Instrument control priority issue
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How is priority indicated in control drawings? If a valve needs to be manually controlled in a DCS as well as subject to interlocking, how should this be represented in the control circuit diagram?
There are two types of manual control: one is a two-position on/off valve, and the other is a control valve with an adjustable opening range from 0 to 100%. When the control valve is equipped with interlocking, it can be manually opened or closed only after the interlocking is disabled. Once the chain is activated, it cannot be turned on or off manually. The control valve is equipped with interlocking functionality; whether it is in automatic regulation mode or in manual mode, once the interlocking conditions are met, a solenoid valve located behind the valve’s actuator will cut off the air supply to the control valve. At the same time, the instrument air inside the valve’s diaphragm or cylinder is released, allowing the springs inside those components to quickly close or open the valve. The interlocked switch is operated by a solenoid valve. The logic here is quite clear; there is no need to specify any priority levels on the DCS.
All of our valves here are electric; there are none pneumatic or hydraulic, so we really need to consider the issue of priorities.
If a quick switch is required, the design will naturally specify it. It should be taken into consideration when selecting a model. Electric valves are rarely found in ordinary chemical and refining plants, unless they are used as isolation valves at the inlets and outlets of certain imported units. I told those upstairs that there’s no need to indicate priorities.