Control system interlock

DCS is a control system that utilizes signals from various sensing elements and control devices such as control valves to regulate parameters such as flow rate, pressure, and temperature； SIS is a professional interlock program; although its operation method is similar to that of DCS, it offers much higher reliability and response speed. The above are my personal views

You have quite a few questions. Answer them one by one. ①What are the connections and differences between DCS and SIS systems? Are the two independent? This is a rather big issue. First: The two are definitely independent. Although you may be able to see information from SIS and DCS on the same computer, they are independent at the underlying level (specifically, their cabinets are separate; to put it figuratively, their \"brains\" are independent). ) Second: The main difference between the two is as follows: a, their functions are different; SIS (Safety Instrumented System, abbreviated as SIS ； Also known as a Safety Interlocking System, it is primarily used for shutdowns (for example: cutting off steam in case of over-temperature). Usually, there’s no need to consider participation after it’s done. DCS (Distributed Control System) is mainly used for control, such as level regulation and temperature control. b. The safety levels differ; typically, SIS requires SIL certification, at least SIL2, while DCS does not have such a requirement. c. Interlocks can also be implemented on DCS, but in more hazardous industries (**there are regulations regarding this, such as in the case of caustic soda, synthetic ammonia, and hydrocyanic acid), SIS must be used. ②Is there a difference between the shutdown functions provided by interlocks on DCS and those of the SIS system? Both can achieve interlocked shutdown, but the parts that shut down vary in the same project. For those that are highly dangerous and of particular importance, SIS interlocks are generally used. ③On DCS, high and low alarms, as well as other types of alarms, can be set up. What is the difference between these alarms and low-level alarms? For example, with a level gauge on DCS, high and low alarms can be configured; is there also a separate low-level alarm for the level? I haven’t seen such a low-level alarm before, but I guess it must be something like an independent level switch combined with an alarm device. If there is both a DCS alarm on the same device (with a level transmitter present at the site), such an alarm usually indicates that this level is very important. The purpose is to avoid common cause failure. For example, a DCS failure or a malfunction in the on-site level transmitter can lead to negligence regarding the low level of liquid on site, thereby causing an accident.

Typically, a tuning fork level switch is installed to provide high or low level alarms; the signals go directly to the SIS system, while the readings displayed on the DCS come from communications with the SIS system

What you mentioned is indeed very common as well. However, in environments with high requirements, common-cause failures must be avoided; that is, the system should not lose its monitoring capability just because one instrument fails. Typically, a level transmitter is used to feed data into the DCS, while a level switch is used for the SIS, with the signals from the SIS being transmitted to the DCS for display and alarm purposes.

By the level low alarm I’m referring to is something like this: on the DCS, the bottom of the reactor is indicated by a single dot; in reality, there’s a device about the size of a fist installed on the pipeline at the bottom of the reactor (with no display at all). When the reactor has finished discharging its contents, that dot on the DCS turns red, and then the switch valve at the bottom of the reactor closes automatically. One more question: DCS sets high and low level alarms through level gauges. What specific measurement and signal transmission does the SIS system use on-site to trigger a shutdown?

Could the low-level detector I’m talking about be a fork switch? Are the signals that enter the SIS system those they measure and transmit? What about other types of signals, such as temperature and pressure?

The liquid level low-level detector you mentioned is a type of liquid level switch; however, to determine what specific type of liquid level switch it is, you need to check the nameplate. Generally, SIS systems use level switches as interlock signals; in fact, any instrument that has passed SIL2/3 certification can be used in SIS systems. Therefore, as long as a transmitter meets the SIL2 certification requirements, it can be incorporated into an SIS system. Thus, both pressure and flow values can be transmitted to an SIS system using 4-20mA signals, as well as digital signals.

DCS belongs to BPCS. It is recommended that the original poster take a look at the LOPA content to understand.

The \"GB/T 50770-2013 Code for Design of Safety Instrumented Systems in Petrochemical Industries\" specifies that interlocks generally do not use digital signals. In actual production, we have a deep understanding of this as well; it can be said that we truly despise level switches and pressure switches!

As stated in “the northwest corner of the city wall,” GB/T 50770-2013 indeed specifies that analog measuring instruments should be used. Here, “yi” means: a slight degree of choice is allowed; this should be done first whenever conditions permit.