Dissolved Oxygen in Boiler Feed Water

Dears

DO2 (dissolved oxygen), is one of the important factor in BFW to be removed before fed to boiler. We have usual water tratement chemical as O2 scavanger (By GE BEtz), this is being dosed in Deaerator neck. We analyse the DO2 at BFW pump suction point, with online analyser. The online analyser always showing very high values. Its known that, any air ingress from fittings or so will be read by this analyser.

I want to know, more about factors contributing to high DO2

1) Can design of the deaerator affect DO2? I know that, the equilibrium DO2 content in water at 106-107 degC can not exceed beyond 40-50ppb. Is it correct? If yes

can we say that, irrespective of deaerator mechanical/stripping section design, if water is at so high temperature, DO2 can not be more than 40-50ppb?

2) For this DO2 traces of ppb level, scavending chemical is being dosed, and target DO2 in BFW is 5-7ppb. Is the analysing point for DO2 at BFW pump suction is correct? It is usual practice to have it at pump suction. But my understanding says, it should be somewhere downstream just before entry to economiser or so. any comment?

3)At the end how one can assure that, the system don't have any localised/pitting corrosion anywhere? In other way, how to ensure no dissolved oxygen in the system, if the online analysis is unreliable?

4)What is advisable frequency to inspect utility boiler, boiler furnace, deaerator?

Hope my explanations elongated the questions, but shall help to understand the concern clearly.