Installation of SIS and DCS for emergency discharge of spherical tanks in the tank farm
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Our company has ethylene and propylene storage areas as well as 1-butene storage areas. The overpressure emergency relief system for the spherical tanks in the ethylene and propylene tank farm consists of two valves: a control valve that is controlled by the DCS, and a shut-off valve that is controlled by the SIS. The two valves are connected in series, with the shut-off valve located near the spherical tank. For the overpressure emergency discharge in the 1-butene tank farm, there are also two valves: the control valve is connected to the DCS, while the isolation valve is connected to the SIS. Two valves in parallel. I would like to ask fellow netizens why separate design is necessary, and what is the basis for such a design?
I think the first design is standard, as SIS is designed out of safety considerations, assuming that the DCS may not be reliable. The second design involves parallel connections; then what’s the purpose of the SIS shut-off valve? It depends on whether these two parallel paths lead back to the same pipeline or go to different places. If they go to different places, then the two designs in question cannot be compared. The SIS shut-off valve in the second design serves to cut off flow downstream in emergency situations, preventing 1-butene from being released into the downstream pipelines； If two streams merge into one pipe, then... I hope someone with expertise can answer this; I’d like to learn about it too
No one has responded for several days. Then I’ll do it. The SIS valve meets the SIL rating requirements and is used for safety protection. DCS valves are used for regular operation. They can be used in parallel; the SIS is activated only for emergency interlocks. During normal production, no manual operation is required (although this valve usually comes equipped with buttons on the control panel to allow manual activation in emergency situations). During DCS operation, normal production activities such as tank cleaning and transfer require human intervention when necessary.
As for series and parallel. It is also necessary to consider the process flow, process interlocks, and the locations of valve failures, among other requirements. From a safety perspective, as long as the safety functions can be met, it is acceptable (the corresponding SIL level also needs to be considered).
My understanding is that, regardless of whether the valves in the DCS are open or closed, in the event of an accident, those in the SIS must be shut off to prevent any flow. In parallel, regardless of whether the DCS valves are open or closed, the SIS valves must be opened during an accident to enable emergency discharge.
May I ask if the valves on both cans are FO? Is the interlock logic such that both the control valve and the shut-off valve are opened, or is it only the shut-off valve that opens?
Please provide a local PID diagram, or answer the questions regarding the upper floors. Thank you
Regarding the issue raised by the original poster, it depends on the specific manufacturing process. From what the poster has said, it’s all overpressure protection. From a safety perspective, parallel operation offers higher safety. They are designed separately; DCS and SIS instruments cannot be used together. Control valves are used for normal pressure regulation ； The emergency relief valve serves as overpressure protection for the spherical tank.
What certificates and documents are required to design safe valves?
The process flow must undergo HAZOP analysis to determine the SIL level of that circuit. The isolation valves in the SIF (Safety Instrumented Function) circuits should have a SIL level that corresponds to the SIL level of their respective circuits, and these valves must possess certifications that meet the required SIL standards
In addition to these, some time ago **they issued a self-inspection document to us, which contained relevant items