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Crude Unit Start-ups: The Results of a High Liquid Level

 

Summary

Pressure-measurement techniques can avoid the dangers of rapidly changing flow rates and levels during start-up. Also, column internals directly above and below flash zones need to be designed for higher uplift forces.

 

Text

National Petroleum Refiners of South Africa Ltd (Natref) revamped its crude unit in 2002 to increase capacity. The start-up and operating procedures were revised to reflect extensive flow scheme changes, which included many new pieces of equipment. Nonetheless, even after careful planning, abnormal events can take place during start-up, such as a high liquid level. Level instrumentation alone is not enough to avoid this problem and its consequences. However, pressure is a simple, yet often overlooked, measurement that operators and start-up personnel can rely on instead.

Start-up can be dangerous at times, as flow rates and levels are changing rapidly and several things are occurring at the same time. Many crude units experience internal damage during start-up, which most engineers associate with wet steam; for example, pressure surges from the rapid expansion of water into superheated steam. However, a high liqiud level occurs so often it is the most common cause of internal damage during start-up. It occurs when high-velocity steam in the bottoms of two-phase feeds from the transfer line generates sufficient forces to dislodge equipment from the stripping, flash or wash sections.

When crude unit stripping sections are damaged during start-up, atmospheric crude and vacuum column distillate yield losses can be as high as 4 and 3% of whole crude, respectively. Furthermore, when flash section internals do not function properly, the distillate product can be contaminated with...

 

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Tags: CDU, VDU