Issues with electric door control in DCS systems
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Our company’s desulfurization unit uses several electric gate valves from Globe Valve Company, and when planning the documentation required, it was specified that a 4-20mA valve position feedback signal should be available. As a result, the electric actuator that arrived was of the DQTYPE type, manufactured by Changzhou Lanling Valve Wired Company. During individual valve testing, it was found that the valve could be controlled, but there was no valve position feedback. The manufacturer said it was because the valve position feedback module on the valve did not have a 24V power supply, stating that it should be provided by the DCS system (the system from Beijing Xinhangzhi). The instrument technicians all said that the DCS control cabinet cannot provide a 24V supply externally, and it is not standard to use an external 24V power source for valves; the 24V DC power should be generated by the valve manufacturer itself through voltage reduction from 380V, and then supplied to the feedback module. I consulted the manufacturer, who said that the price they offered was only sufficient to cover one feedback module; the power supply module is very expensive and would require an additional cost (which is difficult for our company’s procurement department to accept). An electric gate valve costs around 30,000 to 40,000 yuan, but it is unable to provide feedback on its valve position due to the absence of a 380VAC/24VDC transformer. I would like to ask experts how such a problem can be resolved.
How can a DCS not have 24V DC power? I have no experience with DCS in Beijing. It’s unclear. Please ask someone who has used the system before to answer.
Just use the feedback module as a four-wire instrument. DCS systems generally have an external power supply for field instruments, right?
3# liuyun001: We use two-core cables for ours; it’s too troublesome to change them to a four-wire system now! And it’s unprecedented in our company!
2# tonysun12: Even though DCS has 24V DC, it is used only to power its own modules. When powering external modules as well, it operates on a two-wire system; there is no such thing as a four-wire system!
All DCS systems are equipped with a set of DC 24V distribution cabinets that supply power to four-wire instruments and similar devices; there is plenty of backup, and it is sufficient to provide power for the position feedback of electric valves through fuses
Using AC220V\DC24V power supplies on site can also solve this problem
DCS systems generally have 24V DC power supply.
Use the module as a four-wire instrument; the power line consists of two wires, and the signal line also consists of two wires.
1. First, the cables need to be re-routed. Two-wire system solves the output issue; feedback requires a separate wire. 2. I remember that the system of Zhejiang University’s Zhongkong has a dedicated PAT module that can solve this problem. The system manufacturer can be asked to provide such a functional module. It seems other manufacturers also have them.
The 24VDC power supply from the DCS system can be used as the operating power supply. If the DCS system does not have external three-wire or four-wire instruments, that is, if there is no 24V power supply, it is possible to add one; a 24VDC 10A DC power supply is quite inexpensive.