A brief discussion on the selection and use of steam control valves

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Due to the high temperature and pressure of the medium used in steam control valves, careful consideration is required when selecting them. If the wrong model is chosen, failures will occur frequently during operation, severely affecting the proper functioning of the equipment and systems. Next, we will discuss the selection of steam control valves and some issues that need attention during their use through an example. A large chemical manufacturing company needs to use steam to control the temperature of its reaction tanks. The steam comes from the waste heat generated by a thermal power plant; its temperature is around 200°C and its pressure is around 1.2 MPa. Steam control valves are required 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 down occurred 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 control valve is quite large, which places 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 locations where pressure measurements are taken during system design (selecting points where pressure changes rapidly). There are also issues with the PID parameters set for the pressure control module in the control system, leading to excessive sensitivity of the entire system. Even slight changes in pressure cause the regulator to send out control signals to drive the control valve, resulting in frequent movements of 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 puts excessive strain on the control valve, preventing it from fulfilling its regulatory functions. 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, thanks to its balanced structure, can **counteract the unbalanced forces generated by the steam flowing through it, 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-filled 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, if we consider the optimal solution, given the actual operating conditions and requirements here, it would be more appropriate to use a pneumatic double-seat control valve; it wasn’t adopted simply because it is inconvenient for the user to install the air supply pipelines. It is evident that for 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 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. This post was last edited by hk6666 on 2008-7-23 10:25.]