Please your help with this question:
We wish to control the outlet temperature of a boiler. We will measure the outlet temperature, and send this signal to a controller. The controller gives a 4 - 20 mA output signal. This signal will be used to control a modulating valve, which will open/close, so as to increase / decrease gas that is being burned, to heat up the boiler. The greater the amount of gas, the greater the amount of heat generated. The smaller the amount of gas, the smaller the amount of heat. The valve is FAIL SAFE (i.e. a bigger signal will cause it to open, and a smaller signal will cause it to close. If it loses it’s signal, it will revert to the closed position). Please determine what the action of the controller should be (i.e. DIRECT or REVERSE / INDIRECT).
What you need is a reverse-acting controller. Such that when the temperature is rising (if for example the current temperature is around 50% of the transmitter range and thus equal to 12 mA), the controller should respond to close the control valve (as closing the valve requires 4mA as your fail-safe configuration). Conversely, when the temperature is falling, the valve controller should respond to open the valve (valve opens on 20mA signal).
There are 2 direction parameters for a control loop:
1- Control Action - Direct or Reverse
2- Output Action - Direct or Reverse
Since when PV is above SP, positive error, you want the loop to reduce fuel flow, the control action is reverse.
Since the control valve will open with increased signal and close with reduced signal, the output action is direct.
What was specifically being asked about is the action of the Temperature Controller, whether it should be direct or reverse-acting.
The straight answer is, it must be set to reverse-acting considering that the control valve closes at 4 mA and open at 20 mA.
A reverse-acting controller will have a decreasing output with an increasing input, while a direct-acting one will increase output with increasing input.
The control action has nothing to do with the action of the control valve.
If the process variable (PV) is below SP and increasing, or not, you still want to increase temperature, so with a negative error you require reverse control action. So not about increasing or decreasing PV, but about where is the PV in respect to the SP (setpoint).
Here is an exercise that I use to determine what the controller action needs to be that give a negative feedback loop.
1) What happens to the Process variable when the control valve opens?
If it increases mark it (+) if negative mark it (-).
2) What is the fail position of the control valve?
FC or Fail Close or Air to Open mark it (+) / FO or Fail Open or Air to Close mark it (-).
3)Controller Setting. Direct = (+) / Reverse = (-).
This mark should be determined by taking the previous two marks and use this one to make an odd number of (-) marks.
In this case, they would be 1)(+) 2)(+) 3)(-) or Reverse.