It's been many years since my chemistry in power plants, but as I recall, silica is a cation and your anion bed won't remove it... your initial <2 ppb concentration will concentrate in the boiler drum which your blowdown is suppose to remove -- I would suspect that by recycling your boiler blowdown through an anion bed instead of a mixed bed (or two-bed), you are re-introducing the silica into your boiler stream... also, not all silica remains suspended, so any cycling, any maintenance activites, any distubance in the system will affect the silica levels in the boiler water (also check for leaks in your sample cooler; I assume you've done all the routine calibrations, etc on your analyzer, checked blank samples, etc...)...

It is possible that a normal reaction in the anion bed is: SiO2 + 2NaOH ==> Na2SiO3 + H2O (like I said, my experience is several years old and my notes are buried at home...) -- do you monitor silica from the anion bed of the blowdown stream?  If not, it is possible that you are not getting the regeneration of the resin as desired or that it needs regenerated on a more frequent basis.. silica is usually the first ion that is released when a demineralizer is nearing exhaustion --

One other item:  who supplies / regenerates your anion bed? In house or outside vendor?  We dealt with a vendor to suppy mixed bed bottles for a process and we kept having problems with silica slippage .. found out that the vendor didn't deal with customers who had concerns with silica and wasn't doing the regeneration needed to maximize silica removal -- we'd get maybe a 10 - 20% run on the bottle and silica would start to give us problems... if you have a similar arrangement, visit with the vendor -- it maybe you aren't getting the runs that you are expecting?  [We ended up changing vendors because the first just wasn't equipped to do the complete regeneration] -- it took several conversations before we got the first vendor to reveal this situation...

Thank you pablo02 for your help with my question. From what I understand from our station design manuals, the anion resin is supposed to remove blow down impurities, mainly silica. We do not use a cation bed and our water is conditioned using AVT. Our demin is made using R.O. and EDI. We monitor silica at the inlet of the anion resin and at the outlet. Typical values are 200ppb on the inlet and 0.0ppb on the outlet. What is confusing me is that following a minor plant outage the readings on the inlet now exceed 1200ppb and the life span of the anion resin has been reduced as a result. The silica concentration at the anion inlet is not getting any better even after replacing the resin twice, by which time all of the water in the system would have passed through a number of times.  Again I am at a loss because we checked the demin supply and found silica concentration less than 2ppb, so the silica can not be originating from here. The online silica meters where also checked and calibrated. Manual grab sampling also confirmed values. There are no other sources of water as the ST condenser is an air cooled type, with no inleakage. The resin is regenerated by outside with no problems. The anion resin appears to be doing its job, i.e. removing the silica, the problem is the silica content at the inlet is always high and this is reducing the operating life of the resin. Thank you again for your help.

Probably a longshot, but have you considered that the silica may have been leached from blasting residue (grit) in the boiler?

There has also been a reported instance where plant workers have used silicon based compounds to seal vessel doors - which can also lead to high silica readings.

How new is your installation?  Silica is an everyday contaminant -- it's in the dust, it's on the workers' shoes and clothing...  after any shutdown, silica is known to to give a significant spike in your analysis -- do you record the silica values over time...  is it possible your anion unit is not adequately sized to handle the silica load (after shutdowns, etc.)???

[I once caught our maintenance people hooking up service water to wash down the boiler internals during an outage -- fortunately I caught them before they turned the water on -- we had an immediately impromptu class on rinsing out boilers for the entire maintenance crew...]

Double check what maintenance was done during the outage if the silica level is abnormal from other shutdowns..  If they rinse down with condensate (or demin water), did they use dedicated hoses or heavily rinse out the ones used prior to rinse down?

Again, I'd double check the regeneration procedure -- make sure the vendor knows your requirements for silica... don't take the salesman's word for it... get a copy of the procedure and have someone else knowledgeable in this field review it...  do an audit of their regeneration...