The SIL1 level isolation valve can be shared by both SIS and DCS – how should this be understood?
Thread Content
According to specification 50770, for SIL 1 safety instrumented functions, the control valves can be shared with the basic process control system; it must be ensured that the safety instrumented system takes precedence in terms of operation. ”How should this sentence be understood? I currently have a SIL1-level isolation valve and a radar level gauge that are interlocked within the SIS system; however, there is also a double-flange level gauge in the DCS that needs to be interlocked with this isolation valve as well. What should I do? Should I install two solenoid valves on the isolation valve? One solenoid valve is connected to the DCS, and one solenoid valve is connected to the SIS?
A few years ago, SIS and DCS shared one solenoid valve, with the DOs of SIS and those of DCS connected in series. We are now using dual solenoid valves.
In my opinion, control valves should not be used as shut-off valves; solenoid valves should be added to achieve interlock functionality, with the interlock signals provided by the SIS and the control signals provided by the DCS.
When sharing is involved, it is necessary to ensure that the SIS system takes precedence in its operations; that is, when the interlock conditions for shutting off a valve within the SIS system are triggered, the DCS system is prohibited from increasing the opening degree of the control valve or from using solenoid valves to command full opening of the control valve. Only for SIL 1 levels, where systematic safety risk analysis (such as LOPA) shows that the impact of risk factors is minimal and the requirements are not high, is such \"sharing\" not prohibited; the terminology used in standards is usually \"it is advisable to have separate installations\", meaning that such \"sharing\" is not recommended.
If it is a control valve, an electromagnetic valve can be used; if it is a shut-off valve, two electromagnetic valves can be connected in parallel.
A few years ago, I worked on a project in which valves and radar level gauges were interlocked within the SIS (both the valves and the radar met SIL1 requirements). When the interlock conditions were triggered by the double-flange system, the DCS sent an interlock request to the SIS (interconnection between systems was achieved through hardwired connections), and it was the SIS that operated the valves. Priorities can be set at the SIS to ensure the precedence of the radar level gauge interlocks.
Just like this, we installed two solenoid valves as per the customer’s request: one connected to the DCS and the other to the SIS, in series.
Currently, in line with the increased requirements of **chemical safety standards, SIS and DCS valves, regardless of their SIL level, should not share the same sensing elements and actuating elements.
Given the current situation, although there are no regulations requiring separate installation, it is still best to use valves individually.