1、Fire Prevention and Foamsystem The selection of the proper storang tanks,their spacing and proper design,is the best safeguard against fire hazards.There are three basic conditions that must be for fire to start: (1)Fuel must be present in dispersed form as a vapor. (2)Air (supplying oxygen for comb
2、ustion) must be presernt in the right proportion with the fuel vapor to form a maxture that can be ignited. (3)A source of ignition (high temperature and energy sufficient to initiate the chemical action of combustion) must be present. Starting a fire requires all three essentials existing at the sa
3、me time and at the same place. Extinguishment usually requires that either the fuel or air be eliminated. When water is applied directly to an oil fire it often does more damage than good, since burning oil f；oating on the water surface may actually spread the fire. Water is of assistance in keeping
4、 adjacent tanks and buildings cool. Water fog,a finely divided spray, is useful in shutting off air from burning oil surface and cooling it below ignition temperature.Water fog is effective on viscous oils or high flash oils in areas that are within the range of the spray. A standard fire stream is
5、rated at about 250 gpm using a 1.125 in. nozzle and 2.5 in. smooth rubber lined hose. This standard stream will require a nozzle pressure of about 45 psi with a hydrant pressure of 105 psi using 400 feet of hose between hydrant snd nozzle. A main distribution pressure of 125 psi for hose streams and
6、 foam generation is adequate. When pressure is inadequate, fire pumps are installed. Pumps in the 500 to 1500 gpm are normally used. Standby pumps, usually diesel engine driven are provided for use in large facilities or when electrical power may be lost during a fire. A.Foam Fire Fighting Systems T
7、he principal of foam application is to cover a burning oil surface with abblanket that will exclude air and smother the. Here are some basic definitions. (1)Foam Concentrate-a chemical which when mixed with water can be acrated and expanded to form foam. (2)Foam Solution-the mixture of foam concentr
8、ate and water in proper proportions, usually about 3-10% foam concentrate. (3)Foam the aerated mixture of foam solution that is made up of air filled bubbles that form a blanket to smother fire. The most pertinent developments with respect to types of foam include the following: (1)protein foam conc
9、entrate (2)fluoroprotein foam (3)aqueous film forming foams(AFFF) AFFF was developed by the U.S. navy for crash fires and is very effctive on spill fires.When AFFF is combined with dry chemcial powder (Purple K) it is very effective in extinguishing spill fires. Mechanical foam is made when its thre
10、e parts: water, air and foam concentrate (chemical compound) are brought together in proportionate and thoroughly and turbulently mixed. The basic method of combining three components is with a “ foam maker” . Water under pressure via pipeline or hose is proportioned by an internal orifice. The wate
11、r moving at high velocities draws the foam concentrate into the water stream resulting in a soluting of 3%-10% foam. This solution also moving at high velocities creates a vacuum and draws air into the solution. The foam solution now expands 8to10 times. The foam itself is a tough composition entrai
12、ned with bubbles. The effective use of foam requires its placement on the burning liquid surface as a stable blanker, and it must therefore possess certain desirable physicl properties for this object to be achieved. 1. Fluidity An important property of foam is that of being able to flow easily over
13、 the surface of oil and around any obsturctions which may be in its path and so exclude air. An index of fluidity is given by a torsional vane type of viscometer which measures the shear stress of the foam (in bynes per square centimeter). A “stiff” foam would have a shear stress of around 1000 dyne
14、ss/ sq cm, whereas a foam produced by the normal foam-making branch pipe would have a shear stress within the range of 150 to 250 dynes/sq. cm. 2.Expansion Mechancical aeration of the aqueous foam solution expands the solution into a foam which is less dense than oil, so that the foam can float on t
15、he oil surface. Expansion ratios of 6:1 to 12:1 represent the normal range obtained from conventional foam-making equipment, depending upon the appliance and foam solution used. The expansion ratio is measured by comparing the volume of foam produced with the volume of solution required to produce i
16、t. Thus assuming an expansion ratio of 10, one unit volume of solution will expand on aeration to ten volumes of foam. 3. Drainage rate The heat-resistance of a foam blanket is dependent upon the stability of the bubble formation and the retention of water in the foam. Liquid drains away in time fro
17、m the foam as a result of bubble breakdowm and gravitation between the bubble walls. The time taken in minutes for 25 per cent of the total liquid content to drain away under standard conditions of test is known as the 25 per cent drainage rate. Since it is not possible to produce a foam which has a
18、ll the desired properties, the experienced fire-fighter will utilize his foam-making appliances to the best effect according to the cirumstances. For example, varying the amount of foam compound introduced into the water stream will affect the expansion ratio, shear atress, and drainage rate, and fo
19、rmulations of foam compounds can be selected to give low (6 to 7 ), intermediate (8 to 12 ) or high expansion (12 to 16) foams. There are three main ytpes of foam extinguishment systems. The fixed system consists of storage tanks of the foam concentrate, foam pumps and proportioning system, as well
20、as pipelines to various equipment or structures, and foam discharge devices. It is a centralized foam solution system, that will produce foam only at specified locatoins. The fixed systems have been used where large storage tanks are in use such as large marine terminals or bulk storage plants. The
21、advantages of a fixed system are the following. (1)It is fast to operate. (2)It requires low manpower. (3)It makes efficient use of foam. The disadvantages include the following: (1)low flexibility (2)no mutual aid back-up (3)high maintenance costs and high initial capital investment The semi-fixed
22、system is the same as the fixed system with respect to foam discharge devices except that there is no central foam solution production and storage. Instead, mobile foam-proportioning trucks or trailer mounted units are used to produce the foam solution. A back-up truck is usually used for resupplyin
23、g the units with foam concentrate. The advantages of this system are as follows: (1)flexibility to cover the entire plant (2)mutual aid possible (3)low maintenance costs (4)low capital investment The disadvantages of this system are as follows: (1)a time delay to operate of 5 to 15 minutes (2)manpow
24、er requirements are in the 2-4 man range Portable systems may be a combination of mobile and hand transportable equipment. The foam discharge devices may be foam towers, monitor nozzles or foam nozzles. This ststem is especially suited for spill fires. Some of the advantages of this system are: (1)f
25、lexibility (2)mutual aid (3)low maintenance costs The main disadvantages are: (1)high manpower requirements (2)not effective for large cone tanks. B. Foam Making Devices Both the fixed and semi-fixed methods use the same foam making devices, usually at the base of the tank. One system consists of a
26、pipeline called a foam lateral, starting outside the dike and running ti the base of the tank. Then a foam maker, that is, an aerator for the foam solution, is situated at the tank base. The expanded foam is then piped up the outside of the tank through foam riser piping to a foam chamber. On cone roof tanks, chambers are installed to introduce the cirumference of the
