毕业设计资料_钱杰_3外文翻译

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 combustion 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 same 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 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 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 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 The 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 concentrate 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 concentrate 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 three 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 water 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 entrained 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 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 dyness 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 the 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 it 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 from 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 all 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 formulations 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 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 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 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 resupplying 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 manpower 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 flexibility 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 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 shell just below the roof angle Typical chamber requirements are as follows Tank Diameter Chambers 65 ft or less 1 65 ft 117 ft 6 in 2 117 ft 140 ft 3 140 ft and over 4 Another method of applying foam is subsurface or base injection Floating roof tanks are a special case of fire extinguishment system application Fires in this type of tank are and when they do occur they are limited to a flash fire around the circumference between the seal and tank shell Usual practice is to rely on portable systems to extinguish these fires However with the installation of tanks in the 300 ft diameter range portable systems are less efficient The general procedure is to install a foam dam about a foot or so from the seal area and the installation of small foam makers and discharge devices on back boards around the shell Experience from past tank fires has led to the following conclusion 1 It appears that subsurface injection is viable for extinguishing fires in large light fuel cone roof tanks 2 Large amounts of foam concentrate are needed to fight a fire involving several tanks so mutual aid planning may be warranted between other tank farms munic ipalities and foam manufacturers 3 Water supply systems for tank farms are basic to successful fire fighting operations 4 Mutual aid planning with respect to the following is essential for efficient operations 1 all parties to use a common radio frequency 2 logistics planning for fire fighting personnel and equipment i e food and fuel 3 a fire control center for coordinating purposes 泡沫消防系统泡沫消防系统 选择合理的油罐 油罐间距和正确的设计 这是最好的防火安全措施 火灾的发生必 须三个基本条件 1 燃料必须以分散状态存在 如蒸气 2 空气 提供燃料用的氧气 必须以恰当的比例与燃料构成能引燃的混合物 3 必须存在引燃源 足以引起燃烧这种化学反应的高温和能量 起火需要的上述三个基本条件必须同时存在 切断燃料或空气通常就能灭火 如果直接将水泼于油类火灾上 非但无益 反而有害 因为浮在水面上燃烧着的油实 际上只能使火灾蔓延 水有助于保持邻近油罐和建筑物的冷却 水雾 即细分散状的水花 有助于使燃油表面于空气隔绝 并使油冷却到燃点以下 水雾仅对水雾覆盖范围内的粘性 油或高闪点油有效 当使用 9 8 英寸喷嘴和 5 2 英寸光滑橡胶衬里软管时 标准灭火水流流量是 250 加仑 分 该标准水流要求喷嘴压力约为 45 磅 英寸 若喷嘴于消防栓间采用 400 英寸软管 2 则要求消防栓压力为 105 磅 英寸 为满足软管水流和泡沫的生产 干管的分配压力达到 2 125 磅 英寸 便足够了 2 压力不够时 就要安装消防泵 通常泵流量在 500 1500 加仑 分之间 大型站库或火 灾中有可能断电时 常采用由柴油机驱动的备用泵 A A 泡沫灭火系统 泡沫灭火系统 应用泡沫的原理是在燃油表面上覆盖一层能 隔离空气并窒息火焰的覆盖物 以下是某 些基本定义 1 泡沫剂 一种于水混合后能充气并膨胀生成泡沫的化学剂 2 泡沫溶液 泡沫剂与水按恰当比例混合的混合液 通常含有 3 10 的泡沫剂 3 泡沫 泡沫溶液内充空气后的充气混合物 能形成窒息火灾的覆盖层 泡沫类型方面最实际的发展包括 1 蛋白泡沫剂 2 氟蛋白泡沫 3 水成膜泡沫 AFFF 水成膜泡沫时美国海军为扑灭突发性火灾而开发的 对流散火灾非常有效 当水成膜 泡沫与化学干粉 Purple 联合使用时 对于扑灭流散火灾非常有效 当把水 空气和泡沫剂 化合物 按一定比例混合 并充分搅拌就可生成机械泡沫 将这三种成分混合在一起的基本方法是使用 泡沫产生器 在压力下通过管道或软管的水 用内部孔板来调节比例 高速流动的水将泡沫剂吸入水流中形成比例为 3 10 的泡沫溶 液 这种溶液仍以高速流动形成真空吸入空气后 泡沫溶液膨胀 8 到 10 倍 泡沫本身是一 种夹带气泡的粘稠混合物 有效地使用泡沫要求泡沫以一种稳定的覆盖层覆盖在燃烧液体的表面 为达到这一目 的泡沫必须具有某些所希望的物理性质 a 流动性流动性 泡沫的一个重要性质就是能很容易的在油表面上流动 并绕过流道上的任何障碍物 以隔绝空气 用一种测量泡沫剪切应力 以达因 厘米 表示 的扭片型粘度计可测得粘动 2 性指标 流动性差 的泡沫剪切应力约为 1000 达因 厘米 而普通泡沫枪生成的泡沫的 2 剪切应力在 150 250 达因 厘米 范围内 2 b 膨胀性膨胀性 水成泡沫剂溶液经机械充气后 使溶液膨胀形成一种比油轻的泡沫 从而使泡沫能浮 在油面上 根据所采用的器械和泡沫剂溶液 常用泡沫生产设备获得的膨胀比范围为 6 1 至 12 1 比较所生产的泡沫体积与生产该体积泡沫所需溶液体积就可测出膨胀率 若假 设膨胀率为 10 单位体积溶液将充气膨胀为十倍的泡沫体积 c 析水率析水率 泡沫覆盖层的隔热性能与泡沫层的稳定性及水在泡沫中的存留量有关 泡沫的破灭和 泡沫壁间的引力作用导致液体不时从泡沫中析出 在标准试验条件下 总液量中 25 液体 析出所需时间 以分表示 称为 25 析水率 由