Underground Air Line

[kmdigital]

Don't use any kind of plastic or PVC pipe unless you are using an oilless air compressor. With a standard air compressor there is always a slight oil mist in the air stream, and it will cause the plastic to deteriorate over time.

Years ago a lot of people were using PVC as air lines due the fact that it is rated at 180 psi or so in 3/4" sizes. But after several years they found out that the stuff would blow apart, with potentially dangerous results, so much so that mfgs very specifically tell you NOT to use PVC pipe for airlines. Below ground a failure wouldn't be catastrophic, but fixing the leak would be a pain.

If your usage is no more then one or two tools at a time, an airhose inside a larger pipe sounds like a really good solution. If you have a large compressor and plan on running multiple tools you will need a larger pipe to carry enough air.

 

[mjncad]

I use 1/2" Type K soft copper line for an underground air line. Type K is the heavy wall stuff, and is meant to be buried. I wouldn't use threaded pipe because it is such a pain to install and leak-proof. Black pipe (natural gas) will rust quickly underground. Galvanized will rust eventually depending on soil conditions.

The other thing I do is that I make sure the pipe lays in a bed of pea gravel or squeegee. If you need to dig it up and fix it the gravel makes life easier; plus it has some give in shifting soils. Where I live we have mass quantities of bentonite, or claycrete as I call it. I nailed a PVC J-box with the brush cutter one time. Since the conduit was in a bed of pea gravel, the repair was done in about 1/2 an hour. If I had to dig in the claycrete, clean the conduit, etc I would have been out there for 2 or more hours cussing my head off.

This is what we did at the renewable energy lab for underground utilities. Electrical & telecomm was in SCH40 PVC conduit underground. If it crossed under a driveway, we switched to SCH80 for the short stretch. Water ran in PVC pipe. Natural gas or propane ran in plastic pipe rated for gas. Lawn irrigation used the black poly pipe.

Everything I was taught about air piping is don't use plastic unless it is specifically rated for air service regardless of the air quality.

 

[EddieWalker]

Since it's only 50 feet away, I'd just dig a shallow trench about a shovel blade deep. Lay the hose in the ground and bury it. I wouldn't put any money more money into it than the price of the hose. If in a decade, the hose breaks, just do it again. Anything you use, from PVC to copper and steel will break in time. I just replaced a copper line for a client that had a tree root tangle around it and caused it to break. Go cheap, keep it simple and be done with it in a few hours.

Eddie

 

[mjncad]

If in a decade, the hose breaks, just do it again. Go cheap, keep it simple and be done with it in a few hours.

Eddie

I wish I will be younger in a decade so doing it again won't be as painful.

Granted, any kind of piping material will break if enough force is subjected to it and tree roots qualify as a significant force; but I would rather try (keyword "try") to do the job once so I can do other things with my time.

 

[BobReeves]

I have buried 650 feet of grey plastic conduit for various runs to the shop and out buildings over the last two years. I vote for the air hose in conduit, easy to do with a rented trencher and easy to service..

If you forget a pull line just tie a rag to the end of your pull string and suck it through with a shop vac. Have a 200 foot run of 1 1/2 to the shop for video, alarm and phone lines, was able to get a pull string through even though it already had wires in it.

 

[LarryD]

kmdigital, I agree that the pvc will deteriorate over time but have to disagree that failure wouldn't be catastrophic, as shown in the attached photos.

This is the result of an 8" HDPE airline burst 8' under a 15" thick taxiway. While the line pressure was only about 120psi, I wouldn't even care to guess what the cf from the 8 storage tanks was. They are about 2 stories tall and used to airstart aircraft. I've even heard tell of another failure that launched a valve cover through an aircraft wing.

Did I mention the taxiway was 24 days old, three days use after the 21 day cure?

 

[mjncad]

LarryD:

Dang! That's some serious CFM's for shop air PSI. I'd hate to have been around when that puppy let go. An 8" air line?! That is a serious "huffer" to start up a plane.

The Mythbusters made a chicken cannon out of an air tank, some 8" or 10" steel pipe and a butterfly valve. Although they goofed when they were testing small aircraft windshields as their junk yard windshields were not bird strike rated; they still did serious damage with the pressure as low as 20 PSI. It got real messy (cool) when they ramped it up to about 100 PSI.

The Mythbusters just had a segment on tornado force winds blowing straw or reeds through a palm tree trunk. The highest recorded wind speed was about 318 MPH and they made a breach loading gun consisting of a 3" or 4" SCH40 steel pipe tank connected to an 80' barrel made of 1/2" copper plumbing pipe. They pressurized the tank to 80 PSI and their quick release valve launched the reeds at 320 MPH. They could only embed the reeds; but when they switched to piano wire, the wire went through the palm tree trunk, a piece of 3/4" plywood and embedded itself in the concrete block wall.

Yes boys and girls...compressed air is a wonderful thing; but it can also do some serious damage.

 

[CurlyDave]

My compressor is in my wood shop in the basement of my house. I would like to run an underground line about 50' to an outdoor woodshed. The idea is for conveince only. What type of material/line should I use?

I would look at this a totally different way. I would run electricity to the shed (which is useful for a lot more things than compressed air) and then spend ~$100, maybe even less) on a cheap portable air compressor which I would keep in the shed.

I bet this ends up being less expensive and much more useful...

 

[northof54]

I think Dave wins the prize- much simpler, more cost effective. Do you already have power run?

mike

 

[firemanpat2910]

I was going to reply last week that Ive never had problems with the pvc air lines we put in at my buddies place, Wouldnt you know it I heard a leak in the attic of the garage this weekend, and the compressor would come on every hour to refill good thing we put a shutoff right on the compressor. I also agree with Dave