SIS system and DCS
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I work in the field of process engineering and have no knowledge of electricity at all. I’m working on a project related to waste gases in the petrochemical industry; the client wants a PLC cabinet to be installed on site for control, with signals also being sent to the DCS and SIS systems. The on-site equipment is equipped with pneumatic on/off valves for steam valves, while FC is used for the rest, and FL for everything else. Under normal circumstances, we connect all signals directly to the DCS. If the SIS is also connected on-site at the same time, does that mean that the information related to the instruments and valves involved in process control as well as safety protection should be fed into the SIS? To achieve operation of both DCS and SIS simultaneously, can the solenoid valve on a pneumatic valve supply 4–20MA signals to both DCS and SIS? Does a pneumatic valve require one additional DO and two additional DI inputs? Can this be achieved by adding just one additional AI input to the instrument? Or does the pneumatic valve need an additional solenoid valve?
This is too complicated; a systems engineer needs to design it for you first
There are many problems here, and it’s impossible to explain them in just a few words. Let me explain it briefly. The specifications do specify whether a SIS and a DCS can share the same control valve and the same signal point, but based on current industry practices, SIS systems of any SIL level need to be separated from the DCS. If sharing is possible, then both DCS and SIS can use the same point; analog signals and digital signals are processed differently: analog signals can pass through a safety barrier with one input and two outputs, connected to both the DCS and the SIS ； The DO points can all be connected to the same relay contact to control a valve in parallel. Alternatively, as you mentioned, the on-site valve can be controlled via two parallel solenoids for the air supply – one controlled by the DCS and the other by the SIS. However, both of these methods present issues related to priority that are difficult to handle ； The DI points are basically dry nodes; it is sufficient to connect them directly to the DCS and SIS via separate lines, with no conflicts. Another approach is to handle it through software; the signals can be connected to the DCS, and communication can take place between the DCS and the SIS. This method is the most cost-effective, and it also makes it easier to manage priority issues. I hope this can help you.
Since a SIS system is installed, it must be separated from the DCS system. The SIS system requires separate on-site shut-off valves, and the interlock signal points must use a two-out-of-two configuration.
DCS and SIS should obtain signals independently, that is, by adding another solenoid valve.
Pneumatic valves with dual solenoids can be used, and the series/parallel connection of the solenoids is determined based on the control requirements
For the input signal, it depends on whether your SIS and DCS are from the same manufacturer; if so, it goes into the SIS system, while the DCS can read the points from the SIS. If not, there are two other options: one is an on-site signal that enters the SIS, and after passing through the safety barrier it is divided into two paths – one going to the SIS and the other to the DCS. One approach is to set up a separate point for the SIS and DCS on-site, with each going into its own system. You also have a set of PLCs on site; if one signal needs to be sent to three systems, it becomes even more complicated
The interlock signals are fully fed into the SIS, after which the SIS and DCS communicate via Modbus
DCS and SIS cannot share a valve; two separate series connections are required
Assuming it is an RTO furnace, it is necessary to clarify which instrument devices are controlled by the PLC, DCS, and SIS respectively. This requires the equipment, process, and instrumentation parties to sit down and discuss it. The human-machine operation interface is usually located in the DCS; therefore, the signals should go directly to the DCS, rather than being \"connected\" to the DCS via a PLC, so that the DCS receives secondary signals. The same principle applies to SIS signals as well. I’ve encountered quite a few similar cases: initial planning was inadequate, on-site operation was inconvenient, so modifications were made a few months after commissioning, and in the end the PLCs were moved to the control room.