It is not advisable to rely on valve control for the long-term regulation of process medium flow rates
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It is not advisable to rely on valve control for the long-term regulation of the flow rate of process media. In a certain oil atmospheric and vacuum distillation unit, a perforation in the pipeline behind a valve occurred during a viscosity reduction incident, resulting in a fire that forced the atmospheric processing section to shut down. The material of the accident line is grade 20 steel, with an original thickness of 7 mm; after 2 years of use, the corrosion rate reached 3.5 mm/year. Accident analysis: The corrosion in the residue system is mainly high-temperature sulfur corrosion. The sulfur content of the residue is 1.2%. The flow state has a significant impact on high-temperature sulfur corrosion. When the flow rate increased from 0.32 m/s to 0.8 m/s, the corrosion rate of carbon steel increased by a factor of 1.62. The sludge flow rate in the accident section was 0.3 m/s. Under normal operating conditions, the valve opening was less than 1/2, resulting in the smallest fluid cross-section and the highest flow rate (about 0.8 m/s) at a certain distance behind the valve. This caused severe corrosion of the pipeline in that area, leading to perforations. Take measures: More corrosion-resistant materials should be selected. For example, Cr5Mo can be used for pipes, while type 188 stainless steel can be used for elbows and pipes downstream of valves.
For the control of process fluid flow rates, relying solely on valves over the long term may lead to increased corrosion in certain sections of the pipes due to changes in hydrodynamic conditions, as illustrated by the accident at the oil facility mentioned above. The increased flow velocity downstream of the valve leads to severe local corrosion, ultimately resulting in pipe perforation. Therefore, in addition to choosing corrosion-resistant materials such as Cr5Mo alloys and 188 stainless steel, other measures must also be considered, such as improving flow patterns, applying anti-corrosion coatings, conducting regular inspections and maintenance, and providing sufficient safety margins during design, in order to ensure the long-term safe and stable operation of the system. .