Vessel purge steam

On one coker, the combination tower trays were repeatedly upset due to water accumulating in the tower during short unit outages. After considerable investigation, the source of this water was found. Purge steam was being used under vessel relief valves to prevent the relief valve inlet lines from coking up. While the unit was out of service for several hours, it would cool off. The steam condensed and formed potentially explosive pockets of water. 

Water addition to vessels that have been steam-purged If vacuum conditions can develop, then either the equipment must be designed for vacuum conditions or a vacuum relief system must be instiled. 

Cleaning, washing, steaming-out and purging of equipment, vessels, and pipelines in preparation for maintenance can create problems. Often equipment cleaning does not attract much attention and is frequently carried out by individuals with the least amount of technical training. A few problems that have been reported will follow. 

Warm vapors sometimes condense inside closed vessels the resulting may collapse the vessel. Such incidents have occurred when a vessel was purged with steam, then closed in without venting. Similarly, a distillation column may go to vacuum if reboil fails but condensing continues. 

To a steam-heated autoclave are added 950 gm (28.00 moles) of sucrose, 3 gm of 1,2,4-trimethylpiperazine catalyst, and 60 ml of distilled water. The autoclave is purged three times with nitrogen gas and heated to 100°C. Then the addition of 1550 gm (27.0 mole) of propylene oxide is started. After 1 hr the temperature is raised to 115°C. At this temperature the addition is carried out at approximately 90 psig. The addition of the remaining propylene oxide requires about 10.25 hr. The reactants are then heated to 120°C for another 45 min. The reaction mixture is cooled to 60°C and the liquid product is blown into a clean vessel. The volatiles are removed under reduced pressure varying from 80 mm Hg to a final 2 mm Hg over a 5-hr period. The product has MW 839. 

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