CHEMICAL ENGINEERING OF OIL & GAS
Experimental study on the gum in low proportion methanol gasoline for vehicle
Zhang Xinzhuang, Zhang Juanli, Huang Yong(Research Institute of Shaanxi Yanchang Petroleum (Group) Co., Ltd., Xi’an 710075, Shaanxi, China).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp543～546,2012（ISSN 1007-3426, IN CHINESE）
Abstract: The gum content variation with time of the low percentage methanol gasoline used by cars and the influence of modified additives to generate gum tendency were studied. Based on 90# and 93# gasoline, the gum content in the different storage time of unmodified M5, M15, M25 methanol gasoline and the gum content of modified M15 methanol gasoline formulations mixed the preferred additives were measured by using jet evaporation method. The results showed that the gum content of the low percentage methanol gasoline increases with the increasing of methanol incorporation; the gum content of the low percentage appears visible changes with the extension of storage time; the preferred modified additives component B and D will lead to increased gum content of the low percentage methanol gasoline formulations. The research result provides a certain experimental basis for preparation, storage and use of the low percentage methanol gasoline.
Keywords: methanol gasoline, gum, storage, additive, experimental study
Optimization and study of CO2 low temperature fractionation purification process
Lin Mingzhen1, Jia Jianchang1, Wang Chong1, et al（Shengli Engineering & Consulting Co., Ltd., Process and Pipe Design Station, Dongying 257026, Shandong, China）.CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp547～550,2012（ISSN 1007-3426, IN CHINESE）
Abstract: The traditional low temperature fractional distillation process was optimized, and based on the actual project data, the process flow before and after optimization were simulated by HYSYS software. Meanwhile, the two technologies were compared from parameters index and equipments numbers. The results show that the technology after optimization has the advantage of simpler flow, less equipment quantity, lower energy consumption and less cost. The results of HYSYS simulation show that the suitable work pressure of low temperature fractional distillation tower is 2.3 MPa, and the suitable plate number is 12.
Keywords: low temperature fraction, optimization, CO2 product, cooling load, HYSYS simulation
Application of large scale wet-processing sulfur forming process in Puguang natural gas field
Xia Li1, Qiu Yanhua1,Chu Songyuan1,et al(Sinopec Zhongyuan Oilfield Puguang Branch, Dazhou 637156, Sichuan, China).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp551～553,2012（ISSN 1007-3426, IN CHINESE）
Abstract: The process flow and main equipments of large-scale wet processing sulfur forming unit in Puguang natural gas processing plant were introduced. The key technology of sulfur forming is the control of the water content of sulfur products and formation amount of fine sulfur powder. Analysis result indicates that the main factors affecting the water content of sulfur are the water temperature of forming tank, vibration amplitude of vibrating dewatering screen, residence time of sulfur particles and the height of sulfur particles in the forming tank. And the primary factors affecting the formation amount of fine powder sulfur are temperature of liquid sulfur entering the forming plate, liquid level of liquid sulfur on the forming plate, blockage of sieve pores and so on. Depending on the analysis of above influence factors, corresponding actions should be taken to optimize the process flow of sulfur forming.
Keywords: large-scale wet processing sulfur forming process, water content of sulfur, formation amount of fine sulfur powder
Study on converting production program of low freezing point diesel into aviation kerosene in hydrocracking unit
Zhang Xuejia (Refinery of Daqing Petrochemical Company, Daqing 163711, Heilongjiang,China).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp554～557,2012（ISSN 1007-3426, IN CHINESE）
Abstract: The production situation of hydrocracking unit in Daqing petrochemical company refinery is introduced and its feasibility of producing aviation kerosene is analyzed. The factors influencing product quality and adjustment methods are given and the major factors are considered so comprehensively for producing qualified aviation kerosene. Then, a series of relevant countermeasures and solutions are presented. Finally, the aviation kerosene produced in the hydrocracking unit passes the national certification, and the economic and social benefits have improved.
Keywords: hydrocracking, low freezing point diesel, aviation kerosene, production program
Design and development of software for the conversion of three distillation curves of petroleum fraction
Xia Jing，Niu Genlin，Ren Huishu(College of Chemical Engineering, China University of Petroleum, Qingdao 266555, Shandong, China）. CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp558～562,2012（ISSN 1007-3426, IN CHINESE）
Abstract: Based on ASTM distillation curve of petroleum fractions, some other distillation curves were calculated and converted using the models reported in literatures. VB 6.0 was employed as the developing platform to design software that could successfully convert ASTM distillation curve into TBP distillation curve or EFV distillation curve, and could calculate EFV curve in different pressures, which greatly improved the efficiency and accuracy of conversion between distillation data. The test results of cases in the references show that this software can meet the demand of conversion among three distillation curves accurately and fitly.
Keywords: petroleum distillation, distillation curve conversion, software development
Operation summary of 200×103 t/a sulfur recovery unit
Liu Yang1, Li Fazhang2(1.Refining Department of Tianjin Petrochemical Co. Ltd., SINOPEC, Tianjin 300271,China;2.Chengdu RINGT Technology Development Co.,Ltd.).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp563～566,2012（ISSN 1007-3426, IN CHINESE）
Abstract: This paper summaried the occurred problems of Tianjin petrochemical 200×103 t/a sulfur recovery unit since its star-up. The result shows that the lower load start-up and the fluctuation of sour gas, etc. appeared in the operating process of large-scale sulfur recovery unit, especially at the initial stage of the unit operation. To ensure the unit smooth running, the precise regulation of the ratio of sour gas and air, and the optimizing operation of the amine regeneration unit etc. can obviously reduce the interference of the operation fluctuation. Otherwise, by the research on the various processing parameters and the qualities of products, it can be seen that the contents of COS and SO2 in the outlet of the reactor were quite low (almost unmeasured) during the operation fluctuation, the pH value of cold water was above 8, and the waste gas emission ρ (SO2) was between 100 mg/m3 and 300 mg/m3, which indicate that the catalyst CT6-5B has high anti-interference performance and activity.
Keywords: sulfur recovery, sour gas, catalyst, hydrogenation
Effect of excess air ratio on natural gas engine fueled by diesel fuel
Song Jiantong1,2, Zhang Chunhua1, Chen Jinzhu3（1. School of Automobile, Chang’an University, Xi’an 710064, Shaanxi, China;2. Automobile Engineering School, Beijing Polytechnic, Beijing 100026, China;3. Shaanxi Fast Auto Drive Group Company, Xi’an 710077, Shaanxi, China）.CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp567～569,2012（ISSN 1007-3426, IN CHINESE）
Abstract: To study the application of Compressed Natural Gas (CNG) for diesel engines, a turbocharged diesel engine has been converted into a dualfuel natural gas engine fueled by diesel fuel. CNG was injected into the intake pipe via a gas injector. The engine, which is operating in dualfuel mode, has been tested at the speed of 1400 r/min, 1600 r/min and 1800 r/min. The pilot injection of diesel fuel is about 20% of maximum fuel delivery per cycle. This paper analyzed the effect of excess air ratio on the engine performance and exhaust emissions in all tested operating conditions. Experimental results show that, with decrease of the excess air ratio, brake power increases at different speeds, brake specific fuel consumption decreases, NOx emissions increase, HC and CO increase at λ>1.6 while they decrease sharply at λ<1.6.
Keywords: excess air ratio, diesel oil, natural gas, dual fuel, engine
Influence of trace heavy hydrocarbon components in condensate gas on the final boiling point of stabilization light hydrocarbon
Zhou Yingyong1, Liu Wu1, Li Jing1, et al(Tarim Oilfield Company, PetroChina, Kuerle 841000, Xingjiang, China).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp570～572,2012（ISSN 1007-3426, IN CHINESE）
Abstract: The final boiling point is one of the quality indexes of stabilization light hydrocarbon to control the content of heavy hydrocarbon components. High contents of heavy hydrocarbon components will lead to higher temperature of final boiling point, which may affect the product quality. If the rich heavy hydrocarbon components in the condensate gas cannot be controlled effectively, it will lead to the final boiling point exceeding standard. This article analyzed the influence of trace heavy hydrocarbon components in condensate gas on the final boiling point of stabilization light hydrocarbon, ensured the source of heavy hydrocarbon components and the cause of final boiling point exceeding standard, and put forward some suggestions.
Keywords: condensate gas, stabilization light hydrocarbon, final boiling point, heavy hydrocarbon, component
Analysis and improvement on liquefied gas tower of light hydrocarbon recovery unit
Zhao Xiangpo, Wan Xiumei, Yao Lin, et al (Natural Gas Purification Plant of Central Sichuan Oil and Gas District, PetroChina Southwest Oil and Gasfield Company,Suining 629000,Sichuan,China).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp573～575,2012（ISSN 1007-3426, IN CHINESE）
Abstract: light hydrocarbon recovery unit at Nanchong has gone through triple technological transformation, but it is still imperfect, and the liquefied gas tower is hard to keep steady operation balance in summer. The tower is in conditions of ultra-high tower pressure, difficult operation, low yield and high cost. This paper explored and analyzed the problems of the unit, and proposed a solution to the problems.
Keywords: light hydrocarbon recovery unit, liquefied gas tower, condenser, analytic demonstration
Process reform and effect assessment of molecular sieve regeneration process at Yakela gas gathering & processing station
Zhou Guoying1, Xie Shuang2, Yang Gang1 (1. Yakela Gas Production Plant Northwest Oilfield Branch Company of Sinopec, Kuche 842017, Xinjiang,China; 2.Exploration and Development Institutet Northwest Oilfield Branch Company of Sinopec, Urumqi 830011, Xinjiang,China).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp576～578,2012（ISSN 1007-3426, IN CHINESE）
Abstract: This paper introduced molecular sieve dehydration process of Yakela Gas Gathering & Processing Station. In the actual production, due to the effect of regeneration gas flow, temperature, etc., the cycle of molecular sieve adsorption became longer, dehydrating effects was poor and cold box stuck frequently. Therefore, after transformed the existing process, the molecular sieve adsorption cycle was shortened and the dehydration effect had improved, which not only increase yields, reduce labor intensity, but also save the electric energy.
Keywords: Yakela, natural gas, molecular sieve, absorption, regeneration, heating, cold blowing
Experimental study on wormhole distribution and etching pattern in naturallyfractured carbonate formation
He Chunming1, Guo Jianchun2, Liu Chao1(1.Gradute Institute of Southwest Petroleum University，Chengdu 610500, Sichuan, China; 2.Petroleum Engineering Institute of Southwest Petroleum University，Chengdu 610500, Sichuan, China).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp579～582,2012（ISSN 1007-3426, IN CHINESE）
Abstract: The main control factor of acid loss during acid fracturing treatment is acid loss through acid wormhole. Acid excessive loss seriously restrained the propagation of hydraulic fracture, which diminished the chance to connect the cavecleft system and the effectiveness of acid fracturing. This thesis utilizes the long core plate flow experiment, which modeling two dimension flowing and reaction of acid in fracture. The experiment indicates that the number and scale of acid wormhole is determined by acid flow rate, while the acid wormhole place is controlled by the distribution of natural fracture. Through the way of changing the aperture of fracture modeling the etching patterns of core with different aperture of fracture find that mainly etching patterns include: 1 mm aperture fracture corresponding to whole rock beam homogeneous etching; and 1 mm～0.5 mm aperture fracture corresponding to heterogeneous etching of most rock beam; and when the aperture of crack is around 0.1 mm the etching pattern is correspond to local deep and narrow dissolution channel. The test outcome has a guiding significance for adapting loss control measures in acid fracturing treatment.
Keywords: acid fracturing, acidizing wormhole, natural fracture, leakoff, etching pattern
Corrosion and protection of oil pipe steel P110 in multicomponent thermal fluid
Ma Zenghua, Sun Yongtao, Liu Haitao, et al(China Oilfield Services Limited, Tianjin 300450, China).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp583～585,2012（ISSN 1007-3426, IN CHINESE）
Abstract: Corrosion behaviors of oil pipe steel P110 were investigated in multicomponent thermal fluid at 40 ℃～280 ℃ by weight-loss method with autoclave, and corrosion inhibition of four high temperature inhibitors were evaluated under different temperature. The results show that there are two maximum for the corrosion rates of steel P110 at the testing temperature, 60 ℃ and 180 ℃ respectively. Corrosion inhibitors HGY710 and HGY701A are suitable for the corrosion protection demand of multicomponent thermal fluid; the protection efficiency is higher than 85% with 200 mg/L dosage. The protection effect for oil pipe is very well with 750 mg/L inhibitor HGY-710 in thermal exploitation production.
Keywords: multicomponent thermal fluid, corrosion, high temperature, corrosion inhibitor, oil pipe steel
Synthesis and the surface chemical properties of tetramer sulfonate surfactants
Zhou Ming1, Zhao Yanfeng1, Luo Haoyu2, et al(1.School of Materials Science and Engneering, Southwest Petroleum University, Chengdu 610500, Sichuan, China;2. Research Institute of Exploration and Development, Tarim Oilfield Company, PetroChina, Korla 841000, Xinjiang, China).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp586～589,2012（ISSN 1007-3426, IN CHINESE）
Abstract: A tetramer sulfonate surfactant TSS (Ⅲ) was prepared by etherification, ring-opening reaction and sulfonation reaction, using pentaerythritol, epichlorohydrin, dodecanol, 1,3propane sultone as main raw materials. Elemental analysis and IR characterized the structures of tetramer sulfonate surfactants. The surface chemical properties under different temperature were studied by surface tension measurement. The results show that the surfactant TTS (Ⅲ) has a lower cmc (critical micelle concentration) and more excellent surface activity. The cmc and surface tension (γcmc) is 6.85×10-5 mol/L and 25.59 mN/m respectively at 40 ℃. With temperature increasing, Amin and ccmc/c20 increases, but Γmax decreases.
Keywords: sulfonate, oligomeric surfactant, synthesis, surface chemical properties
Synthesis and performance evaluation of αsulfonic acidlauryl acyl oxygen ethyl-dimethyl lauryl ammonium bromide
Wu Yongbing，Liu Yanmei，Men Yanping（Yanchang Oilfield Group, Yan’an 717208, Shaanxi, China）.CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp590～593,2012（ISSN 1007-3426, IN CHINESE）
Abstract: In this paper, using raw materials of lauric acid, chlorosulfonic acid,N,N-DMEA and bromdecane, synthesized a new type of amphiprotic gemini surfactants, i.e. α-sulfonic acid-lauryl acyl oxygen ethyl-dimethyl dodecyl ammonium bromide. And characterized the structure of the final outcome by the infrared spectrum, and evaluated the surface activity, compound property, emulsification and foamability of the synthetic.
Keywords: sulfonation, quaternization, esterification, emulsification, complex formulation, foamability
Research on the corrosion behaviour of X80 steel in simulative soil solution
Wang Xia1, Zhang Renyong2, Shen Shaofei1(1. Southwest Petroleum University, Chengdu 610500, Sichuan, China; 2. China Petroleum Engineering Co., Ltd. Southwest Company, Chengdu 610017, Sichuan, China).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp594～596,2012（ISSN 1007-3426, IN CHINESE）
Abstract: The corrosion behaviours of original X80 steel, normalized, quenched and tempered X80 steel in Yingtan and Korla simulative soil solution were studied by weight loss. The polarization curve of original X80 steel in two simulative soil solution was measured. The result of weight loss shows that the corrosion rates of X80 steel samples with different metallographic structure decreases firstly and then tend to steady with time passed by. The peak corrosion rate of these samples takes place in early corrosion stage. The corrosion rate of X80 steel in Yingtan simulative soil solution is lower than that in Korla’s. The original X80 steel occurs active dissolve in both of Yingtan and Korla simulative soil solution via the polarization curves. The corrosion rate of X80 steel in Korla simulative soil solution is high due to the more negative corrosion potential and larger corrosion current.
Keywords: X80 steel, corrosion, simulative soil solution, microstructure, polarization curve
Research on the acid systems to adapt special lithology
Zhong Cheng, Liu Xuandong, Han Yuting, et al(Southwest Petroleum University, Chengdu 610500, Sichuan, China).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp597～599,2012（ISSN 1007-3426, IN CHINESE）
Abstract: In order to develop the acid systems for three special lithology rocks containing Yi-2 well volcanic rocks, Liping-1 well limestone rocks and Xi-3-69 well dolomite-mud rocks from Chagan oilfield, the dissolution rates of different special lithology cores in different acids are measured to determine the main acids, then the acid addictives including corrosion inhibitor, iron stability agent, clay stabilizer and discharge aiding agent are selected. In this way the acid systems adapting for three special lithology rocks are developed and they all have good acidizing and retarding effects by measurement of dissolution rates.
Keywords: special lithology, dissolution rate, acidizing, acid addictive
Suggestions on revising national standard GB 18047 （2）——Concurrently discuss new edition of ISO 15403
Chen Gengliang(Research Institute of Natural Gas Technology, PetroChina Southwest Oil & Gasfield Company, Chengdu 610213, Sichuan, China).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp600～604,2012（ISSN 1007-3426, IN CHINESE）
Abstract: According to new edition of ISO 15403 and ISO/TR 15403 published in 2006, a series of suggestions for revising enforced national standard Compressed natural gas as vehicle fuel(GB 18047) are put forward in this article: (1) the content about purpose and target of GB 18047 should simply be clarified in chapter 1 (scope); (2) the index of water dew point should be revised to that of water content; (3) the index of gross caloric value should be cancelled; (4) the expression of Wobbe index in paragraph 5.6 should be revised and renamed to Wobbe index limits,the content of Appendix B should be rewrited; (5) Appendix A should be rewrited, the antiknock properties, AVL method for calculating methane number (MN), EN 589 method for calculating octane number and so on should be added.
Keywords: compressed natural gas, combustion properties, Wobbe index, combustion potential, methane number (MN), MON
Detection of electron donor in polypropylene alloy catalyst within reactor
Wang Dandan，Hao Ping，Song Sainan，et al （Petrochemical Research Institute of Lanzhou Petrochemical Research Center, PetroChina, Lanzhou 730060, Gansu, China）.CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp605～607,2012（ISSN 1007-3426, IN CHINESE）
Abstract: Gas chromatography was used to analyze the electron donor content in the polypropylene alloy catalyst within reactor. Dibutyl phthalate was chosen as the internal standard, and n-heptane as the solvent. OV-1 capillary column was used as the column and nitrogen as the carrier gas. The temperature of column was 200 ℃, and the temperature of injection port and FID detector was both 250 ℃. The injection amount was 0.2 μL. The results show that the measured components are separated well, the analysis can be determined within 10 mins.The method is sensitive, fast and convenient, and can be used for the detection of electron donor in polypropylene alloy catalyst within reactor.
Keywords: electron donor, polypropylene alloy, catalyst
UASB/hydrolytic acidification/SBBR process for wastewater treatment of natural gas purification plant
Guo Jun（China Petroleum Engineering Design Co.Ltd. Southwest Company，Chengdu 610017，Sichuan, China）.CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp608～611,2012（ISSN 1007-3426, IN CHINESE）
Abstract: Based on the conventional natural gas purification plant sewage treatment, through the summary of experience and process optimization, this paper proposes the UASB/hydrolytic acidification/SBBR processing technology, and takes a practical application in a natural gas purification plant in Chongqing. According to the water quantity and water quality situation, the specific process method is determined. The main structure design, operation results of the process and economic analysis are discussed in this paper. The results show that all the water quality indexes by the process accord with the primary standard of GB 8978—1996 integrated wastewater discharge standard, the process is feasible and successful,and has a certain inspiration and reference significance for industrial wastewater treatment with similar small flow rate and high load.
Keywords: natural gas purification plant, UASB process, hydrolytic acidification, SBBR process, wastewater treatment
Research on effects of residual polymer on the corrosion and scaling of the oilfield produced water
Li Guichuan1,Gao Jianchong2,Liu Yigang2,et al（1. China United Coalbed Methane Co., Ltd. Taiyuan Branch, Taiyuan 030000,Shanxi;2. Tianjin Branch of CNOOC China Limited, Tanggu 300452,Tianjin, China）.CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp612～615,2012（ISSN 1007-3426, IN CHINESE）
Abstract: The effects of residual polymer on corrosion and scaling of the oilfield produced water were studied through gravimetric method combined with calcium carbonate precipitation method and conventional electrochemical etching method. The results show that the residual polymer with lower concentrations can accelerates the Ca2+ to the scale in the form of precipitation slightly, mainly increased the amount of hard scale attached to the vessel wall, little effect to the soft scale in the produced water. When the residual polymer concentration is greater than 200 mg/L, the polymer chelating solubilization to calcium is in the ascendant, and gradually reduced the scaling tendency of the solution. The residual polymer has corrosion inhibition, the corrosion rate can reach 59.22% with polymer concentration of 50 mg/L, it has a role of chelating the corroded sewage and dispersing the solid suspension. There is a synergy effect between the residual polymer and the field antiscaling agent, corrosion inhibitor, preventing scale and corrosion mitigation is better than that of excluding polymer sewage, it is of positive significance for sewage reinjection.
Keywords: residual polymer, corrosion, scaling, sewage reinjection
Foundation of energy benchmarking system for CNG station
Lu Jianbo1, Zhang Yu2, Yang Hui3, et al (1.Research Institute of Natural Gas Technology, PetroChina Southwest Oli & Gasfield Company, Chendu 610213,Sichuan,China; 2.Quality & Environmental Protection Agency of PetroChina Southwest Oli & Gasfield Company, Chendu 610000,Sichuan,China; 3.Natural Gas Company of PetroChina Dagang Oilfield Branch, Tianjin 300270, China.).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp616～618,2012（ISSN 1007-3426, IN CHINESE）
Abstract: Taking the production and sales of CNG in PetroChina Southwest Oli and Gasfield Company as example, the article analyzes the present situation of energy consumption of CNG devices and important factors affecting energy consumption in the important process. Through comparative analysis and screening, the index of benchmarking can be determined, and the system of energy efficiency benchmarking in CNG production and sales business has been established preliminary. It is clearly that the measures of energy-saving and potential-excavating can be formulated in preliminary, and the value of energy-efficiency can be gotten through calculation with the field energy consumption data of 3 CNG stations, and a comparison can be made, finally a disparity can be found out.
Keywords: CNG devices, index of energy-efficiency benchmarking, compressor, fine management
Initial analysis on the application of Chevron SID in project
Ma Ji(PetroChina Southwest Oil & Gasfield Company, Sichuan Northeast High Sour Gasfield D&C Project Division, Chengdu 610051, Sichuan, China).CHEMICAL ENGINEERING OF OIL & GAS, VOL.41, NO.6,pp619～621,2012（ISSN 1007-3426, IN CHINESE）
Abstract: Application of SID (Safety in Design) system is one of the features of Sichuan northeast high sour gas field project. SID introduces ergonomics engineering philosophy to engineering design to assist the realization of zero accident and zero injury goal of work site. This article introduced characteristics of SID system and its application in Sichuan northeast high sulfur project, and analyzed how Chevron implements SID to every phase of project execution. Learning from the advanced philosophy of Chevron on safety design will enrich and improve the safety management system of similar domestic projects in engineering design, construction and operation.
Keywords: SID, specification, workflow, ergonomics, safety