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Compressed Air Piping-Material? 2

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kepharda

Mechanical
Mar 8, 2006
81
I am working on the piping design for a small production facility. The Process Engr. who is in Switz. constantly suggests to the manager to do things that may work, but are not in my mind very prudent.

The latest is that he wishes to utilize Carbon Steel pipe for an Instrument Air Piping System. He has suggested (rather strongly) that I don't use copper or stainless or even galvanized piping.

My obvious concern is corrosion in the line causing problems downstream in the sensitive instruments and actuators? Is my concern misguided?

thanks all suggestions,

dave
 
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First, it is always wise to install a filter regulator on the air line to the instrument. Also, make sure the air is dried and cleaned. If you do this, then carbon steel is fine. From the regulator to the instruments can be cooper or SS, depends on sour gas in the air to use cooper. Most galvanized pipe doesn't always get a good coating internally, so I don't feel its worth the effort unless its off shore or in a wet climate. The only other problem with cooper is that it bends or get kinked easily.
 

Dave:

Dcasto has given you some sound and valued advice on how to ensure a secure and reliable compressed air supply system.

I believe we are both assuming what you haven’t stated: That this is an application of compressed air for pneumatic instrumentation and limited to only external instrumentation – i.e., it will be used only for external instruments and not for direct “bubblers” for level detection nor on food-grade products or fine chemicals. This means your product air dew point should be somewhere between -40 and -50 oF. This being the case, like dcasto says, carbon steel is the material of choice and will do very well in this service. Copper and stainless would be even better for ensuring better quality, but the tradeoffs are that the copper is structurally very weak (as dcasto states) and the stainless will cost far more in materials.

I judge galvanized piping as a totally wrong application that is doomed to failure. I can’t imagine how and why anyone would apply this type of weak and inherently flawed piping. It is weak because it is totally susceptible to corrosion, flaking, and pin-hole defects - depending on the skill with which it is dipped. And once a pin-hole (or even smaller holes) has been established, the corrosion starts immediately. There is no way on this earth that anyone can guarantee that there is no defect or flaw in a given galvanization – hot-dipped or not hot-dipped. And then there is the real hard issue of how does one inspect the surface that is really supposed to be doing the “protecting”: the internal pipe surface! To date I’ve never heard or read of anyone daring to state that the internal surface of a galvanized pipe can be safely guaranteed not to have defects or flaws - because it can't be inspected. And that’s not all. Most, if not all galvanized pipe is threaded. The moment it is threaded, the galvanization has been compromised and breeched. There is no longer any “protection”, since the galvanization has been stripped and removed at the weakest point – the threads! This methodology or system of ensuring corrosion protection for a pipe has all the ingredients for its own demise.

I’ve also used Schedule 80 Brass Pipe, but for ultra-high purity air at -100 oF. However, this was for very high-purity air at 2,000+ psig. I don’t believe your application warrants that.

That leaves carbon steel as the best and most obvious choice – with the caveat, as dcasto states, that the air be in a relatively dry state with a dew point as stated above.
 
Yes, this is an application of compressed air is for pneumatic instrumentation and limited to only external instrumentation – i.e., it will be used only for external instruments and not for direct “bubblers” for level detection nor on food-grade products or fine chemicals.

The pipe sizes are relatively small 2 inch header and 1/2 to 1 inch drops connecting various external instrumentaion. The structural strength shouldn't matter much as it is all indoors installed on a equipment platform, --no big spans to support.

Maybe it is just my opionion, but the install costs of carbon steel, (threading, cutting, and fittings) seems like alot more expensive than sweat fit copper components.

thoughts?

thanks,

dave
 
I would never use uncoated CS for any small bore ( < 3", maybe even < 4") instrument air lines, especially in front of the regulator. The regulator is susceptible to dirt/corrosion products as much as anything downstream of it. Galvanized piping gets mixed reviews in this service, I don't like it.

In theory the line should never see moist air; in practice there are any number of ways it can get in, but most of the damage is done during installation. Mill scale is going to be present, unless pretty elaborate cleaning is done. Any welding would have to be TIG, and threads are another source of debris and compounds.

Copper is the normal piping I've seen and spec'd, SS is more economical for anything less than 1" if Swagelok-type joints are used. Copper should be spec'd as CFOS (cleaned for oxygen service) if you really want a clean line. Brazed (silfloss) joints are better than soldered on larger stuff.
 
Montemayor: usually I defer to your wisdom and experience, but I don't agree with you on this one!

For (small) instrument drops, flexible lines are king. We use Legris insert-type fittings with polyethylene or nylon tubing, or rubber hose with internal reinforcement and SAE swivel type fittings when a more robust installation is required. Multi-drop manifolds are available in anodized aluminum etc. and these work really well if you've got a lot of users within a short throw of one another.

For instrument air headers, if your air is good and dry, anything other than brittle-fracture plastics like PVC will do, and carbon steel is cheapest. Threaded below 2", Victaulic roll-grooved above that would be my choice unless a customer specified otherwise, in which case it's their money and their call.

The trouble is, air dehydrators and filters are notoriously poorly maintained in the field, which temps people to use materials other than CS. Others merely install drip legs and accept the risk of minor interior rusting, or of mill scale becoming disengaged from the wall and gumming up users, so they put a filter/KO unit on every user or group of users and call it a day.

If it's an outdoor installation, external corrosion is the issue for CS, and painting is often offered as the solution. Anybody who has painted pipe knows it's a bastard from an installation cost perspective- you need to build the piping, take it out, blast it and paint it, and then re-install it- or worse yet, paint it in place- that means both a bad job AND high labour cost. Regardless how you slice it, painting pipe eats man-hours for breakfast!

Copper tubing is great if you silver braze, but that defeats much of the fabrication benefit relative to threaded carbon steel- and the cost of copper has shot up recently to deter even the die-hard copper-lovers. Sweat-soldered copper would require the use of water-soluble flux followed by a water wash and a scrupulous air dry to make me happy- otherwise, flux propagation down the line will eventually corrode the tubing from the inside out AND run the risk of corroding sensitive components in the instruments themselves. Some worry also about the mechanical vulnerability of copper in a plant environment and want it supported so frequently that you'd quickly beg for an alternative.

So galvanized is a compromise some people use. No need to paint the exterior (saving big $$$ and time), and less need to worry about the interior from a corrosion perpsective. Some will consider dispensing with the point-of-use filter/KO units if you use galvanized, which is a huge dollar savings. Others worry about the risk of an errant piece of teflon tape, threading cuttings/burrs or the odd chip of zinc flaking off and gumming up the works, in which case the point of use filters stay- but galvanized is STILL better than painted carbon steel! Personally, I've seen teflon tape cause far more problems than mill scale or zinc fakes, so spec'ing anaerobic pipethread sealant and NO teflon tape downstream of the filters takes care of that.

As to the corrosion protection offered by galvanized- the whole reason one uses zinc galvanizing is that unlike paint, such a coating is a sacrificial anode- it protects across pinholes or other small defects until all the zinc in the local area is gone. Good pipefitters (properly supervised) use cold galvanizing spray to patch up the threaded areas after they cut the threads. And though the threaded fittings are not fully hot-dip galvanized on all surfaces, in practice they last quite well in much more severely corrosive services than instrument air due to the galvanic protection.

The trouble is, most people want to save money by using cast brass valves in their galvanized piping- that's a no-no, because now you have a 1.5 volt battery driving corrosion if condensate happens to accumulate at the joint between the brass and zinc-coated components. Now your caveats about galvanizing apply big time! But you'll see that the instrument air pipe specs for many of the major chemical and petroleum companies permit this practice... The cast stainless 1000 WOG ball valves out of China are so cheap these days that they're an obvious alternative. Although a galvanic driving force for corrosion between the zinc and stainless steel exists in theory, in practice the lower electrical conductivity of the stainless seems to limit galvanic currents and hence the use of stainless components in galvanized air and water lines doesn't seem to result in significant galvanic attack.

Stainless pipe and fittings look nice and are clean(er) but are pretty much a waste of money. The fitting surface finish as threaded is poorer than for carbon steel in both fittings and on pipe, which can lead to difficulties in sealing (solved by good fitting technique and the right thread sealant). But the cost of stainless is not warranted except for process air streams or perhaps offshore etc.- especially now at $15/# for nickel. And Swagelok fittings become so expensive beyond 3/4-1" OD tube size that they're not worth considering either, except perhaps for small drops from main headers in which case it's a convenience issue- much quicker to install and alter later than threaded CS.
 
Dave,

I will throw my 2 cents worth in.

In my industry, the instrument air piping is almost always carbon steel. Our I/A systems usually produce air at -60 °C dew point, give or take.

From the header to the individual instruments is usually via stainless steel tubing (and yes, Swagelok fittings).

We do not allow yellow metal on our facilities/sites, so copper anything is out.

The main piping, I have heard from others in this forum, are sometimes done in stainless. I have personally never seen such a site - then again, maybe I need to get out more.

We don't use PVC, or any plastics in I/A service. Not sure why, but haven't seen it. We do use it in water service (along with fibreglass reinforced plastic) though although that is a bit off topic.

Anyhoot, CS for I/A piping is pretty common for me.

"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?
 
Stainless steel dairy tubing is cost effective for compressed air. No painting, clean, no rust to clean when new. Yes if you go for Sch10s it starts getting costly but not for thin wall tube.

PE can be used but in the event of a fire it will melt and provide oxygen to enhance the blaze.

Carbon steel is a pain as you have a maintenance headache in having to paint it routinely.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
 
It would help if we knew what type of production facility the OP was about? Interesting how the applications differ here!

I should have clarified my comments about SS; I was talking about SS tubing, not SS pipe.

For main headers, contractors tell me Sch 10 SS is economical (total installed cost) compared to copper at about 3", surely at 4", if the copper has to be brazed (not soldered).

 
It appears that the only consensus derived in this post is that galvanized pipe is not the preferred choice and from there all other material is used depending on industry/client/specifier's choice/individual pet peeve and familiarity with material
 
stymiedpiper: nope. The only consensus is that there IS no consensus! Reviewing specs will show you that in a hurry. While one company A's specs will INSIST that something be done a particular way, company B's specs will insist that it absolutely NOT be done that way.

The key in cases like materials selection for IA service is not to over-specify. That's where people get into budgetary trouble without knowing it. Some people hard spec' a single option when there are a number of possible alternatives, any of which will produce acceptable results. Sub-optimal installation cost results.

PVC pipe is definitely out, Ashereng- it is NEVER permitted for instrument air or any other pressurized gas due to the risk of brittle fracture with injuring schrapnel. PE or nylon tubing for instrument drops are a different matter entirely than PVC pipe for headers. These materials have their limits in regard to temperature etc. but for indoor plants they're the greatest thing going.

As to stanier's comment about PE tubing potentially melting and fueling a fire- to my knowledge, fires aren't generally limited by an absence of air, and as they burn convection fans them by amounts of air far greater than your IA system will ever discharge. I want the spring fail valve actuators on my critical services to return to failure position in a fire LONG before the valve bodies themselves get hot, so stainless steel tubing for instrument air drops are another no-no in my books.
 
moltenmetal
My point is that other than yourself the contributors to the post do not favor galvanized steel in this application. Hence, the consensus (majority of contributors)have expressed their opinion that galvanized steel is not their preferred choice of material.
 
Thanks guys, this is for a Toner Recycling Plant, the compressor is fairly small 300 SCFM, 100 psi, and I calculated the total length of the header and I only came up with about 60 ft of 2 inch. So, I am going to keep my copper spec. I also told him I would provide a Carbon Spec. as an option and which ever contractor he chooses could quote both options.

I also spoke to my Instrument guys and they had very little good to say about Carbon Steel Instrument Air Piping, they have had lots of trouble with teflon tape and mill scale clogging the filter regulators and or instruments. So I passed along the specifics to the Owner and he seemed O.K. with it. In fact he told me his current line already is copper, so there isn't anything corrosive at the site to copper itself, I think the only issue is the recent spike in copper price.

thanks,

dave
 
BigInch: Ashereng also said "Not sure why"- that's why I pointed out the problem with using PVC. I also pointed out that not all "plastics" are created equal in regard to this service.
 
stymiedpiper: Dow Chemical amongst numerous others call out galvanized pipe in their instrument air pipe spec. Am I still just a lone voice crying in the wilderness?
 
Molten
We will see what other industries respond to this post and their comments.
I personally have not encountered galvanized IA piping in the steel processing industry. Perhaps galv is more common amongst chemical plants or other industries.
As I had previously stated, to date, the majority of the contributors to this post are not in favor of galv IA piping.
 
molten,

We don't use PE or nylon either. When I said "We don't use PVC, or any plastics in I/A service.", it was to convey the idea that in my industry, we do not use any type of "plastic" (in the generic everyday connotation/sense of the word) for I/A piping.



"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?
 
For what its worth, the majority of major petrochemical, pulp and paper and power projects, with which I have been involved, have specified screwed, galvanized carbon steel for AI. The second most specified materials have been stainless steel, both butt welded and with mechanical couplings. Lagging far behind were copper and carbon steel.


 
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