In many process control application where it is a desire to have multiple control valve, respond to the output of the controller.
When two different control valve is used to command from the same controller is called a split range type control valve.
In many application where the different product needs to be added and then mix up them to get desire consistency.
There are three types of split range mode available. Which one to choose depends on an application requirement.
1. complementary mode
2. exclusive mode
3. progressive mode.
Let understand each in detail to clear all doubt which pops out from your mind.
(1) Complementary mode.
- In an application where two-valve used to serve mixture like base and pigment mix up to form paint.
- Both base and pigment valve operates on the same 3 to 15 psi control signal. Here is a condition is that on the valve is air to open and another valve is air to close.
- In the following graph, you can see how both valves operate from a single control signal.
- Here, when a control signal of 3 psi is applied to both valve Base valve remain fully open and same with pigment valve which will stay fully close.
- At 6 psi, the base valve will stay 75% open and pigment valve will open 25%.
- At 9 psi both valves will stay 50% open.
- At 12 psi, the base valve will stay 25% open and pigment valve will open 75%.
- At 15 psi, the base valve will reach to fully close and pigment valve will reach the fully open position.
(2) Exclusive mode.
- In exclusive mode when in an application where valve need to operate such a way that when control signal between 3 psi to 9 psi is applied, a valve will operate from fully open to fully close (100% to 0%) and for another valve control signal is from 9 psi to 15 psi will operate valve from close to open (0% to 100%).
- So, both valve will remain close at 50% (9 psi).
- For example in food processing industry after production cleaning need to be done. Acid and Caustic is used to clean the machines and pipelines. Now there is a PH transmitter is installed to maintain the PH of both caustic and acid, in order to any changes in PH level will give a command to a valve which is put in parallel operate simultaneously based on programming.
- In such an application you can use exclusive mode type split ranging.
- Have a look into the below graph to understand it.
- Here, you can see that when cleaning using acid is in the process the acid valve is fully closed and while caustic cleaning acid calve will remain closed. In such an application you can use exclusive type split ranging.
(3) Progressive mode.
- In an application, we are using only acid or caustic for cleaning purpose. Here we have to know the value of PH of the acid.
- To control PH for the larger and small value we use two-valves one is for when PH is higher and another is for a small value of PH. When PH goes larger or smaller both valve as per their use keep PH at its set point which you can see in the below graph.
- It can handle larger varying condition due to two-valve application.