Flow measurement importance in fire streams

Flow measurement importance in fire streams

WHEN WE ARISE in the morning and step in to wash our faces, or fill the coffee pot with water, we are not much concerned with the pressure the water works pump house or in the distribution main. In fact, it seldom enters our mind, but the quantity water flowing out of the tap is vital importance to us. If the rate of flow is slow or ineffective, it delays us in accomplishing a specific task. It is not only aggravating and annoying, but it may affect our entire day’s routine work or pleasure. In contrast, the fire service has been taught to think and function using “pressure” as a criterion for operation and this concept of arbitrary pressure, as used in the over-all system, seldom indicates the correct flow.

Watch water flow

One of the great errors in judgment in water application occurred during World War II when the SS Normandie fire, out of control, was deluged with huge quantities of water causing the ship to capsize, later being declared a total loss. On this occasion, emphasis was placed on the fire stream pressure and range wherein the quantity of water flowing was inadvertently overlooked in the excitement. Few ships, if any, have ever been sunk by fire. In such cases they are usually sunk by fire fighters applying too large and ineffective quantities of water into the hull structure and disregarding the removal of the accumulated water. It has always been considered good damage control practice to remove as much water as is being applied in fire fighting. To accomplish this, one must know the total flow with a reasonable degree of accuracy in order to capably institute removal procedures.

The nozzle through which the extinguishment fluid flows is the only device which directly contributes to the effectiveness and the rate of fire suppression. The purpose of the nozzle is to efficiently convert the total energy of the fluid flowing in the system to kinetic energy at the throat of the nozzle where the forces of the fluid are transmitted to the burning material in various forms, such as solid streams, foam, fog, spray, etc. It is therefore necessary to be able to control, at will, the total energy at the nozzle in order to obtain the desired stream characteristics. Retarding and extinguishment of fire can occur only when the coolant is capable of carrying away heat at a faster rate than that which is generated by the burning materials.

Considering the desired stream characteristics as a departure point for all practical problems in fire stream hydraulics, it can be reiterated that the elevation of the nozzle, the length of the hose lay, the size of the nozzle, the system losses, the source of water supply, and the operating rate of the apparatus are all supporting factors which contribute proportionally to the nozzle performance. The operating rate of the pumping apparatus cannot be correctly controlled until the quantity of fluid flowing in the system has been reasonably established. Applying the customary “practical guess” increases the probability of accidents. The calculation or determination of routine hose lay problems must commence with the establishment of the flow characteristics at the nozzle exit.

It has been quite obvious for many decades that the hydraulics of fire streams and the associate measuring devices have been neglected and have not kept pace with modem engineering, apparatus development and nozzle design. This situation has, no doubt, been brought about by the lack of technical knowledge and practical understanding, lack of demand for measuring instruments and the complexity of design of an economical, simple measuring device. Few departments, if any, have accurate measuring devices other than for test purposes and apparatus trials, and even these units are incapable of monitoring a continuous flow cycle.

The pitot tube which was developed in the same era as the steam pumping engine is still considered as a modem, usable, measuring device in the fire service although it is obsolete as the steamer itself. This measuring device has many built-in errors due to lack of understanding of “the impact tube theory” which inevitably creates discrepancies in design and in commercial manufacture. Furthermore, this type pitot tube is limited to use with conventional jets and is capable of indicating only the velocity head developed at the nozzle exit. Using proper numerical coefficients the velocity head is customarily expressed in terms of flow.

The contracting jets produced by newly designed nozzles have improved stream characteristics, greater range and are considered superior to the conventional type conical nozzles now in use. However, there are no economical measuring devices for use with this equipment. Rotating multi-jet nozzles, hollow stream jets, contracting jets, variable spray and other varieties of trade name nozzles cannot effectively utilize the pitot tube for flow measurement.

Therefore it appears that only one type of flow measurement device is economically available to fire service and its application is limited. During a recent field demonstration of a pumping apparatus the manufacturer casually announced, “This engine is now pumping 1,200 gpm.” Observing the adjustable type of nozzle (variable flow), for which the pitot gage is unsuitable for nozzle measurement, the fog pattern, the suction lift of the pump, the hose layout and the lack of measuring equipment, it was virtually impossible to give a reasonable estimate of the performance of the apparatus. It appeared that the observers accepted the manufacturer’s statement, in most cases with no reservation of doubt. This policy of “seeing is believing” without a yardstick of measure indicates a lack of technical knowledge in modem fire department operation.

It is apparent that a major question arises: Why is it necessary to measure the quantity of water flowing in a fire stream? A parallel question would be to say: “Why is it necessary to measure the nozzle pressure? In answer to the latter question, it is believed that there would be multiple explanations, and as many answers as there are people participating in the particular discussion. Pressure components are considerably easier to measure and more people are familiar with pressure gages because of their universal usage in all types of fluid systems. Also, pressure gages are economical and easy to install. Due to this ease of installation, a convenient location is sometimes selected rather than the proper instrument standard prescribed position, thus compounding errors.

FLOW MEASUREMENT

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Pressure gages on occasion are very misleading and accidents have occurred resulting in death or injury and damage to apparatus because the pressure indicator did not adequately picture the correct hydraulic situation. On several occasions, apparatus has sustained pump and engine damage because the operator could not visually observe the fire stream on a long hose layout, or a line entering the interior of a ship, and therefore operated the engine at a high output rate, not realizing that the nozzles were shut off.

Governor controls and relief valves regulate pressure only—NOT FLOW—and therefore only provide partial short-period protection to the equipment. Under a “no flow” condition, the mechanical energy developed by the engine is transmitted to the pump where it is converted to unwanted heat which must be disposed of immediately. The accumulation of this unwanted heat rapidly increases temperatures which may cause distortion of the pump casing, bent shafting, damaged bearings, worn seal rings and packing. The only positive method of preventing damage is for the operator to know the quantity of fluid flowing as measured by direct readout flow gages installed at the pump discharge gates and by maintaining proper engine output to satisfy the operational requirements.

Many people are taught that pressure is the contributing force that causes the nozzles to break loose from their secured position and thrash violently in the surrounding area. This is not likely. It is the rate of energy release at the nozzle throat that produces an unstable reactive force which raises havoc. This reactive force is a function of the quantities of fluid flowing and the jet velocity. To consider the pressure component only (partial pressure), provides a sense of false security which eventually may lead to trouble and ineffective fire extinguishment.

Important factors

Referring to the necessity for measuring the flow of fluid it is often remarked: What difference does a few gallons of water per minute make when fighting a large fire? The direct answer is: None. It is the transition of the water from a confined state to a dispersed state that is important. To accomplish this objective, the jet horsepower, the desired flow distribution pattern, the reactive forces and the flow rate for efficient application must be established and adhered to. In addition, if the above information was unnecessary, we shouldn’t require modern fire apparatus and could revert to our old standards of operation and the “practical guess.”

Furthermore, it is necessary to upgrade our technical knowledge and to improve our training procedure to conform with modern apparatus, structures and ways of living. The engine horsepower and torque of the apparatus has generally doubled in the past years and the pump rating in both output and energy gradient have increased proportionally. Operational requirements now demand intelligent personnel capable of understanding and mastering simple hydraulic situations and problems which occur using high-energy machinery. To effectively operate emergency equipment it is mandatory that the operator be able to accurately and quickly measure the essential elements of a hydraulic system and rationally express these direct readout gages in numerical dimensions of tolerance, performance and human endurance. To accomplish this objective the engineering departments of the fire service must be equipped with simplified modem measuring devices, schooled in their usage, and then the applied technology be put into daily practice.

November Cover Credit

The cover photo on the November 1960 issue of FIRE ENGINEERING and the photographs which illustrated the accompanying New Haven factory fire article were taken by Lieutenant Alfred Bysiewicz, New Haven Fire Department photographer.

To rapidly predict the characteristics of fire streams, a nomograph showing the predominant hydraulic relations is illustrated here. By establishing any two of the eight variable functions, the remaining six values can be ascertained. The operation of such a graph is extremely simple and is accomplished by establishing any two known numerical values and projecting a straight line through these points, intersecting all of the scales. Direct readout at the intersection shows the correct numerical characteristics.

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