How to Troubleshoot Your Process Heating System
It is 3 a.m. and you, the head plant engineer, are soundly asleep in your warm bed. Then, the phone rings. It is your third-shift electrician on the line, telling you that your vital process heating system has just cratered, shutting down the main assembly line. What do you do?
A. Hang up the phone, roll over and go back to sleep. You'll be fired tomorrow anyway.
B. Put on your clothes, grab some java, and jump in your car to drive the 40 miles to the plant and try to save the day.
C. Call the guy who sold you the system so he can be awake at this hour, too.
D. Calmly troubleshoot the system with your electrician over the phone.
Options A and C might be tempting, at least at 3 a.m. And Option B might be what you would've had to do before reading this article. But with a little knowledge, you can choose Option D every time.
A process heating system has a beginning, middle and end. Start trouble-
shooting at the beginning and work your way through each component until you reach the end. Remember, rarely does just one problem bring a system down. Usually, a combination of things is to blame. A blown fuse can kill power to the heater, but a shorted heater can cause a blown fuse. If you find and fix one component, continue on until the end to be sure you find the true source of the failure. But, where do you start?
Get ControlBefore beginning any work, remind your electrician to follow all local plant and Occupational Safety and Health Administration (OSHA) safety rules while troubleshooting the system. There will be times when he or she may be working with energized circuits.
Once you have given all the appropriate safety warnings, tell your electrician to gather his tools. He will need a multimeter that works, a 500 or 1,000 VDC insulation-resistance test meter, a thermocouple test set, screwdrivers, an adjustable wrench, a 0.375" nut driver for the heater terminal pins and any required safety gear.
To troubleshoot the system, start with the main power feed and check to be sure that the breaker is on. This may sound foolish, but in many plants, more than one system is fed from the same breaker. Someone could have tagged out the breaker to work on something else, not realizing he is shutting down the entire plant.
Next, check the heater control panel. Is the main disconnect in the “on” position? Is the heater “power on” switch in the “on” position? Check the heater high-limit light to see if it is energized. If it is, the system was shut down due to an overtemperature condition. Inspect your valves to see if they are open and you have the proper flow of material through your heater. This is the most common cause for an overtemperature trip.
Check whether your process temperature control displays the proper setpoint and is functioning. (I once got a call at midnight from a plant that was down. I drove to the plant, and the temperature control was dead. Upon further inquiry, the electrician admitted to pulling the control out and playing with the switches inside to see what would happen.)
The next step is to turn off the control panel disconnect and shut off power inside the panel. Use your multimeter and check to be sure you have the correct voltage coming into the panel.
Next, check for loose wires or broken connectors. Take your meter and check your fuses. If one is blown, do not replace it -- just yet. There is a reason why the fuse blew. Use your meter and check for short circuits on both the line and load sides of the fuses. If there is no short, go ahead and replace the fuse.
Now, it is time to inspect your thermocouples. There should be two: one for the process control and one for the high limit. Connect each to your thermocouple test set and check the reading. If the process is still warm and you are reading ambient temperature, you have a secondary junction in your thermocouple and it will have to be replaced. If the reading goes to zero or shows a full scale error, you have an open thermocouple, which also must be replaced.
Next, use your multimeter to check for the proper phase-to-phase resistance at the point where the heater leads terminate in the control panel. If there are no blown fuses and all of the resistance readings are correct, odds are that the heater is fine.
It is time to check the power-switching devices. The easiest way to check the power circuits is to run the power output from the temperature control to 100 percent. You can do this by temporarily raising the control setpoint by 100oF (55oC). Once the temperature control has responded, you will hear the contactors pull in. If you have SCRs, check whether the light is cycling on the firing card. If not, the firing package should be replaced. Then, check the amp draw on each circuit. If you have a low amp reading but the correct resistance reading, then the SCR has blown open.
Don't Stop YetIf everything in the panel is functioning fine, it is time to move on. Follow the electrical run from the control panel to the process heater. Check to be sure there are no breaks in the line; in particular, pay close attention to the thermocouple runs. This may sound silly, but anything can happen.
Open the terminal housing cover of the heater. Check for loose wires and broken connectors. Look for rust and corrosion on the jumpers and connectors. Make sure the housing is free from moisture -- moisture means death to electric heaters. The refractory insulation absorbs the water and creates a short to ground.
Next, disconnect the wires to each heater circuit. (Helpful hint: Unless you have the heater wiring diagram in your hand, draw one before you start removing the wires. If you do not, it will be extremely difficult to wire back correctly.) Now, take out your resistance tester and check the resistance of each circuit to ground. If you are at 1 Mohm or less, you have a bad circuit. If you have a bad circuit, remove the jumpers and test each element. One or two of the elements may be bad, but you might be able to wire around these and get your process back up and running. If your heater shows an open circuit, it needs to be removed and examined for failure due to excessive heating, scaling or corrosion. If you do not find your problem until you reach this point, I hope you have a spare heater on the shelf because you are going to need it.
If to this point you find absolutely nothing wrong and you are still scratching your head, there is one more thing to check. Rewire the heater and energize it until it is up to operating temperature. Then, quickly turn it off and check the resistance of each circuit again. The weakest point on an electric heater is where the nickel-chromium heating wire attaches to the terminal pin. When that connection fails, it may check out fine when the heater is cold, but as the element heats up, the wire will pull away from the pin, creating an open circuit. When the heater cools, the element contracts, and wire makes contact with the pin again.
You now should have successfully identified the problem and fixed your process heating system. The line is running again. You are the hero, and it is time to go back to sleep. Good night!