How long b4 pitting with high DA O2? 2

[Paperdude]

We are starting up a new watertube boiler (250psi, 100kpph) and feedwater system. The DA seems to be under-sized and is only getting down to 100ppb O2. How long can we run with high O2 before pitting the economizer? We are adding a scavenger but I'm not sure it's enough.

 

[TomWolfe]

It depends on several operating variables such as water temperature, pH, type and amount of passivator/oxygen scavenger and mineral content of the feedwater. I don't know how long it would take for pitting problems, but there have been situations where HP boilers (>1500 psig) operated 18 mos without pitting failure.

 

[jproj]

Paperdude:

I'm not sure how long your economizer has, but I may be able to help with the DA... What is the O2 content after chemical addition? Normally DA's are designed to remove O2 down to 7 ppb. If you're around there after chemical addition you'll be just fine (if you don't mind the chemical costs).

What type of DA do you have (e.g. spray, spray-tray). If you have a trayed unit, is it counterflow or parallel flow?

There are actions you can take to increase DA performance that may be more cost effective than chemical addition, depending on your unit. If you could post some operating information I may be able to help..... things like:

Rated design flow (normally listed on the DA nameplate)
Actuall flow
Operating pressure
Storage tank temperature

Normally a failsafe way to increase DA performance is to open the vent. While this will cause a greater steam loss, it will reduce the outlet O2 concentration.

jproj