Compressed air delivery system
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Appropriate piping for shop air distribution lines. January 15, 2003
Question
What is the best type of pipe to use for a compressed air distribution system - copper, galvanized, PVC, etc? Our shop has grown and is now in need of upgrading from the "old rubber hoses on the floor" method.
Forum Responses
From contributor H:
I believe that most shops use black pipe but I would sure try to buy it from a North American producer. Some of the stuff that I have seen from offshore looks scary.
There are some tips that the automotive and machine shops use to cut down condensation in the lines and make it less noisy in the shop. I put them on this board about a year ago. They may be in the Knowledge Base. If you are running finishing equipment off your shop air, this will probably make your life a lot easier, particularly during the hot, humid summer months.
From the original questioner:
I did purchase an air dryer to keep the condensation out of the lines, but I will still look for your previous post in the archives.
My big question is on using PVC. My brother is a mechanical engineer and thought that sched 250 PVC should be fine, but I have yet to see a cabinet shop using PVC for their compressed air.
From contributor H:
Request that he run Barlow's Formula equations for the applicable sizes in black pipe and PVC. That will keep him busy while you are running the black pipe. LOL. If this doesn't work, remind him that the pressure is not constant and the pipe has a strong tendency to vibrate with the compressor. Ask him to check the fatigue data and confirm his thinking. By that time, you should be more than done. Then tell him that since he didn't do any real work, it's his turn to buy at the Dr. Pepper stand.
Black pipe comes in 10 foot, 20 foot and nipple lengths. You can sometimes get longer lengths off the shelf or get your supplier to cut and thread. A plumbing supply house is a better choice because they will tend to stock a higher quality product for this.
A couple of things to remember--
1) Nothing really dries the air in a compressor line. It removes it by separating the water or water vapor that has condensed from the process stream.
2) Empty your water trap and filter often.
3) Install tees right before the horizontal run to your air outlet. This gives an area for large water drops, crap, crud and corruption to settle *before* it gets to your air tool or whatever.
4) See instruction 2, above!
5) This isn't what is posted in the Knowledge Base. That has to do with where and how to install the compressor for the best noise reduction. And it has a neat method for reducing the water that would otherwise get to your tools or project.
6) See instructions 2 and 4. Really, I have seen a lot of messes because someone says that they have a water trap or filter and both are *full*.
We ran PVC in our last shop and it worked fine except for one thing. When we made the transition to brass fittings and quick connects, the PVC female fittings had a tendency to crack from over threading. You don't feel the resistance when tightening into PVC like you do when putting together black pipe. After a couple of replaced fittings we figured out the right amount of pressure to apply, and it worked great. It is very inexpensive to run PVC and also very easy to cut in line to add more air outlets. Our new shop already had copper lines running in it, but if we had to put in our own, we definitely would have run the PVC again. We ran schedule 40 and had line pressure of 175 lbs. Had no leaks in PVC.
Our wood shop and metal shop ran black pipe for compressed air supply. We ran the pipe ourselves (the guys were familiar with the basics of threading) and did many of the things outlined in contributor H's posts. We run 2 industrial size compressors all day with no trouble, and have done so for years. Adding new lines is not a problem.
I have no experience with compressed air in PVC, and am not an engineer, so I will not argue the numbers or his experience. I do know that there is a lot that can, and does, happen to utilities in a working shop that is not considered when evaluating a material. My opinion is it doesn't pay to cut a few dollars on what should be a very long term installation.
There is some new PVC piping that is supposed to be okay for air lines but I hear it costs as much as black pipe. Less labor to install, though. PVC in general is not acceptable by OSHA and many local codes. Lucky for you this is an all new installation so you can do it right the first time. Make sure you pitch the pipe in the proper direction and have your drops go up, then down. Get good instructions because running air lines isn't the same as water pipe. I'm having to redo our system because the last guy didn't understand air lines and screwed everything all up.
In addition, at the low ends and tees, use a bleeder valve. It looks like a tire stem and operates the same way - push in and air will blow out. Make sure they are pointing down. This will help bleed off excess water that collects in piping.
From contributor M:
If it were my shop, I would install only copper pipe. Why? Because condensation is the biggest problem in compressed air systems. If it is not all removed (which it cannot be unless you spend a small fortune), that water will lie in your piping and over time will rust black pipe. This sludge will end up in your piping and if you intend to use this for finishing...
Further, the time and money you will save on installation should be reason alone. Copper can handle anything you will throw at it in a real world application and can take standing water. Take the money you saved and buy a good refrigerated drier which actually does what it says. It cools the air as it goes through and condenses the moisture to droplets before removing it - *much* more efficient. Also read rules 2 and 4 again. The piping arrangement with the tees is right on the money, but add blow down valves at the bottom of those tees to facilitate removal of said crud and corruption.
My research shows only type L and K copper pipe is acceptable. Type M is too light a gauge as per my local air compressor guys. Also, OSHA says PVC is a no go. There is a reference to it on the OSHA site.
From contributor L:
We use copper. If it gets hit it bends, while PVC cracks or shatters. Copper doesn't rust and is easy to work with. We ran a loop system with short branches. Put a few ball valves in for isolation if you need to work on the system and still use part of it. We have a refrigerated drier. The only problem it's had is kicking out when it gets really hot. We filter the air going into the compressor room and have an air-to-air cooler after the compressors, then a coalescing filter, then the refrigerated drier, then the main regulator. We also have filters ahead of the expensive equipment and another coalescing filter at the paint booth. All airdrops go up off the main lines before going down. The main lines slope to drain valves. The compressor manufacturers can provide you with information booklets. I would locate my compressor room on an outside wall so I could pull in and exhaust outside air in the summer. I’d exhaust into the shop in winter to save the heat.
From contributor M
Contributor L, check the condenser fan for the refrigerated drier. Maybe give the blades a little twist to increase the pitch and move a little more air across the coil and use that super duty air supply system you have to blow out the fins on the condenser (if you haven't tried this already).
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor A:
I have worked in two shops that used sch.80 PVC for air lines. Worked great at first but after a few years, both failed. There was no warning in either case, just instant failure. Beleive me, when lines break with 175psi in them, it is very scary. I would not recommend the usage of PVC for air lines.
Comment from contributor B:
If you want to get rid of vibration from your air compressor, go to an industrial supply or farm implement company and have them make you a 4 ft hydraulic hose. Use that to connect the compressor to the manifold system. Burst pressure is about 3800 psi on a 3/4 inch 2 wire hose.
Comment from contributor C:
Ah, the eternal PVC versus metal pipe debate. First of all, all PVC pipe manufacturers insist that their product not be used for compressed gasses. However, there is plastic pipe rated for compressed gas service in harsh (corrosive) environments. The price is much more that the most expensive metallic piping system.
The reason that PVC pipe is not to be used for compressed gasses is its tendency to fail explosively (high speed flying debris that can and has killed). That being said, black steel is used as it is cheapest of metallics. Its drawbacks are the fact that it corrodes on the inside. This scale ends up in your point of use filters or the finish of that project you have been working on. Copper (thick wall best choice) is the next in cost, and in my opinion the best choice, as it is easy to work with if you know how to sweat copper joints. Brazing or silver soldering for very high pressures. Most any threaded adaptors can be had and installed easily.
Galvanized is not worth mentioning due to cost, but it is a very good choice for corrosion resistance.
The most important thing is to control the amount of water that ends up in the lines once they are installed. Refrigerated dryers are great as far as they go. For the ultimate, a dessicant dryer cannot be beat. The tradeoffs are as follows... Refrigerated dryers cost you for the energy that they use (many times the purchase cost over the life of the dryer). Dessicant dryers cost you 30% larger compressor for the regenerant cycles and the dessicant has to be topped up periodically. Drops should come off from the top of the main line and a drip leg won't hurt either (with stopcock to drain any condensate). Slope the main lines to another point and install a drip leg and drain there, hopefully at the lowest point in the system.
Comment from contributor D:
Anything but PVC! I thought I could get by cheaply using PVC for a moisture trap, so I made one out of 4" PVC and end caps. It worked for years, then my wife bumped it while sweeping, and it exploded! She was not physically hurt, but it was a long time before I got her to help in the shop again.
Comment from contributor E:
I am a professional, registered engineer and an avid woodworker.
1. The basic answer is: Copper pipe (or brass) is the best material to use in this environment. It is easy and cheap to install - much easier than black iron. All you need to know is how to silver-solder the pipe and fittings.
2. I've never heard of "Schedule 250" PVC pipe; beware of any such claims. It's not the thickness of the pipe; it's the adhesive or glue used in joining the pipe and fittings that represents the weakest joint. And this type of joint is impossible to predict or calculate.
3. Do not rely on so-called "galvanized" pipe. What is conventionally called galvanized pipe is merely galvanized coated (not "hot-dipped") externally. Hot-dipped is the only credible type of protection - and it is subject to visual and mechanical surface inspection. I have never heard nor seen any pipe hot-dipped internally; I can't imagine how anyone could inspect the pipe internally! And if it can't be inspected, it doesn't warrant any interest in any application.
4. Some of the best suggestions are given by previous contributors - such as:
a) Install a compressor aftercooler and water trap to remove the bulk of the condensed water in the compressor's discharge.
b) Install the product air piping always inclined or sloped in the direction of flow, with a copper trap & drain at the lowest points.
c) If possible, use a refrigerated dryer or adsorber type - this is the absolute best way to condition your air supply. The adsorber type is the best and the recommended type if you're really looking for quality air for fine work.
d) Always drain your water traps on a frequent schedule or simply install automatic float drainers which can be bought and installed for less than $200.
Doing your installation with copper pipe is much safer and gives you a constant, reliable system that has mechanical integrity. This is what is needed to maintain a woodworking shop in a congenial and enjoyable state. We certainly don't want to introduce safety issues or hazards.
Comment from contributor F:
Using plastic pipe in any compressed air system is strongly discouraged in pressures over 60psi. Don't use it. For a wood or cabinent shop blowing off sawdust it's fine. OSHA sets this limit at 30psi. The problem with the plastics is the additive packages in today's compressor lubricants cause the inside of plastic pipe to deteriorate, become brittle, and cause shrapnel type projectiles in high pressure failures. Cases have been documented of shrapnel penetrating drywall 30' away. Review OSHA.org. Aluminum pipe is the best, and goes up without chemical glues (plastic) or fire (copper) and is expandible when your shop grows.
Comment from contributor R:
PVC used for airlines is against the law. OSHA could chain and padlock your doors until you replace it with explosion-proof material (galvanized pipe, blk iron or copper).
Comment from contributor N:
You should never run PVC for air-lines. I work for a major plumbing distributor and we recommend black iron for air lines. With the way the plastic market is the black iron isn't that much more expensive and you don't have to worry about it cracking or shattering like you do with the PVC.
There is another type of plastic that is approved but it is harder to locate. One example is Acrylonitrate-Butadiene-Styrene. The bottom line is if you want to use a plastic pipe check to make sure it is marked "approved" for compressed air supply.
Comment from contributor S:
My job is to install compressed air systems and the majority of our installations are using Polypropolyne compressed air pipe with socket fusion joints. This product is designed for these applications and can handle the pressure and has high impact resistance. It’s less expensive than the next best product which is copper. Black steel pipe may be less but it will take twice as long to install rust internally and leak over time if screwed together. It is a very costly and time consuming job to add to later on.
Comment from contributor J:
I've worked in the compressed air industry for a long time and PVC is unacceptable for compressed air - illegal in many states. Copper and steel are among reasonable solutions, but I recommend Transair pipe systems. These are push-to-connect aluminum pipe systems that require very little time to install with moderate material costs. The system is versatile - and much easier to install than steel or copper.
Comment from contributor G:
I've used PVC for air system and have experienced breakage, which distributed chards of plastic all over the shop. I highly recommend being careful if you are going to use PVC for air lines. Furthermore, PVC expands and contracts at a different rate than copper and iron pipe. If a combination of PVC and metal is used, always create joints installing PVC into, rather than over a metal nipple. If the PVC is installed over metal, then the temperature cools down and the PVC becomes overly stressed resulting in cracking and failure.
Comment from contributor H:
The Plastic Pipe Institute in their recommendation part B recommends that plastic piping such as PVC used to transport air should be protected with a shatterproof encasement because of PVC's brittle nature. (OSHA) has stepped in and regulated against using PVC in these applications. So you may want to check with local and state and (OSHA) before proceeding.
Comment from contributor I:
I tried PVC once for an airline in my shop. It worked for a few years without fail until one day I bumped it with ladder while cleaning the workshop. It literally exploded in to hundreds of sharp fragments. One of those fragments had to be removed at the local emergency ward. It was described by the attending doctor as "just like a broken tip of a knife blade buried just at the bone in my left thigh". I never realized the pain until I got over the loss of hearing in my right ear from the blast. Yeah, PVC was less expensive and easier to install - so much for less expensive and easier.
