FOREGROUND HYDRAULICS In Your Head

FOREGROUND HYDRAULICS In Your Head

HYDRAULICS

Proficiency in the mental arithmetic of hydraulics will pay dividends on the fireground and in the boardrooms,

AFTER A $6 MILLION fire disaster shocked the Wabasca, Alberta Fire Department into realizing that the traditional approach is ineffective for fires involving modern materials, every aspect of the department’s operations was subjected to close engineering scrutiny. This was done ten years ago, and to such good effect that fire losses were dramatically reduced.

One facet of Operations examined was fireground hydraulics. Hydraulics books have been published, it les-of thumb devised, slide calculators bolted onto pumpers, and electronic geniacs invented. Nevertheless, in the bitter cold of winter, or in the searing heat of a fIre (or the heat of the mat-or’s office. where you’re pleading for large-diameter fire hose). members of the fire department that can pert~rm painks~. hydraulics calculations can usually be counted on the fingers of one hand, Personal frustration in trying to use the calculating aids in snow, fire, and municipal offices led to a system of hydraulics that employs, mental arithmetic. Only one number Or factor needs to be remembered for each size of hose. in use (See Table 2. p 83). First, Lst’s review some simplified theory.

CONTINUITY OF FLOW

The basic principle of flow continuity is this: The water that goes in one end of a hose or pumper must come out the

The simplest hydraulic system on any fireground is that of a flow through single hoseline. If the nozzle s flowing 300 gpm every minute, that 300 gpm must be flowing from the pumper

A more complicated situation is that of a pumper feeding several lines. If the pumper is supplying two 300-gpm handlines and one 1,000-gpm master stream. the supply hose must be flowing 1,600 gpm We can’t use 2 1/2-inch supply hose because a only flows about 300 gpm. Why waste a 1,500-gpm pumper (or even a 500-gpm pumper) by feeding it with small hose? That is like using a 25O-horsepower outboard motor on a canoe.

FOG NOZZLES AND NOZZLE PRESSURE

Fog nozzles need a pressure of 100 psi at the nozzle tip to work properly. The original fog nozzles, patented in 1863 by Dr. John Oyston, arc often misleadingly called “constant gallonage nozzles.” The name implies that they hold the flow constant at the rated gpm. regardless of nozzle pressure. They do not. They are, in fact, nozzles With no flow control other than “on” or “off?”* For either a straight stream or a fog pattern, the rated gpm flows only when the pressure at the nozzle inlet is 100 psi. At other nozzle pessures, flow varies directly as a function of the square root of the actual nozzle pressure though it may affect the nozzle pattern adversely. A better name is “single gallonage” nozzles.

Chief C. H. McMillan of the Gary, Indiana Fire Task Force realized the need for variable flow dependent On fire size-up while maintaining a constant nozzle pressure of 100 psi for a good pattern and effective reach. He developed the automatic master stream nozzle in 1969. If the nozzle pressure increases above 100 psi, the baffle plate aperture will open and gpm will increase to maintain the pressure at the nozzle at 100 psi.

*Nozzles with only twist control for on/off do give an untrimmable straight stream at low flow rates

FIREGROUND HYDRAULICS

In theory, handline flow rate can be controlled by the pump operator changing the engine pressure, but during a fire the difficulty of communicating with the nozzleman makes this impractical. If the nozzleman attempts to adjust the flow with his ball valve, increased turbulence will adversely affect the nozzle pattern.

The most advanced type, the controllable-flow automatic nozzle, was developed in 1973. It solves the flow-control problem by replacing the turbulencecausing ball valve with a slide valve that, because of its symmetry, does not introduce turbulence. The flow can therefore be controlled directly by the nozzleman from 50 gpm for overhauling to 350 gpm for darkening down large fires. In addition to being flexible, it is highly cost-effective, since a pumper doesn’t need to carry many different 1 1/2and 2 1/2-inch nozzles. It also automatically maintains the nozzle pressure at 100 psi.

In our calculations, therefore, we can correctly assume there to be “100 psi at the nozzle” whenever using automatic nozzles.

FRICTION LOSS

Flowing water rubs against the hose lining and adjacent water molecules, so energy is lost in overcoming this friction as it is converted into heat. In a hoseline, this loss of energy translates into a loss in pressure along the length of the hose stretch.

To compensate for this pressure loss and drive water through the hose, the pressure at the pumper (engine pressure, or “EP”) has to be greater than the 100 psi nozzle pressure that’s needed. This is why setting the throttle on a pumper so the discharge gauge shows 100 psi gives pathetic results with the old so-called “constant gallonage” fog nozzles, unless you use large-diameter hose. Automatic nozzles give an adequate pattern, but at a low flow that is ineffective for controlling any structural fire larger than a tiny bedroom fire.

Pressure drop due to friction is additive along the length of hose. If the hose length is doubled, the friction loss doubles. For ease of calculation, tables express friction loss (FL) in psi in 100-foot sections of hose. For a 2 1/2-inch hoseline flowing 200 gpm, the friction loss will be 10 psi per 100 feet (the first figure to remember). The total friction loss (TFL) along the entire length of hose is equal to the length of hose (L) multiplied by the friction loss (FL) per 100 feet of that particular size of hose. That is, TFL = L x FL/100. Thus, for our 500 feet of 2 1/2-inch hose that’s flowing 200 gpm:

TFL = 500 ft. x 10 psi/100 ft.

TFL = 50 psi.

It follows that since the engine pressure must equal the sum of the nozzle pressure (NP) and the total friction loss (EP = NP + TFL), the engine pressure must be set at 150 psi to flow 200 gpm over 500 feet with 100 psi of pressure at the nozzle. If the engine pressure is less than 150 psi, the nozzle will flow less than 200 gpm.

FIREGROUND HYDRAULICS

This equation for engine pressure works just as well for delivering water from a hydrant through a hoseline to a pumper (simply substituting different terms for the same physical action: “engine pressure” becomes “hydrant pressure,” and “nozzle pressure” becomes pressure into the pumper). To get even 200 gpm at, say, 10 psi to a pumper from a hydrant 800 feet away, the hydrant pressure must be at least 80 psi + 10 psi = 90 psi, since TFL = 800 x 10/100 psi.

VARIATION OF FRICTION LOSS WITH FLOW RATE

Friction loss increases very quickly as the flow(gpm) increases. It varies as the square of the flow. That is, if gpm doubles, friction loss quadruples. Therefore. if we want to get 400 gpm through 2 1/2-inch hose, the friction loss will be 40 psi per 100 feet.

In the same way, if the flow is halved, friction loss will be reduced to one quarter. For example, the friction loss for 100 gpm is only 10 psi/4 = 2.5 psi per 100 feet. Theoretically, a 90-psi hydrant could supply 100 gpm to a pumper 3,200 feet away. Double the friction loss results from increasing the flow 1.4 times, since the square root of 2 is 1.4. That is, for 280 gpm, the friction loss will be 20 psi. For half die friction loss, the flow must be reduced to 70%, since 0.7 squared is 0.5. For 140 gpm, the friction loss is 5 psi for each 100 feet of hose. This allows us to construct Table 1 to compare with measured values.

It is seen that the calculated values are close enough to the measured values for any foreground situation.

Example 1

If we have a hydrant with a pressure of 70 psi, what flow will 2 1/2-inch hose supply to a pumper 1,200 feet away? With a pumper inlet pressure of 10 psi and no friction loss in the hydrant and water mains (a “stiff” hydrant), the maximum friction loss in the hose is 70 – 10 = 60 psi. This is 5 psi per 100 feet, corresponding to 70 percent of 200 gpm, or 140 gpm.

Example 2

To deliver 300 gpm to 400 feet of 2 1/2inch attack hose, friction loss is roughly 20 psi per 100 feet (corresponding to 280 gpm), and 4 x 20 = 80 psi for 400 feet, so the engine pressure must be 80 psi (TFL) plus 100 psi (NP), or 180 psi.

To calculate friction loss for other hose sizes, one must remember the flow percentage compared to 2 1/2-inch hose for the same friction loss. These are shown in Table 2.

Referring back to Example 1, calculate the hose size needed to utilize frilly a 1,500-gpm pumper, fed by a stiff hydrant 1,200 feet away. A 2 1/2-inch hose will supply 140 gpm (FL = 5 psi per 100 feet). Roughly ten times that flow (10 x 140 = 1,400 gpm) is needed, supplied by a six-inch high-volume supply hoseline.

Suppose we are making the case to the city council for a five-inch supply hose for our new 1,000-gpm pumper. We need 500 feet of hose. Our hydrants will flow 1,000 gpm at a residual pressure of 40 psi. The friction loss must therefore be less than 40 psi/5 psi = 8 psi per 100 feet. The 2 1/2-inch hoseline will flow 200 gpm with FL = 10 psi/ 100 feet, or 140 gpm with FL = 5 psi, so, splitting the difference, it will flow about 170 gpm with a friction loss of 8 psi per 100 feet. The pumper needs 1,000/ 170 or 6 times the flow of a 2 1/2inch hose. Five-inch hose is needed.

An economy-minded council member who has done his homework points out that 4-inch hose costs only 60% of the cost of 5-inch, and surely it flows almost as much water. Goodbye fiveinch hose? Mental arithmetic comes to the rescue. Since four-inch hose flows only about 600 gpm (170 gpm x 3.4), the effectiveness of the S 100,000 pumper would be reduced to 60% capacity, thereby wasting, say, S 20,000 in addition to placing large buildings at risk. Five-inch hoseline is cost-effective in this circumstance—four-inch is not. The situation—and maybe your next fire-threatened shopping plaza—is saved.

What about the 300-foot preconnect with the 125-gpm nozzle? If departmental policy dictates that engine pressure cannot exceed 170 psi, what size attack hose is needed? The friction loss will be 170 (EF) – 100 (NP) = 70 psi, or 23 psi per 100 feet. A 2 1/2-inch hose could flow roughly 280 gpm (corresponding to 20 psi per 100 feet). The preconnect needs 125/280 = 0.4 of that flow. That means 1 3/4-inch hose is needed. The old 1 1/2-inch hose could flow only 280 x 0.25, or about 70 gpm at an engine pressure of 170 psi.

The above examples are typical of the calculations needed on the foreground. They were all performed without the aid of tables, calculator, or pencil and paper. Without being able to perform such calculations, firefighters are working blindly, unable to master the tool they use most. A constant awareness of hydraulics is a first step toward improving effectiveness, since fire suppression capability is directly related to our ability to maximize the gpm applied to a fire and minimize the time to apply it. If you discipline yourself to perform the mental arithmetic on every fire hydraulics situation you encounter, soon you will develop such a “feel” for flow that you’ll always know what is happening.

A new awareness of hydraulics was one of the many factors that transformed the Wabasca FD from a department that rarely saved a building into a department that rarely loses one.

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