Why are some flow control loops FIC positive and others negative?
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Looking at the PIDs, in the flow control circuits, the control valves are all of the FC type. Some of the FIC valves are marked with a plus sign, while others have a minus sign – is there any difference between them? Why distinguish between positive and negative?
The operation mode of a valve also determines its function; for example, there are valves that operate when air is present, and those that shut down in case of an accident – these are referred to as FC valves. There are also valves that open in case of an accident, known as FO valves. The choice of such valves is primarily based on process safety considerations