SIS interlock logic
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The principle of a shut-off valve controlled by two solenoids
SIS interlock logic refers to the Safety Instrumented System, which is used to ensure the safety of industrial processes and equipment. In the use of cut-off valves, in order to ensure their reliable and safe operation, two solenoid valves are typically used to control their opening and closing. When one of the solenoid valves fails, the other solenoid valve will automatically close the shut-off valve, thereby ensuring safety. This is the application of SIS interlock logic. .
Do the two solenoids control a shut-off valve (on/off valve) in a interlocked manner, and does it only operate when power is lost to both of them?
Yes, usually when two solenoids are used in interlocked control of a shut-off valve, a \"cross-power-loss protection\" function is required. In other words, if either of the two solenoid valves loses power or fails, the shut-off valve will automatically close to ensure safety. If there is no cross-power failure protection feature, the situation can become dangerous, as if both solenoids lose power or fail simultaneously, the shut-off valve cannot be closed, thereby creating a safety hazard. .
Is the solenoid valve connected in series or in parallel?
Thank you for the answer from above; I’ve learned something
It is determined by the SIL level of the safety instrumented system and the requirements of the verification results; a single solenoid valve does not have a sufficient SIL level, so two solenoid valves are required. Taking ASCO as an example, the SIL level of a single solenoid valve is only SIL2; therefore, two solenoid valves need to be used together, and with this combination the SIL level can reach SIL3.
It might be necessary to take a look at the PID diagram; the solenoid valve here is likely used to control the flow of instrument air, and the opening and closing of this air flow is what enables the valve to open or close.
Thank you to the two people above for their answers; they were very useful, I’ve learned something from them!
First, it is necessary to determine whether the air circuits of these two solenoid valves are connected in parallel or in series. In a series connection, if either solenoid valve loses power or air supply, the valve will close or open accordingly. In a parallel connection, the loss of power in just one solenoid valve will not cause the valve to move; only when both solenoid valves lose power at the same time will the electric valve close or open
Please refer to GB50770; it explains clearly that for high safety levels, a series connection is used, so that if one circuit loses power, the valve will operate； High availability is series-connected; the valve will only act when two power supplies fail. It’s not possible to tell from the logic diagram whether it is in series or parallel. It is necessary to check the SRS or valve specification sheet.