Regarding safety instrumented systems
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I have a question: one of the specifications for petrochemical safety instrument systems states that pneumatic actuators should be used rather than electric actuators. I would like to ask what the reason for this is? Is it useful for everyone when designing? Are 380VAC electric valves useful? Do all the power supplies for instruments and valves in the SIS system need to be provided by UPS?
There may be the following reasons: 1. Most control valves in petrochemical enterprises are pneumatic control valves； 2. The sites of petrochemical enterprises are mostly hazardous locations (which is the reason mentioned in point 1), and pneumatics have inherent explosion-proof properties ； 3. Pneumatic control valves operate faster than electric control valves ； 4. The actuation of electric control valves requires a high-power power supply, which is unsafe when installed on-site.
Hello, we have already installed electric valves on the gas pipeline. According to the regulations, can it then not be integrated into the SIS? If it is to be included, must pneumatic valves be used? Also, the decision regarding whether to incorporate something into the SIS system is generally based on certain criteria. Our valve is used for coal gas entering the plant area; the subsequent sections have been designed according to the SIS classification standards. Since this criterion wasn’t mentioned in the SIS classification, does that mean it’s not necessary to implement it?
To address your question, it’s necessary to consider things from a fundamental perspective. You need to understand that the purpose of installing a shut-off valve is to enable rapid interruption of the flow of material. Therefore, there are two requirements: one is the time it takes to close the valve, which generally needs to be less than 3 seconds; the other is the amount of leakage. Therefore, electric actuators generally cannot meet the requirement of a closing time of 3 seconds, so they cannot be used. Strictly speaking, the level of SIS systems is higher than that of DCS systems; since DCS systems require UPS power supply, SIS systems must also be powered by UPS. It ensures that the system operates accurately at critical moments.
I would like to ask: According to the design specifications for power supply in the chemical industry, instrument power is classified into two categories – primary important loads and tertiary loads. Primary important loads are to be supplied by UPS. Does this rule also apply to 380VAC electric valves?
In the design specifications for power supply of chemical process instruments, in the section on conventional power sources and UPS systems, the voltage is specified to be 220VAC or less. Electric valves operating at 380VAC should not be subject to this rule; I’m not sure if that’s correct
What I mean is that it’s incorrect to use 380VAC electric valves in the SIS system; safety instrument systems use quick-acting valves, and your electric valve won’t work for that purpose.
SIS definitely requires a separate UPS; its function is to initiate an emergency shutdown in cases of power or gas outages at the site. Electric valves cannot serve as a means for such shutdowns, that’s how I understand it. Then, whether it is necessary to incorporate it into the SIS depends on the protection layer identified through HAZOP analysis; if the protection layer is sufficient, there is no need to include it in the SIS.
Because it needs to be fail-safe, it’s easier to do it pneumatically than electrically
It is recommended to check the relevant specifications; it is written clearly there