Dissolved O2 removal from boiler feed water 9

[gugubarra]

Our company has signed a pledge to reduce energy consumption. I am looking at using waste steam to pre-heat the feed water to the deaerator. I undertand that there is a general belief of a maximum temperature limit- about 20 oC temperature difference between the incoming feed water and the outgoing deaerated boiler feed water has to be maintained for efficient O2 stripping with steam. I wonder why this has to be so. If the feed water is preheated to a higher temperature than normal boiling point of 100 oC, it should flash (to the extent dpendent on the temperature difference) as it comes off the sprays. This steam flashing should be as effective as using raw steam to strip the O2. It should eliminate the use of raw steam for stripping. Anyone has experience in this please comment.
Thanks.

 

[b8checker]

KeItSiSt
If condensate and makeup water mix in a stainless steel atmospheric surge tank before being pumped to a deaerator, is it also necessary that the pump discharge line be stainless steel? If so, why? Should the wetted parts of the pump be stainless also?

b8checker

 

[KeItSiSt]

b8checker:
No, on both accounts. The surge tank needs protection because of the temperature changes that are occuring with the incoming water and the fact that the water in the surge tank is exposed to atmospheric conditions. As water temperature changes occur more or less gasses become dissolved in the water. As water cools the saturation point for dissolved oxygen (and carbon dioxide) is greater meaning the water can hold more dissolved gasses. For example, water at 0 psig and 178F will have a dissolved oxygen content of approximately 2.9 ppm and water at 0 psig and 118F will have a dissolved oxygen content of approximately 5.7 ppm. As water temperature increases it cannot hold as much dissolved gasses and is "driven" out. In a surge tank when mixing returning condensate and make-up water the temperature of the make-up water is increased releasing the gasses. Raising the water temperature increases the reaction rate. In the pump and piping the water temperature does not increase significantly alhtough some pump experts might disagree about the pump statement. However, for this application I have seen many carbon pumps used for a very long time. Although I believe attacking still takes place it is not as troublesome as in a surge tank or boiler tube because of the steady temperature, the lack of residence time, and thickness of materials involved. I do know of some installations where stainless piping and pumps were specified and supplied. In my opinion it's not worth the extra cost.