A valve is controlled by both the SIS and the DCS for sequential control
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This post was last edited by Qingse Nianhua on 2018-12-4 at 12:43. A valve is equipped with two solenoids, and these two solenoids are connected in a redundant manner on the side of the valve. First, both solenoid valves are connected to the SIS system, and then linked to the DCS system via a hard wire; in other words, the DCS outputs a dry contact DO to the SIS, thereby enabling control of both solenoid valves simultaneously. How can we now make this valve capable of receiving both SIS interlock control and DCS sequential control? That is, under normal circumstances it is controlled by the DCS for sequential operation; when the liquid level is high, the SIS interlock takes over to close the valve, and control by the DCS is no longer applied. Once the liquid level returns to normal, the SIS must be reset before the DCS can take control again.
Then, interlock control shall be carried out only on the SIS, while the DCS will be used solely for signal display and alarm functions
Then how can the valves be turned on and off in normal situations?
I’m a bit confused about this issue. The client requires that, when the normal liquid level has not reached the high-level interlock point, the solenoid valve should be controlled in sequence by the DCS; in other words, the logical control is to be carried out on the DCS. How do you expect to transmit the sequential control of the DCS to the SIS system using a hardwired connection? Does this mean that the control program also has to be implemented on the SIS? And the final control command still has to be issued by the SIS system? Then, after the level interlock is activated, control should also be switched to the SIS system. Logically, the SIS system is only meant to perform simple and straightforward interlock shutdown controls, without any associated trigger logic programs. The sequence control program should be implemented on the DCS; in this way, at least the signals indicating when the valves are fully open and fully closed must also be fed into the DCS ； Sequential control must take into account the possibility of solenoid valve failures; when the DCS commands solenoid valve A to operate, the valve should close automatically as a result of interlocking. If the valve does not close automatically, it indicates that there is a failure in solenoid valve A, and in such cases, solenoid valve B should be activated through interlocking. Because these two solenoid valves are redundant, with one in use and one as a backup.
Isn’t it simple to connect the output relay of the SIS in series with the circuit of the DCS control valve?
This post was last edited by feng*aosa on 2018-12-4 at 17:17; a normally open valve was selected. If the valve is air-actuated, a pneumatic three-way valve can be installed in the control air line to reverse the control signal. The solenoids of DCS and SIS are connected in series in the valve control air circuit, forming an \"AND gate\"” ； Under normal conditions, the SIS solenoid valve is in the open position, and the valve is controlled by the DCS solenoid valve ； When in series, the SIS solenoid valve closes, while the DCS solenoid valve does not affect the valve status regardless of whether it is closed or open. Logically, DCS and SIS perform their respective functions, with the final output being subject to a \"AND\" operation.
Yours is a method. But can control be achieved logically when solenoid valves are connected in parallel?
The valve on/off feedback is connected to the DCS. The solenoid valve is connected to the SIS. The DCS outputs a dry contact signal to the SIS DI card for sequential control of the valve.
As you requested, the condition of high liquid level is incorporated into the output from the DCS; when the liquid level is high, the circuit on the DCS side is disconnected under any circumstances.
Are there any other better ways to achieve this using software logic, without changing the existing hardware wiring?