Post on 28-Apr-2015
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A UTC Fire & Security Company
The Design and Construction of Fire Fighting MonitorsFixed and Mobile, Manual and Remote Control, Water and Foam
IntroductionMonitors spend most of their lives static and lifeless. But when a fire is detected they can often be the only practical way of applying foam or water to the fire. While simple in principle, monitors are sophisticated pieces of engineering made to deliver a specific performance after long periods of inactivity. Like many engineering challenges the design of a monitor can take many forms depending on the specific hazard it is intended to protect and the mechanism and method of operation the designer uses to achieve the final layout. When designing a monitor the manufacturer must balance performance, operational life and ease of use against cost. The installation of fixed monitors, or the provision of mobile or portable monitors, is usually the outcome of a careful analysis of the fire risk and the realisation that without planning in advance fighting any subsequent fire will present difficulties. It is essential therefore, that monitors are robust and will have a long service life, even under adverse conditions.
ApplicationsFixed monitors are found wherever there are substantial Class B fire risks while mobile or portable monitors are often used to protect multiple risks by moving the monitors around the site. Nearly all industrial fire hazards are candidates for monitor protection, but some of the more common applications are: Refineries Fuel distribution depots Chemical plants Warehouses Helicopter landing pads Aircraft hangars Loading jetties Process plants Industrial process areas Shipping Vehicle-mounted
Fixed or mobile?While many monitors are permanently fixed to pipework and designed to protect specific installations, it is sometimes more convenient to mount monitors on trailers that can be moved from hazard to hazard. In addition, smaller monitors can be designed to be moved by hand and placed on the ground to provide a rapid response in the event of a fire. However, mobile monitors require a water supply, usually provided by hoses or portable pumps. The jet reaction force for a portable monitor can vary from a few kg, for a small ground monitor, to over a tonne for a larger trailer-mounted unit. Any portable monitor must be secured so that it cannot move once the full water flow and pressure is applied. Small, hand-wheel portable monitors are specifically designed to be easy to manoeuvre and carried over rough terrain. To resist the jet reaction forces portable ground monitors are provided with a method of stabilising them on soft ground. Larger monitors are usually mounted on trailers. In addition, the trailer is often fitted with outriggers to provide stability. Water tanks on the trailer can be filled to provide additional weight for stability. Extra tanks can also be specified to provide foam concentrate.
Angus monitor with optional foam induction system anchored on soft ground with bipod mechanism
Angus trailer-mounted monitors with self-inducing foam cannons
Trailer-mounted monitors provide a useful addition to the armoury of equipment a fire service can draw on should a large fire occur. The mobile monitor can be used to protect locations inadequately covered by fixed monitors or provide cooling to equipment adjacent to the fire.
Monitor DesignDesign is always a compromiseThe layout of the pipes that make up a monitor must serve several functions. They must contain the water while allowing the jet to be moved in both the horizontal and vertical planes; they must be strong enough to resist the pressure and reaction forces generated by the water; and they must be robust enough to allow the mounting of additional items such levers, gearboxes, hydraulic actuators and nozzles. All of this must be achieved with a design that is cost effective, has an acceptable pressure loss, will resist corrosion and is not too heavy. Like many engineering designs a typical monitor is a compromise between cost, weight and performance.
BearingsThe normal practice is to support the two parts of a monitor, where it moves in the horizontal or vertical plane, using a double ball race. A shaft seal is positioned between the ball races and the waterway to retain the water under pressure. Plugs are machined into the outer casing to allow the ball bearings to be inserted during the manufacturing process and can be fitted with grease nipples to allow lubrication during manufacture or maintenance.
Pressure lossesIf too much water is forced around too many tight bends at too high a speed there will be an unacceptable pressure loss between the monitor inlet flange and the nozzle. The result will be that the water or foam jet will not travel as far as it could. Some designs use a very compact layout forcing water to turn 90 bends and splitting the water into two paths which meet again at the outlet. While the dual path waterway layout is compact its pressure losses can be unacceptable where the supply pressure is limited. Monitor type Single waterway fabricated stainless steel Single waterway cast bronze Pressure loss at 1000 l/min 0.2 bar (3.0 psi) Typical nozzle throw 38 m
Sharp bends and merging flows cause turbulence
Some designs include a second seal to prevent dirt and dust entering the bearing assembly. However, this can cause difficulties since the bearing chamber becomes a sealed space with no room for expansion. Like most engineering designs the layout of the bearings is a compromise. The wider apart they are the lower the forces on each bearing race. However, widening the bearing spacing also increases the overall size of the unit. To resist the loads steel balls are used. These are normally a high grade stainless steel such as SS316. When the monitor body is bronze the steel balls rest in tracks machined into the casting. However, when the monitor is fabricated from stainless steel pipe and the bearing tracks are incorporated into the fabrication it is essential to select a material for the steel balls that does not react with the steel of the bearing races or the balls may pick up causing the bearing to seize.
0.3 bar (4.5 psi)
37 m 33 m
Dual path waterway 1.0 bar (15 psi) cast aluminium
In addition, the turbulence created when the two water streams meet usually adversely affects the nozzle performance and limits its range still further. Waterways manufactured from cast bronze usually have tighter bends than those manufactured from steel since it is difficult to fabricate tight bends in steel tube. As a result, stainless steel monitors generally have a lower pressure loss than the equivalent cast bronze monitor while the dual waterway designs have the highest pressure losses of all. Water takes on a spin when negotiating the bends in a single or dual waterway pipe. By the time it has negotiated both the horizontal and vertical joint bends the spin generated can cause a reduction in throw. The spinning water expands the jet stream creating greater friction as it passes through the air. To reduce the spin cast bronze or iron waterway monitors often have vanes or blades cast into the tube to reduce the spin in the water stream.
Flow straightners in pipe reduce water stream turbulence and rotation
Nozzle flow at 7 barBall Bearings Shaft Seal
Total reaction force at tip
Typical side force on monitor handle with 5 of misalignment in the horizontal plane 6 kg 11 kg 14 kg 19 kg
2,000 l/min 4,000 l/min 6,000 l/min 8,000 l/minFitting Plugs for Bearing Insertion
140 kg 240 kg 310 kg 425 kg
Misalignment is more common in fabricated monitors. Unless great care is taken during the welding, and special techniques and jigs are used, the welded pipes can easily distort. A worm and wheel gearbox drive is generally used for control in the vertical and horizontal planes for monitors where the flow is 3,000 l/min or more.
Gearbox actuation Reaction forcesIt is important to arrange the layout of a monitor so that the thrust or reaction force caused by the water leaving the nozzle or foam cannon is directed through the pivot point in both the horizontal and vertical planes. Control of the vertical and horizontal movement using a worm and wheel gearbox is recommended for two reasons. Firstly, it is easy for the operator to set the monitor in position. The handle loads are low and control is precise. Secondly, a worm and wheel gearbox, if correctly designed, will resist any out of alignment forces. Provided the worm drive angle is less than 20 it is possible for the worm to drive the wheel, but it is impossible for the wheel to drive the worm due to friction in the gearing. The mechanism is therefore intrinsically safe and any out-of-balance forces in the monitor cannot move it off target.
Out-of-balance forces cause the monitor to rotate or move out of alignment
If the monitor is distorted and the reaction force does not pass through the pivot there will be a sideways or vertical force on the monitor. If the out-of-line force is not restrained the monitor will tip up or down or spin in the horizontal plane. The consequences of this can be serious. If the monitor is spinning, often the only way to stop it may be to turn off the water at its base. If over 80 kg of monitor body is spinning at 2 or 3 revolutions per second it can be dangerous to climb underneath the spinning body to turn off the water! It could also damage or bend the pipework or other structures. The forces exerted by the jet are considerable and even a minor misalignment can result in a large side load. If at the same time there is a side