Seeking help with DCS control
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In daily production, is it in line with design specifications to use DCS level interlocks to control the opening and closing of control valves?…
SH/T3007-2014 5.4.3 Storage tanks for liquids of toxicity grades I and II, storage tanks for flammable liquids of categories A B and A C with a capacity of 3000 cubic meters or more, and storage tanks for other liquids with a capacity of 10,000 cubic meters or more shall be equipped with high-level alarms and interlocks; the high-level alarm shall trigger the interlocked closure of the control valve on the inlet pipeline of the storage tank.
There should be no problem; I’ve used it before, with interlocking based on the liquid level in the device and the opening/closing of the valves.
According to standard specifications, safety and control should be separate systems, with all safety functions to be handled by the SIS system. However, in many countries, control and security functions are not separated; the DCS handles both control functions as well as security interlock functions!
It is recommended to add another level sensor for redundancy
Let’s analyze each specific issue individually. What is the liquid level of the equipment? Does it fall under the category of processes that are considered critical or high-risk? Have HAZOP and LOPA analyses been conducted? Do the results of these analyses require the implementation of SIFs? If it’s not necessary, I think this interlock function can be implemented using DCS.
It should be possible, right? What system are you looking at? Large systems generally control the liquid level to stay at a certain value.
It is likely that the requirements for safety are not met; the DCS level interlock serves as an auxiliary element in the control system’s safety interlocks, while the SIS system is the actual safety interlock system. The two systems are independent.
No problem. If it does not proceed to the SIS after SIL analysis, it is a common practice to use on/off control via switch valves on the DCS side. For example, in the liquid separation tank at the compressor inlet, there is a high level interlock connected to the SIS system. Normally, a small amount of liquid accumulates there; therefore, it is possible to set a level threshold on the DCS side – for instance, to open the drain valve when the level reaches 50% and to close it when it reaches 20% – thereby enabling intermittent drainage.
If it’s just DCS, using control valves for the process and DCS to control the on/off valves for interlocks is no problem. If the design includes DCS and SIS, the valves of the SIS cannot be controlled by the DCS; they must be separated
I learned from this… The container itself isn’t very large… After consulting a design company, they said it’s possible.