Study On The Migration Characteristics Of Volatile Gases From Leaked Oil In The Central Drainage Pipe Of Floating-roof Tank

Floating-roof tanks are important strategic facility for oil storage,with a single tank capacity of up to 150000 m~3,effectively alleviate the impact of oil supply interruptions and price fluctuations on national energy security and the stability of the petrochemical industry.The central drainage pipe is an important drainage device in the floating-roof tank,which effectively prevents the accumulation of rain and snow from causing the floating plate to be overloaded and overturned.However,the damage of the central drainage pipe caused by the corrosion of the storage liquid,the subsidence of the storage tank,the uneven force and other reasons lead to the oil leakage(oily vapor is easily generated when oil is leaking slightly,referred to as volatile gas from leaked oil).The oil and gas which enters the external environment along the central drainage pipe will cause water and air pollution.During maintenance,if the gas in the central drainage pipe are not discharged in time,it may even cause safety accidents such as burning and explosion.Therefore,carrying out research on volatile gas diffusion from oil in the central drainage pipe is of great engineering significance for formulating safety pre-warning measures and guiding later maintenance and repair.This paper took the gas generated by the volatilization of oil when the central drainage pipe leaked as the research object.Based on the computational fluid dynamics method,the different influencing factors on the law of the diffusion of gas in the central drainage pipe was analyzed.Then,a laser detection experiment device for gas diffusion in the central drainage pipe was built to verify the feasibility of the laser detection of volatile gas diffusion in the central drainage pipe of the floating-roof tank.Finally,a study on the influence of ventilation on the gas dispersion effect in the central drainage pipe was carried out.The specific research contents are as follow:(1)A mathematical and physical model of gas diffusion in the central drainage pipe of the floating-roof tank was established.The mixed gas of propane,butane and pentane was used to simulate volatile gas with complex composition.The influence of environmental temperature,leak position,leak velocity,gas composition and leak aperture on the diffusion of volatile gas in the central drainage pipe was studied,and the influence of different factors on the alarm time was analyzed.The research results showed that the alarm time for the three environmental temperatures of 303K,308K,and 313K were 561.5s,553s,549.5s,respectively.The alarm time for the upper,middle,and lower leak positions were 561.5s,1023.5s,1869.5s,respectively.The alarm times for the three leak velocities of 0.2m/s,0.5m/s,and 1m/s were1333s,561.5s,and 277s respectively.When the oil and gas components are different,the alarm times were 561.5s,565s,564s.The alarm times of the three leak apertures of 1mm,3mm,5mm were 2668.5s,921s,and 561.5s,respectively.It can be seen that the leak velocity,the leak aperture and the leak position had a greater impact on the alarm time,while the environmental temperature and gas composition had a small effect on the alarm time.(2)Based on the principle of tunable diode laser absorption spectroscopy(TDLAS)technology,experiments had verified and analyzed the feasibility of laser detection on volatile gas leakage in the central drainage pipe.A set of monitoring system suitable for the leakage of the central drainage pipe of the floating-roof tank was designed,which mainly included the duty room monitoring system,the field control system and the signal transmission module.(3)A model for gas leakage from a central drainage pipe under ventilated conditions was established,and the simulation studied the influence of the ventilation velocity and the leak position on the gas concentration distribution in the pipe and the concentration change of the monitoring point.The study found that the leak position and the ventilation velocity had a greater influence on the gas diffusion in the central drainage pipe under the ventilation state.(4)The influence of reducing tube and the installation angle of the the ventilation pipe on the gas displacement effect in the central drainage pipe was studied.The results of the study showed that the structure of the convergent and divergent tube had little influence on the gas displacement effect,while the angle of the ventilation pipe had a greater impact on the range of the vortex influence zone near the vent.The dispersing effect of the gas in the central drainage pipe can be effectively improved by controlling the angle of the ventilation pipe.The research results of this article will provide certain guidance for the leak safety detection of the central drainage pipe and the formulation of the ventilation and dispersal plan during the maintenance process.