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Accidental release source terms are the mathematical equations that quantify the flow rate at which accidental releases of air pollutants into the ambient environment can occur at industrial facilities such as petroleum refineries, petrochemical plants, natural gas processing plants, oil and gas transportation pipelines, chemical plants, and many other industrial activities. Governmental regulations in many countries require that the probability of such accidental releases be analyzed and their quantitative impact upon the environment and human health be determined so that mitigating steps can be planned and implemented. There are a number of mathematical calculation methods for determining the flow rate at which gaseous and liquid pollutants might be released from various types of accidents. Such calculational methods are referred to as ''source terms'', and this article on accidental release source terms explains some of the calculation methods used for determining the mass flow rate at which gaseous pollutants may be accidentally released. ==Accidental release of pressurized gas== When gas stored under pressure in a closed vessel is discharged to the atmosphere through a hole or other opening, the gas velocity through that opening may be choked (i.e., it has attained a maximum) or it may be non-choked. Choked velocity, also referred to as sonic velocity, occurs when the ratio of the absolute source pressure to the absolute downstream pressure is equal to or greater than (+ 1) ÷ 2 ) ''k''÷(''k'' - 1 ), where ''k'' is the specific heat ratio of the discharged gas (sometimes called the isentropic expansion factor and sometimes denoted as ). For many gases, ''k'' ranges from about 1.09 to about 1.41, and therefore (+ 1) ÷ 2 ) ''k''÷(''k'' - 1 ) ranges from 1.7 to about 1.9, which means that choked velocity usually occurs when the absolute source vessel pressure is at least 1.7 to 1.9 times as high as the absolute downstream ambient atmospheric pressure. When the gas velocity is choked, the equation for the mass flow rate in SI metric units is:〔''Perry's Chemical Engineers' Handbook'', Sixth Edition, McGraw-Hill Co., 1984.〕〔''Handbook of Chemical Hazard Analysis Procedures'', Appendix B, Federal Emergency Management Agency, U.S. Dept. of Transportation, and U.S. Environmental Protection Agency, 1989. Also provides the references below: – Clewell, H.J., ''A Simple Method For Estimating the Source Strength Of Spills Of Toxic Liquids'', Energy Systems Laboratory, ESL-TR-83-03, 1983. – Ille, G. and Springer, C., ''The Evaporation And Dispersion Of Hydrazine Propellants From Ground Spill'', Environmental Engineering Development Office, CEEDO 712-78-30, 1978. – Kahler, J.P., Curry, R.C. and Kandler, R.A.,''Calculating Toxic Corridors'' Air Force Weather Service, AWS TR-80/003, 1980. ( Handbook of Chemical Hazard Analysis, Appendix B ) Scroll down to page 391 of 520 PDF pages.〕〔("Risk Management Program Guidance For Offsite Consequence Analysis" ) U.S. EPA publication EPA-550-B-99-009, April 1999. (See derivations of equations D-1 and D-7 in Appendix D)〕〔"Methods For The Calculation Of Physical Effects Due To Releases Of Hazardous Substances (Liquids and Gases)", PGS2 CPR 14E, Chapter 2, The Netherlands Organization Of Applied Scientific Research, The Hague, 2005. (PGS2 CPR 14E )〕 or this equivalent form: For the above equations, it is important to note that although the gas velocity reaches a maximum and becomes choked, the mass flow rate is not choked. The mass flow rate can still be increased if the source pressure is increased. Whenever the ratio of the absolute source pressure to the absolute downstream ambient pressure is less than (( ''k'' + 1 ) ÷ 2 ) ''k'' ÷ ( ''k'' - 1 ), then the gas velocity is non-choked (i.e., sub-sonic) and the equation for mass flow rate is: or this equivalent form: The above equations calculate the initial instantaneous mass flow rate for the pressure and temperature existing in the source vessel when a release first occurs. The initial instantaneous flow rate from a leak in a pressurized gas system or vessel is much higher than the average flow rate during the overall release period because the pressure and flow rate decrease with time as the system or vessel empties. Calculating the flow rate versus time since the initiation of the leak is much more complicated, but more accurate. Two equivalent methods for performing such calculations are presented and compared at (www.air-dispersion.com/feature2.html ). The technical literature can be very confusing because many authors fail to explain whether they are using the universal gas law constant ''R'' which applies to any ideal gas or whether they are using the gas law constant ''R''s which only applies to a specific individual gas. The relationship between the two constants is ''R''s = ''R''/''M''. Notes: * The above equations are for a real gas. * For an ideal gas, ''Z'' = 1 and ''ρ'' is the ideal gas density. * 1 kilomole (kmol) = 1000 moles = 1000 gram-moles = kilogram-mole. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Accidental release source terms」の詳細全文を読む スポンサード リンク
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