Steam Tracing Sleeved HDPE Piping

[RJB32482]

Folks,

Our plant is looking for an option to put a 4" HDPE pipe in a 6" carbon steel line. The 6" carbon steel line is currently in service. They want to reutilize the 100 PSIG steam tracing on the outside of the pipe.

My main concern is the HDPE piping only has a rating of 140F and the 100 PSIG is way more than that (338F). In case there is stagnent water flow (4" HDPE piping is for water) the steel pipe can heat up to above 140F and melt the piping if any part of the HDPE pipe is touching the steel pipe wall.

Anyone have any thoughts on this or documentation/articles that can help? Or past experience with steam tracing steel pipes with HDPE pipes in them?

Thanks in advance!

 

[LittleInch]

"They want to reutilize the 100 PSIG steam tracing on the outside of the pipe." - Why??

If you're basically using the steel pipe as a duct, what is the point of the heat tracing?

To me it all depends on how long the system is stationary and can heat up the water in the HDEP pipe, but looks like a risk too far to me. I have a PE100 softening temperature at 125C, as opposed to the long term rated temperature which you have. It would still melt eventually but might not be so bad.

sounds a bit silly to me.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[bimr]

The manufacturer's maximum temperature for heat tracing of common HDPE pipe is 120 Deg F.

http://plasticpipe.org/pdf/tn-27-faq-hdpe-water-transmission.pdf

 

[RJB32482]

They do not want to remove the 6" steel line. They want to install a 4" HDPE pipe in the 6" steel pipe. It is all related to cost

The person recommending the job thinks the steam tracing (that has no temperature control) will never heat the pipe inside the steel pipe to 120 degrees F. This might be the case in flowing water, but stagnant water can heat up. I'm more worried about the steel pipe getting above 140F and any contact will the HDPE pipe will cause major issues.

From an engineering standpoint, it just looks like a bad idea since we can never say without certainity the HDPE pipe will not be exposed to temperatures above 120F.

The point of heat tracing is if the temperature outside is low and the water is stagnant, they dont want it to freeze.

 

[bimr]

One problem that you will experience is thermal expansion. In response to changing temperature, unrestrained polyethylene pipe will
undergo a length change. Anchored or end restrained pipe will develop longitudinal stresses instead of undergoing a change in length. This stress will be tensile during temperature decrease, or compressive during temperature increase. If the compressive stress level exceeds the column buckling resistance of the restrained length, then lateral buckling (or snaking) will occur. While thermal stresses are well tolerated by polyethylene pipe, anchored or restrained pipe may apply stress to restraining structures.
The resulting stress or thrust loads can be significant and the restraining structures must be designed to resist the anticipated loads.

The coefficient of thermal expansion for DriscoPlex® high density polyethylene pipe material is about 9.0 x 10-5 in/in/°F. This coefficient results in an approximate expansion for pipe of 1/10/100, that is, 1 in for each 10° F change for each 100 ft of pipe. This is a significant length change compared to other piping materials and should be taken into account in piping system design. A temperature rise results in a length increase while a temperature drop results in a length decrease.

http://www.performancepipe.com/en-us/Documents/PP814-TN Thermal Effects.PDF

You can get around the thermal issue if you use the flow tite pipe. This is basically a pipe lining process with the inside pipe OD being slightly larger than the ID of the exterior pipe. The result is a lined pipe and you will not have a problem with the temperature of lined pipe which can be steam traced.

http://www.insituform.com/CompanyIn...rate/Files/United/United Family Brochure.ashx

 

[mbt22]

Even with flowing water, the HDPE conductivity is an awful lot less than the steel pipe it's inside. Depending on the insulation, mightn't the steel pipe itself (now full of nice insulating static air) get hot enough to degrade the HDPE?

Matt

 

[LittleInch]

I get the fact that they don't want to remove the steel pipe and essentially use it a duct to hold a smaller PE line.

The rating at 120/140 F is for a 50 year life. PE can operate at higher temperatures and pressures, but will not last as long or could creep.

Your limit of 140F therefore might be seen as too conservative, but you're right - the whole thing is just a bad idea...

I would change the heat tracing to electric myself to reduce the temperature, but it all depends on the number of times you have stagnant flow, what happens if it leaks inside the steel pipe, how long it is, what is the impact of a leak, how long will it take to repair etc etc. Given the extra insulation in the pipe from air and the PE, could you not just leave it and then apply an occasional burst of steam when it gets cold AND the pipe is stagnant?



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MJCronin]

You might want to consider using a forced circulation water heating system instead of a steam system.

Hot water can be circulated through the steel pipe with the HDPE piping temperature closely controlled.

Steam can still be used to keep the system warm through use of a direct injection steam heater system.

http://www.pickheaters.com/

PROS: Close temperature control
No steam traps, no steam hammer, no loss of internal piping


CONS: A circulation pump and possibly an expansion tank may be also required.
Recirculation piping required




MJCronin
Sr. Process Engineer