Steam heating PID systems

maintpro

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Join Date
May 2003
Location
Osceola, IN
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I have a wax melting process wich utilizes a steam heated system controlled by a TI 545 PLC. My steam valves operate at 0 - 20 psi.
I use a Cutler Hammer PanelMate 1500 to make adjustment to and monitor my kettle temperatures.
The system operates so that the steam valve for each kettle is open proportional to the wax temperature.
I know that when my kettle temperature is low (80 deg f.) and my setpoint is high (250 deg f.) that my valve is open 100% with 20 psi and the outputs are at 20ma. The valves will start to close as the temperature in the kettle climbs to 250 degrees and the PLC outputs decrease as well. If the kettle goes overtemp (by 20 deg f or more), the valves will go into a "lockdown mode" and the PLC outputs will be at 4ma and the air pressure on the steam valve will be at 0psi.

My question is,why is it that when the Kettle temp is the same as the preset temp, the PLC output will be at 7ma and the air pressure on the steam valve is at 9psi. Is this to maintain heating and help hold temperature or is the system malfuctioning. I have been having a lot of trouble with the kettles overheating and have yet to find an answer to the problem.
 
Re: Steam controller

Finally a PLC I deal with every day.
Do you know wether you are using a SFPGM (Special Function Program)
or a LOOP (PID) to control this output.
It sounds as though the output is working correctly since it is able to change the full 4-20mA signal, so it would appear something in the logic is telling it to hold at 7mA.
To confirm that you do have full control of that output, you can force a value (MAKE SURE IT IS SAFE TO DO SO) into that output. Change this value through the spectrum and make sure you get the correct mA reading.
If this all works correctly, you know it is not the card or the valve, and thus bust be in the program.
Find the logic that controls that output and if you need more help, re-post.

I hope this helps. I'm new to this too.
 
Even though your kettle has reached set point, it still requires some heat to maintain the current temperature. Insulation slows down the rate of heat loss, but it doesn't stop it.

If you are overshooting the setpoint, maybe someone has changed the PID loop tuning. I've never used the 545, but in the 555 & 565's this is easily done.

Have you had to reload your program from disk? It's possible someone tuned your loops and didn't make a back up. Then when the program was loaded from disk, it did not have the latest tuning parameters.
 
You seem to have a PID controlled system.
There will be an output at setpoint to maintain the system temp as indicated by previous posts.
One thing to check is that your temperature feedback is accurate, using a calibrated temp probe.
If your RTD or T/C is failing or the outside of the well is built up with material (wax) you may have a longer feedback delay than previously tuned for. This would cause overshooting of setpoint.

Brian
 
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