Improper selection of steam electric control valves can lead to serious consequences

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　　Steam electric control valves are mainly divided into steam electric temperature control valves, electric pressure control valves, and electric flow control valves. Due to the high temperature and pressure of the media they handle, careful consideration is required when selecting such valves; otherwise, failures will occur frequently during use, severely affecting the proper operation of the equipment and systems. Next, we will use an example to discuss the selection of steam electric control valves and some issues that need attention during their use. 　　There was once a large chemical manufacturing company that needed to use steam to control the temperature of its reaction tanks. The steam came from the waste heat generated by a thermal power plant; its temperature was around 200°C and its pressure was around 1.2 MPa. It was necessary to use steam-electric control valves to maintain a constant steam pressure, with an inlet pressure of 0.8–1.0 MPa and an outlet pressure of 0.4–0.5 MPa. At the beginning of the selection process, since the site parameters were not clearly understood, an electric single-seat control valve was chosen. It started to malfunction frequently after short use; issues such as the motor of the actuator burning out, the control module being damaged, and the reduction gears breaking were occurring repeatedly. 　　Only after sending people to the site for observation and analysis was the cause of the fault identified. It turns out that the pressure difference between the inlet and outlet of the steam electric control valve is quite large, which results in excessive load on the actuator during operation; in other words, it’s an overload situation. As a result, the reduction gears cannot handle such heavy loads and get damaged several times, while the motor and modules also get damaged. Moreover, users make improper choices regarding the pressure measurement points during system design (selecting locations where pressure changes rapidly), and there are also issues with the PID parameter settings of the pressure control module in the control system. This leads to excessive sensitivity of the entire system: even minor changes in pressure cause the regulator to send out control signals to drive the control valve, resulting in frequent adjustments by the valve and thus oscillations. This in turn causes oscillations throughout the entire system. Not only does this prevent the controlled variable from remaining stable, but it also places excessive strain on the control valve, preventing it from fulfilling its regulating function. 　　After consulting with the user, it was decided to use an electric sleeve control valve instead of a single-seat control valve, as the spool of the sleeve valve’s balanced structure is capable of **cancelling out the unbalanced forces generated by the steam flow, thereby reducing the load on the electric actuator and facilitating the long-term, stable operation of the control valve. The reason for not choosing a balanced single-seat control valve is that the guiding seal ring of a single-seat valve cannot withstand long-term wear in steam environments, and since the system does not have strict requirements regarding the leakage rate of the control valve, a sleeve valve is more than sufficient for such conditions. At the same time, the pressure sampling points need to be reset. After replacing the sleeve control valve, during system commissioning, the PID parameters should be adjusted by reducing the proportional band and increasing the integral time, thereby slightly adjusting the system’s response time, so that the fluctuations in steam pressure remain within acceptable levels for use. After such adjustments, this problem was basically resolved. 　　In fact, from an ideal perspective, given the actual operating conditions and requirements here, a pneumatic double-seat control valve would be a more suitable choice; it wasn’t adopted merely because it is inconvenient for the user to install the air supply pipeline. 　　In summary, for steam electric control valves, while selection is important, installation, commissioning, and maintenance are even more crucial. Many failures occur because users are not familiar with the performance of these control valves as well as their characteristics regarding use and maintenance. You cannot isolate the control valve from the control system; it must be considered and adjusted as part of the entire system. Moreover, the control valve is a dynamic actuator that is prone to various failures, which is something manual valves do not experience. Furthermore, with the increasing use of fieldbus control valves, the tasks that were previously carried out by the system have been delegated to these valves, which places higher demands on their operation and maintenance.