Basics of Split-Range Control in Control Valve Applications ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Basics of Split-Range Control in Control Valve Applications

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In many process control applications in industry, it is sometimes desirable to have multiple control valves respond to the output of a single common controller. Control valves configured in this way to follow the command of a single controller are said to be split-ranged, or sequenced.

Split-ranged control valves may assume different forms of sequencing. Common modes of control valve sequencing seen in the process industry are:  complementary, exclusive, and progressive.

Complementary Split-Range Control
With this form of split-ranging, there is never a condition in the controller’s output range where both valves are fully open or fully shut. Rather, each valve complements the other’s position.  A typical example of complementary split-range control is a situation where two valves serve to proportion a mixture of two fluid streams, such as where base and pigment liquids are mixed together to form colored paint as shown below:
Example of complementary split-range control.


Both base and pigment valves operate from the same controller output signal. While the pigment valve is Air-To-Open, the base valve is Air-To-Close. The following table shows the relationship between valve opening for each control valve and the controller’s output:
Controller Output (%)
I/P Output (PSI)
Pigment Valve (Stem position)
Base Valve (Stem position)
0
3
Fully Closed
Fully Open
25
6
25% Open
75% Open
50
9
Half-Open
Half - Open
75
12
75% Open
25% Open
100
15
Fully Open
Fully Closed

Exclusive Split-Range Control
The nature of valve sequencing in this type of split-range control 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.

This type of 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%.

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

The basic operating principle of the above process is:
  1. A pH analyzer monitors the pH value of the mixture and a single pH controller commands two           reagent valves to open when needed.

  2. If the process pH begins to increase, the controller output signal increases as well (direct action)         to open up the acid valve.

  3. The addition of acid to the mixture will have the effect of lowering the mixture’s pH value.

  4. Conversely, if the process pH begins to decrease, the controller output signal will decrease as      well, closing the acid valve and opening the caustic valve.

  5. The addition of caustic to the mixture will have the effect of raising the mixture’s pH value.
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 of 9 to 3 PSI. The table below shows the relationship between valve opening for each control valve and the controller’s output:

Controller Output (%)
I/P Output (PSI)
Acid Valve (Stem position)
Caustic Valve (Stem position)
0
3
Fully Closed
Fully Open
25
6
Fully Closed
Half - Open
50
9
Fully Closed
Fully Closed
75
12
Half - Open
Fully Closed
100
15
Fully Open
Fully Closed

Progressive Split-Range Control
This form of split-range control for control valves is used to expand the operating range of flow control for some fluid beyond that which a single control valve could deliver. In this type of control, one of the valve usually a small valve opens gradually and becomes fully open at 50% of controller output while the large valve will remain shut at until the controller output goes beyond 50% when it starts opening. Both valves become fully open when controller output is 100%.

An example of progressive split-range control is a pH control process where the incoming liquid always has a high pH value, and must be neutralized with acid as shown below:
An example of progressive split-range control

The PH of the incoming water to be treated is measured by the analyzer, AT. As the output of controller AIC increases, the small acid valve starts to open and becomes fully open at 50% of controller output. Meanwhile the large acid valve will remain shut until controller output goes beyond 50%. At 100%, both small and large acid valves are fully open to ensure that the PH of the incoming water is neutralized.

Controller output and valve status for proper sequencing of the small and large acid control valves is shown below:

Controller Output (%)
I/P Output (PSI)
Small Acid Valve (Stem position)
Large Acid Valve (Stem position)
0
3
Fully Closed
Fully Closed
25
6
Half Open
Fully Closed
50
9
Fully Open
Fully Closed
75
12
Fully Open
Half Open
100
15
Fully Open
Fully Open