CONDUCTIVITY PROBE MELT DOWN

[DRUMLINE]

Hi guys,
I have been the level instrumentation industry for 25 years now and never seen anythin like this.

This is a stainless steel conductivity probe installed in a water column attached to a boiler drum. Obviously this is nothing to do with the water as its only the tip section that has melted. I can only think that there is a major issue with earthing and stray voltage somewhere. The current and voltage applied to the tip of the probe should only be 6 volts and microamps of current.

I have asked the guy on site to check everything for earthing issues and stray voltages. Whatever it is, it must be a huge issue to melt the metal like that.

 

[IRstuff]

That's not just "stray" voltage; it took some serious amperage to melt steel.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[Compositepro]

Could be caused by EDM (electrical discharge machining) due to constant static sparking from liquid surface to ground at the probe. Are there high velocity inlet pipes nearby?

If that is the problem, one solution would be to surround the probe tip with a pipe (guard ring) that does not interfere with the liquid level.

 

[DRUMLINE]

The probes are in a water column so no high velocity inlet pipes, columns are filled by condensing steam. The body of the probe is earthed to water column, the tip that is melted, has 6 volts ac with 60 microamps appied to it. The water bridges the gap between the tip and body to complete the circuit.

Really this should be destroying the electronic unit attached to the probe.

 

[itsmoked]

It was probing for conductivity and found it!

Is the melted tip isolated from the threads?
Or, are you saying that short gap is what the conductivity is measured across?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[danw2]

I've seen the trim in valves installed in blow-down lines erode due to cavitation because of the high pressure drop across the trim. Was this probe in a blow down line?

 

[DRUMLINE]

The tip of the probe is connected to a stud that runs through the body. The stud is connected to the electronic unit. Tip and stud are isolated from the body (which is earthed to the column and forms the earth for the electronic unit.

There appears to be no blow down as it is only the end of the probe that is trashed. It would seem that the electronic level indicator used is not a mainstream hydrastep / Aquarian etc. I am thinking that they have a DC voltage and very high amps rather than AC voltage in microamps. This is causing the tip to spark across to the body of the water column and potentially stroying both.

 

[IRstuff]

The lowest breakdown voltage for air is around 350V for a 0.1-in gap at about 0.2 Torr. Even still, if there were breakdown, it couldn't sustain the current required to melt steel.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[DRUMLINE]

To be honest I am at a loss. Other probes, the tips look completely rusted away, which indicates galvanic corrosion through a DC voltage applied to the probe. The whole melting thing is another story all together. I think we will have to see what they come up with on site.

 

[danw2]

Other probes, too?

Long shot, but years ago a vendor making carbon probes for heat treat furnaces used a carbon steel welding rod to weld the hastalloy or 440 SS (or whatever nickel alloyed SS the probe tube and end was at the time) and the welds failed in a fairly short time.

If these probes are the same brand, model and vintage are failing, could it be product materials issue ?

 

[DRUMLINE]

The probes are all the same, same material as thousands of other probes we manufacture. Everything is material certed so its not the problem.

 

[Latexman]

How about a pic of an identical probe that is undamaged?

Good Luck,
Latexman