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Exclusive Control Valve Sequence

Other applications for split-ranged control valves call for a form of valve sequencing where both valves are fully closed at a 50% controller output signal, with one valve opening fully as the controller output drives toward 100% and the other valve opening fully as the controller output goes to 0%. The nature of this valve sequencing is to have an “either-or” throttled path for process fluid. That is, either process fluid flows through one valve or through the other, but never through both at the same time.

A practical example of this form of split-ranging is reagent feed for a pH neutralization process, where the pH value of process liquid is brought closer to neutral by the addition of either acid or caustic reagent liquids:

Exclusive Valve Sequencing

Here, a pH analyzer monitors the pH value of the liquid solution and a single pH controller commands two reagent valves to open when needed. If the process pH begins to increase, the controller output signal increases as well (direct action) to open up the acid valve. The addition of acid to the mixture will have the effect of lowering the mixture’s pH value. Conversely, if the process pH begins to decrease, the controller output signal will decrease as well, closing the acid valve and then opening the caustic valve. The addition of caustic to the mixture will have the effect of raising the mixture’s pH value.

Both reagent control valves operate from the same 3 to 15 PSI pneumatic signal output by the I/P transducer (AY), but the two valves’ calibrated ranges are not the same. The Air-To-Open acid valve has an operating range of 9 to 15 PSI, while the Air-To-Close caustic valve has an operating range of 9 to 3 PSI. The following table shows the relationship between valve opening for each control valve and the controller’s output:

Valve Sequencing table

Again, we may express the two valves’ exclusive relationship in the form of a graph, with colored stripes representing valve opening:

Two Valves opening

Exclusive-sequenced control valves are used in applications where it would be undesirable to have both valves open simultaneously. In the example given of a pH neutralization process, the goal here is for the controller to add either acid reagent or caustic reagent to “push” the pH value either direction as needed. However, simultaneously adding both acid and caustic to the process would be wasteful, as one reagent would simply neutralize the other with no benefit to the process liquid itself.

Also Read : Types of Valve Actuators

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