Painted Pipes and Galvanized Pipes

Please see the following standards.

http://www.techstreet.com/cgi-bin/detail?product_id=32189

http://www.techstreet.com/cgi-bin/detail?product_id=960763

Internal coating is applied to some types of pipe carrying potable water. The coating is cement. Rubber lining is sometimes used for pipes carrying aggressive fluids. Galvanising (zinc coating) is also used for some fluids.

<...what the reason we use internal painting?...>

The original poster is in the best position to advise.

<...what about technical comparison betwen internal painting & galvanizing?...>

Sometimes, instrument air is carried in galvanised steel piping, also various classes of water. The galvainising protects the pipe against corrosion, and lowers the risk of corrosion products being carried along with the fluid.

<...I want to transport LPG liquid & vapour, what do you think, should i use internal painting? ...>

The piping must be selected from the Piping Standards applicable to the end Client's operations. All producers of these materials will have strict criteria to be followed, that have been developed from experience and the occasional mishap. In this application, the selection is not a matter of global opinion and chance replies from an engineering forum with anonymous respondents!

The post of PWSlack stated (perhaps based on no little experience) that the lining for potable water piping is "cement". While galvanized water piping has indeed also been used for potable water piping, and for various reasons is still used in parts of the world (indicating perhaps at least some degree of practical effectiveness), the following should also be explained.

In the United States and many other countries materials to be in contact with potable water are now regulated to various extents. The website e.g. at http://www.nsf.org/business/newsroom/waterworks98-1/ states (with copy apparently dated more than 10 years old),

"If your water utility is not currently requiring suppliers to meet these standards, it should be. According to a 1997 survey conducted by the Association of State Drinking Water Administrators, 41 states require compliance with ANSI/NSF Standards 60 and 61 through legislation, regulation or policy. Twenty-nine states have enacted legislation or regulations, and 12 require compliance by policy. Three other states have proposed legislation to establish compliance requirements."

For curiosity's sake, I just stepped through current listings available at http://www.nsf.org/Certified/PwsComponents/Listings.asp?TradeName=&StandardExt=PRP&MaterialType=&ProductType=&PlantState=&PlantCountry= and I think I found only one listed supplier of galvanized water pipe (and that manufactured outside the USA).

I will leave it to others to explain all the reasons why this is the case, but I did notice the quote at http://www.nsf.org/consumer/plumbing/index.asp under the heading, Galvanized Pipe:

"Galvanized metal is still used for some residential purposes, such as well casings, although it is not frequently used inside the home today. Rust buildup can occur inside of small-diameter galvanized pipe over time, causing the water flow to become restricted. In some situations, the water coming from the faucet can appear rust-colored when the tap is first turned on, as the scale can discolor the water or break loose from the inside of the pipe."

Beyond that, I would only state that galvanized pipes (and for that matter many other thin coatings) do not have the proven record of successful longevity in (particularly in some aggressive) poitable water exposures as does cementlining (whose service record now spans far more than a hundred years). I believe the galvanizing layer is incidentally normally quite thin (e.g. on the order of only 3 thousandths of an inch or 75 micrometers thick, whereas cementlinings may be twenty times or more this thickness in even small pipes (and as such are not just a sacrificial barrier).

Obviously the pipe should be painted / galvanized from inside to avoid rusting.

But painting internally is difficult for long length pipes. Even galvanizing or any electochemical coating is not possible from inside. (I have not understood, why the pipe doesn't get plated from inside in any electrochemical process, where pipe is dipped completely in the respective liquid)

For longer length pipes, hot dip galvanizing is possible.

All pipes are painted to avoid corrosion resistance and also to know about the kind and hazardous characteristic of the mediyum travelling inside the pie. For painting procedures and coloursyou can refer ASME standards.

galvanised pipes are used for potable water, instrument air & plant air piping only. Neverthless the galvanised pies are used in any other medium pipelines.

Internal coatings are also provided when the mediyum is highly corrosive and in the longpiperun of the refinery anticipating that the pipe thickness may go off drastically. this is the governing principle and you can also find the rubber lined pipes used to carry hazardous fluids.

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The pipe doesn't get plated from inside in any electrochemical process, where pipe is dipped completely in the respective liquid?

Even when you use the dip rocess for pipe during the drying process the pipe surface area would not be totally exposed to the dry light and the paints which got coated used to wipe off from the surface not providing the uniformity.

internal painting / coating is used to protect from corrosion. the same thing is for galvanizing.

http://www.galvanizeit.org/aga/about-hot-dip-galvanizing/how-long-does-hdg-last/

http://www.galvanizeit.org/aga/about-hot-dip-galvanizing/other-corrosion-protection-systems/

the type of protection you can use depends on many factors, temperature, pressure, fluid corrosivity, design life....

S

corrosion prevention

I am not aware of any corrosion problems internally for schedule 40 welded pipe for LPG use. External painting with epoxy type paints with zinc additives are very good for long term protection.

As far as I know schedule 40 is not galvanised it would interfere with the welding process.

most LP lines are either copper or black iron. when buried they are copper. in commercial applications(inside a building) they black iron for strength.

I do not want to challange your (and Abdel's.... who is really master in his subjects) statement.

But I have some question to ask.

I had fitted LPG gas kit to my car. Whenever I servised my car and cleaned the gas filter, I used to get ample rust and fine dust from the filter.

Where from this comes? Service station expert also say that it is not a problem of my car, but it is from LPG itself.

When we start to proceed new design for an LPG pressure vessel fabricated from high tensile alloy steel such as ASME SA-517, with a life time duration not less than 30 years, we select the corrosion allowance to be 1 mm (only 1 mm). That is mean the expected corrosion shall be 0.033 mm/year.

And for carbon steel material such as ASME SA-516, I select only 2 mm for corrosion allowance, without need for any internal protection. In practical, we have a lot of LPG vessels fabricated since 50 years without noticing any reduction in vessel wall thickness.

So, in nature there is no ideal condition where you can't prevent corrosion completely to be nil, but you can attain a situation where there is a min. corrosion rate. And in processes where corrosion is very low, it will be costly to protect these surfaces without significant gain.

Good answer Premium.

Watch out for H2S, it will crack your sphere.

http://products.ihs.com/bs-seo/00293957.htm

I think I made that point in 6.