Carbon steel heat exchanger passivation beneficial? 1

[homeslice]

I'd like some opinions on the benefits/pitfalls of passivating new carbon steel heat exchangers. I'm concerned, since they're not stainless steel, that this is not really beneficial, but a waste of time. Most water treatment specialists have promoted this. But won't the chemical just form a layer of rust with the steel?

 

[metengr]

Depending on the specific application, the waterside surfaces of most new boiler and/or heat exchangers containing carbon steel components are passivated to avoid iron oxide transport problems. This is a large benefit. Passivation is done deliberately to minimize the formation of rust from exposure to atmospheric corrosion, and to assure protection of the carbon steel surfaces prior to placing the components into service. Magnetite that is formed as part of the passivation process protects the tube surfaces from further atmospheric corrosion (provided there is no contact with oxygenated water during storage of the heat exchanger) and corrosion after the heat exchanger is placed into service.

If you don't passivate prior to placing the heat exchanger into service, iron oxide (rust) will form and dissolve into your feed water system. This could create corrosion problems elsewhere in your system.

 

[EdStainless]

The benifit on CS that I have seen is more a fuction of truely cleaning the surface and starting it all off with as good of a surface film as possible.
The issue of getting the unit clean is a big one. Often various parts of the unit will have different surface conditions and this can aggivate corrosion.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[oldcorreng]

Carbon steel used for heat excahngers is very susceptable to standby corrosion and pitting when new or following use. When excahangers or coolers are fabricated in a shop they are usually hydrotested with water of whatever quality is available then drained. Corrosion will begin at this time. If you have your own people witness the hydro, perhaps you can have them get the fabricator to flush and hold an inhibitor in the equipment for a period of time, usually several hours, then drain it. This may be an additional cost to the project but well worth it.

Your water treatment supplier is correct in recommending you to flush and hold the equipment with an inhibitor, typically 3X the normal dose during operation before putting it into service. In some cases, particularly high pressure BFW exchangers, acid cleaning is needed, to prevent minute amounts of iron from reaching the boilers, which is usually followed by a neutralizing and passivating step.

In the case of new carbon steel boilers, an alkaline boil out is performed. This removes oils & grease from rolling tubes in place and to promote a dense protective oxide film.

All of the above are for short term periods prior to putting the equipment into service. If you plan to keep the equipment in standby for weeks or months, then you need to devise a long term standby program such as nitrogen capping, a suitable wet method, cryogenic or dehumidified air. Keep in mind environmental conditions such as rain, humidtiy and deposit composition for instance when you have the fire side of a boiler in standby. All will need frequent monitoring and inspection.

 

[rmw]

I have seen a multitude of high pressure heat exchangers with nitrogen blanketing systems installed at initial construction for long term storage/outage conditions, and I have yet to see one being used.

I have also seem a multitude of CS tubed exchangers being retubed or replaced with stainless.

If the plant doesn't use the protections that the engineers provide, then they are worthless. It gives CS as a tubing choice a bad rap.

rmw

 

[unclesyd]

You might want to give these people a call

http://www.cortecvci.com/Publications/Lay-Up/lay-up.html