Control Valve Feedback Operation

Dear Guest

your question indicates that you are really not experienced in DSC.

In Any DCS there is a graphix in the software....designer must have to design that graphix ( 0% close ...to 1005 open).

Trained Operator can easily understand about valve condition by seeing that tagged graphix.

Thanks for this suggestion but sir my aim is to how control valve sent this signal or whiich one electro-mechanisam using for given feedback signal to operator console indication that valve is @ 50% open ....till i khow this work is done by positioner's mechanisam that give feedback signal to operator console but i don't khown in deep about this procedure that which kind signal is going from valve & in which parameter it convert and how it reaches @ DCS console in 50% value indication , actually i m interested in how signal flow from control valve is released & reach on graphics of DCS ...?

Thanks in advance

There are two signals, then. One of them drives the valve to its desired position, which is an output. The other sends the valve's actual position back to the DCS, which is an input.

Or, if there is only one signal, the output signal is displayed on the screen, instead.

The key thing is that it is not usually the valve position that is important. The important thing is the process parameter that the valve is associated with. The valve is only one element in a control loop that has a number of other elements in it. A typical feedback control loop consists of:

• some form of measuring instrument for the process parameter
• some form of set-point setting device
• a comparator
• a 3-term controller, either physical or virtual
• a modulating device (in this case a valve)
• the process itself
• varying external influences on the process

Focussing solely on the valve is to ignore the remaining elements and their contribution to the whole. Valve position feedback will confirm that the valve has travelled to the chosen position. It will not in itself cause the process parameter to approach, or stay at, its set point, which is what usually matters.

Assume the valve has a positioner that can supply a feedback variable representing the valve's true position to the DCS.

The DCS has its I/O, inputs and outputs.

The I/O can be analog 4-20mA (one variable per pair of signal wires; ignore HART for a moment) or a digital fieldbus like Foundation Fieldbus or Profibus (multiple variables per pair of signal wires).

The DCS sends the output signal to the valve positioner and the DCS reads the valve position from positioner.

If the I/O is analog, one pair of wires is needed for each variable (HART can send multiple variables as digital 'on-top-of' its analog 4-20mA, but it's debateable whether it qualifies as a Fieldbus).

If the I/O runs on a digital protocol, then one pair of wires is used to read or write as many variables as needed the fielddevice, or to multiple field devices on the same comm link.

Please, first read mechanical/electical conversion, Pneumatic to electrical converter...then you will come to know the operation of valves etc.