HDPE is subject to creep especially at temperatures greater than 70 degF if I recall correctly.  Have you contacted the manufacturer yet?

What is the compressed air used for? Just curious.

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I have not spoken to the manufacturer yet. That's my next step...

I do not see this pipe going over 80°F.
Thx for your reply!

This pipe will be used to connect a compressor to a bollard system. Basically muscle air to raise the bollards.

Neat. I always thought bollards were fixed to the ground. I didn't realise you raise/lower them.

Anyhow, back to HDPE. I don't know whether oxygen will attack it, although I suspect, given long enough, most material will oxidise.

I guess what I am saying too is contact the vendor.

In my industry, almost all of our air lines (instrument and utility) are steel. Carbon steel for most headers and distribution, and SS to the instruments. I don't know why, but that's what we do. Most of our IA system is 100 psi, maximum of 120 psi.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
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OK, thx for the input. I'm trying to aviod CS because of the coating and cathodic protection involved...

If I can't go with HDPE then I will go with SS.

...it requires a 2" line from compressor to bollards. I was suprised as to how well the bollards hold up against impacts!

I recently had a piece of HDPE fail and the shrapnel was really scary (the furthest piece was 60 yds away).  Failure of HDPE is always in brittle-fracture so it is pretty scary stuff in gas service.

Have you looked at the new spoolable composite piping?  This stuff has a layer of HDPE, a layer of kevlar, and a layer of carbon fiber.  Pressure rating is similar to steel, corrosion resistance and surface roughness are similar to SDR7 (if you disregard the weld seams in the SDR7).  It comes on spools in 1-inch through 4-inch.  I would have no qualms about using it in air service.  Take a look at http://www.futurepipe.com/usa/index.asp.   Future Pipe purchased Hydril's spoolable composite line and has some good products.  I've run them in natural gas and water service with good results.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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The harder I work, the luckier I seem

Has anyone looked into fiberglass piping? I would think 2" buried would be a good application.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
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was just curious, are you sure the temperature range of the air piping coming right off the compressors is not "going over 80°F" (for some reason this is surprising to me)?

The air coming out of the compressor often goes into a dry air receiver. The receiver may allow the temperature of the air to cool down to below 80°F.

Also, if the pipe is buried, chances are that the soil temperature will be lower than 80°F, and depending on thermal conductivity of the pipe wall and soil, may act to further cool the air?

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
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I've used FRP (fiber-reinforced plastic) for gas service for many years.  The stuff comes in 30 ft sticks that are screwed together.  It is fine for gas service up to whatever pressure it is rated for (you can get it in several classes).  The biggest downside to it is that it is not able to handle mechanical shocks.  More than a few times I've had to cut out a joint and add a repair joint simply because a rock fell off the spoil pile and bruised it.  Every bruise will eventually be a leak.

Below-ground installation is about twice the price of installation of steel.

David

If it's safeguarded above ground and the majority of it is buried, I'd have no problems whatsoever in using an appropriate schedule of HDPE pipe for 80 F compressed air at 170 psig.  Take care at the joined ends.

The continuous coil nature of the material is a definite benefit for a buried piping system.

Recent experience would suggest that you avoid FRP piping below 2".  Even at 2", all those buried joints would be worrisome.  Small-bore FRP piping and fittings are expensive because they're difficult to make properly.  We had a significant number of defective fittings supplied to us, and not by some fly-by-night supplier either: this was a major manufacturer of FRP piping components, and their fittings leaked through the body like a soaker hose.  

The low pressure buried natural gas line that runs in front of my house is heavy schedule HDPE pipe.  HDPE tubing is also used by the thousands of miles to connect instrument air users to supply headers etc.

Thanks for all of the input.

Like Ashereng said. The air coming out of the compressor goes through an aftercooler, a pulsation bottle, a dryer and a filter. All of this piping will be galv. CS thus helping dissipate the heat. When it goes underground the temperature of the ground will help cool the pipe and not let it get too hot.


After speaking to the vendor the following was noted:

Experience has shown that HDPE can be successfully used in air systems. There are factors that must be evaluated. These are:

-Air temperature- often several lengths of metal pipe are needed from the compressor to transfer heat away from the air. - CHECK!
-Filtration to remove lubricate. - CHECK!
-Limited life (a good life expectation is ten years.) -MMM???

I will be installing SS pipe. This section of pipe will be in a well traveled area and I cannot afford for this pipe to fail.

Thanks for all of your input. Hopefully this thread helps someone in the future...

There are other options also:

- Fusion-bonded epoxy external and internal coating on CS: shop-applied, bulletproof IF done properly and your contractor is skilled at installing it
- Plexcoat (HDPE extruded coating applied to CS) or its equivalent

Not sure I understand your cooling problems.  If your compressor and aftercooler are working properly, you should have near-ambient air leaving your dryer skid or at least air within 15°-20° of the ambient dry bulb temperature.

Galvanized CS is no long-term panacea.  We have gobs of those systems out here (central Cali) and plenty of them have internal corrosion after a few years.  It only takes one or two dryer malfunctions for water to get in there, and then it's just a matter of time before internal corrosion sets in...  If you are that critical, it might pay you to run SS the entire way.  Use sch10S buttwelded to keep the cost down.  If you have long runs, you can get DRL's from a special mill run and that will save a lot of welding $$$.

Thanks!
Pete

Guaranteed, the low pressure gas main in front of my house has a design life of greatly longer than 10 years.

Sounds like some butt-covering on the part of the pipe manufacturer to me.  But of course you can go with stainless, provided you have an open trench to lay it in.  The other benefit of the polyethylene material is that it can be installed with trenchless technology (hydraulic rams etc.)

What do you mean by a "hydraulic ram" to install polyethylene pipe?

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
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No cooling probs here. I don't see temperature as the driver to discard the HDPE pipe.

The fact that oxygen permiates into the wall of HDPE is the concern. Yellow HDPE pipe used for gas does not have a problem because there is no oxygen in Natural Gas.

I see what you are saying about internal corrosion. I had not thought about that. Either way, galv. CS or plain CS would still have to be coated and wrapped, as well as a cathodic system installed... in terms of cost and time, I believe stainless is a better option.

Trenchless technology is a neat idea. Although digging a trench is not a big deal in this project.

Stainless steel seemed like overkill at first.. but with the options I have, it's looking better and better.

FYI, at the bollards we are using Duratec pipe. It's an aluminum pipe with HDPE lining on the inside and the outside. So the Aluminum holds the pressure while HDPE protects against corrosion. Check it out:

http://www.ipexinc.com/Content/EN_CA/2_0_Products/2_1_Industrial/2_1_4_2_Duratec.asp


Once, again. Thanks for all of your input.

Ashereng:  I'm no expert at trenchless work by any stretch, but this is a common method used in laying underground utility lines in soils.  You basically dig a small trench at each end of the run, use a specially designed hydraulic ram rig to push a rod through the soil from trench to trench, attach the pipe coil to the other end, and pull the rod back out again.  That's how both the new gas main and the drops serving each house were installed in my neighbourhood without digging up the whole street and every little old lady's prized peonies etc.  They seemed to have no problem at all with the local dense clay till soil, even though it basically takes either machinery or a pickaxe and lots of cursing to dig anything below about a food in depth in my neighbourhood.

Anonymous:  In a buried pipeline I doubt there's much risk of HDPE degradation due to reaction with air.  If oxygen diffuses through the HDPE wall, so what?  You'll lose a trivial amount of oxygen from your air into the soil, and there's no UV light to do any embrittlement.  

There's been lots of crosslinked HDPE used in hydronic heating systems over the past fifty years as well as the buried low pressure NG lines I mentioned.  But again, I'm not in that biz, so what do I know?  SS is safe, so go for it.  It's only money...

Bare stainless steel may not prove so safe without CP. If chlorides concentrate through cyclic exposure to moisture and drying, pitting can be rapid. Replaced many a mile of buried 304 S.S. pipe at a well known California US Air Force base over 20 years ago. Had the same problem with 316 in natural gas service. Millions of dollars lost.   

you may wish to also see thread at http://www.eng-tips.com/viewthread.cfm?qid=142251

Interesting.

I am not familiar with that method.

We usually plow it in:

http://www.spiderplow.com/

Or we dig an open trench and spool it in:

http://www.fiberspar.com/Assets/PDF/FieldNotes7CDA.pdf


Much faster than the hydraulic ram method I think.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

Take a look at Asahi America, product called Air Pro.  It is a HDPE product specifically designed for compressed air. They have had considerable experience and success in the railrod industry with this product.  It may be worth giving them a call as I believe it would be considerably less costly than SS!  www.asahi-america.com

Thanks for the heads up stanweld. I did think about the chlorides in the ground but since we are using clean fill and the soil is not contaminated, I believe the SS is still the safe.. or safer.. way to go. ...and yeah, it's only money right?  :/

jdarco: I had not heard about this pipe. I will take a look. Thx for the link.

Check this out...

http://www.cpchem.com/hb/getdocanon.asp?doc=138&lib=CPC-Portal

here's another link to a similar pipe like AirPro by ASAHI.

http://www.vinkkunststoffen.nl/Default.aspx?ID=6556