Laying of High Temperature Thermocouple and Ignition Cables

[tahirinst]

Dear Electrical / Instrument Engineers,
We need to lay high temperature Ignition and thermocouple cables ( non armoured ) from flare ignition panel to flare stack ( approx 100 Meter distance )
Can we lay these cables under ground ( burried ) using 2 inch PVC conduit pipe ( separate conduit pipe for each cable). Please advise your expert opinion.
Data Sheet of thermocouple and Ignition cable is attached.

 

[danw2]

I have at least three experiences that I remember (over 30 years). The phone call sounds like this, "Your controller/indicator/analog input is crap. It reads a fixed 15 Deg C all the time, and it won't go any higher. Even when we swap it out it reads the same low value."

Upon investigation, it turns out that the thermocouple wire is in a buried conduit and the conduit is full of muddy water. The external insulating jacket was damaged and will leak water. The T/C wires have fiberglas insulation which is porous to water. The water seeps through and forms a false junction in the buried conduit so the analog input 'sees' a false junction at underground soil temperature or thereabouts depending on how well the conductivity of the electrolyte in the water makes the 'junction' along the thermocouple wires.

I'm sure that there are means of creating waterproof buried conduit, but it's outside my realm of experience. I can only report on the typical thermocouple failure mode when the conduit fills with dirty water.

 

[djs]

Actually the thermocouple wire in the conduit should be what is called thermocouple extension wire. This wire is commonly supplied with a PVC insulation which should be water tight.

 

[MikeHalloran]

"High Temperature" wire is typically insulated with fiberglass.
If you use a thermoplastic like PVC as insulation, it will keep out water only until it melts or burns.

From the perspectives mentioned here, it would make sense to use High Temperature wire, above ground, from the flare to some intermediate junction box, some distance from the flare, where the wire can be connected to thermoplastic insulated wire, and then run underground in a thermoplastic conduit.

OR, you purge the conduit continously forever with dry air or dry nitrogen at a pressure above the pressure in the surrounding dirt. ... with an alarm for low pressure, suggesting that the gas supply is off, or there is a big leak somewhere.





Mike Halloran
Pembroke Pines, FL, USA