So, your control system is crippled. No problem -- you are the person who is ready today because you got ready yesterday. Here's what to put in place at the design and installation stage of your process.
Too late? You inherited it? Join the club. Even if you had been there, you probably would not have been on the procurement team.
Review Your Troubleshooting ToolsHave all of these items at the ready: Clipboard, multimeter, existing control and display instrumentation, digital thermometer and probe, clamp-on ammeter, flashlight, schematics and manuals, records and logs of normal behavior, and chart recorder.
Identify and label items such as control zone IDs, temperature controllers, sensors, plugs and sockets, heaters, heater contactors, SCRs, control valves, meters, indicators, control knobs, switches and pushbuttons, electrical disconnects, fuses and breakers (with ratings), indicator lights, terminals and blocks, supply voltages and wiring identification.
For controllers, list settings that can vary by batch or job such as temperature, speed and current.
Record adjustable controller parameters. These do not normally need resetting, except for major changes in process dynamics or when you replace a controller.
Note what to expect of any change or adjustment you make.
Know where to find the wiring and piping drawings, and keep them up-to-date after changes. Vet them and make payment to suppliers conditional upon acceptable documentation.
Know how to open enclosures without shutting down. If you cheat a door-latch isolator, be aware of live and dangerous parts inside. When checking, use insulated test probes.
Put a permanent sticker on the control cabinet showing names and phone numbers of contractors and suppliers who can help with parts or troubleshooting.
Identify spares. For removed parts, attach a "what's wrong" label, then repair promptly. Only good parts belong on the spares shelf.
Separate and label controllers that are appearance and plug-in compatible but differ functionally. Separate high-speed semiconductor fuses from regular HRC types.
The most important diagnostics aid is transparency of the process by means of indicators or screens showing what the process is doing, allied with experience and records of the normal behavior pattern.
For observing a control loop in action, make sure you can see the following data at all times: process temperature and setpoint temperature as well as the final controlled output. This could be heater current, valve position, pump speed, etc. Fit ammeters on every electrical heater; otherwise, check heater currents with a clamp-on ammeter. Lights showing the on/off state of contactors, solenoid valves or blowers are useful aids.
Normal Behavior of a PID Control LoopIf temperature is well below setpoint, the heat will be on steady at its maximum value. Cooling (if fitted) will be off. If temperature is well above setpoint, the heat will be off or at minimum value. If fitted, cooling will be full on. For a quick test, take the setpoint up and down through the indicated temperature and watch the output change.
If temperature is anywhere near setpoint, the output state is not totally predictable with PID controllers. A small overtemperature does not necessarily take all the heat off, nor does a small undertemperature necessarily bring heat on. That can be all right - the controller's PID action will slowly work it out and settle down, delivering just enough heat or cool to hold the correct temperature.
A look at potential problems will prepare you for the real thing.
Temperature indication stays abnormally low, but the process overheats. Check whether the sensor is pulled away from the heat or if stray wire strands are bridging the sensor's wiring. If the temperature indication goes below room temperature, check for crossed thermocouple wires. Be aware that the red is negative convention on North American thermocouple extension wires. Some controllers can be configured to sense an abnormally low or negative reading, cut the heat off and show an under-range message.
Zone temperature comes up normally at startup, then the process slowly gets too hot, yet the controller indication stays normal. Caused by the incorrect use of thermocouple wire, this error can vary during the day from not noticeable to some 100°F too hot.
In the next issue, I will look at problems peculiar to electric heating.