Author Argues There Can Be No Damage to Hose in Service as Water Pressure is Greater Than That of Inflation of Tires

WHILE the arguments of Mr. Slauson in the following article are interesting, we fear that fire chiefs generally would hardly agree with him, but will contend that no vehicle under any circumstances should be permitted to drive over fire hose in service at a fire. It strikes the Editor that the objection would be that it is hard to discriminate between the light touring car and a heavy truck or that equipped with solid tires. There would hardly be time to explain matters to the aggrieved truck driver, who, while being refused the right to drive over the hose, saw a passenger car chauffeur allowed this privilege. It would also, with the average intcllegence of the truck driver, be hard to convince him of the rights and urongs in the matter from a scientific standpoint. The article, however, is presented to give chiefs an opportunity to express their opinions on the subject.

How many times have you been held up by an excited fireman who warned you not to drive your pneumatic tired car over a hose extending from a fire engine or hydrant to a burning building on the other side of the street. Possibly, you yourself have hesitated to drive over such a hose for fear that you might burst it, reduce the flow of water, or otherwise damage the hose or its service. Under ordinary conditions, however, any effect whatever on the stream of water or on the hose itself would be impossible if only a pneumatic-tired passenger car is considered.

In the first place, any fire hose is built to withstand probably from 200 to 500 pounds pressure and even more. Probably the minimum pressure which the fire hose will carry will be 60 pounds from a hydrant, and if the water is pumped from a fire engine, the pressure will probably be in the neighborhood of 100 pounds per square inch, at the least. Therefore, a weight or pressure in excess of this sixty or one hundred pounds per square inch on the hose itself will be necessary even to begin to flatten and bend it and thus reduce the flow of water.

We have shown that the weight of any pneumatic tire on the road is exactly the inflation pressure carried by that tire. Few tires are pumped higher than 60 or 70 pounds, and thus practically all pneumatic tires are “softer” than any fire hose carrying an adequate pressure of water. This means that the “give” or indentation will be in the tire rather than in the hose; there can be no distortion or flattening of the hose under these conditions. But even if the hose should be carrying only a small stream of water at low pressure so that it will be flattened by the wheel of a passenger car, the pressure which would be built up momentarily beyond the average would be no greater than the pumping pressure of the engine. The very fact that the hose is sufficiently soft to flatten indicates that this pumping pressure is very low and probably would not amount to one-tenth of the maximum necessary to burst or otherwise injure the hose.

It may be a difficult matter to convince a fireman or fire chief that a heavy touring car can be driven over a line of hose carrying one hundred pounds pressure of water with absolutely no effect on the hose itself or on the stream of water which it is throwing. But, in fact, the rear wheels of that heavy touring car can be rested on the hose with absolutely no ensuing effect.

And yet the reasons are simple. The one hundred pounds per square inch pressure in the hose represents that amount of force applied to each square inch of the inner surface— upward, downward and at all sides. Sixty, seventy or eighty pounds applied on the outside of the hose, therefore cannot serve to overcome the greater internal pressure and make the hose give or flatten by the slightest amount. In fact, such a load as a pneumatic-tired car wheel passing over the hose, in reality serves momentarily to lessen the strain of the confined water at the point of contact of the fire with the hose.

Possibly, these conditions can be outlined better in another way. Supposing a hose carrying a stream of water at one hundred pounds pressure per square inch passes through an air-tight tank. Under normal conditions, there is, as already stated, an effort on the part of the water to escape. This effort amounts to one hundred pounds on every square inch of hose surface and is exerted in all directions. Now, if the air-tight tank, through which this hose passes, is pumped to an air pressure of one hundred pounds per square inch, the outside force exerted on every square inch of the hose will exactly equal and balance the inside pressure and the rupturing tendency, or effort of the water to escape, will be reduced to a minimum. If the air pressure exerted on the outside of the hose is increased to a point above the one hundred pounds internal water pressure, the hose will begin to collapse, for the inside pressure will then become overbalanced by the outside pressure.

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Should Autos Drive Over Fire Hose?

(Continued from page 1055)

It is, therefore, not until the outside weight on each square inch of hose exceeds the internal pressure of the water that a hose can be made to deflect, flatten or collapse. Consequently, as we have already stated, inasmuch as the load on a pneumatic tire per square inch of contact is exactly the inflation pressure carried by that tire regardless of the weight of the vehicle, no tire which is pumped up to a pressure less than the pressure carried in the hose can, in any way, damage the hose or effect the passage of the stream of water through it.—Kant-Slip for January.

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