Derek McFarland
Member
[font=verdana, arial, helvetica]I arrive at a site where I am starting up a used piece of equipment of which a component was a steam to water to product heat exchanger that was controlled by a dedicated PID controller. The site does not have steam, so they installed two on-demand hot water heaters as a heat source, removed the steam control valve and added a mixing valve. Thermocouples from the second water circuit and the product were connected to analog inputs of the PLC-5. The mixing valve accepts discrete open or close outputs and is actually plumbed as a diverting valve where heated water always enters the AB port and A port supplies the heat exchanger and B port recurculates back throught the on-demand heaters. The mixing valve takes approximately 40 seconds to switch from fully open to fully closed or vice-versa.
The customer asks whether I can have it operating by noon!
I devise a cascade PID control system where the product loop is the master and its output is the setpoint of the slave water loop. The heated water source temperature is not controlled by the PLC. The output of this PID system is then used in a Time Proportional fashion where if the PID output is 60%, the mixing valve will be opened for 6 seconds out of ten to allow heated water into the heat exchanger water loop. The time base has not been established, so the ten seconds is an example. I suggest that an analog valve be used so that finer control of the mixing valve can be achieved.
To get things up and running I suggest setting the heated water source close to the product setpoint and fully opening the mixing valve to see what happens. It turns out the mixing valve or plumbing is no good as the on-demand heaters would not operate because of no flow with the mixing valve in any position. The mixing valve is bypassed and the product heats up.
The customer now wants to discuss the control system and my suggestion of the analog mixing valve. They are convinced by the supplier of the valve that it will work perfectly in this application and don't recognize the advantage of an analog valve. They further think that simply turning the pump on and opening the valve when the product is cold and then tuning off the pump and closing the valve when the produt is hot will result in what they want. The product must stay within 4 degrees F of setpoint or bad things happen. After much discussion, I have swayed their opinion to the point that they recognize the need for closed-loop control, but they want me to "role-my-own" with timers that make a variable pulse based upon error and fire either open or close output dependant upon direction of error.
Now, what do you guys think? I think I approached this unexpected situation properly and proceeded in a standard fashion that woud result in the best control possible with what I had to work with.
I know I don't like working with plumbers! [/font]
The customer asks whether I can have it operating by noon!
I devise a cascade PID control system where the product loop is the master and its output is the setpoint of the slave water loop. The heated water source temperature is not controlled by the PLC. The output of this PID system is then used in a Time Proportional fashion where if the PID output is 60%, the mixing valve will be opened for 6 seconds out of ten to allow heated water into the heat exchanger water loop. The time base has not been established, so the ten seconds is an example. I suggest that an analog valve be used so that finer control of the mixing valve can be achieved.
To get things up and running I suggest setting the heated water source close to the product setpoint and fully opening the mixing valve to see what happens. It turns out the mixing valve or plumbing is no good as the on-demand heaters would not operate because of no flow with the mixing valve in any position. The mixing valve is bypassed and the product heats up.
The customer now wants to discuss the control system and my suggestion of the analog mixing valve. They are convinced by the supplier of the valve that it will work perfectly in this application and don't recognize the advantage of an analog valve. They further think that simply turning the pump on and opening the valve when the product is cold and then tuning off the pump and closing the valve when the produt is hot will result in what they want. The product must stay within 4 degrees F of setpoint or bad things happen. After much discussion, I have swayed their opinion to the point that they recognize the need for closed-loop control, but they want me to "role-my-own" with timers that make a variable pulse based upon error and fire either open or close output dependant upon direction of error.
Now, what do you guys think? I think I approached this unexpected situation properly and proceeded in a standard fashion that woud result in the best control possible with what I had to work with.
I know I don't like working with plumbers! [/font]