Liquid Hydrocarbon Permeation and HDPE Pipe

[gazepdapi]

Hi guys

We're having some issues with the inability to fuse HDPE pipe which has been attributed to the natural gas quality, specifically hydrocarbon liquids. These is indication of bubbling of the plastic after preheat making it difficult to get a high quality fusion. I've done some research and can see that it is indeed the hydrocarbon that is causing this issue (

http://www.easternct.edu/sustainenergy/EnergySeminars/documents/CPchemFusionManual.pdf)

I have a few question.

1) Is the permeability of the hydrocarbon dependent on the specific components of the NGL? For example does it matter if it's C3, C4, C5, C6, etc?

2) Other than using mechanical connection methods to join the piping, is there any other way to make the connection? Other than also reducing the HC inside the piping. Maybe some higher grade HDPE?

Any help is appreciated.

 

[BigInch]

It is probably dependent on the molecular weight of the HC; most dispersion transport mechinisms are, but whether that is considered in practice, or if indeed it effectively makes any difference, probably doesn't matter. If it does affect, depth of penetration is secondary, as it can potentially damage the pipe. Even a little penetration is not desirable, since it is the skin of the pipe wall that is most affected during pressure reductions. And it is not only a problem at connections, but can occur all along the pipe wall as well.

you must get smarter than the software you're using.

 

[gazepdapi]

Any response to the second portion? This is my main concern as we have a few places where we are having this issue and I don't want to go down the road of mechanical connection if I don't have to.

 

[Compositepro]

If the plastic has absorbed volatile material then the volatiles will increase in vapor pressure and create bubbles during welding. The only ways to address that is dry the pipe prior to welding or to weld under pressure. Welding under pressure would require the development of new welding processes and equipment. I doubt that you wish to undertake that. Perhaps the equipment has already been developed, as you cannot be the first to want to weld plastic pipe that has already been in service.
Drying the pipe is a long and slow process involving heating the pipe to below melting for an extended time. The process would also need to be validated. The only source of this information that I can imagine is the pipe manufacturer.

 

[rconner]

Some authorities have for many years been warning of various effects of "hydrocarbons" on plastic pipelines. I think the report nine years ago entitled, "White Paper on Liquid Hydrocarbon Drop Out in Natural Gas Infrastructure" and now ensconced e.g. at

http://www.beg.utexas.edu/energyecon/lng/documents/NGC_HDP_Paper.pdf

does a pretty good job talking about natural gas liquids (NGL's) and history concerning same, and I noticed a passage on page 7 of this paper addresses this specific issue to some extent:

"1.4.10 Additional reliability and safety concerns for LDCs and end users due to natural gas liquids include the impact to polyethylene (PE) plastic piping, plastic piping components and current handling / pipe joining methodologies. According to APGA, approximately fifty percent of the typical LDCs distribution system is now comprised of plastic pipe and approximately ninety percent of new pipe installed is now plastic (2003 OPS Annual Report). Hydrocarbon gas constituents that are normally present within historical acceptable levels will have a minimal effect on the long-term strength of the plastic. However, it has been shown that aliphatic gaseous fuels of higher molecular weights (“heavy hydrocarbons”) tend to be absorbed to a small extent by PE. This absorption somewhat reduces the long-term strength of PE pipe materials. Further, if the (NGLs) are routinely present, these liquids can cause a greater reduction in long-term strength up to 40% 7. In addition, it has been reported that during the heat fusion joining of PE piping that has been in service conveying fuel gases that consist of, or include heavier hydrocarbons, the PE surfaces being heated in preparation for fusion on occasion will exhibit a “bubbly” appearance. The bubbling is a result of the rapid expansion (by heat) and passage of absorbed heavier hydrocarbon gases through the molten material, which could compromise the fusion joint if not properly recognized 8 9 10."

I know that at least many years ago it was common practice in natural gas systems to try to remove as many of these liquids as possible, e.g. by way of "drips" or drip pots, though this report indicates this practice has apparently been discouraged in many areas since the 1990's.

It should also be understood other authorities have explained for many years that "hydrocarbons" can potentially be near everywhere else in the external e.g. soil and construction environments( and be absorbed into pipes from these sources as well), as e.g. I think explained quite well at

http://www.google.com/url?sa=t&rct=...uoDIBA&usg=AFQjCNGMl9TypxLJbSfGRYoC3Ou7wDSuOg

, with incumbent effects on plastic piping and liabilities etc.

All this considered and given this body of knowledge, you might want to get the promoters/purveyors of the plastic piping on record with regard to aspects of these issues, including the following:

1. How can I recognize when the pipe has absorbed an unsuitable level of undesirable or prohibited contamination from whatever source, BEFORE I produce any defective fusion joints (or lateral connections, etc.?
2. If I do not see alarming external "bubbling" on the weld bead, how do I know/be assured that there are no unseen or unknown defects within the weld or fused area caused by same that I cannot see?
3. In the event that the extent of the contamination of the pipe is undetermined or the pipe cannot be replaced before being put back in service (e.g. in emergency circumstances etc.), what EXACTLY does the pipe manufacturer mean by alternative "suitable mechanical connection methods" (particularly given the sort of unGodly complexity of accomplishing correct fusions in all manner of field and weather circumstances, even without any contamination, represented by all the caveats etc. in the 30-some page document you linked to in the OP!)