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怎么扫线才能扫干净？ 以下是关于吹扫的一段英文资料，可以参考： purging before start-up before any inflammable material can be introduced into the plant at start-up, all the air must be purged out to prevent the formation of explosive or combustible mixtures. this includes the flare header. purging the air is carried out immediately before the start-up itself. all the instrumentation, relief valves, control valves, etc must be in place. the plant is mechanically complete, catalysts are loaded and the last of the instrumentation checks are completed. purging of all process lines and equipment is carried out with nitrogen. purging of the steam, nitrogen and water systems is achieved by using the medium itself. the purging of air from the plant is carried out at atmospheric pressure with nitrogen. it is continued until the concentration of oxygen in the vented gas falls to less than 0.8 vol%. then, by pressurisation with nitrogen, the oxygen concentration can be further reduced. pressurisation from atmospheric pressure to 0.340 mpag, will reduce the oxygen concentration to about 0.2 vol%. the rate of injection of the purging nitrogen should be low, so that the air being displaced is not mixed with the incoming nitrogen by unnecessary turbulence. the purging velocity in process piping should not exceed 1 m/s and should be considerably less through large vessels. the velocity in large vessels is usually limited by the maximum possible supply rate of purge gas. generally, it is best to purge from top to bottom of a vessel if the purge gas is less dense than the original gas. however, in the case of nitrogen displacing air, the difference in density is so small as to make this advantage insignificant and the most convenient method of flowing through the vessel should be selected. whilst purging is in progress, it should be checked that all places where a `dead pocket' could exist are vented or that flow is directed through them, e.g. pipework bypasses, both sides of column sump dividers, isolated side streams, process drain and vent points. the purge rate should be set within the limits of the supply system and of course the rate will be a compromise for systems having various cross sectional areas. it is usually found that at least four times the system volume of nitrogen is required to bring the oxygen content down from 21% to less than 0.8% during the low pressure purging. purging after shutdown after shutting down the plant for a maintenance operation, the vessels and lines are full of combustible and toxic materials. before any part of the plant can be opened up all liquids must be drained, it must be purged out with nitrogen and before anyone is allowed to enter a vessel the nitrogen must be thoroughly purged out with air. after shutdown the actions taken will depend to some extent on the section and area of the plant. for example: a) gas filled lines and vessels will be vented down to the flare system before purging with nitrogen. b) the columns must be filled with nitrogen as they cool down and vapour condenses. this is to prevent a vacuum being pulled and the potential of oxygen ingress into the vessels. however, all vessels and lines which contain process liquids must be drained down into suitable containers and purged out with nitrogen. even after efficient draining, vessels with complex internals will contain small amounts of trapped liquid in crevices. this liquid will not always be removed by purging with cold nitrogen. it could later be a health hazard or produce an explosive mixture. vessels which present this problem should be steamed out with lp steam for a few hours after purging with nitrogen. on cooling, allow the vessel to refill with air and run the condensate to the dirty water sewer. the vessel is then ready for purging out with air. distillation columns are a special case and should be steamed out by filling with condensate and using the reboiler and condenser. water should be purged continuously to the oily water sewer and replaced by more condensate. purge streams should be taken from the reflux line and from the column sump. the condensate must be cooled with water before running to the sewer. on cooling allow the system to fill with air and run the condensate to the oily water sewer. the system is then ready for purging with air. after shutting down, the section of plant to be opened up must be positively isolated from any other part of the plant containing hydrocarbons. the isolation must be with blinds. valves check valves, etc are not sufficient. when purging out hydrocarbons with nitrogen, attention should be paid to the difference in densities as this is usually significant. purge from top to bottom when the nitrogen is less dense than the hydrocarbon and vice-versa. before any person is allowed to enter a vessel for any purpose at all without the use of a direct air supply breathing apparatus, the inert gas must be purged out with air and the atmosphere tested by a competent analyst to show that the vessel is completely free of all gases except those that make up air, and that the oxygen of the vessel atmosphere is as near to 21% by volume as the test apparatus will indicate. this test should be witnessed by the plant supervisor, and only when a satisfactory test result has been achieved should a `safe to enter permit' for the vessel be issued查看更多
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