QUESTIONS and ANSWERS – Fire engineering


Questions and answers

It answers questions relating to current fire protection issues.


For the publisher:

Are the following statements true or false? 1. Combustion is a chemical reaction which is always accompanied by light and heat. 2. A chemical reaction must involve oxygen before it can be called combustion or fire.


Reply: The latest edition of Webster’s “New International Dictionary” states the following with respect to combustion:

“Combustion: any chemical process accompanied by the release of light and heat, commonly the union of substances with oxygen; therefore, slower oxidation, as in the animal body.

There are cases when the combustion is not accompanied by light, as in the slow decomposition of wood. Therefore, the answer is “False”. But as we know, combustion produces light and heat, or at least one of the two.

Likewise, combustion as we commonly know it almost always involves oxygen.

Regarding combustion, the “Fire Chief’s Handbook” specifies:

“In the narrowest accepted sense, combustion is simply the oxidation of a chemical combination of the element oxygen with another chemical element or compound. Without oxygen there can be no combustion, and with the exception of some special compounds, such as gunpowder and nitrocellulose, which contain their own oxygen, oxygen is usually supplied by air, which is essentially a free mixture of four parts nitrogen gas. and part of gaseous oxygen. During the process of combining or combustion, a defined and determined amount of heat will be emitted, whether combustion takes a minute or a year.

As generally interpreted, a chemical reaction must involve oxygen before it can be called combustion or fire. We believe that the question “true” should be answered.

Heating of aluminum dross

For the publisher:

How would you handle a case like this?

An open steel gondola car, loaded with about forty-four tons of aluminum slag, is en route between an aluminum company and a smelter about eight hundred miles away.

After driving about 150 miles, the car got so hot that you could barely place your hand outside of the car. and ammonia fumes escaped from the roof of the car. The greatest depth of content was about five and a half feet in the car.


Reply: It is very likely that the heating of the aluminum dross in the open gondola was due to the presence of humidity.

Such dross, when wet, will heat up spontaneously and it is very likely that it will eventually catch fire because of this cause.

Soaking the mass with water would only be a temporary expedient, because then the heating would start again.

The only way to eliminate the problem would be to ship the coated dross so that it is not exposed to moisture.

When it comes to controlling the heater, there isn’t much you can do except remove the material from the car and spread it out to dry.

There is always a danger of aluminum shavings from a machine shop containing oil, cutting, milling and other machines. This greatly increases the risk of fire.

Dangers of polishing and leather dust

For the publisher:

I would like your answer to the following problem. Involved in this problem is a three and a half story structure of timber construction which is located in our commercial section.

The building does not have a cellar. There is a space of about a foot and a half between the beams of the floor and the ground. At the rear of the building is a “well” for an oven. This space, which is located under the whole building, is completely open. The building houses four stores on the ground floor and the upper floors are dedicated to the rented apartments, all occupied. The stores are a hardware store, a shoemaker, the headquarters of the Red Cross and a vacant store. The leather dust and rubber linings from the various machines in the shoemaking are all blown under the building into the small space between the building and the floor.

In our state, we do not have the support of fire laws and we do not have the services of a state fire marshal. However, the city has passed an ordinance under which we conduct periodic inspections of business establishments. We do not have authority that amounts to much; we’re just making suggestions. We suggested that the owner of this shoemaker install a separate container in his workshop to collect this dust as we believe it is a fire hazard. A leather seller spoke to him and advised him that this dust and the way he gets rid of it is not a fire hazard. To prove his claim, he put some of the dust on a hot stove and also hit a match on it and it did not catch fire.

Does the way in which this dust is removed constitute a fire hazard? If so, how can this cause a fire? NS

The deposit of polishers and dust from the shoemaking workshop under the building would constitute a serious fire hazard if the polishers and dust were oily.

A mixture of this material and oil would be liable to ignite spontaneously.

In fact, several fires have been caused in shoe factories from this same source.

The deeper the accumulations become, the more chances there are for spontaneous heating.

On the other hand, if the sandings and dust are dry and therefore remain, and are not at all oily, the risk of fire would be very low.

Impact formula

For the publisher:

What is the formula for finding the rate of speed an object reaches when it falls from a height, and also the force with which it hits the earth? JER

Reply: The formula for finding the velocity of a body falling from a certain height is as follows:

V = 8.02 xh, where V is the speed in feet per second, and h, the height of the object’s fall.

For example, if an object were to fall at 100 feet, the speed would be V = 8.02 x 10, or 80.2 feet per second.

Of course, if a body falls a great distance, the resistance of the air has a material effect on the ultimate speed. For example, if an object were to fall from a plane a few miles high, the resistance of the air would ultimately be so great that it would bring the falling object at a uniform speed. This speed, called critical speed, would naturally be different for objects of different mass.

The term “force” or “force of a blow” when one object strikes another is a misnomer, because the energy of a falling body can only be expressed in foot-pounds. or energy or work. The energy in a body of weight W falling from a height h is (W / 2g) xv2, where v is the speed acquired while falling from height h. If F is the weight of the body, M is its mass, g is the acceleration due to gravity, S is the height of fall, and v is the speed at the end of the fall, the energy in the body just before to hit is FS = 1/2 Mv2 = Wv2 / 2g = Wv2 64.32, which is the general equation for the energy of a moving body. Just as the body’s energy is the product of a force at a distance, so the work it does when it strikes is not the manifestation of a force, which can be expressed simply in pounds, but it is the passing of a resistance by a certain distance, which is expressed as the product of the average resistance by the distance over which it is exerted. If a hammer weighing 100 lbs. falls 10 feet, its energy is 1000 foot-pounds. Before being brought to rest, it should be 1000 ft-lbs. working against one or more resistances. These are of various natures, such as those due to the movement imparted to the struck body, to the penetration against friction, or to the resistance to shearing or other deformations and to crushing and heating to the both of the falling body and the struck body. > The distance over which these resistive forces act is generally indeterminate, and therefore the average of the resistive forces, which themselves generally vary with distance, is also indeterminate.

Discharge at different pressures

For the publisher:

In lesson 5, question 6 of the “Firefighter Promotional Study Course”, the question “Why doesn’t a pump deliver the same flow at 200 pounds as at 120 pounds?” is requested. The answer is given as “It takes more horsepower to discharge a gallon of water at 200 pounds than at 120 pounds.”

Isn’t it a fact that 120 pounds of pressure through 50 feet of 2 1/2 inch hose and a one inch nozzle will discharge about 307 gallons per minute, while 200 pounds of pump pressure through the same arrangement will discharge about 39.s gallons per minute? LMA

Reply: When we were talking about the flow of a pump at different pressures, we had in mind the maximum flow of the pump at a specific pressure.

For example, when a pump is operating at a pressure of 120 pounds, it will give more water than when operating at a pressure of 200 pounds. Indeed, the motor which drives the pump has a certain power limit. He can only give this limited power and no more. If the water is to be discharged at a pressure of 200 pounds, and since each gallon of water discharged at that pressure requires more work on the part of the engine than a gallon discharged at a pressure of 120 pounds, it is obvious that the total discharge at the higher number will be less than the lower number.

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