Discussion on operational issues of air shut-off control valves on DCS screens
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Some of the control valves at our chemical plant are air-operated; the process requirements dictate that the valve should move from closed to open as the DCS signal ranges from 0 to 100. A reverse output has been configured in the DCS system to meet these requirements! There are differing opinions among our colleagues; some believe that such a modification has no impact on the manufacturing process, while others insist that a range of 0-100 should correspond to on-off states! I would like to ask everyone what the advantages and disadvantages of these two methods are, and what impact they have on safety in chemical production?
It has an impact, with different effects in opposite directions; according to the usual way of thinking, 0 should correspond to fully off, while 100 corresponds to fully on.
According to people’s habitual way of thinking, 0∽100 corresponds to on/off. Generally, if there is a corresponding switch, it will indicate the ‘degree of closure’ – for example, on the vent valves of some centrifugal compressors. . .
If the production team insists that you make changes, ask them to submit a written report, have their supervisor sign it, have the factory discuss and approve it to create an official document for archiving, and at the same time require the production workshop to update its own operation manual.
In terms of operation, it is generally set to 0 for off and 100 for on in order to avoid accidental operations; accordingly, the air shut-off valve is configured with reverse action in the DCS
The choice between air-close and air-open depends mainly on the impact of the valve’s opening or closing in a fault condition on the safety of the equipment or process.
This should be chosen based on actual operations, with ease of use and safety in mind…
This mainly depends on the requirements of the process operators. Typically, 0-100 corresponds to fully closed to fully open to prevent accidental operation.
This must have 0 meaning fully off, and 100 meaning fully on. If not, it is more likely to lead to accidental operations that cause danger. Especially during the 0-o’clock shift, the process control system is in complete chaos; asking him to turn everything off when it’s at level 0 and turn everything on when it’s at level 10 is simply ridiculous.
What kind of control valve? Yaskawa’s models come with a manual operator that allows the 4-20MA signal for the opening degree to be adjusted at the control valve itself
A valve opening at which a process parameter remains stable can be set on the DCS screen. Or use the manual valve in front of the automatic valve to adjust the valve opening so as to stabilize the process parameters