Compilation and Analysis of Selection Cases for Control Valves

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• Case 1: In 2005, a DN80 flow control valve for a water treatment system in a power plant – it operated normally when opened, but took an extremely long time to close. The adjustment cannot be completed. Upon checking the calculation sheet for the control valve, it was found that the pressure difference before and after the valve was set at 0.15 MPa, but in reality it was 0.4 MPa. During the valve selection process, the calculation of the unbalanced forces was underestimated, resulting in insufficient thrust from the actuator. After replacing it with an actuator with greater thrust, it is working properly. • Loss incurred: The water supply cannot be regulated, and the cost of replacing the actuator is 16,200 yuan. • Lesson: In p-design, it is necessary to repeatedly verify the parameters provided by the manufacturing process; these parameters must be accurate. If one parameter is not up to standard, the valve cannot function properly. When selecting p, it is necessary to take into account variations in process conditions within a certain range. • Case 2: A user of a certain system in a refinery carried out technical upgrades and needed to regulate the flow of hydrochloric acid. A pneumatic control valve made of WCB+PTFE was selected for use; the sealing method was packing, with PTFE as the material. The valve stem was made of 316L stainless steel. After it was put into operation, everything worked normally. However, 20 days later, the valve actuator moved, but there was no change in the flow rate, and a small amount of hydrochloric acid leaked out. Upon opening the valve, it was found that the internal components of the valve were intact, but the sealing gaskets had been corroded and the valve stem was broken, resulting in valve failure. • Analysis: • During the selection process, the issues related to corrosion by hydrochloric acid were taken into account for the valve body and its internal components; however, insufficient attention was given to the valve stem and the sealing method. Later, the sealing method was changed to diaphragm sealing, and Hastelloy B alloy was chosen for the materials. Since then, everything has been working properly. • Loss: The original valve had to be discarded, worth 21,000 yuan; the new valve was not purchased until 4 months later, during which time production relied entirely on manual adjustments. Production will be halted for 8 hours before and after the maintenance. • Lesson: The selection personnel took into account the issue of medium corrosion on metals, but did not give sufficient attention to the sealing method. A small error in selection led to significant financial losses and disrupted normal production. • Case 3: In August 2002, the smelting plant underwent an electrolysis technology upgrade, which involved automatic control of the heating of the catholyte storage tank. Steam was used as the heating medium, and a pneumatic thin-diaphragm control valve was selected; the diameter of this valve was DN50. After it was put into operation, everything worked normally. However, after 60 days of operation, even when the valve was opened to its maximum position, the temperature remained below the set value. When the control valve was checked, it was found that during the selection process, a steam pressure higher than the actual pressure had been chosen. The minimum steam pressure required by the process was 0.35 MPa, and the diameter of the valve core and seat was DN25. By October, however, the actual steam pressure had dropped to 0.15 MPa, resulting in insufficient heating power and thus an inability to adjust the temperature to the desired level. After recalculation, normal operation was achieved with a DN32 valve core. • Lesson: Although the design and selection personnel did not make any mistakes when considering the valve parameters, they failed to take into account the specific climate conditions in Jinchuan, as well as the fact that steam pressure decreases as temperatures drop in winter. • Case 4: In 1995, in a chlorine leaching process of a certain system at a refinery, while adjusting the chlorine flow rate, the production staff mistakenly identified a titanium valve as a stainless steel one while replacing the bypass valve. Just 10 seconds after chlorine was introduced through the valve, it turned red, resulting in a severe chlorine leak. Fortunately, the maintenance personnel were wearing head-mounted gas masks and the instrument maintenance staff were wearing gas masks, so the poisoning was not severe; they recovered after taking two days off. • Although no injuries were caused in this accident, the large amount of chlorine released led to the evacuation of many people and a 4-hour shutdown of operations, resulting in significant economic losses. • Lesson: • Improper selection of valves in the process flow can not only affect the manufacturing process but also lead to safety accidents. Therefore, careful consideration must be given to the selection of valves; even a small oversight can result in serious safety incidents and significantly disrupt production. • Case 5: In a precious metals manufacturing plant built in 2009, there was one production line that required precise control of the flow rates of three different raw materials. The pipe diameter was DN20, and the flow rate range was 60–300 L/h. During the process calculations, the pressure behind the valve as indicated by the process parameters did not match the actual pressure there. After the system was put into operation, at the maximum flow rate, the valve opening was less than 10%, which made it impossible to control the flow rate. Subsequently, measures such as replacing the valve core and seat and adjusting the pressure behind the valve were taken, which basically resolved the issue, but the regulation quality remained unsatisfactory. • Summary: In the metallurgical process, low-flow regulation is rarely used. However, with the implementation of high-value-added product projects by the group company, such as titanium metallurgy, precious metal metallurgy, metal salts, and fine chemicals, low-flow control is becoming increasingly important. During project construction and technical upgrades, it is necessary to carefully verify the parameters provided by the process during the design phase; if there are any doubts, such verification must be carried out thoroughly for low-flow designs. Examples of both successful and unsuccessful applications of valves within the group company can be found in various factories. The issues discussed in these examples serve to highlight the importance of proper valve selection and the value of appropriate technical handling, thereby encouraging relevant technical personnel to pay sufficient attention to the selection of control valves when working on engineering projects, technical upgrades, and the selection of spare parts.