Painted Weathering steel in sewage treatment station 5

[Italo01]

Hello,

If this forum is not the most adequate to this question, please someone inform me which one is so i can question there.

I was hired to design a steel structure to support a roof and a runway girder. This structure will be located in a sewage treatment station but will not be in constant direct contact with sewage. The runway girder will be used to lift the grill with the wastes one time per day so maybe it will have contact daily with contaminants, although is not immersed.

At first i thought of specify stainless steel but i'm want to know if there are some solution which is adequate and more economical.

CBCA, which is a steel institute in my country(Brazil) has a guide to painting and surface treatment of steel and this guide has a suggestion of a painting stustem for "steel structures which are immersed in effluent and sewage" that is suitable for carbon steel and weathering steel. This system consists of three layers (150µm each) of coal tar epoxy. My first idea is to specify weathering steel with this painting system.

My question is if a system design for steel immersed in sewage is also adequate and economical for steel that has sporadic contact with sewage.

Thanks in advance.

 

[dik]

You need to know what you are protecting it from. Weathering steel can be cleaned and a zinc rich primer or other coating can be used. Careful with tar modified epoxies... many of them are carcinogenic.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[swazimatt]

sulfur dioxide would be one element to protect it from. Gantries i have done at WW pumpstations has been two-part epoxy paint. If maintenance is expected to be non-existent then go for galvanised

 

[Italo01]

Thank you guys for your help.

Dik, all i now is that it will be near a waste treatment plant. Are you aware of standards or supplier's manuals which define the appropriate painting for the specific substances?
With respect to the Tar modified epoxy, i will avoid it due to the carcinogenicity.

Swazimatt, did you decide to use this specific paints due to some recommendation(Guide, Standard) specific to Waste treatment plants?
I'm using this guide from the brazilian steel institue. The guide has recommendations for industrial environment and also recommendations for elements immersed in effluent and sewage( Which is the coal tar epoxy that dik showed to be carcinogenic).
Maybe the painting for industrial environment is adequate, since sulfur dioxide is common in industrial environments. What do you think?

 

[dik]

You have to contact suppliers.

As far as tar modified epoxies, there may be some that are non-carcinogenic... they are excellent coating materials; check with suppliers.

If critical, you need to determine the effluents chemically and check with major/reputable coating suppliers to determine what is best coating system. It's likely a high build zinc rich epoxy with a top epoxy coating plus a high gloss polyurethane top coat. It's costly... or something of that ilk.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[swazimatt]

I didn't use a standard, but as DIK mentioned i worked with a paint/coating supplier. There is also a push for polyurea coatings for wastewater installations with claimed lower maintenance than epoxy

 

[dik]

You need to contact a reputable supplier, or retain the services of a qualified coating specialist, and know exactly what you are protecting it from.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[bimr]

Coal tar epoxy is an obsolete coating. It is still sold, but people that know coatings would not use it.

Your situation is similar to handrails in wastewater treatment plants. These are normally supplied in aluminum or 304SS.

I would recommend the posts be made of aluminum or 304SS. The upper structure can be painted steel. Here are some manufacturers:

3.6 EXTERIOR HIGH-PERFORMANCE COATING SCHEDULE
A. Steel Substrates:
1. First Coat:
a. Sherwin-Williams Company (The); Corothane 1 Gal-Va-Pac Zinc Primer B65G00010 at 3.0-4.0 mils DFT.
b. Tnemec Inc.; Series 90-97 Tnemec-Zinc at 2.5 to 3.5 mils DFT.
2. Second Coat:
a. Benjamin Moore & Co. Corotech V160 Epoxy Mastic Coating @4.6-7.2 DFT
b. Sherwin-Williams Company (The); Dura-Plate 235 Multi-Purpose Epoxy B67W235 at 4.0-8.0 mils DFT
c. Tnemec Inc.; Series N69 Hi-Build Epoxoline II at 3.0 to 5.0 mils DFT.
3. Third Coat:
a. Benjamin Moore & Co. Corotech V500 Acrylic Aliphatic Urethane Coating Gloss or V510 Acrylic Aliphatic Urethane Coating Semi-Gloss at 3.2-4.6 mils DFT
b. Sherwin-Williams Company (The); Acolon 218 HS Polyurethane Gloss B65W611 at 3.0-6.0 DFT.
c. Tnemec Inc.; Series 1074 or 1075 Endura-Shield II at 2.0 to 5.0 mils DFT.

 

[JedClampett]

I'm late to this game, but is this an open frame or are there walls?

 

[dik]

Thanks bimr... I haven't used tar modified epoxies for years (decades?), but they may be a common coating system in some areas. I've used the Sherwin-Williams coating system you note, and have found it to be excellent.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Italo01]

Hello, Thank you all for your help.

Jed, no you're not late. The client is waiting some information about the equipment that will be lifted so started monday the design and i still can make changes. It's a open frame.

Dik, i've asked the client if they can determine the effluent chemically, or if they have already determined, and will check with suppliers to see what they have available for this situation.

Bimr, thank you for your suggestions. I'll check if these suppliers are present in my country (Brazil).

 

[JedClampett]

OK, I'd build a steel frame, coat it with two coats of epoxy and one of polyurethane. The coating will likely last as long as any other commercial or industrial paint. The only place you'll have a minor issue is where gas might not move, like a corner. Open frames are not a problem.

 

[Italo01]

This is the answer of the client about the gases. They say that will see if can determine the chemical composition.

"The decomposition of organic matter produces gases as hydrogen sulfide(H2S), methane(CH4) and carbon dioxide(CO2), among others, which can be present, in large quantities, at the channel entrance."

I really liked the idea proposed by bimr of using stainless steel for the columns and base plates and painted carbon steel for the upper structure, i would have only to carefully detail the connection between them to avoid galvanic corrosion, since the columns will be more exposed to the effluents and the upper structure only to the gases, i think that a painting system adequate to industrial facilites will be enough for the upper structure. Also, the weight incresed for the upper structure considering stainless steel will be greater than for the columns.

 

[dik]

That's it jed... and a zinc rich epoxy is a tad better. You have to be careful with some epoxies, they some are time critical for chemical bonding. In addition the high gloss topcoat is essential because epoxies don't take well to exposure (they dull) and the high gloss provides better topcoat protection than another type of finish surface.

Unless exceptional, I would not be using SS for framing. Are they venting these gases to the atmosphere?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Italo01]

Unless exceptional, I would not be using SS for framing.

That`s because the smaller stifness and nonlinearity of the stress-strain diagram on service loads or for other reason?

Are they venting these gases to the atmosphere?

No, but the roof height over the channel entrance is 6 meters, so i think that there is good natural ventilation.

 

[dik]

They tend to be more costly and some sections are not readily available.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JedClampett]

dik, agree with your zinc rich epoxy.
One more item with stainless; It's hard to weld. The welds need to be cleaned with nasty chemicals (pickling or passivating, I'm never quite sure) or they will get surface rust and you'll get a call from the owner asking you "...why you used stainless, it rusted anyhow?"

 

[dik]

and using non-ferrous tools, too.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

I still don't know if this stuff is being vented to the atmosphere.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Italo01]

I think you missed what i said above:

No, but the roof height over the channel entrance is 6 meters, so i think that there is good natural ventilation.