28 INDUSTRIAL FIRE JOURNAL THIRD QUARTER 2010 Sign up to our emag at www.hemmingfire.com

MONITORS IN THE SPOTLIGHT

A decade ago a 2,000 gall/min monitor was consideredlarge. Now manufacturers are stretching the envelope with5,000 and 8,000gall/min models and the latest designavailable from Angus Fire can handle flows of 15,000gall/min andthrow a jet of foam over 600ft.

In practice a 15,000gall/min monitor is just a stretched 8,000gall/min design. But thats the easy bit. A 15,000gall/min monitorcan generate a reaction force of 4 tons. With forces this big thestrength of the monitor body and the mechanism for directing itbecome critical.

Fixed or mobile monitors?The layout and sizing of monitors to protect a fixed installation isrelatively simple. The maximum distance the jet has to reach isknown, and once an allowance has been made for wind effects, aplan can be drawn up locating the monitors so they can reach allthe risks. All that remains is to install the water and foam piping.

On a large site it may be impractical to mount enough monitorsto cover every risk under all conditions. The answer is for themonitors to be mobile. When there is an incident the monitor canbe towed on a trailer and deployed, hopefully up-wind of the risk!But as soon as we solve one problem we encounter another. Howdo we get enough water and foam to the monitor?

Water main sizingA 4,000gall/min monitor uses a substantial amount of foam andwater. If a site has a 6 fire ring main feeding water from twodirections the velocity in the pipes will be over 23ft/sec; which isabove the limit most system designers will allow. If the water feed isnot a ring main the pipes must be at least 10 diameter.

Even if the fire main can supply the water there is the difficulty ofgetting it from the hydrants to the monitor. A 4,000gall/min

Calculating the jet flowThe new Angus Fire desk-top computer simulator

allows a design to select monitors and foam cannons

under operating conditions. Chartered Engineer and

Product Manager for Angus Fire Michael Mills writes.

monitor will need to be fed by at least three 6 hoses to stop thepressure drop in the supply pipes becoming too high. While manyfire mains have hydrants with dual 6 outlets not many have three.

At a water flow of 4,000 gall/min and an induction rate of 3% amonitor will use 120 gall/min of foam. If the run time is 20minutes it will need a 2,400 gallon foam tanker to keep it supplied.And even if we can supply enough foam there is the difficulty ofgetting the foam into the water stream in an efficient and accurateway.

Foam inductionOn small monitors it is practical to use a foam-inducing nozzle orcannon. The system is simple and accurate, and the pressure lossthrough the inductor can normally be compensated for byincreasing the supply pressure. However, on a large monitor it maybe necessary to use a foam pump, or a mobile foam inductionsystem.

Angus monitors can be supplied with self-inducing aspiratingfoam cannons for flows up to 4,000gall/min. Above this, jet pumpsor portable inductors must be deployed.

A tiger by the tailA large monitor, with a flow of 8,000gall/min will exert aconsiderable reaction force on the mechanism used to direct thejet. Manual operation, using a lever, becomes difficult and possiblydangerous. The solution is to use a worm and wheel gearbox tomove the monitor in the vertical or horizontal directions.

Provided the worm drive angle is less than 20 it is usuallyimpossible for the wheel to drive the worm due to friction in thegearing, but it is possible for the worm to drive the wheel. Themechanism is therefore intrinsically safe and any out-of-balanceforces in the monitor cannot move it off target.

Jet throw calculationsThe distance a jet of water or foam solution will travel is, in theory,simple to calculate. If the flow of water through the monitor isknown and this is divided by the cross sectional area of the nozzle,the speed of the jet can be calculated. If it is assumed that there isno atmospheric drag then the optimal angle for the jet is 45. For aflow of 1,000gall/min and an effective nozzle diameter of 2 theexit velocity from the nozzle is 100ft/sec. If the nozzle is at an angleof 45 the vertical component of the velocity is 70ft/sec. Gravity willslow the jet by 32ft/sec for every second the jet is in the air.Therefore the time that will elapse before the vertical jet stream hitsthe ground is 4.5 seconds (2.25 seconds upwards and 2.25seconds downwards). During this time the jet will travel horizontallyfor 4.5 seconds at 70ft/sec or a theoretical distance of 315ft beforehitting the ground. However in practice air resistance and otherdynamic effects make the actual distance much shorter, typically50% to 65% of the theoretical distance.

Air resistanceThe major influence on jet travel is air resistance. A small diameterjet (typically 1/2 will travel through the air without breaking apart.This is due to surface tension effects and because the turbulence ina small diameter nozzle is usually minimal. However, large jets donot behave in this way. A large jet (typically 3 or above) will oftenspin and contain eddies as it leaves the nozzle. These cause water

Angus remotecontrol monitor

under test.

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droplets to spin off the main jet and cause additional friction losseswhich in turn reduce the distance the jet will travel. It also followsthat, if the major influence on jet travel is air resistance, any headwind will cause a significant change in range.

How far will the jet travel?There have been a number of attempts to generate mathematicalformulas that predict throw. It is relatively simple to generatemathematical formulas for water jets which remain coherent, but itis difficult to generate reliable formulas to allow for a jet whichbreaks up into smaller particles.

Early attempts in the 1900s were inaccurate; especially for largerdiameter nozzles. Later attempts in the 1970s used computers tosolve the equations using the step method. This began to yieldusable results, but computer time was at a premium and tooexpensive for day to day use.

Angus Fire has developed a computer simulation that mirrorsexperimental results for most conditions encountered during firefighting; including wind effects. However, while it is simple to checkthe results of jet throw in still-air conditions against themathematical prediction, it is difficult to cross check the effects ofhead or side winds during field tests.

The use of computer simulations has confirmed a number ofeffects which, while known to experienced firefighters, can now bepredicted using mathematics.

For a given water flow there is an optimum jet size which givesthe maximum jet range.

Above a certain pressure (roughly 170psi, but varies with thejet size) an increase in pressure will lead to a decrease in jetthrow and not an increase.

The effect of wind speed on jet throw is substantial. In additionthe effect of a head wind on jet range is less for every ft/sec ofwind strength than the effect of a following wind.

The Angus Fire desk top computer simulation allows a designerto select monitors and foam cannons or fog/jet nozzles in therange 100 to 9,000 US gall/min; choose the height of the monitorabove ground level; select the angle of the jet (including negativeangles with the jet pointing downwards); and vary the monitor inletpressure. Finally the user can change the wind speed from zero to65 ft/sec either with or against the direction of the jet. When a finalselection has been made the result can be printed out for futurereference.

Improvements in monitor design have continued over the yearswith larger and larger units being produced. However, electronicdocumentation, to enable the user to predict the jet throw and flowunder a range of operating conditions, is a major step forward inthe technology of firefighting monitors.

Sofrad Groupe & Gimaex Group have joined forces to createScorpe Technologies. The new company not only designsand manufactures but also market its products worldwide.Ann Marie Knegt speaks with Managing Director ChristopheCahoreau.

What types of large capacity monitors do you manufacture?In our range of products, we have four different models rangingfrom 2,000 LPM to 8,000 LPM. In the 5,000 LPM models, we canfind the Rockett GF5, which can be used both on the ground ormounted as a fixed monitor, then the Rockett GF5 RF with the samefeatures except from the radio frequency control of the movements of the monitor. We then go tothe Rockett F5 RF, which is a 5,000 LPM fixed mount radio frequency operated monitor. Thehighest capacity nozzle we offer in our range is the Rockett F8 RF, an 8,000 LPM fixed mount,radio-frequency operated monitor. All the nozzles mounted on our monitors benefit from theVelocity Equalizing Nozzle, using a cone shape to restructure the flow, resulting in improved reachof between 15 and 20%.

Which of these are best suitable for industrial use?All of them are suitable for industrial use, depending on the needs of each industry, in terms offlow, but also in terms of monitor control via wireless/manual options.

What are the main requirements of your industrial customers?The radio frequency control of monitors is something that the industrials are really interested in,because it enables better control of the situation by having a global view of the actionsite, and as well protecting the people who can be up to 400 meters away from themonitor with the remote control.

How are customer requirements driving the design of yourmonitors?The input of industrial customers isreally important to us. They mayhave specific needs but most ofthe time the improvementsbrought by the industrial world areinstalled in our entire range. Eachand every bit of information wereceive from our different customershelps to design better products, enablingus to offer more complete and efficientsolutions to many different types ofconcerns.

The three-minute interview:Scorpe Technologies

Christophe Cahoreau,Scorpe Technologies.

Rockett GF5 RFradio-controlledfixed monitor.

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