Furnace Runaway - What fails in PID contollers?

Had what I consider to be a catastrophic failure with a furnace this week. Had a runaway condition and it ramped all the up. I did not notice it in time and lost a fairly important set of gear shafts we made for a customer. Nothing in the shop was damaged, but my pride was wounded and I now have to remake all the gear shafts.

I am just curious as to what causes them to do this. I have already ordered a new controller for it ( newer, too ), but I am still curious.

Open thermocouple. Welded relay contacts or shorted SSR could all cause your problem.............

Tim in D

Tim in D said:

Open thermocouple. Welded relay contacts or shorted SSR could all cause your problem.............

Tim in D

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And on rare cases the actual controller gone nuts. But the causes you mentioned probably cover more than 99% of the cases.

Tim in D said:

Open thermocouple. Welded relay contacts or shorted SSR could all cause your problem.............
Tim in D

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Hmmm... thanks. I checked the t-coupler and the mechanical relay. They seem fine. And there's just not a lot to these things. That's why I went ahead and just ordered a new controller.

MattiJ said:

And on rare cases the actual controller gone nuts. But the causes you mentioned probably cover more than 99% of the cases.

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I have a sneaky suspicion that I found one of those rare cases...

Thermocouples can be sneaky bastards on how they fail. Intermittent open circuit, internally shorted along the lenght, insulation leaking and whatnot.
(One of the most commonly misunderstood and mis-used sensor type in industry is a thermocouple.)

What thermocouple type your oven uses?
If its one of the base metal types (K/N/J) just replace it.

This topic is one I'm really interested in, as my oven's run hot too (burns the 321 stainless bag material). Please keep us updated on what you find, and also let us know where (and what part number) you got the new controller.

Does anyone think that a poorly sealed oven is part of the problem? I've wondered about excess oxygen getting into the hot zone and messing things up.

Let me guess, you didn't have an over temp controller. Look at Watlow's diagrams. In addition to the set point controller, they show an over high limit controller that would take the furnace off line through a mechanical contactor. Milland, you need to confirm your temperatures with an independent instrument. Also, if you have the proper thermocouple material, I use chromel-alumel, oxygen will have no effect.

Tom

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Milland said:

This topic is one I'm really interested in, as my oven's run hot too (burns the 321 stainless bag material). Please keep us updated on what you find, and also let us know where (and what part number) you got the new controller.

Does anyone think that a poorly sealed oven is part of the problem? I've wondered about excess oxygen getting into the hot zone and messing things up.

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Depends on your application. If you are hardening HSS at 2200F I would expect your bags to burn quite easily.

15-20usd for a thermocouple and another 20 usd for hand-held meter gives you independent means to check the oven temperature.

From the limited information provided, I'd guess that the thermocouple is shorted somewhere along the length that is cooler than that at the intended temperature measurement point, fooling the controller into thinking it must try harder.

Opens don't usually cause a problem because all controllers sense open thermocouples.

If the thermocouple is cheap, just replace it. If it's expensive, inspect it inch by inch. If you have the equipment to reweld the wires together (a TIG unit will do), cut the tip off and verify that there is no electrical conductivity between the wires. If shorted, find where and repair. Repeat as needed. When done, reweld the wires togerher at the tip.

Finding the short with tip removed: Use a heat gun walked along the wire. The temp will increase when one is heating the short. If there are two shorts, the pattern may be confusing. But if that's happening, time to replace the wire.

Joe Gwinn

MattiJ said:

Thermocouples can be sneaky bastards on how they fail.

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I completely agree, because that's what the PID uses to determine the actual condition, AFAIK.

I have written a PID using Embedded Linux, but used a chip which converted the analog signal to digital, but it was slightly different. The outcome was the same, it could monitor and maintain constant voltage depending on workload and current.

I don't fully understand how the PID is being used here in regard to the furnace, a water heater uses those crappy thermostats and they ALWAYS go out...wouldn't that copper element be the same for a furnace ? That is what I have on my house heater as well...replaced that a couple times...

I am leaning toward the K couple being okay. It was replaced in the recent past as a matter of general maintenance. I do happen to have another on hand ( bought a few back then ) and can replace it, but I really don't think that's the culprit. It measures the same as the others and exhibited normalcy up until now. I am betting that when I receive the new controller it will work fine. This controller has done some weird things in the past and I always chalked it up to me not paying attention or doing too much at one time and losing track of things.

MattiJ said:

And on rare cases the actual controller gone nuts. But the causes you mentioned probably cover more than 99% of the cases.

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One failure mode is human. Sure the setpoint, ramp rate, hi limit, etc were programmed in correctly. I mean no offense - but this is the failure mode I would probably experience. Sigh.

traditional-tools said:

I completely agree, because that's what the PID uses to determine the actual condition, AFAIK.

I have written a PID using Embedded Linux, but used a chip which converted the analog signal to digital, but it was slightly different. The outcome was the same, it could monitor and maintain constant voltage depending on workload and current.

I don't fully understand how the PID is being used here in regard to the furnace, a water heater uses those crappy thermostats and they ALWAYS go out...wouldn't that copper element be the same for a furnace ? That is what I have on my house heater as well...replaced that a couple times...

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Two different animals. The thermocouple in the gas furnace is used to generate a current to hold a valve open. Thermocouple goes out, the valve releases and shuts of the gas. The materials used in the water heater TC are completely different from the furnace TC wire.

Tom

bosleyjr said:

One failure mode is human. Sure the setpoint, ramp rate, hi limit, etc were programmed in correctly. I mean no offense - but this is the failure mode I would probably experience. Sigh.

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No offense taken at all. In fact, I am oft quick to blame myself. More often than not, actually. But in this case I am sure that it was not me. I verified the set point. 1650°. Furnace was at 2200° when I caught it.

Thanks for the reminder.. A few weeks ago, my furnace started acting goofy, displaying temps I knew were not right. I need to check things out...

Joe Gwinn said:

Opens don't usually cause a problem because all controllers sense open thermocouples.

Joe Gwinn

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I agree that open thermocouples are usually not a problem. But I have also seen (oven) temperature controller configured to go full power if the sensor fails
(that would be preferred mode of action on freezer)

Do you have a PID control but its contacts are controlling a mechanical relay in on/off mode? Is there any sort of transistor anywhere? If so, they would be likely to fail in a closed circuit.

MattiJ said:

I agree that open thermocouples are usually not a problem. But I have also seen (oven) temperature controller configured to go full power if the sensor fails
(that would be preferred mode of action on freezer)

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If that's how the user programmed it, then OK - one assumes that they had a reason. But certainly on modern furnace controllers, open thermocouple is ignored by default, rather than melting the furnace and burning the factory down.

Zahnrad Kopf said:

Nothing in the shop was damaged, but my pride was wounded

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Consider yourself lucky, I had a runaway a couple of years ago.

Don't be Stupid upgrade to STAND ALONE over-temp protection. Use multiple relays or SSRs in series as well, relays will weld shut. Also PM the relays, we just replace them every so often. A few hundred dollars in relays is usually less expensive than the contents of the furnace let alone the catastrophic possibilities.

And test the over temp every so often.

Zahnrad Kopf said:

No offense taken at all. In fact, I am oft quick to blame myself. More often than not, actually. But in this case I am sure that it was not me. I verified the set point. 1650°. Furnace was at 2200° when I caught it.

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Ok, there's another possibility. One poster above asked why a PID controller would be used in a furnace. Two reasons. The first is better regulation - making sure that the furnace temperature gets to and stays at the desired temperature. The second is better servo properties. That is, the controller allows the furnace to better follow a ramp up/down program. And the ramp program can be internal to the PID controller box, or can be received as s setupoint from another controller, or it could be you, just sitting there and increasing the temperature setpoint by a fixed number of degrees every 15 minutes.

To get better regulatory and servo control, the PID controller uses three modes. In fact, they're often called "three mode controllers". Each mode adds to the controller output. The first mode is called Proportional (that's the P). The control output is changed in proportion to the "error". Error is just the setpoint minus the actual temperature. The controller multiplies the error times the gain (alternately, 100% divided by proportional band) and adjusts the output. The gain can be adjusted ("tuned") to match your system to give good regulatory and servo control.

You often can get good control just using P control. But what can happen if things aren't set up properly, or if they are proper, but things change (e.g. you set up the controller in the summer in a 110° shop, and then try to run it in the winter in a 55°shop) and you can get "offset". That is, a stable difference between the actual and setpoint temperatures. You can manually compensate and account for the offset, but another approach is to use Integral (I) control. Integral means area under the curve. That is, the controller keeps a running tally of the graph of area versus time. That persistent offset is integrated and multiplied the Integral Gain (also called "Reset") and the product is added to the controller output. The Reset is also tuned.

For systems with "large mass", like a furnace (where there is a large thermal mass to heat), you can add Derivative (the D) control. This action is the slope of the error curve (kind of - its usually the slope of actual temperature) and multiplies it times the "Rate" or "PreAct" constant. This allows the controller to "pre act", that is, when the P and I control action adds a lot to the control output to warm up the furnace. When the temperature gets close, the D anticipates a large overshoot and adjusts the control to avoid it.

So much for the tutorial (I'm a control systems engineer). One thing that can get you into trouble are your tuning parameters. In almost any realistic situation, setting the P, I, and/or D tuning constants too high can lead you to instability and can result in just the type of overshoot you mentioned (you needn't ask me, but can probably guess how I know this). And tuning settings for a furnace can be especially tricky. How confident are you in those settings?