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fapsa (Industrial) (OP)
30 Apr 08 12:44
Hi, can any one help me with information on how to protect a pipe of PVC from the sun, here in summer we go up to 115°F, is there a paint that we can use of some kind? Or is there another pipe material that can take the heat, sun and handle corrosive material?
Thanks
moltenmetal (Chemical)
30 Apr 08 12:59
Yep, there's a white mastic that can be painted over top of a closed-cell elastomer foam insulation like Armaflex.  The Armaflex gets chalky without the mastic.  No need for the insulation?  Talk to a paint supplier:  PVC can definitely be painted, but you want something without solvents which will attack the underlying PVC, plus something which will have a good bond strength to the PVC.
BigInch (Petroleum)
30 Apr 08 13:10
There's no UV stabilized PVC?

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

Helpful Member!(2)  jte (Mechanical)
30 Apr 08 14:31
The Harrington Plastics engineering guide is a useful reference: http://www.hipco.com/handbook.cfm

Take a look in there under PVC and CPVC. Both require a water based latex paint for UV protection. CPVC will probably give you better overall service. The resistance guide at http://www.hipco.com/chemGuide.cfm is also useful.

jt
Helpful Member!  rconner (Civil/Environmental)
30 Apr 08 15:46
If the conveyed material is in any way hazardous, and you happen to find a way to effectively say by plastering and maintaining at least some sort of paint or mastic over the pipes to protect same some (from at least the UV), if you have not yet done so you might also want to confirm that the pipe has adequate long-term strength and moduli/stability to handle the heat condition to begin with.  While you did not state exactly what kind of "pvc" material this is, if it is plain garden variety unplasticized pvc, the tensile strength of the plastic pipe at the temperature you are talking about may be less than half what it is at normally reported room/lab temperature.  As to whether any other sorts of pipes might be suitable for the service, that might be hard to determine without folks knowing exactly what the ("corrosive material" etc.) service is.   
jdarco (Chemical)
1 May 08 11:29
Most PVC pipe has some UV inhibitor added (TiO2 I believe), but over time the pipe will become chalky on the outside and more brittle.  jte and moltenmetal are correct, use latex paint, not solvent based paint.  If this is an existing line (already weatherd), you may have trouble getting the paint to bond well to the pipe, causing an ongoing maintenance headache!  Depending on the "corrosive material" involved, you may be able to use FRP, or lined steel.

You may want to consider other materials with your high ambient temperature in the summer.  115 Deg outside means the surface of the pipe is likely much higher.  PVC has a maximum service temp of 140 and loses much of its strength and pressure capability at that temp.  CPVC will be better as it is rated to around 200 deg F.

Lastly, make sure you compensate for expansion and contraction in the PVC or CPVC line.  With large temperature swings, it can be considerable compared with metal or FRP.
stanier (Mechanical)
1 May 08 21:42
The other end of the spectrum also need to be considered. Below 5C PVC-U loses any impact strength and becomes very brittle. Try putting a fitting in a domestic refrigerator and hitting it with a hammer. It will shatter into many pieces.

Go to any waste site in an industrial complex and it will be strewn with PVC as pressure pipe, electrical conduits and drainage.

On top of that the "green" credentials of a project have resulted in its ban in many countries.

moltenmetal (Chemical)
2 May 08 7:46
Man, you guys are far too hard on poor old PVC!

True, PVC can be embrittled by prolonged direct sunlight exposure, but that's easily mitigated with coatings with or without insulation.  In low stress applications it's used successfully with direct sun exposure lifetimes of 20-30+ years WITHOUT coatings, and without ill effects.

Yes, it's brittle at low temperatures and reduces in strength as it gets hotter, with an upper limit temperature around 60 C.  It's also essentially immune to corrosion by media that are VERY troublesome for metallic materials of far higher procurement/installation cost, and it's very easy and cheap to install- something that cannot be said of FRP or lined metallic piping.  And unlike HDPE, PP etc., it requires only the most basic equipment and pipefitter training to make sound joints.

PVC and CPVC are materials like any other:  with strengths and weaknesses.  Know these and you can design around them.   
rconner (Civil/Environmental)
2 May 08 9:58
I agree with moltenmetal that there are places for all types of pipe; however, stanier also makes some interesting points.  There was e.g. of course a time when damaged and leftover crops/waste and end of life issues weren't given much consideration in engineering analysis (and still might not be in some parts of the world). I noticed a couple days ago at http://www.environment-agency.gov.uk/news/2031948?lang=_w&region=&projectstatus=&theme=&subject=&searchfor=&topic=&area=&month=  however that that time may be passing in some areas, as I saw a UK (waste) contractor company was fined twenty nine thousand pounds just the other day for dumping some material that had pieces of "plastics" in it!  Other materials appear to have held their value quite well over very long periods of time (in fact, if you now leave your pipe compound unlocked, you just might find that they will "walk off by themselves!)    
stanier (Mechanical)
5 May 08 22:29
Moltenmetal,

I agree with your sentiments. I was just passing on some information.

These days many engineers buy on price alone. I have been involved in many investigations and court cases when the cost of a failure (including the lawyers fees and consequential damages) far outweighs the savings.

Plastics pipe design is actually more difficult than for metallic produicts. The engineer has to know a lot more. The challenge is that many of the design guides, standards and codes lull youinto a false sense of assurance. The only requisite in many is to design for hoop stress and ignore shear, bending, torsion and other stresses. They ignore fatigue affects on thermoplastics and required derating. They ignore the strain rate change in properties and the affects of temperature.

Yes good old PVC is cheap. Certainly it is more prone to failure than other quality thermoplastics and metallic products. Yes it is misunderstood and misused by ignorant people who do not have the time or inclination to spend the time to learn more than what the salesman offers. It earns me a good living when these poor sods stuff up and get sued.

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