Steam pipe protective coating under insulation

[simplemath]

What is an acceptable and economical method of protective coating for steam pipe under insulation (calcium silicate)?

Pipe is for cyclic loading between 450 degF and ambient. Not in high moisture zone.

I was told NACE prohibit hot galvanized coating (zinc based) under insulation. Do not know the reasoning behind it.

Want to know what the general practice is for the case. Thanks.


It boils down to simple math.

 

[unclesyd]

I would give Dampney a call with your requirements. They have a complete line (Thermolux) of high temperature paints and coatings. In a crunch one time we used their stove paint on some pipe.

On steam pipes where we a problem similar to yours we will lag the Cal-Sil and finish it with some type coating. We exclusively use SS weather coating for these pipes. It's the Cal-Sil that will more than likely need the protection from water. If ever the insulation gets wet it needs replacing as it will never effectively dry in service.

http://www.dampney.com/Products/ProductLines.asp

 

[pennpiper]

It has been a few years sense I have looked at things at such a finite level of detail but I really don't remember ever painting any steam lines before insulation was applied.
Why do you think you need to?

 

[simplemath]

Thanks unclesyd and pennpiper.

Forgot to mention the A106B pipe material.

pennpiper: Does pipe need to be galvanized or better without?

I was told I should be concerned about under insulation corrosion. But is corrosion from inside more than from outside for steam pipe?

Thanks.

--It boils down to simple math--

 

[unclesyd]

In your case the material doesn't matter.

Do not use galvanized in Steam Service or any other service above 350F.

Corrosion under insulation is a big problem if one doesn't keep the weather barrier in good condition, which is hard to do in today's economic times. There just any money for maintenance but plenty to replace a failed line.

Generally corrosion or erosion is from the inside is the biggest problem with carbon steels in a poorly operated steam service. If a steam line remains above 100C about the only problem you will see external is you have poorly maintained insulation and drops of water hits the pipe you eventually have trouble.


pennpiper,

At one time our pipe specs called for silicate based coating or aluminizing on any steam service under 300 Lbs. This was in the days of tar paper and chicken wire with "bull" mastic. Never was told why, just do it, yes sir.

 

[RossABQ]

I have also never used any coating on steam lines, at any pressure. I can't imagine anything lasting in cyclic service especially.

Stainless steels that are insulated can be corroded by some types of insulation.

 

[unclesyd]

We coat SS insulted pipes and tanks with a product such as Thrmalox 70C. We also have a large CS thermal reduction unit that we cover with the same coating as the SS. This is the only inexpensive coating we've found that will stand up to the temperature excursions when we have a stinger on the skin.

http://www.dampney.com/Upload/Products/Products72.pdf

RossABQ,
At one time before low chloride insulation became available we would take the Cal-Sil clam shells and lag the inside with A Cloth and coat with Sodium Silicate. We still did this + coating the lines until we lost the A cloth.

Addenda:
All black iron pipe used to come with a heavy coating. On fittings you could tell by the color who made them. All I recall is that Taylor Forge was green.

 

[simplemath]

Thanks all for your valuable inputs.
I would have owner to get invilved and make this decision.

--It boils down to simple math--

 

[denniskb]

Do not use any product containing Zinc in this service.
There is a small temperature range where the galvanic potential between steel and Zinc reverses. If there is moisture (there will be bacause it will condense from the air breathing in and out of the insulation as it heats and cools) and any damage to the paint surface on the pipe you will get very agressive corossion.

NACE have a paper on it. Last I saw it recommended several coats of a silicone based paint and be careful not to scratch it.

Make sure you have inspection panels in the cladding and open them regularly. Then every now and then open a section you have not previously inspected. You will be shocked if you find this damage and will realise the importance of the measures suggested here.

Regards
Dennis Kirk-Burnnand



Dennis Kirk Engineering

http://www.ozemail.com.au/~denniskb

 

[athomas236]

Not a painting expert but we would usually specify the following:

Service - Insulated parts (carbon steel) exposed to temperatures up to 400 deg C

Location - Indoor/Outdoor

In the works
Surface preparation - Blasting to ISO 8501-1 grade Sa 2.5

Surface profile to ISO 8503 ' medium (G,S)

Primer, Weldable inorganic zinc ethyl silicate, 25microns DFT

At site
Surface preparation - Cleaning and derusting mechanical damages to ISO 8501-1 grade Sa 2 or St3

Touch up, Weldable inorganic zinc ethyl silicate 25microns DFT

 

[KevinNZ]

We never had to paint steel steam pipe under insulation. One of our clients does not even paint the pipe and flanges outside the insulation and they have been running the plant for 50 years with no external corrosion problems. the plant is geothermal piping so the atmosphere is corrosive but the as long as the pipe is hot the surface will be dry and not corrode. Let to go cold and then the corrosion rate is high.