Regarding manual operation of instruments
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The interlock valve at the inlet of the backup fan for our heating furnace is manually operated on-site; the air duct has been closed, while the interlock cut-off valve is open. However, the valve status of the inlet isolation valve shown on the DCS in the control room indicated that it was closed. What I want to ask is, if the on-site operator shuts off the valve manually, will the readings displayed on the DCS in the indoor control room be inaccurate? If that’s really the case, what is the reason?
It’s due to the instrument; you can go to the site to verify it and report the situation to the instrument.
Start with the circuit, start with the composition.
It seems this valve has no switch feedback, or the feedback is faulty.
Whether the isolation valve is operated manually or automatically, it does not affect the feedback signal; even when operated manually, the display on the DCS side remains consistent with that at the site. On the part of the poster, it’s likely due to the on-site shut-off valve; perhaps the valve limit switch is out of position.
The most likely cause is an issue with the instrument; there could be three reasons for this: 1. A fault in the microswitch within the on-site transmitter, or problems with the wiring. 2. DCS card channel issue. 3. System configuration issues. For the above issues, you can just check with a meter – it’s very easy to identify the problem!
You can check the instrument circuit or the feedback signals from the valves on site
It depends on the design. If the manual cut-off valve is equipped with on/off limit switches and those switches are functioning properly, the DCS will reflect the actual status of the cut-off valve. If a limit switch is not installed on-site, the valve status shown on the screen is not the actual one; it may be the theoretical status as defined during configuration.
If it’s not a problem related to the limit switch, I suspect there is no limit switch at all or that the wires leading back to the control room are missing; what the original poster sees as open or closed is merely the signal status sent by the DCS, and this does not allow one to determine the actual valve position.
Baidu: \"A limit switch, also known as a travel switch, can be installed on relatively stationary objects (such as frames or door frames, referred to as stationary elements) or on moving objects (such as cranes or doors, referred to as moving elements).\" When the animal approaches the stationary object, the linkage of the switch drives its contacts, causing the closed contacts to open or the open contacts to close. The circuit and motor are controlled by changes in the on and off states of the switch contacts. ”If a limit switch is installed on the valve on your end, it is possible to connect the valve’s open/closed status to the DCS, allowing the valve’s condition to be viewed on the screen ； If it is not installed, what is displayed on the DCS screen will not reflect the actual condition of the valves on site.