Can Electrically Heat Traced Tubing Be Run Inside An RGS Conduit?

[SamuelAaronWard]

Hello! Long time listener, first time caller... I have a client who wants to run electrically heat traced tubing inside a 4" RGS (rigid galvanized steel) conduit underground. I've never done this before, and I've never heard of it being done before. I have concerns - mainly heat dissipation:

1) I understand that the point of electrically heat traced tubing is to use resistive heating as a heat source - and that it [the EHT's insulation] should be able to withstand a certain degree of heat, but I'm concerned about it overheating because it is inside conduit with nowhere for the heat to go - except dissipation through the conduit itself.
2) I'm concerned that with the conduit being buried, that will make it even more difficult to dissipate heat.

I read through NEC Article 427: Fixed Electric Heating Equipment for Pipelines and Vessels, and I didn't see anything relating to running electric heat trace (or electrically heat traced tubing) in conduit.

A little background info... I work for a company that designs metering and regulation skids for the natural gas industry. These skids are typically 50 to 100 feet from an RTU (remote terminal unit - aka PLC, SCADA, etc) / EGM (Electronic Gas Monitoring) building where the GC (gas chromatograph) is located. Samples of the gas from the pipelines are taken with a Genie probe inside of a Pony Box. From the Pony Box to the Mustang P53 panel and from there to the GC, we run electrically heat traced tubing to keep the gas warm - so that the gas doesn't condense and drop out as liquid on its way to the GC. The EHT-tubing is buried between the skid and the RTU building. I've never given that part much thought until today when a client asked if they could run TWO EHT-tubings together in a single 4" conduit. I told the client that I wasn't even sure that, by code, you can run any EHT inside a conduit - much less two! ...So that brings me to running two EHT together in one conduit. Is that allowed? My understanding of electric heat trace (EHT) is that you're not supposed to run multiple EHT cables together / on top of each other - I suppose because they'll melt each other (?) from the combined heat. With regular EHT, if you have to cross them, you cross them at right angles - again, I suppose to 1) cancel magnetic fields / induced currents and 2) minimize contact.

Any ideas on this matter?

Thanks!
Aaron Ward

 

[tom baker]

What is the EHT? Does it use self regulated heat tape?
And your comment “by code”, it’s likely the NEC does not apply as this is for a utility.

 

[SamuelAaronWard]

We use a stainless steel tubing that comes factory heat traced and insulated. I don’t know the watts per foot or anything else about it. I’m assuming that it is self-regulated. We power it with 120VAC and only use about 20 feet of it on our skids. The clients run the EHT tubing from their RTU buildings and power that section of tubing from the RTU building. I’ve never been involved in the installation of that underground run, so I don’t know the details. I’m just wondering if anyone knows what is permissible / recommended practice for in regard to EHT or EHT-tubing In rigid conduit (or PVC).

Regarding “code”… I knew that electric power utility companies aren’t required to follow the NEC, but I wasn’t aware that gas utilities weren’t bound by code requirements. We do follow AGA XL1001 and API RP 500 for haz loc, and I do my best to follow the NEC.

 

[tom baker]

The conduit you run the EHT is a sleeve, you could use plumbing pipe or drain fine. Electrical conduit is used as it has sweep elbows.
I don’t know if the NEC applies to your installation, I don’t recall ever seeing that question here before. Does anyone ever pull an electrical permit?
Interesting application, I had similar for level sending from water reservoirs to pressure transmitters

 

[SamuelAaronWard]

Yeah, I think you might be right about it being a sleeve with sweeping radii elbows. I need to ask some of the construction folks how it’s actually installed.

That said, I’m still concerned about heat dissipation - and if it could be an issue. And particularly, can TWO runs of EHT-tubing be run in a single conduit? I see a couple different potential issues there. Who knows - maybe it’s common practice, and I’m overthinking it!

 

[winnie]

This is an engineering issue, not a code issue.

1) Contact the manufacturer of the heat trace tubing for their recommendations if the tubing is sleeved underground rather than direct buried. There might be no problem with heat dissipation, but instead restrictions on bends or distance to limit pulling tension.

2) Regarding 2 heat traced tubes in a single sleeve, my first thought is to ask the manufacturer if they provide assemblies of multiple tubes with a single heat cable.

3) The section of the electrical code about calculating the ampacity of underground duct bank installations can provide some guidance for things such as the insulation provided by soil. This doesn't apply to your installation as a code, it is just an analogous situation which might help you calculate operating temperature of your sleeved heat trace tubing.

4) Self regulating cable is not strictly temperature controlled. It is basically proportional control with no error integration. If it is very well insulated than the temperature will be higher than target.

Jonathan

 

[SamuelAaronWard]

Great feedback, Jonathan! Thank you! …Yes, after reading the feedback from you and Tom, I think that you guys are right - an engineering issue, not a code issue.

 

[nhee2]

What is the EHT? Does it use self regulated heat tape?
And your comment “by code”, it’s likely the NEC does not apply as this is for a utility.

The NEC generally always applies to natural gas utility reg/metering stations or other facilities, if not by 'law', at least by project spec (in my experience).

However I agree this particular question doesn't really sound like an NEC issue.

 

[SamuelAaronWard]

Yes, my company follows NEC, and customer / client specifications and standards do too.

I’ll follow up with the manufacturer and construction. I’ll update this thread if / when I have more information.

 

[kingpb]

Does not sound like a very efficient way to do this since you will be heating the ground. Why not end the heat trace and just run the power to the next location? That is typically done.