Wednesday, February 9, 2011
Fire sprinkler system
A fire sprinkler system is an active fire protection measure, consisting of a water supply, providing adequate pressure and flowrate to a water distribution piping system, onto which fire sprinklers are connected. Although historically only used in factories and large commercial buildings, home and small building systems are now available at a cost-effective price.
Sprinklers may be required to be installed by building codes, or may be recommended by insurance companies to reduce potential property losses or business interruption. Building codes in the United States for places of assembly, generally over 100 persons, and places with overnight sleeping accommodation such as hotels, nursing homes, dormitories, and hospitals usually require sprinklers either under local building codes, as a condition of receiving State and Federal funding or as a requirement to obtain certification (essential for institutions who wish to train medical staff).
If building codes do not explicitly mandate the use of fire sprinklers, the code often makes it highly advantageous to install them as an optional system. Most US building codes allow for less expensive construction materials, larger floor area limitations, longer egress paths, and fewer requirements for fire rated construction in structures protected by fire sprinklers. Consequently, the total building cost it is often less by installing a sprinkler system and savings money in the other aspects of the project, as compared to building a non-sprinklered structure. In the UK, since the 1990′s sprinklers have gained recognition within the Building Regulations (England and Wales) and Scottish Building Standards and under certain circumstances, the presence of sprinkler systems is deemed to provide a form of alternative compliance to some parts of the codes. For example, the presence of a sprinkler system will usually permit doubling of compartment sizes and increases in travel distances (to fire exits) as well as allowing a reduction in the fire rating of internal compartment walls.
Renewed interest in and support for sprinkler systems in the UK, largely as a result of effective lobbying by the National Fire Sprinkler Network, the European Fire Sprinkler Network and the British Automatic Fire Sprinkler Association, has resulted in sprinkler systems being more widely installed. In schools, for example, the Department of Children, Families and Schools has issued strong recommendations that most new schools should be constructed with sprinkler protection. In Scotland, all new care homes are sprinklered as are sheltered housing and high rise flats. Most local authorities in Scotland have a policy of fitting sprinklers to new schools and to most of their own new buildings.
Each closed-head sprinkler is held closed by either a heat-sensitive glass bulb (see below) or a two-part metal link held together with fusible alloy. The glass bulb or link applies pressure to a pip cap which acts as a plug which prevents water from flowing until the ambient temperature around the sprinkler reaches the design activation temperature of the individual sprinkler head. Because each sprinkler activates independently when the predetermined heat level is reached, the number of sprinklers that operate is limited to only those near the fire, thereby maximizing the available water pressure over the point of fire origin.
A sprinkler activation will do less damage than a fire department hose stream, which provide approximately 900 liters/min (250 US gallons/min). A typical sprinkler used for industrial manufacturing occupancies discharge about 75-150 litres/min (20-40 US gallons/min). However, a typical Early Suppression Fast Response (ESFR) sprinkler at a pressure of 50 psi (345 kPa) will discharge approximately 100 US gallons per minute, (380 litres per minute). In addition, a sprinkler will usually activate between one and four minutes, whereas the fire department typically takes at least five minutes to arrive at the fire site after receiving an alarm, and an additional ten minutes to set up equipment and apply hose streams to the fire. This additional time can result in a much larger fire, requiring much more water to achieve extinguishment.
Sprinkler systems are intended to either control the fire or to suppress the fire. Control mode sprinklers are intended to control the heat release rate of the fire to prevent building structure collapse, and pre-wet the surrounding combustibles to prevent fire spread. The fire is not extinguished until the burning combustibles are exhausted or manual extinguishment is effected by firefighters. Suppression mode sprinklers (formerly known as Early Suppression Fast Response (ESFR) sprinklers) are intended to result in a severe sudden reduction of the heat release rate of the fire, followed quickly by complete extinguishment, prior to manual intervention.
Saturday, February 5, 2011
The concept of fire plugs dates to at least the 17th century. This was a time when firefighters responding to a call would dig down to the wooden water mains and hastily bore a hole to secure water to fight fires. The water would fill the hole creating a temporary well, and be transported from the well to the fire via bucket brigades or, later, via hand pumped fire engines. The holes were then plugged with stoppers, normally redwood, which over time came to be known as fire plugs. The location of the plug would often be recorded or marked so that it could be reused in future fires. This is the source of the colloquial term fire plug still used for fire hydrants today. After the Great Fire of London in 1666, the city installed water mains with holes drilled at intervals, equipped with risers, allowing an access point to the wooden fire plugs from street level.
It has been claimed that Birdsill Holly invented the fire hydrant, but his 1869 design was preceded by many other patents for fire hydrants, and a number of these earlier designs were produced and successfully marketed. Numerous wooden cased fire hydrant designs existed prior to the development of the familiar cast iron hydrant. Although the development of the first above ground hydrant in the USA traces back to Philadelphia in 1803, underground fire hydrants — common in parts of Europe and Asia — have existed since the 18th century.
A hose is attached to the fire hydrant, then the valve is opened to provide a powerful flow of water, on the order of 350 kPa (50 lbf/in²) (this pressure varies according to region and depends on various factors including the size and location of the attached water main). This hose can be further attached to a fire engine, which can then use a powerful pump to boost the water pressure and possibly split it into multiple streams. The hose may be connected with a threaded connection or a Storz connector. Care should be taken not to open or close a fire hydrant too quickly, as this can create a water hammer which can damage nearby pipes and equipment. The water inside a charged hoseline causes it to be very heavy and high water pressure causes it to be stiff and unable to make a tight turn while pressurized. When a fire hydrant is unobstructed, this is not a problem, as there is enough room to adequately position the hose.
Most fire hydrant valves are not designed to throttle the water flow; they are designed to be operated full-on or full-off. The valving arrangement of most dry-barrel hydrants is for the drain valve to be open at anything other than full operation. Usage at partial-opening can consequently result in considerable flow directly into the soil surrounding the hydrant, which, over time, can cause severe scouring. A hose with a closed nozzle valve, or fire truck connection, or closed gate valve is always attached to the hydrant prior to opening the hydrant's main valve.
When a firefighter is operating a hydrant, appropriate personal protective equipment, such as gloves and a helmet with face shield, are typically worn. High pressure water coursing through a potentially aging and corroding hydrant could cause a failure, injuring the firefighter operating the hydrant or bystanders.
In most jurisdictions it is illegal to park a car within a certain distance of a fire hydrant. In North America the distances are commonly 3 to 5 m or 10 to 15 ft, often indicated by yellow or red paint on the curb. In the UK, yellow lines are used to keep cars from parking over underground hydrants. Parking restrictions are sometimes ignored (especially in cities where available street parking is scarce), however these laws are usually enforced. The rationale is that hydrants need to be visible and accessible in an emergency.
To prevent casual use or misuse, the hydrant requires special tools to be opened, usually a large wrench with a pentagon-shaped socket. Vandals sometimes cause monetary loss by wasting water when they open hydrants. Such vandalism can also reduce municipal water pressure and impair firefighters' efforts to extinguish fires. Sometimes those simply seeking to play in the water remove the caps and open the valve, providing residents a place to play and cool off in summer. However, this is usually discouraged as residents have been struck by passing automobiles while playing in the street in the water spray. In spite of this, some US communities provide low flow sprinkler heads to enable residents to use the hydrants to cool off during hot weather, while gaining some control on water usage. Most fire hydrants in Australia are protected by a silver-coloured cover with a red top, secured to the ground with bolts to protect the hydrant from vandalism and unauthorised use. The cover must be removed before use.
In most US areas, contractors who need temporary water may purchase permits to use hydrants. The permit will generally require a hydrant meter, a gate valve and sometimes a clapper valve (if not designed into the hydrant already) to prevent back-flow into the hydrant. Additionally, residents who wish to use the hydrant to fill their in-ground swimming pool are commonly permitted to do so provided they pay for the water and agree to allow firefighters to draft from their pool in the case of an emergency.
Municipal services, such as street sweepers and tank trucks, may also be allowed to use hydrants to fill their water tanks. Often sewer maintenance trucks need water to flush out sewerage lines, and fill their tanks on site from a hydrant. If necessary, the municipal workers will record the amount of water they used, or use a meter.
Since fire hydrants are one of the most accessible parts of a water distribution system, they are often used for attaching pressure gauges or loggers or monitor system water pressure. Automatic flushing devices are often attached to hydrants to maintain chlorination levels in areas of low usage. Hydrants are also used as an easy above ground access point by leak detection devices to detect locate leak from the sound they make.
In areas subject to freezing temperatures, only a portion of the hydrant is above ground. The valve is located below the frost line and connected via a riser to the above-ground portion. A valve rod extends from the valve itself up through a seal at the top of the hydrant, where it can be operated with the proper wrench. This design is known as a "dry barrel" hydrant, in that the barrel, or vertical body of the hydrant, is normally dry. A drain valve underground opens when the water valve is completely closed; this allows all water to drain from the hydrant body to prevent the hydrant from freezing.
In warm areas, hydrants are used with one or more valves in the above-ground portion. Unlike cold-weather hydrants, it is possible to turn the water supply on and off to each port. This style is known as a "wet barrel" hydrant.
Both wet- and dry- barrel hydrants typically have multiple outlets. Wet barrel hydrant outlets are typically individually controlled, while a single stem operates all the outlets of a dry barrel hydrant simultaneously. Thus, wet barrel hydrants allow single outlets to be opened, requiring somewhat more effort but simultaneously allowing more flexibility.
A typical U.S. dry-barrel hydrant has two smaller outlets and one larger outlet. The larger outlet is often a Storz connection if the local fire department has standardized on hose using Storz fittings for large diameter supply line. The larger outlet is known as a "steamer" connection (because they were once used to supply steam powered water pumps), and a hydrant with such an outlet may be referred to as a "steamer hydrant" although this usage is becoming archaic. Likewise, an older hydrant without a steamer connection may be referred to as a "village hydrant."
Hydrant coloring may be due either to purely practical criteria or more artistic. In the United States, the AWWA and NFPA recommend hydrants be colored chrome yellow for rapid identification apart from the bonnet and nozzle caps which should be coded according to their available flow. Class AA hydrants (>1500gpm) should have their nozzle caps and bonnet colored light blue, Class A hydrants (1000-1499gpm), green, Class B hydrants (500-999gpm), orange, and Class C hydrants (0-499gpm), red. This aids arriving firefighters in determining how much water is available and whether to call for additional resources, or locate another hydrant. Other codings can be and frequently are used, some of greater complexity, incorporating pressure information, others more simplistic. In Ottawa, hydrant colors communicate different messages to firefighters; for example, if the inside of the hydrant is corroded so much that the interior diameter is too narrow for good pressure, it will be painted in a specific scheme to indicate to firefighters to move on to the next one. In many localities, a white or purple top indicates that the hydrant provides non-potable water. Where artistic and/or aesthetic considerations are paramount, hydrants can be extremely varied, or more subdued. In both instances this is usually at the cost of reduced practicality.
In Germany, most hydrants are located below ground (Unterflurhydrant) and are accessed by a Standrohr which provides the connections for the hoses.
In the UK and Ireland, hydrants are located in the ground. Yellow "H" hydrant signs indicate the location of the hydrants, and are similar to the blue signs in Finland. Mounted on a small post or nearby wall etc., the two numbers indicate the size of the water main (top number) and the distance from the sign (lower number). Modern signs show these measurements in millimetres and metres, whereas older signs use inches and feet. Because the orders of magnitude are so different (6 inches versus 150 mm) there is no ambiguity whichever measuring system is used.
In areas of the United States without winter snow cover, blue reflectors embedded in the street are used to allow rapid identification of hydrants at night. In areas with snow cover, tall signs or flags are used so that hydrants can be located even if covered with snow. In rural areas tall narrow posts painted with visible colours such as red are attached to the hydrants to allow them to be located during heavy snowfall periods.
In Australia, Hydrant signage varies, with several types displayed across the country. Most Australian hydrants are underground, being of a ballcock system, and a standpipe with a central plunger is used to open the valve. Due to this, hydrant signage is essential, due to their concealed nature.
* Painted markers - Usually a white or yellow (sometimes reflective paint) triangle or arrow painted on the road, pointing towards the side of the road the hydrant will be found on. These are most common in old areas, or on new roads where more advanced signs have not been installed. These are almost always coupled with a secondary form of signage.
* Hydrant Marker Plates - Found on power poles, fences, or street-signs, these are a comprehensive and effective system of identification. The plate consists of several codes; H (Potable water Hydrant), RH (Recycled/Non Potable), P (Pathway, where the hydrant cover can be found), R (Roadway). The plate is vertically oriented, around 8 cm wide, and 15 cm high. Found on this plate, from top to bottom, are the following features:
o The codes listed above, Potable/Non-potable at the top, Path/Roadway on the bottom of the plate.
o Below this, a number giving the distance to the hydrant (in meters), then a second number below that giving the size (in millimeters) of the water main.
o A black line across the center of the plate indicated the hydrant is found on the opposite side of the road to which the plate is affixed.
o Plates for recycled water have a purple background, as well as the RH code, normal potable hydrants are white, with the H code.
* Road reflectors or 'Catseyes' - Almost exclusively blue, these are placed on the center line of the road, usually with little indication on which side of the road the hydrant lies. They are visible for several hundred meters at night in heavy rain, further in clear conditions.
Inspection and maintenance
In most areas fire hydrants require annual inspections and maintenance - they normally only have a one year warranty, but some have 5 or even 10 year warranties, although the longer warranty does not remove the need for periodic inspections or maintenance. These inspections are generally performed by the local municipalities but they often do not inspect hydrants that are identified as private. Private hydrants are usually located on larger properties to adequately protect large buildings in case of a fire and in order to comply with the fire code. Such hydrants have met the requirements of insurance underwriters and are often referred to as UL/FM hydrants. Some companies are contracted out to inspect private fire hydrants unless the municipality has undertaken that task.
Some fire Hydrant manufacturers recommend lubricating the head mechanism and restoring the head gaskets and o-rings annually in order that the fire hydrant perform the service expected of them, while others have incorporate proprietary features to provide long-term lubrication of the hydrant's operating mechanism. In any case, periodic inspection of lubricates is recommended. Lubrication is generally done with a food grade non-petroleum lubricant to avoid contamination of the distribution system.
Occasionally a stone or foreign object will mar the seat gasket. In this case, most hydrants have a special seat wrench that allows removal of the seat to replace the gasket or other broken parts without removing the hydrant from the ground. Hydrants extensions are also available for raising a hydrant if the grade around the hydrant changes. Without extending the height, the wrenches to remove caps would not clear and the break flanges for traffic models would not be located correctly in case they were hit. Hydrant repair kits are also available to repair sacrificial parts designed to break when hit by a vehicle.
Many departments use the hydrants for flushing out water line sediments. When doing so, they often use a hydrant diffuser, which is a device that diffuses the water so that it doesn't damage property and is less dangerous to bystanders than a solid stream. Some diffusers also dechlorinate the water to avoid ground contamination. Hydrants are also sometimes used as entry or exit points for pipe cleaning pigs.
The fire triangle or combustion triangle is a simple model for understanding the ingredients necessary for most fires.
The triangle illustrates a fire requires three elements: heat, fuel, and an oxidizing agent (usually oxygen). The fire is prevented or extinguished by removing any one of them. A fire naturally occurs when the elements are combined in the right mixture.
Without sufficient heat, a fire cannot begin, and it cannot continue. Heat can be removed by the application of a substance which reduces the amount of heat available to the fire reaction. This is often water, which requires heat for phase change from water to steam. Introducing sufficient quantities and types of powder or gas in the flame reduces the amount of heat available for the fire reaction in the same manner. Scraping embers from a burning structure also removes the heat source. Turning off the electricity in an electrical fire removes the ignition source.
Without fuel, a fire will stop. Fuel can be removed naturally, as where the fire has consumed all the burnable fuel, or manually, by mechanically or chemically removing the fuel from the fire. Fuel separation is an important factor in wildland fire suppression, and is the basis for most major tactics, such as controlled burns. The fire stops because a lower concentration of fuel vapor in the flame leads to a decrease in energy release and a lower temperature. Removing the fuel thereby decreases the heat.
Without sufficient oxygen, a fire cannot begin, and it cannot continue. With a decreased oxygen concentration, the combustion process slows. In most cases, there is plenty of air left when the fire goes out so this is commonly not a major factor.
The fire tetrahedron is an addition to the fire triangle. It adds the requirement for the presence of the chemical reaction which is the process of fire. For example, the suppression effect of Halon is due to its interference in the fire chemical inhibition.
Combustion is the chemical reaction that feeds a fire more heat and allows it to continue. When the fire involves burning metals like lithium, magnesium, titanium, etc. (known as a class-D fire), it becomes even more important to consider the energy release. The metals react faster with water than with oxygen and thereby more energy is released. Putting water on such a fire results in the fire getting hotter or even exploding because the metals react with water in an exothermic reaction. Carbon dioxide extinguishers are ineffective against certain metals such as titanium. Therefore, inert agents (e.g. dry sand) must be used to break the chain reaction of metallic combustion. In the same way, as soon as we remove one out of the 3 elements of the triangle, combustion stops.
Based on the combustible material involved, the fire can be classified. In the European Standard "Classification of fires" (EN 2:1992, incorporatiing amendment A1:2004), the fires are classified as:
* Class A fire: Ordinary combustibles such as wood, paper, carton, textile, and PVC;
* Class B fire: Flammable liquids and solids which can take a liquid form, such as benzene, gasoline, oil;
* Class C fire: Flammable gases, such as butane, propane, and natural gas;
* Class D fire: Combustible metals, such as iron, aluminum, sodium, and magnesium;
* Class F fire: Cooking media, such as oils and fats, in cooking appliances;
A fire involving energized electrical equipment is not classified by its electrical property.
In the American standard, fires are classified as:
* Class A fire: Ordinary combustibles such as wood, paper, carton, textile, and PVC;
* Class B fire: Flammable liquid or gaseous fuels such benzene, gasoline, oil, butane, propane, and natural gas;
* Class C fire: Involving energized electrical equipment, often caused by short circuits or overheated electrical cables;
* Class D fire: Combustible metals, such as iron, aluminum, sodium, and magnesium;
* Class K fire: Containing a fat element, such as cooking oil
The oxidizer is the other reactive of the chemical reaction. In most cases, it is the ambient air, and in particular one of its components, Oxygen (O2). By depriving a fire of air, we extinguish it; for example, when covering the flame of a small candle with an empty glass, fire stops; to the contrary, if we blow over a wood fire, we activate it (by bringing more air). In certain torches, we bring dioxygen to improve combustion. In certain very particular cases (such as explosives like aluminum), the oxidizer and combustible are the same (i.e.nitroglycerin, an instable molecule made of one oxidizing part linked to a reducing part). Reaction is initiated by an activating energy, in most cases, it is heat. Several examples include friction, as in case of matches, heating an electrical wire, a flame (propagation of fire), or a spark (from a lighter or from any starting electrical device). There are also many other ways to bring sufficient activation energy including electricity, radiation, and pressure, all of which will lead to a temperature rise. In most cases, heat production enables self-sustainability of the reaction, and enables a chain reaction to grow. The temperature at which a liquid produces sufficient vapor to get a flammable mix with self-sustainable combustion is called its flash-point.
Extinction of the fire
To stop a combustion reaction, one of the three elements of the fire-triangle has to be removed:
* Suppression of the Combustible: by closing of the valve fueling the combustion, creating sufficient distance between fire and flame, exhausting hot smoke (containing unburned elements), …
* Suppression of the Oxidizer (also known as choking): by the use of a carbon dioxide fire-extinguisher, a blanket, or spraying sufficient water on a solid combustible (water vapor removes fresh air) …
* Suppression of the Activation Energy (cooling down): by spraying water in a mix of air + combustible particles), net absorbing the heat ("Davy" miner lamp), exhausting to remove hot smoke, …
Role of water in fire-fighting
Water can have two different roles:
1. In the case of a solid combustible, the solid fuel produce pyrolyzing products under the influence of heat, commonly radiation. This process is halted by the application of water, since water is more easily evaporated than the fuel is pyrolyzed. Thereby energy is removed from the fuel surface and it is cooled and the pyrolyze is stopped, removing the fuel supply to the flames. In fire fighting, this is referred to as surface cooling
2. In the gas phase, i.e. in the flames or in the smoke, the combustible can not be separated from the oxidizer, the only possible action consists of cooling down. In this case, water droplets are evaporated in the gas phase, thereby lowering the temperature and adding water vapour making the gas mixture non combustible. This requires droplets of a size less than about 0.2 mm. In fire fighting, this is referred to as gas cooling or smoke cooling. There also exist cases where the ignition factor is not the activation energy. For example, smoke explosion is a very violent combustion of unburned gases contained in the smoke created by a sudden fresh air input (oxidizer input). The interval in which an air/gas mix can burn is limited by the explosivity limits of the air. This interval can be very small (kerosene) or large (acetylene).
Role of water additives
The role of water in extinguishing a fire can be summarized as follows: The main effect is cooling down the fire by absorption of heat energy either at the fuel surface or in the gas phase. A contributing effect is diluting the atmosphere by adding vapor and thereby removing oxygen from the fire The main limits to the use of water are directly linked to the physical-chemical characteristics of water itself: - Water can’t be used on certain type of fires :
* Hydrocarbon fires (B class) - as it will only spread the fire because of the difference in density
* Metal fires (D class) - as these fires produce huge amounts of energy (up to 7.550 calories/kg for Aluminum) and water can also create violent chemical reaction with burning metal (by oxidization)
* Fat fires (F class) - as vapor will carry and spread burning oil everywhere.
Since these reactions are well-understood, it has been possible to create specific water-additives which will allow:
* A better heat absorption with a higher density than water
* Carrying free radical catchers on the fire
* Carrying foaming agents to enable water to stay on the surface of a liquid fire and prevent gas release
* Carrying specific reactives which will react and change the nature of the burning material
Water-additives are generally designed to be effective on several categories of fires (class A + class B or even class A + class B + class F), meaning a better global performance and polyvalence of the fire-extinguisher.
Chemistry of Combustion
Combustion is a chemical reaction in which complex molecules are broken down into smaller, more stable molecules through a rearrangement of atomic bonds. A major component of the chemistry of high-temperature combustion involves radical reactions. However, it is possible to consider combustion as a single overall reaction.
Example : H3C-CH2-CH3 + 5O2 → 3CO2 + 4H2O
Carbon dioxide and water are more stable than oxygen and propane. Combustion is an oxidation-reduction reaction, meaning oxidization of a combustible by an oxidizer; • combustible is being oxidized during combustion, it is a reducer as it loses electrons; • Oxidizer is the part being reduced; it is an oxidizer as it gains electrons. As with any chemical reaction, a catalyst encourages combustion and as it has a high activation energy level, the use of a catalyst enables working at lower temperature. This leads to more complete combustion as in the catalyst of the exhaust of a car, where catalytic metals burn residues contained in the exhaust smoke at lower temperature than in the engine. Concerning solid combustible, the activation energy allows for vaporization or pyrolysis of the combustible. Gas produced will then mix with an oxidizer resulting in a combustible mixture. If the energy produced by the combustion is higher or equal to the quantity of energy required for the combustion, the reaction is then self-sustainable.
Produced energy and calorific power
The quantity of energy produced by the reaction is higher than the quantity of energy requested to start it. The quantity of energy produced by the combustion is given in Joules (J); it is the enthalpy of the reaction. In the application domains (oven, burner, engine with internal combustion, fire-fighting), we use the notion of calorific power, what is basically the enthalpy of the chemical reaction per unit of weight of combustible or the obtained energy given by the combustion of one kilogram of combustible, expressed in kilojoules per kilogram (kJ/kg or kJ•kg-1). Combustion of hydrocarbon produces water in its vapor form. ; This water vapor contains huge amount of energy and this parameter has to be taken into account in a specific way to evaluate correctly the calorific power. We define: • Superior Calorific Power (SCP): « Quantity of energy produced during a complete combustion of a combustible unit, water vapor is said condensed and heat collected »2. • Inferior Calorific Power (ICP): « Quantity of energy produced during a complete combustion of a combustible unit, Water vapor is said non-condensed and heat not collected »3. Difference between ICP and SCP is the latent heat of water vaporization (Lv) multiplied by the quantity of produced vapor (m), what equals +/- 2 250 kJ•kg-1 (this value is influenced by pressure and temperature). We have the relation SCP = ICP + m•Lv.