Drainage system is an indispensable part in the normal production and operation of every enterprise. In view of the fact that raw materials, auxiliary materials, semi-finished products and finished products used in chemical production are mostly flammable or combustible substances, and many materials are toxic and corrosive, the drainage system of petrochemical enterprises needs to consider corresponding safety measures in design to prevent safety accidents. Combined with relevant codes and practical engineering experience, this paper discusses the problems that should be paid attention to in the design of drainage system in petrochemical enterprises and the safety measures that should be taken.
1 Properties and characteristics of drainage in petrochemical enterprises
Petrochemical enterprises usually have six major areas: process unit area, raw material and finished product storage area, transportation loading and unloading area, public facilities area, auxiliary production area and management area. See table 1 for the drainage characteristics of each area.
2 factors leading to unsafe drainage system
The factors that lead to unsafe drainage system generally include: ① the drainage quality is not recognized, and some sewage and wastewater that should have been pretreated or specially set up are directly discharged into the production drainage system of the whole plant without effective treatment; (2) The underestimation of sudden working conditions leads to the outflow of sewage and wastewater, leading to environmental pollution and even large-scale explosion; ③ The material, laying method and water seal of drainage pipeline are not fully considered; (4) Ignoring the daily maintenance management after the production and operation of the enterprise, resulting in safety accidents.
3 drainage system design
3. 1 The drainage system is set according to the principle of "sewage diversion and high-quality drainage". Petrochemical enterprises should set up different drainage systems, such as domestic sewage system, production wastewater (including initial rainwater) system, clean wastewater and rainwater system, and other drainage systems can be added or merged according to different treatment requirements. 3. 1. 1 Domestic sewage system An independent drainage system should be used for domestic sewage [1]. When the guard room is far away from the factory domestic drainage system, and the drainage does not affect the treatment effect of production wastewater, it can be directly discharged into the factory production wastewater system after passing through the septic tank. Isolation measures such as water seal must be set before water inflow [2] to avoid toxic and harmful gases and combustible gases in the production wastewater system escaping back to living facilities and causing safety accidents.
3. 1.2 The design scheme of pretreatment, collection and disposal methods should be reasonably determined for the production wastewater (including initial rainwater) system in combination with the nature, concentration, water quantity, drainage frequency and characteristics of the drainage site. If there is gas in the production wastewater pipeline that can cause explosion and fire danger, it will be discharged from buildings, equipment areas, tank farms and other places connected to this pipeline network; The intersection of branch pipes and main pipes in the whole plant; When the length of branch pipes and main pipes in the whole plant exceeds 300m [1, 3-4], water sealing measures shall be set. In addition to process units, tank farms and other places, in butyl five factories, public works and other places where the drainage itself does not contain flammable substances, in order to ensure the safety of the drainage system, water-sealed wells [4] should also be set before the drainage flows into the production wastewater pipe network system to prevent dangerous gases from escaping back. Initial rainwater should be considered in general open areas such as tank farm, pump area, process unit area and loading and unloading station. The polluted area of initial rainfall should be reasonably determined in the design, and the initial rainfall depth should be determined by 15 ~ 30 mm [2]. When a fire accident occurs, emergency measures should be taken to avoid the impact of wastewater discharge on the water environment.
Fire dike, cofferdam, initial rain pool and fire-fighting wastewater pool can all be used to store fire-fighting wastewater. The calculation of fire-fighting wastewater should include fire-fighting water consumption, material leakage, rainfall and the amount of wastewater remitted to the fire-fighting wastewater collection system during the accident [5]. 3. 1.3 clean wastewater system Although the clean wastewater in the public facilities area and the auxiliary production area is not dangerous in itself, and the danger category in the drainage area is mostly D-E, it is often discharged into the production wastewater pipeline of the whole plant due to the cost of the pipe network and the occupation of the project, which is easy to ignore the safety problem and cause an explosion accident. Therefore, safety measures such as water sealing and cooling should be taken before the drainage in the above areas flows into the production wastewater of the whole plant. The pollution of clean wastewater can't be ignored. The drainage of open circulating cooling water system includes system sewage, backwashing drainage in the process of bypass water treatment, replacement water in the process of cleaning pre-membrane, overflow and emptying water in the pool, etc. In the closed system, the circulating cooling water of high-concentration chemicals will be discharged during commissioning, shutdown or emergency. Disposal methods should be determined according to emission standards and water quality, and priority should be given to reuse. Waste water that cannot be reused can not be discharged until it reaches the standard.
3.2 Layout of drainage system
3.2. 1 Reasonable planning of drainage path should ensure the smooth discharge of sewage and wastewater, and at the same time consider the influence of daily maintenance and accidents on surrounding water bodies. It is suggested in the specification that pipelines conveying easily sinking media, toxic media and corrosive media and pressure flow sewage pipelines should be laid overhead [1, 6], so as to facilitate maintenance and timely discover potential safety hazards. The production waste water pipe containing flammable liquid cannot be laid vertically under the roadway and process pipe gallery [1, 3, 6], which can not only reduce the fire risk caused by automobile exhaust, but also avoid the influence of road excavation on traffic during maintenance. The fire-fighting wastewater pool should be set in the low-lying area at the end of the pipe network in combination with the floor height of the factory, otherwise it will bring trouble to the emergency disposal. For example, when some old factory buildings are rebuilt, the fire-fighting wastewater pool is set at the starting point of the pipe network, and the connecting pipe between the pool and the pipe network is set above the existing pipe elevation, while the whole floor in the factory area is lowered from the starting point to the end point of the pipe network, which is very unfavorable for the collection of fire-fighting wastewater. When an accident occurs, after the cut-off valve at the end of the pipe network is closed, the water level can not flow back to the wastewater pool until it rises above the elevation of the connecting pipe.
3.2.2 Correct Selection of Discharge Methods Sewage and wastewater can be discharged by gravity flow in pipeline, open ditch, pressure rise or a combination of the above forms. No matter which way is adopted, it must be determined after comprehensively considering the characteristics of drainage water quality and the location of drainage source to ensure timely, effective, safe and reasonable drainage. Production wastewater and oily wastewater containing combustible substances shall be discharged through pipelines or culverts [3,7]; Clean rainwater can be used for open ditch or pipeline drainage; For overhead pipelines, pressure lifting drainage should be adopted. Attention should be paid to open ditch drainage: ① In order to avoid secondary accidents caused by volatile harmful substances, when the rainwater drainage system is also used as a fire accident water collection system, the open ditch form cannot be adopted; (2) When open ditches are used for indoor drainage, and the open ditches should be controlled within 30m, each open ditch should be discharged into the production wastewater system separately, and the open ditches should not be connected by concealed pipes; (3) In order to prevent explosion caused by rapid expansion when cryogenic liquid leaks and gasifies, open ditch drainage is adopted within 30m of loading and unloading port of cryogenic tank farm.
3.3 Drainage system facilities design
3.3. 1 There is no ladder in the corrosive sewage well [1 6], and similar places: initial rain pool, accident pool, etc. Ladder is an accessory component in wells and ponds, and it is often handled casually, which brings security risks to future maintenance work. If the effect of anti-corrosion measures is uncertain, the correct way is not to erect ladders. 3.3.2 The sewage pipeline system containing volatile toxic, harmful and combustible gases in the manhole should not be provided with manhole [1]. When connecting new and built pipe network systems, special attention should be paid to the elevation of the connection point to avoid the drainage containing the above sewage falling into the water. 3.3.3 Several positions that are easily overlooked when setting water seal: ① Trench with combustible gas, liquefied hydrocarbon and combustible liquid pipelines [3]; (2) a water inlet pipe and a water outlet pipe [3] of the grease trap; ③ The backwater of gravity flow circulating water is at the general outlet of the process unit [6]. 3.3.4 The manhole cover and Zuo Jing shall be sealed, and there shall be no holes in the manhole cover [1, 3].
In order to ensure the organized discharge of combustible gas in the pipeline and reduce open flame contact, exhaust pipes are set in the inspection wells and grease traps at the top of branch pipes and main pipes in Class A and B device areas and tank farms. Similar places that need ventilation measures include septic tanks and cooling pools [8]. 3.3.5 Pipeline materials and anti-seepage pipeline anti-seepage are easily overlooked in engineering design, and their practices are closely related to pipeline materials [9]. The choice of pipeline material should not only be based on the water quality of sewage and wastewater [10], but also consider the anti-seepage mode of the pipeline, and be determined after combining the construction difficulty and project cost. 3.3.6 Block valve drainage system needs to pay attention to the valve type when selecting the on-off valve. Many factories use gates when switching pipelines. This valve has a certain leakage, that is, it cannot be completely cut off, and there will be a small amount of overcurrent. If it must be completely cut off, it is more appropriate to choose knife gate valve. In order to ensure personal safety and timely and effective emergency operation, the remote control function should be considered in the drainage cut-off valve at the discharge outlet of the factory and within the range of less than 15m from the tank farm and device area.
This paper introduces the content of "drainage system design and safety measures in petrochemical enterprises". Welcome to Zhong Da for more information.
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