What are the reasons why automatic cascade control of the instrument cannot be activated?
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What are the reasons why the DCS upgrade, which was originally capable of enabling automatic cascade control, cannot be put into use in the field?
Was the entire DCS system upgraded, or was it just an update to a single control loop?
It’s too vague; it’s impossible to tell! Is it just a software upgrade, or a software and hardware upgrade? Were there any changes to the instruments related to this circuit during the upgrade? Are the readings of the relevant instruments normal after the upgrade? Is it impossible to connect to all circuits, or is just one circuit not working? Have there been any changes to the settings for the proportional, derivative, integral values, as well as the upper and lower limit valve positions in this circuit? Also, did the operation fail after it was started, or was it impossible to conduct inspections before starting? Before starting operation, since the measurement value is at the upper or lower limit of the range and does not change, it’s possible that as soon as you make a setting, the judgment function of the cascade control loop will automatically be disabled.
There are also requirements for cascade control; it is necessary to wait until the system becomes more stable before enabling cascade control. If you don’t wait for the system to stabilize, the fluctuations will be too large and cascade control cannot be activated. Additionally, it is necessary to check the upper and lower limits of cascade control, and only after confirming those ranges can it be enabled
Pay special attention to the changes before and after the major overhaul, including software, hardware, and on-site instruments. It can only be determined based on the examination results
The key to solving this problem is to see where changes were made. Modern DCS systems basically all have PID parameter auto-tuning capabilities. However, the original parameters depend on the DCS performance: some have default values built into the machine, while others require manual input. For complex control loops, the initial values of PID parameters differ from those of single-loop PID parameters, and they often need to be set manually. Some machines return to their default settings after a power outage and restart, which causes the self-tuning process to fail due to exceeding its capabilities, resulting in the rejection of the cascade control setup.
Some of the PIDA parameters have changed while others remain unchanged. When asked, the instrumentation team replied that it was just a DCS upgrade and nothing else has been altered. There is no simple way to put the cascade control into use; it simply cannot be activated. It has nothing to do with stability.