The issue of DCS and SIS jointly controlling electric valves
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A certain factory originally had a DCS temperature control system; for the temperature of the reaction vessel, interlocks were in place to shut off the feed valve as well as to adjust the cooling water flow to the jacket, with all of these valves being electric. Under normal conditions, the opening of the kettle temperature control valve is adjusted; in case of overtemperature, the feed valve is shut off and the chilled water control valve is opened fully. Now, a SIS system needs to be added. Logically, the control of shutting off the shut-off valve and fully opening the control valve should be incorporated into the SIS system. How can this be achieved? Please advise, experts
This post was last edited by jlshnlhj on 2018-5-9 at 16:17. The question is not very clear: since it mentions \"the opening degree of the temperature control valve under normal conditions,\" is this valve a pneumatic valve for the chilled water control valve with a 4-20ma signal? “What kind of valve is a “feed cut-off valve”? Is it an interlock solenoid valve? “What does “All valves are electric” mean? Does it refer to the interlock solenoid valve? So is the feed valve a 4-20ma pneumatic valve or xx? Is the interlock to fully open the chilled water control valve and shut off the feed valve? Is that so? If that is the case, then the solenoid valve cuts off or connects the signal to the pneumatic valve diaphragm head, allowing control as needed: so that the air pressure signal at the diaphragm head is 0% (the solenoid valve vents the signal from the diaphragm head) or 100% (the solenoid valve connects the air supply to the diaphragm head).
They are electric control valves and electric shut-off valves
Suggestions: 1. The kettle temperature should be integrated into DCS and SIS. Temperature control is carried out in the DCS, while interlocking is implemented in the SIS (taking into account whether a two-out-of-two or three-out-of-three scheme is required). 2. The actuation commands for the shut-off valves are sent to the SIS (two DO points), while the on/off feedback from the shut-off valves is sent to the DCS (two DI points) for display. 3. The control valve should be connected to the DCS, not to the SIS. Because the control valve is electric, and it is not convenient to perform PID control with SIS. If it’s a pneumatic control valve, that’s convenient – choose an FO, connect the solenoid valve to the SIS, and the rest to the DCS.
So, if it is desired for DCS to perform PID control on the control valve, and SIS sends a full-open signal to the control valve during interlock operations, is there any way to achieve this?
This is quite difficult, because your control valve is electric and can receive only one control signal (AO); moreover, there are no solenoid valves, so it can be integrated either into the DCS or the SIS. If the control valve is a pneumatic valve, DCS can perform PID control on it; during interlock operations, SIS sends an open signal to the control valve via solenoid valves.
This post was last edited by jlshnlhj on 2018-5-10 09:16: “So, if one wants DCS to perform PID control on the control valve, and SIS sends a full-open signal to the control valve during interlock operations, is it possible to achieve this?” It should be possible. Let’s sort this out first: 1. For the original pot temperature control, the chilled water control valve is an electric control valve that operates on an AO 4-20ma signal, with 20ma corresponding to full opening. 24v power supply? 2. The feed valve is an electric cut-off valve, powered by 24V; does it operate on signal receipt or loss? 3. During interlock of SIS, the chilled water control valve should be fully open at 20 ma; should the feed valve electric cut-off valve be powered on/off? If that is the case, SIS can be implemented on the output circuit using a relay DO.
The 3 points you mentioned are correct. The main issue is how, during interlocking, the SIS can give priority to the DCS and send a full-open signal to the chilled water control valve; I’m not quite sure as to how this is achieved
This post was last edited by jlshnlhj on 2018-5-11 09:04. Here’s a schematic diagram to illustrate the principle temporarily. It would be best to add a relay to provide a 250-ohm resistive load to the DCS AO when it is disconnected. Additionally, the +24V ground should be shorted to the “-” terminal of AO. Additionally: In the diagram, “+24v” should be replaced with “current value required for interlock”, which can be provided by the SIS or DCS.
Thank you for such a clear schematic diagram. By the way, I just checked and it seems that Rotork’s electric actuators have an ESD function, which allows a digital signal to be used to open or close the valve with the highest priority; I’m not sure if other brands also have this feature
“The electric actuator seems to have an ESD function; in fact, there is a solid-state relay inside it.