Feedback issues with pneumatic control valves
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The equipment used before was of two-wire type; the workers described it as having feedback via those two wires, and its main function was to automatically control the on/off state of the valve based on flow rate. I provided a four-wire unit, but only two wires were connected on-site, so remote switching is not possible; I’m considering either buying a new unit or making adjustments on-site. There are two problems now. First, are there two-wire valve positioners with feedback? I asked several companies, and all of them offer four-wire models. Secondly, how can a four-wire system be adapted to a two-wire setup in the field to enable remote adjustment? I hope you can all offer me your guidance. Figure 1 shows the scene, and Figure 2 shows my product.
There are no 2-wire instruments with feedback; if feedback is required, it must be a 4-wire system. Because 2 are given and 2 are feedback. You just need two wires to create a control loop in the DCS.
Oh, it must be that the on-site engineer wasn’t clear; they kept telling me that the two wires needed to have feedback
Two-wire system is also acceptable, but the locator must either be based on a field bus or support the HART protocol; HART signal acquisition is used in the circuit to enable reading of the valve’s response information
The person upstairs said that HART can be used, after all HART reads data over two wires; however, a HART communication card has been installed at the control side
The two-wire feedback means that the feedback signal circuit requires power from the DCS; in other words, the feedback signal circuit consists of only two wires, with no separate power supply circuit. This is something different from the control signal circuit