Problems in the production and engineering design

Problems existing in the production and engineering design of carbon dioxide fire extinguishing system

carbon dioxide fire extinguishing system can be divided into: carbon dioxide fire extinguishing system with extinguishing agent stored at room temperature and carbon dioxide fire extinguishing system with extinguishing agent stored at -18 ℃ - 20 ℃; Traditionally, the former is called high-pressure carbon dioxide fire extinguishing system and the latter is called low-pressure carbon dioxide fire extinguishing system according to different storage pressures

low pressure carbon dioxide fire extinguishing system is composed of carbon dioxide extinguishing agent storage device, selector valve, pipe, nozzle and control system. The extinguishing agent storage device shall be equipped with storage containers and container valves. It is composed of safety pressure relief device, pressure gauge, pressure alarm device and refrigeration device, and the storage container shall be provided with thermal insulation measures. Low pressure carbon dioxide fire extinguishing system is widely used in the protection of industrial production devices because of its large storage capacity of extinguishing agent, flexible application, easy maintenance and other advantages

when formulating the code for design of carbon dioxide fire extinguishing systems (GB 50193-93, hereinafter referred to as the "code"), there is still a blank in the low-pressure carbon dioxide fire extinguishing system, neither products nor engineering applications; Therefore, there are no requirements and regulations for low-pressure carbon dioxide fire extinguishing system in the "specification". Later, Shanghai Volkswagen introduced a 13T low-pressure carbon dioxide fire extinguishing system from figgie company of the United States, Shanghai Baosteel introduced a 15t low-pressure carbon dioxide fire extinguishing system from King Kerr company of Denmark, Qinhuangdao Power Plant and other plants also installed low-pressure carbon dioxide fire extinguishing systems, and Xi'an 524 plant and other plants also successively developed low-pressure carbon dioxide fire extinguishing systems. In just a few years, the manufacturers of low-pressure carbon dioxide fire extinguishing systems in China have begun to take shape

at present, the low-pressure carbon dioxide fire extinguishing system produced in China has not only large capacity products of more than 1t, but also small capacity products of less than 500kg; There are vertical, horizontal and cabinet types of storage devices; In terms of insulation measures, there are vacuum accumulation insulation method and polyurethane foam insulation method; In terms of release control mode, there are time control, flow accumulation control and liquid level control. Judging from the production capacity of existing manufacturers, it can meet the market requirements; However, the sales price is not low enough, and there are still some problems in product quality, design and installation, which affect the popularization and application of low-pressure carbon dioxide fire extinguishing system

1 problems in the product manual

1.1 description of nozzles

carbon dioxide nozzles are divided into full flooding system nozzles and local application system nozzles. The application parameters of the sprinkler in the total flooding system are mainly the sprinkler population pressure Pt and sprinkler flow Qi; In addition to Pt and Qi, the application parameters of the sprinkler of the local application system also increase the sprinkler protection area at0. The sprinkler protection area is: the area that can extinguish the fire within 20 s under the condition that the chord height of the oil pan rod (the distance from the liquid level to the edge) is 150mm, and the n-heptane in the pan or 70 # gasoline in accordance with GB 489 does not splash, and the corresponding sprinkler flow is its Qi value. The protection area of overhead sprinkler varies with the installation height, and the protection area of grooved sprinkler varies with the flow of sprinkler. In addition, the nozzle of high-pressure system cannot be used in low-pressure system, because the injection rate of high-pressure system and low-pressure system is different under the same nozzle population pressure. Therefore, the characteristic curve of the nozzle should be given in the product manual of the low-pressure carbon dioxide fire extinguishing system. Unfortunately, the product specifications of the existing low-pressure carbon dioxide fire extinguishing system manufacturers do not give the nozzle characteristic curve; This cannot help but make people doubt whether its nozzle parameters have an experimental basis

1.2 equivalent length of pipe accessories

equivalent length of pipe accessories is an essential parameter for pipe calculation. It is obtained through tests. Some domestic manufacturers of low-pressure carbon dioxide fire extinguishing systems have not given, and some have given theoretical values. All these affect the accuracy of design calculation

1.3 problem of residual carbon dioxide in storage container

because the value of residual carbon dioxide in storage container MS varies with the product structure, the specific value is not given in the "specification", which should be found out from the product manual of low-pressure carbon dioxide fire extinguishing system. Due to the small change of displacement, some instructions do not even give such easy to measure data

in a word, the above problems need to be solved, and the engineering data should be supplemented and improved through a large number of tests and experiments; In addition, the manufacturers of low-pressure carbon dioxide fire extinguishing systems should also make further efforts to improve product quality, reduce prices by reducing costs, and promote the popularization and application of low-pressure carbon dioxide fire extinguishing systems in China

2 design and installation problems

with the popularity of low-pressure carbon dioxide fire extinguishing systems, their applications in engineering are also increasing, not only in power systems and chemical systems. Low pressure carbon dioxide fire extinguishing system is used in industrial fire protection such as metallurgical system, and low pressure carbon dioxide fire extinguishing system is also used in building fire protection. For various reasons, there are also some problems in the design and installation

2.1 design method problem

the design calculation of low-pressure carbon dioxide fire extinguishing system includes: carbon dioxide design consumption m, carbon dioxide storage MC, pipe node pressure PJ, etc. The calculation sequence of node pressure PJ can be divided into positive sequence (forward flow direction) and reverse sequence (reverse flow direction) according to the flow direction. Only the node pressure PJ downstream of the step-down link in the local application system area method is calculated in reverse order, and others are in positive order. There are graphic and analytical methods in both positive and negative order. The author believes that analytical method should be advocated, because analytical method is convenient for computer calculation and easy to control the calculation accuracy, while graphical method is time-consuming and difficult to control the accuracy. Indeed, the graphical method is given in the code for design of carbon dioxide fire extinguishing systems (GB 50193-93), because computers have not been popularized and ISO 6183 is equivalently adopted. The calculation method of pressure drop of two-phase flow of carbon dioxide in pipeline began with he -- globalization of resource allocation has become a new way for manufacturing industry to cultivate competitive advantage. In his doctoral thesis, sson J.C. proposed that two-phase meet the use demand? Therefore, the flow formula - that is, the formula in the "specification" (4.0.5), and later Cardox described the curve cluster based on it - that is, appendix C0 in the "specification" was not popular in the past, and people used to use graphical method; Now computers are very popular, so when you can use analytic method, you should try to use analytic method first

2.2 two phase flow formula problem

the formula given in code for design of carbon dioxide fire extinguishing systems (GB 50193-93) is not exactly the same as that given by ISO and NFPA:

q2=0.8725 • • d5.25 • y/(l+0.04319 • d1.25 • Z) (1)

Q2 =0.8725 • • d5.25 • y/(L + 0.04319 • d1.25 • z) (2)

formula (1) is given by ISO, NFPA and other standards.Yes, The unit of y value is bar • kg/m3, and the coefficient is; Formula (2) is given in GB 50193-93 standard, where the unit of y value is MPa • kg/m3 and the coefficient is. Some people have noticed this, while others have raised questions. For example, comrades from Shenzhen Shengjie fire engineering company and Chengdu Helida fire engineering company have answered this question twice by the "standard" management group. However, some people still didn't notice that they only changed the unit of Y and ignored the coefficient value

2.3 pipe inner diameter problem

in the above formula, the pipe inner diameter D should adopt the actual size, not the nominal size, and the actual size should be selected according to appendix J of the revised "specification". Similarly, when calculating the residual amount of carbon dioxide MR in the pipeline, the pipeline Volume VI should also select the inner diameter D value of the pipeline according to appendix J of the revised "specification"

2.4 injection time of total flooding fire extinguishing system

the value of injection time t of total flooding fire extinguishing system, "the specification" points out: "the injection time of carbon dioxide in total flooding fire extinguishing system should not be greater than 1min. When extinguishing solid deep fire, the injection time should not be greater than 7min, and the concentration of carbon dioxide should reach 30% within the first 2min". Some people still don't know this well. They should first see whether the suppression time is given in Appendix A of the "specification". If it is not given, it will be treated as a surface fire, generally taking 1min; If the suppression time is given, it will be treated as a deep fire. For deep fire, the longer the spraying time is, the better the fire extinguishing effect is. At this time, the substances with the material coefficient KB value less than 3 can be taken; Equal to 2 times KB divided by 0.8584 concentration 30%, corresponding to material coefficient 0.8584); For substances with a material coefficient KB value greater than or equal to 3, take t equal to twice the KB value (so that the injection time is not greater than 7min)

2.5 pipe diameter coefficient value problem

the value range of 1.41-3.78 is given in the "specification", which is based on Appendix C. Appendix C is only suitable for interpolation, not extrapolation, so the upper and lower limits are given. The analytical method can be exempted from this limitation, but it is more economical to turn the discharge outlet downward according to the above value

2.6 elevation problem

there is no limit on elevation in the "specification", but experience shows that it is not easy to meet the population pressure condition of sprinkler when the elevation exceeds 50m. The first point is that some people have noticed it based on engineering practice, and some people may not have noticed it, but they may encounter it in future work

2.7 nozzle specification problem

the nozzle specification code given in the "specification" refers to the specification code of a standard nozzle with a flow coefficient equal to 0.98; If the nozzle of the manufacturer is not converted into such standard nozzle data (it is OK if there is no requirement in the product standard), the nozzle specification should be determined according to the nozzle population pressure PR of the manufacturer - the spray rate curve of unit orifice area

2.8 installation problems

low pressure carbon dioxide fire extinguishing