixed body, the former will travel only a limited distance before coming to rest. In other words, the energy of the moving body has been entirely used up in overcoming the friction between the two surfaces, and this energy has been transformed into heat. How far the moving body will travel depends upon the amount of energy transmitted to it and the smoothness of the two surfaces or amount of friction between them. In the flow ot liquids through pipes, the loss in pressure is also partly due to the friction of the liquid against the inside of the pipe and the lost energy of motion is transformed into heat. The particles of liquid strike against the rough projections on the inside of the pipe and bound into the stream, interfering with the movement of other particles. Another important factor causing friction loss is the movement of the particles of liquid against and over each other in the process of flowing. They offer a certain resistance to movement which varies with the nature of the liquid. This property is indicated by viscosity, and we find that viscous liquids which offer considerable resistance to flow, such as molasses or heavy oils, produce much greater friction loss in flowing through pipes than thin liquids such as water or gasoline.
Work of Ellis and Freeman
The laws governing the flow of water through steel and iron pipes were on account of the importance of the matter given considerable study during the nineteenth century, but it was not until 1878 that any real comprehensive tests were conducted on hose streams. At that time George A. Ellis. C ity Engineer and Water Registrar ot Springfield. Massachusetts, made such experiments and prepared a set of tables on “Fire Streams.” Ow ing to the complete and convenient form in which this data was presented, the tables found very general use and were accepted in text books, pump makers’ catalogues and various other publications until about 1888. At that time Mr. John R. Freeman, then Chief Engineer of the Inspection Department of the Associated Factory Mutual Fire Insurance Companies undertook a very complete and accurate series of experiments on the Hydraulics of Fire Streams.” Mr. Freeman in making tests on the delivery of pumps had found what he considered wide discrepancies between Mr. Ellis s tables and what he concluded should be the delivery of new pumps in perfect condition. He therefore concluded that accurate experiments on hose streams should be carried out.
I will not take the time to discuss the details of Mr. Freeman’s experiments. It any of you are sufficiently interested, you can find a full description in the transactions of the American Society of Civil Engineers, volume 21, November. 1889. It will briefly state, however that the tests consisted essentially of forcing water through hose of different types, diameters and lengths, catching the water in a specially constructed tank, the capacity of which was accurately determined. and measuring the depth of water to thousandths of a foot. The pressure was read at a point near the hydrant and at the base of the playpipe by special mercury gages indicating the pressure to hundredths of a pound. The high accuracy of the tests was attained by careful arrangement of the apparatus and design of the measuring instruments and the elimination of all sources of error of such a magnitude that they would affect the result as reoorted. The investigation was so well carried out that the data obtained has been accepted ever since without question by hydraulic engineers, and no doubt is established for all time. A few other tests have been conducted since, repeating more or less of the investigation which was carried on by Mr. Freeman, but these have served only to confirm the original figures.
“In view of the great advantage of the 3-inch hose, it is hard to see why fire departments in the larger cities have been so slow to adopt it. Its use is increasing, and this is largely due to the praiseworthy campaign which the National Board of Fire Underwriters has conducted, but there is still a large number of cities where 3-inch hose could be adopted to advantage.
Two Important Points Established
Now what does this ail mean in every day fire fighting? Two important points were clearly established as a result of the Freeman investigation: first, that the friction loss in fire hose follows the same law as in pipes varying inversely as the fifth power of the diameter, and second, that the character of the water way in the hose has a big influence.
“The great effect of the diameter on friction loss in hose is a matter with which most of you are undoubtedly entirely familiar, but there is still room for improvement in applying this knowledge among certain fire departments.”
Taking these two points up in reverse order. Previous, to these tests, some manufacturers claimed that corrugations in hose were an advantage since the water would slide through more easily on the pockets of water formed in the corrugations. Others did not take such an extreme view, but no one apparently appreciated the great disadvantage of corrugations in hose. Experiments with different kinds showed surprising results, however. Two and one-half inch unlined linen hose which may he considered to have about the roughest interior of any hose in use showed a drop in pressure per 100 ft. with 250 gallons per minute flowing of over 32 pounds. Two and one-half inch cotton rubber lined hose with a badly corrugated interior showed 25 1/2 pounds, while good cotton rubber lined hose showed but 14 pounds. The importance of this can he quickly appreciated if we consider a 500 ft. line of hose. In order to deliver an average good hose stream of 250 gallons per minute which with a 1 1/8-inch nozzle requires 45 lbs. at the nozzle, a pressure of 205 pounds will be required at the hvdrant or pumper with linen hose. 162 pounds with poor cotton rubber lined hose and but 115 pounds with smooth cotton rubber lined hose. No further argument is needed to emphasize the importance of a smooth waterway.
Effect of Diameter on Friction Loss
The great effect of the diameter on friction loss in hose is a matter with which most of you are undoubtedly entirely familiar, but there is still room for improvement in applying this knowledge among certain fire departments. The actual figures for different diameters with varying flows and lengths of hose are available to all in fire stream tables published by different insurance organizations, but there is a great difference between 2 1/2-inch and 3-inch hose. With a 250gallon stream, the friction loss in 2 1/2-inch hose per 100 ft. is 14 pounds, whereas with 3-inch hose it is less than 5 pounds. Taking a 300-gallon stream, which more clearly represents what fire departments are likely to use. the friction loss in 2 1/2-inch hose is 20 pounds, while in 3-inch hose it is but 8 pounds. Let us consider for a moment a 500 ft. line of hose, not to take an extreme case, with a nozzle
and a pressure of 60 pounds at the nozzle delivering357 gallons per minute. To produce this stream with 2 1/2-inch hose, 211 pounds will be required at the hydrant or pumper, while with 3-inch hose only 120 pounds will he needed.
Cities Slow to Adopt 3-Inch Hose
In view of the great advantage of the 3-inch hose, it is hard to see why fire departments in the larger cities have been so slow to adopt it. Its use is increasing, and this is largely due to the praiseworthy campaign which the National Board of Fire Underwriters has conducted, but there is still a large number of cities where 3-inch hose could be adopted to advantage.
It is appreciated, of course, that the 3-inch hose is in the nature of heavy artillery, and there is a legitimate use for smaller hose in the smaller fire departments or for part of the equipment in the larger departments. In this connection it is of interest to note to what an appreciable extent an increase in diameter, even as small as 1/8-inch, will decrease the friction loss. Taking again a 500 ft. line with 1 1/4 nozzle and 60 pounds pressure at the nozzle with 2 1/2-inch hose 211 pounds pressure is required at the hydrant or pump, while with 2 5/8-inch hose approximately 175 pounds would be necessary, a gain of 36 pounds.
The use of 2 5/8-inch hose in place of 2 1/2-inch has been quite general in private factory equipment, but for some reason or other fire departments have adhered to the smaller hose. This is probably due to the fact that the matter has never been properly presented to them. The 2 5/8-inch hose has much in its favor and nothing of importance against it. There is practically no difference in cost or weight, and 2 1/2-inch couplings can be easily adapted to fit the larger hose.
(Continued on page 1115)
What Friction Loss in Hose Really Means
(Continued from page 1094)
Advantages of 2 5/8-Inch Hose
The most important objection to the 2 5/8-inch diameter has come from the manufacturers on the ground that it is wasteful to manufacture two sizes of hose differing by only one-eighth of an inch. This position is fully justified and is in line with the efforts being made in all branches of industry toward standardization and elimination of unnecessary sizes. In carrying out this program, however, care should be taken that the more desirable sizes are retained and the others discarded. In view of the great advantage of the 2 5/8-inch diameter, there is no reason why this oversize hose could not be accepted as standard, eliminating the 2 1/2-inch, and if fire departments more
generally specified the 2 5/8-inch hose, this desirable result would undoubtedly be obtained.
( Paper read before the annual convention of the International Association of Eire Engineers at Richmond. Ya.)