Galvanizing piping after welding 2

[sider]

We have a requirement from the customer to install a chilled water system with galvanized steel, and such that we first have to weld and connect steel piping and after the hot works have been done, dismantle the piping and take it for galvanization, after which the installation should be assembled again.
Of course, such a procedure I believe is much more expensive and time-consuming than just connecting already prepared galvanized piping which could be threaded and all necessary fittings installed. Could someone clarify this requirement? Is there any specific reason for such a request?

 

[TugboatEng]

Threaded fittings have poor corrosion resistance. The threads are not galvanized.

 

[EdStainless]

they don't want the ID galv do they?
They aren't concerned about the Zn oxide getting into valves and regulators?

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P.E. Metallurgy, consulting work welcomed

 

[bimr]

Yes, and the moisture from the chilled water will enhance the corrosion on the threads.

 

[EdStainless]

So even with threads and welds galvanized I would think that they will have OD rusting in a few years once the Zn is consumed.
How long the ID holds up is completely a function of their water treatment.

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P.E. Metallurgy, consulting work welcomed

 

[Dhurjati Sen]

This is a typical catch 22 situation.
The customer wants to prevent the galvanization damage due to heat generated by welding and suggests complete galvanization after welding.
You want threaded piping to cut costs, but that goes against the design.

General practice is as follows:
1. Galvanization.
2. Welding.
3. Repair of damaged galvanization locations by painting.

DHURJATI SEN
Kolkata, India

 

[DekDee]

I agree with Mr Sen

 

[LittleInch]

For chilled pipe I guess the issue is corrosion under insulation or just plain wet pipe due to condensation??

Client probably thinks it will make his pipes last longer, but you might find it cheaper overall to just buy stainless steel....
Also if the inside isn't protected you can get corrosion and dirty water circulating.

If galvanising and welding I'm pretty sure you need to make sure the weld area is free of the galvanising material as it contaminates the weld material if not removed or not present on the weld area. Hence it takes time and money to do this and it all adds tot he total cost.

Threaded fittings are one way to avoid it but has lots of downsides as well.

what other types of protective coating have you considered and costed?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TugboatEng]

Stainless Steel is also subject to corrosion under insulation in this application. It is not a good choice.

 

[LittleInch]

Above 50C and in chloride atmospheres maybe, but this is chilled water?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[TugboatEng]

Anywhere the insulation is wet and in contact with the piping creates conditions that promote crevice corrosion.

 

[FacEngrPE]

The following are my opinion. The advice will not apply to all use cases. You are responsible for validating suggestions against your intended application.

Outside of pipe a number of good corrosion under insulation coatings are available that can help here. A web search will find a number of suppliers that can be vetted.
A well managed water chemistry program can control corrosion in an unlined or ungalvanized iron pipe system better than an untreated galvanized pipe system without water treatment.
Ring groove connections can eliminate most of the the pipe threads, and do not have the corrosion issues mentioned above.

There are a number of plastic materials that can be used on chilled water systems. Some are stiff and can be brittle. Others are flexible and ductile. UV resistance is available. Joints can be mechanical / Glue / Welded.

https://plasticpipe.org/

has a free handbook that can be useful for exploring this option.

 

[EdStainless]

For insulated chilled service SS is often used but it requires external protection under the insulation.
Nothing comes free or easy in this service.
As much as I like SS this is CS service.
Externally coated, and well controlled water chemistry.

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P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

This system must use large diameter piping if CPVC or copper alloy are not the #1 and #2 options?

 

[sider]

Hi all,

firstly thanks for all comments and opinions.

Some of you have proposed other solutions, which if you ask me, are valid and could be used to install chilled water piping.

However, we are designing the additional capacity for the already built plant based on the already agreed on parameters/materials which the customer defined in their tender document, and unfortunately, the piping must be done according to those requirements. This leaves me with only slight adjustments which can be made in terms of design.

The issue I was facing was the galvanizing after welding which could inflict high costs (installation, welding, disassembly, transport to galvanization plant, assembly again,...).I believe you get me much-required info about decisions that need to be made when specifying piping material and taking into account water chemistry and corrosion under insulation. However, eventually, the agreement was made, because the installation has to be disassembled and assembled again, that threaded piping and fittings could be used, and no pieces will be welded.

They aren't concerned about the Zn oxide getting into valves and regulators?

How big of a problem is this?

 

[3DDave]

Galvanzing doesn't mean only electrodeposition (plating) or hot dipping. See

https://galvanizeit.org/designing-for-hot-dip-galvanizing/inspection/touch-up-and-repair

 

[IFRs]

can you use flanged joints?