Is There Any Advantage in Using 3-Inch Hose ?
Pointing Out Where This Type Is Better and Where It Is Less Useful Than the Smaller Varieties—Computing the Friction Loss
THE question as to whether the larger size of hose can be used by fire departments to best advantage is one which is being debated by chiefs throughout the land. The following paper presents the subject in a fair and at the same time analytical way that should make it a valuable addition to the writings on this important subject:
In the light of all which is now accepted as good practice, we have first to agree that water, when delivered in the form of fire streams, is the medium best adapted for coping successfully with the major work that fire departments have to do under present day conditions. Approaching the topic, namely, “The use of three-inch hose,” there needs to be emphasized initially, that an analysis which is to afford satisfactory deductions, must necessarily take into account all of the various factors concerned with the production of effective fire streams.
Aside from the hose, quite a number of other things enter for consideration and at the outset therefore, we shall also have to concede, that the hose used by firemen—regardless as to its kind—constitutes but one link in a chain of other equally important essentials and which collectively serve to create the so-called fire stream. Customarily designated and referred to in terms expressing the nominal inside diameter in inches, any sort of fire hose is to be regarded as a form of flexible conduit designed for the purpose of conveying water and, as to the various sizes in common use, the greater footage, by far, is undoubtedly in the kind known as “two and one-half-inch hose.”
Advantages and Disadvantages of Three-Inch Hose
That the use of three-inch hose does afford distinctive advantages cannot be denied, but the proposition needs to be qualified by also pointing out the disadvantages which attend. Helpful under certain conditions of usage, it is undoubtedly true that occasions are more frequently encountered where the larger hose imposes a rather needless burden and becomes a handicap to firemen. The subject will entail comparison with results which are attainable with hose of the more conventional diameters and, an equitable presentation of the points which are involved, demands consideration of the fundamental principles associated with fire streams, because primarily, the purpose of putting any hose into use, is to achieve the jet, which finally passes into space from the tip of the nozzle and thereby realizes the effect which is sought.
Must Be Conversant with Hydraulics
Fire streams, when effectively sustained, will use up a large volume of water quickly, but it should be self-evident, that all the water in the world—at sea level—would be of no avail for putting out fires occurring at any higher elevation. Water neither runs up hill of its own accord, nor does it ever stop running of its own accord. Like all material substances, water moves only under the influence of some compelling force, and when not in motion it remains dormant only as the result of forces which are acting to produce and preserve equilibrium. Since the state of motion as well as the state of rest, are brought about by the action of forces, it must follow, that any and all causes which have to do either with starting, accelerating, maintaining, retarding or totally arresting or defeating the flow of water, are to be regarded as factors which essentially enter and have a place in every problem related to fire streams.
Moreover, and because of the influence of those factors as reflected by the characteristics of a fire stream, those who are directing the work, should be conversant with the underlying principles. Fire department heads especially, ought to be able to differentiate intelligently as between conditions which make for higher efficiencies and the conditions which subtract from that objective.
Pressure Is Primal Cause of Flow
Primarily, well fortified with a lasting source of supply, we may next consider the most dependable and appropriate methods for getting the water into motion when needed for fire purposes. Pressure is the primal cause which produces flow and one fact in this connection, that frequently appears to be overlooked is, that the development of fire streams is really initiated at a point back as far as the place where the water first begins its flow. In this connection and with reference to the force of gravity and some other hydraulic terms, it should be understood that gravity head and pressure, although differently expressed, are nevertheless somewhat synonymous. Thus, .the perpendicular measurement, from the surface of a body of water to some given point situated at a lower level and this distance, when expressed in terms of feet, is representative of what is known as the “Head.” And, with respect to “pressure,” it can be demonstrated by tests made with fresh water at normal temperature, that 2.31 feet of head, is the equivalent of one pound pressure. Likewise, a head of 23.1 feet equal 10 pounds pressure and 231 feet of head, will register 100 pounds on any ordinary pressure gauge.
“That the use of three-inch hose does afford distinctive advantages cannot be denied, but the proposition needs to be qualified by also pointing out the disadvantages which attend. Helpful under certain conditions of usage, it is undoubtedly true that occasions are more frequently encountered where the larger hose imposes a rather needless burden and becomes a handicap to firemen.”
Applying More Direct Pressure
Conversely, we can reduce pressure readings to their equivalent, expressed as “head,” by simply multiplying the given pressure by 2.31. As an example, 100 pounds pressure as observed at gauge, becomes 100 times 2.31 and the product, which is 231 represents the head corresponding to a pressure of 100 pounds. Reference to water pressure, always means “pounds pressure per square inch of surface”; it means also that all surfaces impressed by a fluid are subjected to force acting with the intensity and in the manner indicated.
Although the force of gravity acting upon water, always manifests pressure in manner explained, yet, it is possible—as by means of pumping apparatus— to apply pressure to water more directly and without actually raising the water to any such height as the equivalent term of “head” may indicate.
It is important not to confound “static” pressure readings—taken when water is at rest—with the lesser pressures which may occur at the same point of observation at times when water is flowing. As an example, a closed hydrant may indicate an exceptionally high pressure, yet, that same hydrant—by reason of many faults which a static pressure reading fails to reveal—may be of little or no use for fire service.
As to the Use of Three-Inch Hose
Going into action at fires, water is sought at the nearest source. But as between where water is supposed to be and the point where its delivery is to be effected, any fire department is likely to encounter quite a variety of widely varying conditions. In some instances, everything is quite favorable; more frequently perhaps, the circumstances attending make the work slower and more arduous, with an outcome that subtracts discouragingly from the sum of fire department efficiency. To what extent a more general use of three-inch hose tends to promote the effectiveness of a fire department involves considerations which are debatable. Statistics as to where and how much three-inch hose is in actual service at this time, are not available. Nevertheless the fact that such hose is in use, must be accepted as evidence indicative of opinions which are favorable. It has been said, that “opinions” are sometimes based upon convictions which are bred from evidence that is delusive or only partially true. However, personal preferences are not being sired here, nor will any other person’s opinions be questioned. The purpose rather, is to simply present such facts as are pertinent to the topic in an impartial manner. There may be many interested in the subject, who will agree that no horizontal rule is applicable to the proposition; their position is not without merit, because the environment and conditions generally vary appreciably in different communities.
The Matter of Friction Loss
Supporting the use of larger hose, one advantage claimed and which is beyond dispute, relates to the marked decrease of such pressure losses as necessarily occur between the points where water first enters the hose and finally leaves at the nozzle. The so-called “friction loss” here indicated, seriously impairs the efficiency of a fire stream by using up within the hose, much of the initial energy and of which enough must survive—as pressure—a.t the point of discharge if the fire stream is to be effective.
A good engineering axiom to observe, is that “all work should be accomplished with the least expenditure of force.” Interpolated from earlier papers— by the author—the original references pertaining to 2 1/2-inch fire hose only, are now supplemented by similar data relative to three-inch hose. The text as here presented, therefore, affords information in a form facilitating comparisons as to the probable friction losses to he expected in hose of either size.
(Continued on page 115)
What Advantage in Using 3-Inch Hose?
(Continued from page 110)
The conventional way of expressing friction loss— which is the loss of pressure manifested when water is flowing through the hose—is in terms of pounds per square inch for a unit length of 100 feet, and for any given rate of flow stated in gallons per minute. It is this drop in pressure which constitutes friction loss and the loss will always vary somewhat with the quality or make up of the hose in use.
Departures from the nominal diameter and the fact that some makes of hose present a smoother water way than others are causes which prevent formulating a fixed co-officient which can be applied to formulae which is otherwise adaptable to determining the friction loss in hose of the size which happens to be up for investigation.
Computing the Friction Loss
From what has just been stated, it will be evident that published fire stream tables and the formulae from which their figures are derived should be accepted only as estimated approximations of results attainable in actual practice. However, within the range of such rates of flow and also within the limited length of hose leads ordinarily employed in fire service, the tabulations and rules available for computations may be accepted as being fairly accurate for the purpose which they are intended for.
By actual tests, it has been found, when water is forced through 2 1/2-inch fire hose at the rate of 240 gallons per minute, the pressure loss attending—due to internal frictional resistances—will average about 14 pounds per 100 feet of hose length. Now as we lengthen or shorten a hose lead and, as we increase or diminish the rate of flow, there are two things occurring and firemen should keep them in mind.
First: That friction loss in hose increases directly with the length. This means, that if the drop in pressure for 100 ft. of length is 14 pounds, the loss will not vary much from 28 pounds for a length of 200 ft. or 42 pounds for 300 ft. and so on or as the case may be.
Second: The effect of varying the rate of flow follows a different and more intricate law; thus, if the drop is 14 pounds for 240 gallons flowing and we double the flow, the pressure loss will mount as the “square of the times” and be four times 14, or 56 pounds when 480 gallons are passing.
A rule for computing friction loss in 2 1/2-inch fire hose, appearing on page 23 of the Red Book published by the National Board of Fire Underwriters is reproduced as follows:
Approximate pressure loss in each hundred feet of 2 1/2-inch hose equals 2 where Q is the quantity in gallons divided by 100.
By the same authority, the loss for fire hose which nominally has an inside diameter of 3 inches, may be approximated by dividing the loss as ascertained for 2 1/2-inch hose by 2.6
Approximations for friction losses—as used by the Ahrens-Fox Fire Engine Company-—afford substantially similar results, viz:
For 2 1/2-inch hose L equals .00024 G1
For 3inch hose L equals .000096 Ga
G representing the number of gallons flowing per minute. L stands for friction loss in pounds of pressure.
Importance of the Proper Sized Nozzles
While friction loss in the hose tends to detract from attaining the best effects at the nozzle, yet, the size and calibre of the stream has to do with the question because primarily, the size of the nozzle bore fixes the quantity of water demanded. Obviously. the larger the nozzle, the greater the volume of water required to build up an effective jet velocity, and the more water sent through the hose, the greater becomes the friction loss.
Failure to provide and use nozzles with bores appropriate to the varying conditions which commonly attend, is largely responsible for many fire stream disappointments and it unquestionably is true, that many situations could be saved and the work of firemen enhanced if this particular phase of the subject were better understood and acted upon.
Characteristics of a Fire Stream
From what has been presented it will be evident that the characteristics of a fire stream involve at least three interlocking factors, namely:
- The pressure which survives at the nozzle.
- The volume of water discharged from the nozzle.
- The relation of the nozzle bore to a and b.
The jet developed as the result of the joint action of the factors here indicated may be divided into four classifications which are defined as follows:
Weak Stream:—Where the nozzle pressure is less than 24 pounds and the jet velocity is under 60 feet per second.
Fair Stream :—Where the nozzle pressures range upwards from 25 to 54 pounds with a jet velocity of from 61 to 90 ft. per second.
Maximum Stream:—Where the nozzle pressures approach 97 pounds and the jet velocity is close to 120 feet per second.
Excessive Stream:—Where the nozzle pressure exceeds 97 pounds and the jet velocity is greater than 120 feet per second.
Nozzle Regulates Strength or Weakness of Stream
Streams falling in the class last mentioned are wasteful in power. In other words, the useful effect is disproportionate to the energy expended. Streams of this sort should be avoided. The logical remedy is a large bore nozzle which, at the lower pressure following the change will deliver a smoother and more effective jet.
Reversing the proposition; a stream which is weak may be intensified by the substitution of a smaller bore nozzle; the effect here, will be to increase the jet velocity without adding to the volume of water passing through the hose.
Fortified with such facts as have been hereinbefore presented, it will be possible to trace the probable difference in the initial pressures required—at the pump or hydrant—to properly support two streams which are identical; one stream from a single lead of 21/2-inch hose and the other from a single lead of three-inch hose; the lengths in either case to be the same.
Assuming the streams are to be sustained at maximum pitch, the nozzle pressure will be fixed at 97 pounds, thereby affording a jet velocity of 120 feet per second. Using 1 1/4-inch smooth bore nozzles— which is a popular and well established size—the volume of discharge in each stream will be about 454 gallons of water per minute.
Pressure at Point of Entrance
For the various length of hose leads indicated, the pressure required at the point where the water enters the hose will be approximately as tabulated as follows :
A study of the figures as here presented reveal a number of things which are well worth thinking about.
Advantages of Three-Inch Hose
Most directly associated with the topic, there is to be noted the lesser initial pressure recorded in favor of the three-inch hose. The figures appearing show, that the initial pressures for the same effect at the nozzle are only about 40 per cent, of the pressure required for 2 1/2-inch hose and it must be allowed that this saving in working pressure represents a valid argument offerable in behalf of the use of three-inch hose.
A further advantage incident to the lesser pressure loss in 3-inch hose is, that it opens and prepares the way for larger nozzles and greater fire streams. This of course, is contingent upon whether the necessary water is at hand and the working pressure can be augmented to serve the increased demands.
To present all sides fairly, there needs to be pointed out, that two leads of 2 1/2-inch hose, siamesed in parallel, are more effective in keeping down friction loss and economizing on initial pressure than is true of the single three-inch lead. Doubling up in the manner here indicated, cuts the velocity of flow through the hose squarely in two with the result that the friction loss drops from 100 per cent, for a single lead to about 25 per cent, for two leads in unison.
Compared with the foregoing, the single 3-inch lead stands with a value of 40 per cent.
Disadvantages in Heavier Weight
In debating this phase of the subject, it may be held that double 2 1/2-inch hose leads are the more cumbersome. Undoubtedly this is true, but, on the other hand, perhaps, the most vulnerable point for an attack on the three-inch hose issue centers about the greater weight involved with its use.
Along the line of portability, we are confronted with following:
Weight of one length, 50 ft. 3 inch Cotton Jacketed Hose —74 pounds
Weight of one length, 50 ft. 2 1/2-inch Cotton Jacketed Hose —55 pounds
Difference representing the greater weight of 3-inch hose —19 pounds .
Difference expressed in percentage—approximates—35%.
The cross sectional areas of the 2 1/2 and three-inch hose, comparatively are as 25 is to 36 and additional data follows:
3-inch hose—water content—per foot of length 0.367 gal. 2 1/2-inch hose—water content—per foot of length 0.255 gal. One 50 ft. length 3-inch hose—contains 156 pounds of water One 50 ft. length 2 1/2-inch hose—contains 108 pounds of water. Difference in water weight 48 pounds. Gross weight, one length—50 ft. 3-inch hose—filled 230 lbs. Gross weight, one length—50 ft. 2 1/2-inch hose—filled 163 lbs. Difference in gross weight—lines filled with water—68 lbs.
There is here to be observed, that filled with water, the excess weight of a single fifty-foot length of three-inch hose as compared with a single fifty-foot length of 2 1/2-inch hose, will approximate about fortytwo per cent. It is not logical to assume that the heavier hose can be handled as readily as is desirable without some increase in man power. Fire departments as a rule are never over manned at best, and fires cannot be fought effectively by holding streams too long in fixed positions. Man power is an expensive item, and as applied to the purpose of handling heavier fire hose, it would become an item representative of an ever present and never ceasing fixed charge. Therefore, and from an economic point of view, there may be many who will be hesitant as to whether the benefits to be derived would justify the greater cost.
More Mains and Hydrants Needed
As a further item for consideration, comes the greater first cost of the larger hose; this item, however, along with some others that might be mentioned are not so important. From the viewpoint of those who believe in spending money for anything which affords real and lasting benefits, there would be sound wisdom in at least investigating if it might not pay to invest in such more permanent betterments as more iron pipe laid in the ground. In practical fire-fighting, ideal working conditions seldom obtain. Responding to calls, fire departments must accept conditions as found. It is the rule, rather than the exception that fire hydrants are placed much too far apart and too often also, water mains are inadequate and fall short of coming up to fire service demands.
Because of such conditions as here noted, fire engines are too frequently seen in operation; straining valiantly, but with little effect, at distances which are all too remote from the scene where the water is supposed to perform a mission in the form of a fire stream.
A point therefore, which appears to be entirely pertinent is whether or not, 2 1/2-inch hose really does represent the greatest general weakness with which a fire department may happen to be afflicted. The answer to this question however, is not a part of the topic.
(From a paper read before the Illinois Firemens Association at its annual convention at Aurora, I11.)