Temp. Transmitter Head Residue

[macmet]

Hello All,

I'm not 100% sure this is the right forum to post this, but it seems like the best place to start.

I'm on-site commissioning a boiler and I have a number of thermocouples/transmitters throughout the unit and piping. Two of these are installed on the bottom of two different outlet pipes - so that the head is directly below the probe. These two pipe circuits are completely independent.

So far we've only commissioned one of these circuits in the boiler, and only one of these TT's is in service. This TT is providing curious data, so today I went to check the wiring in the head and it had quite a bit of white residue build-up or "gunk" inside it. This residue is like a grainy, white paste. It is on the cover, and the wires not being used. I've never seen this before, so, I checked a few other TT's on the same circuit nearby and there was no evidence in any of them of any of this white residue. They were all clean.

Then I decided to check the other TT that is in the bottom of the other circuit, and it had the same build-up. This time some water even dropped out. IT was a small amount, probably just enough to fill the cover, but it definitely caught me by surprise.

Can anyone tell me what this is? I have never seen this build-up up before and I asked the (new) controls tech on-site and he has never seen it before either. The build-up is not on the transmitter part itself.

Thanks in advance for any feedback.

 

[danw2]

Could the white stuff be a thermal conductivity paste, which ranges in color from white to clear or in-between, which is used to fill any gap between the tip of the T/C sheath and the end of the bore in the thermowell in order to maximize heat transfer from thermowell to thermocouple?

If the insertion is "upside down", then gravity will pull any residual thermal paste down along the thermocouple sheath and it could accumulating in the head?

The water is a real concern.

There's always the possibility of condensed humidity, but I would think that the ambient temperatures in a boiler room are not likely to drop below ambient dewpoint, but could that be the case somehow? Some source of cold air hitting the head and condensing air humidity?

The other option is a leak. I can't ever recall a thermowell leaking internally, but on occasion a threaded thermowell doesn't get tightened and will leak at the threads, and this is a commissioning, could that might be the case? Being upside-down, the cover/cap on the T/C head might catch any leakage depending on the style of the head and let it collect.

And I have seen installations where the T/C was held into a pipe by a threaded compression fitting which could leak.

 

[macmet]

I'm not sure about any thermal paste being applied, but that would definitely make sense. No one here was on-site when they were installed so I'm having a hard time finding out if anything was applied. That would be completely logical.

In terms of water, I'm not really sure where it came from. I think if it was a leak through the TW there would have been more of it. This seemed like the cap had somehow just filled up and it dumped out when it was inverted.

This unit is outside and it is currently extremely hot outside, but when the units were wired it was cooler.

 

[lacajun]

The head should be sealed from the thermowell. I can't think of a reason for conductivity paste to be used in the head. Sounds like something went wrong in the installation. Since it's outside, the head should be rated for it, e.g., NEMA 4 or 4X. I never specified a material be put in the head for any reason.

Pamela K. Quillin, P.E.
Quillin Engineering, LLC

 

[danw2]

Yes, when the threaded bushing in the head threads into the well, that combo is "tight".  

But spring loaded assemblies are loosey-goosey because the element sheath has be able to be moved by the spring.

 The space at the top of the well and bottom of head (upright orientation) is common to both components.  The head would only be sealed from the well if the element sheath were brazed to the bushing.  Thermal goop could easily move from the well to head in a spring loaded assembly.

 

[lacajun]

I missed the OP calling it spring-loaded. My fault for not reading closer.

Pamela K. Quillin, P.E.
Quillin Engineering, LLC

 

[danw2]

@ Pamela
Nothing wrong with your reading; spring-loading wasn't mentioned.

I'm just assuming spring loaded because that means the well is 'open' to the head and spring loaded assemblies are commonly used with thermowells.

 

[macmet]

Hi All,

It is indeed spring loaded. I have some pictures too which I will upload tomorrow. The TT is working alright, but I'm still curiuos to what gunk is.

Thanks for all the feedback so far.

 

[macmet]

Hi All,

Please see attached photos of the heads in question. Like I said, they both work, but I'm curious about what this stuff is.

Cheers

 

[macmet]

Looks like only one of the photos uploaded.

Please see the other photo in this posting.

I think they're pretty self explanatory, but let me know if you have any questions.

 

[danw2]

It sure looks like thermal paste. They're typically an oil base with metal oxides, which is apparent when the can has been on the shelf too long and the mixture separates, like oil based paint does over time.

Note what Omega says about their thermal paste:

in their spec sheet here:

http://www.omega.com/manuals/manualpdf/m0066.pdf

It appears that the installer came from the school of "if a little bit is good, lots is better".

Although The spec sheet omits mention of vertical 'upright', I suspect that's what they mean and they're aware of the effects of process heat and gravity on the paste and where it ends up in horizontal or upside down vertical installations. I think the stuff is thermal goop and just requires paper towel clean-up.

Could the "water" you mentioned be the oil base from the thermal compound? I doubt that there are water based compounds given the use of thermowells at elevated temperatures.

If it was real water, that's still an issue.

 

[macmet]

Ya, I guess it could have been. It felt like water to me thought when it was all over my arm.

I checked the one that dumped the water today after running yesterday and there was no water in it. I don't think it could be from the steam itself. Must have got in there during installation, whether it was water, or the oil base of thermal compound.

 

[ScottyUK]

It doesn't look like the heatsink compound we use in power electronics - more like a light silicone grease. Heatsink compound is finer in texture, brilliant white, totally opaque and fairly viscous. It also has the ability spread like plague - a milligram could cover an entire workbench in white gloop. Could an instrument tech have smeared silicone grease on the thread of the t/c head to prevent the cover seizing on? Some silicone greases seem to separate into a thin fluid and something translucent and slightly lumpy. An over-zealous application would result in your mess.

 

[macmet]

I feel like it probably wasn't anything that was put on the threads. I did not see any indication of the residue on any of the other thermocouples I looked at. I feel like it has to have something to do with the "upside down", vertical, position.

 

[hacksaw]

seal pour fittings and drains are commonly used in the conduit where high humidity is encountered, give that a try

beyond that the material you describe is similar to the soap used to pull wire and cables in long runs