5 replies to this topic

Dear friends,

I would like to know about the action of control in operational parameter, when have to considered the reverse action & direct action.

[Zauberberg]

In most simple words, direct action means that increase in value of controlled variable causes control valve to open, i.e. increasing the percentage of controller OP. Reverse action does the way opposite.

For example, if you are controlling vessel pressure by evacuating part of the gas stream to downstream unit, increase in system pressure will cause PCV to open more and allow for higher gas flow out of the system.
Reverse action can be seen, for example, in systems where vessel level is controlled by manipulating the flowrate of inlet stream while excess feed flow is sent through the other line to, let's say, storage tank (usually controlled by separate PC control loop). Now, if upstream flow increases, level in the vessel will go up and LCV will close a little bit - in order to maintain the same level (set point). Here you can see how increase in measured variable (level) causes controller OP to decrease.

This is true for AFC/AO valve and opposite for AFO/AC valve.

Direct acting means increase in process variable (increase in devation from setpoint) will increase in controller output (Electric signal or Instrument air flow) and visa versa.
And opening and closing of valve will depend on valve type AFC or AFO. (AFO valve normally use for cooling water, antisurge valve etc.)

Reverse acting means decrease in process variable will increase controller output.

[fallah]

Here you can see how increase in measured variable (level) causes controller OP to decrease.

And vice versa?

[djack77494]

And vice versa?

Yes. Though the system has been adequately described in previous posts, I like to look at a system that has both direct and reverse acting valves for illustrative purposes. I'm going to introduce even more complexity by including split range, but I think it will serve to illustrate some key concepts in process control, and is therefore worth the extra complexity.

Tank blanketing systems are often designed with both types of control valves. Consider that as the tank's internal (vapor space) pressure increases above its setpoint, the blanketing controller's signal, assuming it is proportional to the tank's pressure, also increases. The direct acting vent valve will open, trying to lower the tank's pressure. Let's say this action is successful, and for various reasons the tank's pressure drops back to setpoint and then continues to drop even further (i.e. below setpoint). The controller's output has moved in conjunction with the pressure (downwards) causing the direct acting vent valve to be closing. But, we also have a reverse acting blanket gas valve. As the controller's output decreases, this valve will start to open, coming to its full open position at minimum controller output (corresponding to minimum tank pressure).

We would probably not want both valves open simultaneously since that would mean that our valuable blanket gas is being admitted to the tank and immediately vented. Instead, we would likely prefer that the blanket valve is closed at tank pressures above our setpoint, and that only the vent valve is active in controlling the tank's pressure. At pressures below the setpoint, the opposite is true. Here we'd use the blanket gas valve to control the tank's pressure while keeping the vent valve closed. This use of a single controller's output to operate two control valves in this fashion is refered to as split range control.
Regards,
Doug

[JoeWong]

Well... we have discussed this issue some time ago... we found that it can be done in many ways... the instrument & control engineer will choice the method as he/she wish... reverse acting FC versus direct acting on FO valve...

One way to minimize the potential of simultaneous closing/opening of control valves could be having gap set point, time gap for valve action, dampen the opening / closing of control valves, etc... subject to studies...