Thermocouple Question

[Gage]

Wow this is an occasion...I'm asking for help lol, second time ever.

Just wondering if any of you have any advice as to which type of thermocouple to choose here.

Scenario:

I have several Gas-fired furnaces, all of which include both Natural Gas and Endogas, currently we have been using Type K with Inconel sheath. Now the problem isn't with the sheath, I believe it's the type, and the reason I'm asking before I test the theory is that is by no means an easy task removing these thermocouples and changing the instrument configurations to another type as a few of the machines are ran by old T/Cs / O/Ts that don't have modern thermocouple types configured.

So...What is actually happening here is that the thermocouples are breaking rather regularly, once a month or so, and when doing so cause catastrophic problems for the production line, obviously I can do a few shorts to override it and bring it off another input but when me or my partner are not around it's a nightmare. I've managed to link this to furnace breakdowns in general, such as roof fan distortion damage or brickwork collapsing, no matter the fault, it happens when the furnace is cooled to low temperature (20-30 Degrees Celsius) and then after repairs...heated back up to the general working temperatures of 750-1000 Degrees (C). Obviously the thermocouples do not like this cooling down and heating back up, it is not the sheath which is at fault, it is the actual metals at the junction inside which are breaking down. The furnaces generally stay at 750 Degrees - 900 Degrees (C) until the furnace breaks down...that's just how things are here, the thermocouples don't mind sitting at those temperatures for long periods of time, there have been periods with no breakdowns where thermocouples have lasted 3-4 months. Like I say, if a breakdown occurs and the furnace is cooled to ambient and then back up to working temperature after a day or so the thermocouples die.

Onto the question...has anyone had a similar issue to this? And if so which thermocouple did you try and did it work?
Now I was thinking of changing the majority over to type N as they have a little more tolerance for higher temperatures, although K are rated around the same, we have better luck with type N in vacuum furnaces, they breakdown maybe half yearly mostly yearly. Is this worth a try or am I going to get similar results? These thermocouples are around £60 a pop and there are quite a few to change. Obviously I will try it in one furnace for theoretical purposes. One issue I can't really get around is the fact that we have a mixture of old Honeywell instruments and more modern Eurotherm instruments in some furnaces, however some are completely Eurotherm, the issue being the Honeywell instruments are not new enough to have Type N thermocouple inputs.

Just thought I'd ask here out of curiosity.

Thanks.

Warning not proof-read and probably never will be.

 

[Marvo]

It's a bit of a puzzler. The K-type thermocouples we use are rated to 1400 celsius for short periods and 1000 celsius for constant use so I'm not sure why they'd fail. Even the cooling and reheating doesn't make sense because I assume that this would be over a long time frame being a furnace application.

Have you tried k-types from different manufacturers? Have you had the failed units post-mortemed by the manufacturers? How sure are you that it's a temperature related problem and not chemical or other issues?

It's about time you asked a question rather than just answering them by the way

 

[Gage]

It's a bit of a puzzler. The K-type thermocouples we use are rated to 1400 celsius for short periods and 1000 celsius for constant use so I'm not sure why they'd fail. Even the cooling and reheating doesn't make sense because I assume that this would be over a long time frame being a furnace application.Have you tried k-types from different manufacturers? Have you had the failed units post-mortemed by the manufacturers?

Yes, it's the junction simply breaking down to open circuit. And currently we get ours from TC Direct, we've used others from other suppliers and for customer service and quality they've done us well. And well yes, the furnaces are at 750-1000 Degrees (C) 24/7 until a breakdown which requires the machine to be cooled. They are very much abused lol. The reason I'm thinking type N is because in our Vacuum Furnaces these thermocouples (which have very basic sheaths) are constantly cycled between 500-1250 Degrees (C) and then quenched down to ambient via Nitrogen gas injection. And I've only had to change these thermocouples maybe a dozen times, and that's only because they don't meet Aerospace Standard 2750E after a test I do, even then, usually they are all within spec for the standard of Aerospace, and these thermocouples take some serious abuse and bending. With an inconel sheath they may just be invincible... lol. The environment inside these gas-fired furnaces is very rich in carbon and occasionally a lot of cracking ammonia, but the inconel protects the thermocouple from this like I say, it is the junction.

 

[Marvo]

I'm not familiar with TCDirect products but maybe try one or two K-Types from a good manufacturer. Can you replace just the thermocouple junction or must you replace the entire Iconel casing?

 

[Darkwood]

Rapid Nitrogen injection cooling .... that's one hell of a thermal shock and aint surprised you have issues... as this is an expensive problem is it an option to look at other methods of temp' sensing like remote sensing using laser measurement

 

[Marvo]

I'm assuming the Nitrogen cooling is only the vacuum furnaces not the gas-fired.

 

[Gage]

Rapid Nitrogen injection cooling .... that's one hell of a thermal shock and aint surprised you have issues... as this is an expensive problem is it an option to look at other methods of temp' sensing like remote sensing using laser measurement

I'm looking into that now however I'm doubtful because of the fittings needed and water/air cooling which will be necessary for them.

Also I think you've misunderstood lol, the problem isn't with the Vacuum Furnaces, I was praising the Thermocouples in them hence my interest in changing the Gas-fired furnaces to a similar type. Also it is not rapid nitrogen injection, it is a modulated injection of nitrogen GAS, which is then circulated via a cooling fan. Although the Vacuum furnaces do cool semi-fast, not rapid. And usually the quench is set at a dwell aswell, so it does not try to go from for example 1000 degrees to 30 as soon as possible, usually hold backs are also included etc. Don't have time to fully explain the system though lol

- - - Updated - - -

I'm assuming the Nitrogen cooling is only the vacuum furnaces not the gas-fired.

Yes, the Gas-fired furnaces have their own sealed quench chamber.

 

[Gage]

I'm not familiar with TCDirect products but maybe try one or two K-Types from a good manufacturer. Can you replace just the thermocouple junction or must you replace the entire Iconel casing?

Might have to give this a shot, but it's guna take a while to get our specifications built and set up a new supplier and we have a good relationship with TC in that they stock our exact custom spec thermocouples for when we need them, and quality is generally great, we get our type N's from them too. But I suppose I must explore all options. And yes the whole thing must be replaced as one.

 

[Darkwood]

I'm looking into that now however I'm doubtful because of the fittings needed and water/air cooling which will be necessary for them.

Also I think you've misunderstood lol, the problem isn't with the Vacuum Furnaces, I was praising the Thermocouples in them hence my interest in changing the Gas-fired furnaces to a similar type. Also it is not rapid nitrogen injection, it is a modulated injection of nitrogen GAS, which is then circulated via a cooling fan. Although the Vacuum furnaces do cool semi-fast, not rapid. And usually the quench is set at a dwell aswell, so it does not try to go from for example 1000 degrees to 30 as soon as possible, usually hold backs are also included etc. Don't have time to fully explain the system though lol.

Not really my field just throwing in a suggestion ill leave the furnace talk to the more familiarised of them and sit back and add to my ever expanding learning curve

 

[Knobhead]

Thermocouples were just regarded as plant consumables on our kilns and furnaces. Change it and be done.

In the combustion chambers two or three a week wouldn’t be unusual. The atmosphere in them was highly acidic as it was burning the volatile exhaust from iron production.

 

[Gage]

Thermocouples were just regarded as plant consumables on our kilns and furnaces. Change it and be done.

In the combustion chambers two or three a week wouldn’t be unusual. The atmosphere in them was highly acidic as it was burning the volatile exhaust from iron production.

Yeah basically are here too but like I say, it's the fact that when they go when I or my workmate aren't around it causes absolute havoc. By not around I mean either at the pub or asleep.

 

[Knobhead]

Simple answer.

Work shifts. At least you know when you'll be at work.

 

[malcoa]

Although this doesnt directly anwer your question, I used to have similar issues with both K type and like you i tried N type with a similar result, I put the thermocouples on plugs and trained the furnace operators to change them routinely (in your case perhaps on cooldown) and had it wrote into standard operating instructions. you just need to stock ready assembled units. I purchased all tc's (type K 6mm) from RS components and fitted carbon sheaths around them as they were in moulten aluminium. my headache disappeared...an option is just hang an N type at the side of the working K type and check it after a cooldown or two, that way you dont have to go to the expense of changing controllers for the purpose of trying somethig out...just an idea

 

[Knobhead]

Malcoa has it right with the plugs, sockets and training the production workers.

Be honest do you have to do anything other than reconnect the new unit? Check it’s working is about the sum total.

Didn't know there was more furnace men on here. Who's going to the brew house with the bucket?

 

[Gage]

Although this doesnt directly anwer your question, I used to have similar issues with both K type and like you i tried N type with a similar result, I put the thermocouples on plugs and trained the furnace operators to change them routinely (in your case perhaps on cooldown) and had it wrote into standard operating instructions. you just need to stock ready assembled units. I purchased all tc's (type K 6mm) from RS components and fitted carbon sheaths around them as they were in moulten aluminium. my headache disappeared...an option is just hang an N type at the side of the working K type and check it after a cooldown or two, that way you dont have to go to the expense of changing controllers for the purpose of trying somethig out...just an idea

The idea was I'd just choose one Thermocouple to change then if the theory proved correct I'd change the rest. But the idea of having ready made thermocouples inside of the furnace ready and just change over the plugs is not an option as they would also break. The thermocouples themselves simply need to be put in the furnace and plugs plugged in. However due to a few safety systems, while a job is in process if the temperature drops below 760 degrees (C) our furnace automatically shuts down and purges with nitrogen to push out all the endogas. As endogas does not light until 750 degrees (C) and this would inevitably make the furnace a bomb! My point being we have to short the T-couple to the O/T T-couple during the change and must do this EXTREMELY quickly before the furnace cuts out. The operators simply could not be trusted to do anything of the sort. Again, the sheaths are not the issue, inconel more than provides enough protection for the thermocouples, it's the junctions inside the sheaths actually breaking down. We also use type K 6mm.

Shift work isn't an option for me or my partner or the company. It just won't happen. I'm not so much bothered about the T-couples going when I'm not there, well I am, but it would be nice if they broke a little less often anyway! There's also the fact that while they're broken and not replaced, during that time the furnace is not being regulated as it should and work has occasionally been affected by this and needed a rework.

 

[Gage]

Malcoa has it right with the plugs, sockets and training the production workers.

Be honest do you have to do anything other than reconnect the new unit? Check it’s working is about the sum total.

Didn't know there was more furnace men on here. Who's going to the brew house with the bucket?

Never been near a kiln though.

 

[Richard Burns]

Is the problem with the sheath expanding and contracting against the thermocouple, because really even though type K may be a bit susceptible to this sort of thing I would have thought they would survive for a few months at least even if they are cooled and heated. Are the breakdowns when it is rapid cooling but shutdowns are slow controlled reductions?
Type N may help but I think there is something else odd here, though if bits of furnace are falling on them this could cause problems of a different nature!

 

[Gage]

Is the problem with the sheath expanding and contracting against the thermocouple, because really even though type K may be a bit susceptible to this sort of thing I would have thought they would survive for a few months at least even if they are cooled and heated. Are the breakdowns when it is rapid cooling but shutdowns are slow controlled reductions?Type N may help but I think there is something else odd here, though if bits of furnace are falling on them this could cause problems of a different nature!

There isn't a way to rapidly cool the furnace. It just has a fan blow through it and it takes about 24-30 hours to cool to ambient. And inconel is highly resistant to expansion/contracting/distortion. But my theory is that the actual junctions themselves do not like being cooled to ambient after so long at High temp. And when heated back up, start to crack.

The thermocouples are embedded in the wall of the furnace however not inside the hearth/chamber. Nothing ever falls off and hits them but the walls do expand and contract. However again...the sheath is perfectly fine when these thermocouples break.

 

[Marvo]

The more you say about them the more I'm convinced you should try another manufacturer. It does sound at least partially like a quality issue.

 

[Gage]

The more you say about them the more I'm convinced you should try another manufacturer. It does sound at least partially like a quality issue.

All their other types of thermocouples are perfectly fine, absolutely fantastic in fact. The price is right. The service is right. It would seem odd if every single one of these thermocouples they send us is dodgy. (I've received close to 30). And it's not as easy as just finding another supplier lol.