Q&A Compilation Post for the Control System Technology Discussion Area — No Replies Allowed
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The following is a temporary compilation of posts made for forum development, intended to facilitate everyone’s search for the information they need; please do not post new replies~~~~~~~~ If you have any suggestions or errors that need to be corrected, please contact us via message or post a reply appropriately (replies will be deleted after some time, and valid replies will earn points in other posts)~~~~~~~~~~~~~~· It is being continuously improved and updated~~~~~~~~~~~~~~~~~~~ Issues related to instrument wiring: Everyone is welcome to share their opinions, including those from production sites, design teams, and installation crews. Currently, for the wiring from the field instruments to the DCS and ESD systems in the control room, should it be done on a one-to-one basis, or through field wiring boxes connected by multi-core cables? Is that way good? Floor 1: Strange shutdown phenomenon due to the compressor oil pressure interlock: In our plant’s compressor oil pressure interlock shutdown system, the transmitters used are Yokogawa EJA530 models, while the safety barriers are from Shanghai Chenzhu. The paperless recorder used is the MC200 model from Hangzhou Pangu. The shutdown threshold for oil pressure is set at 0.12 MPa. When the oil pump was operating normally, the system suddenly shut down. Checking the paperless recorder showed that the oil pressure was initially at the normal level of 0.33 MPa, but then it dropped suddenly to 0.08 MPa, further dropping to 0 MPa, before rising back to 0.08 MPa and finally returning to the normal level of 0.33 MPa. The entire process, from when the oil pressure dropped to when it returned to normal, took four seconds. The sampling interval of the paperless recorder is 1 second, so it should be able to record the entire fluctuation in oil pressure accurately. Initially, we suspected that some interference at the site was causing these fluctuations. After consulting Yokogawa, they said that the transmitters were affected by interference. A 47 uF capacitor was connected in parallel in the transmitter’s output circuit, but this did not have a significant effect. No electrical interference sources were found at the site, and other transmitters there were working properly. Could there be other types of interference that could affect Yokogawa’s transmitters? Experts, please help determine this.Floor 2: Are explosion-proof instruments used on-site? Is it still necessary to use isolated safety barriers in the DCS? Under what circumstances are safety barriers not needed? What effects can occur if safety barriers are not used?
Floor 3: This post was last edited by Zikong Hai on 2009-2-25 at 16:53
1-to-1 configuration, facilitating subsequent maintenance and reducing the number of contacts. But the cost is slightly higher; it’s better for the construction party to use a one-to-one approach. Generally, for small-scale use, a 1-to-1 setup is adopted ； For the construction party, it is better to use on-site junction boxes along with multi-core cables, as this helps save costs ； Large-scale systems use on-site junction boxes ；
It’s impossible for the lubricating oil circuit system to reduce the oil pressure from 0.33–0.08–0–0.08–0.33 MPaG in just 4 seconds! After all, the compressor should have a high-level oil tank as well as a secondary pump. The most likely explanation is that the wiring is loose; it’s important to check whether the terminals of the safety barrier, transmitters, and wiring boxes are securely connected. Check whether the signal wires and control wires are in the same conduit or use a multi-core cable, with the control wires, interlock wires, and signal wires combined together. Over time, aging of the wires can also lead to a decrease in insulation quality. You can check if there are any construction works on that day. Paperless recorders have poor interference resistance, and the interference generated by operators using their phones can cause the high-pressure machine to shut down! For the interlock related to oil pressure, you can use 1 – three transmitters to implement a two-out-of-three trip mechanism ； 2. Increase the damping of the transmitter ； 3. DCS can lower the oil pressure and cause the system to shut down after a 5-second delay. Any of the above will do. The damping of the transmitter seems to be adjustable manually on the transmitter itself.
1. Determine whether the instrument is of the flameproof type or the intrinsically safe type; instruments of the flameproof type do not require a safety barrier, while those of the intrinsically safe type must be equipped with one. 2. It can be determined based on the on-site conditions; if there are flammable or explosive substances near the instruments at a certain location, a safety barrier must be installed. Either Zener or isolated type is acceptable; the isolated type is recommended. Conversely, there is no need to install a safety barrier. For example, if the environment near a workstation used to measure dynamic wind is dynamic wind itself, these points can be connected directly to the DCS cards without going through a safety barrier.