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[-G-]

So a bit of background on the situation...

 

Not long ago the plant started back up after a 9 week maintainance shutdown. The HRSG is a dual pressure boiler, producing high pressure steam at 130 bar and intermediate pressure at 15 bar.

 

Chemical treatment is amine (corrosion inhibitor) and carbohydrazine (O2 scavanger) to the feed water tank, which is fed to both boiler drums. The intermediate pressure drum is additionally dosed with a caustic/phosphate blend to further increase pH and reduce the risk of flow accelerated corrosion.

 

It is common that during start-up we get a lot of magnetite (water side oxide coating) come through our sample lines to our on-line pH and conductivity analysers. The majority of this is blown through the sample bypass to drain until the samples run clear, but even then residual magnetite will still be present to some degree for a few days.

 

For the first three days of operation we ran with very high conductivities in the feed water and both boiler waters. This was initialy dismissed due to contamination in the sample lines. Additionally before we started online we had high DP accross the feed water pump suctions strainers, which turned out to be blocked with iron filings and a couple of aerosol cans!! So the general impression was that there's likely residual crap in the system and it will take a while for our conductivity values to come down into spec. This completely diverted attention to what was actually happening!!! The actual cause of the high conductivities was that the continuous blowdown line was blocked (again likely with crap from the maintainance work) which meant that the blowdown tank was backing up and overfilling! There is a line connecting the top of the blowdown tank to the feed water tank (to recover heat/steam from the blowdown) which is the only point for the blowdown to escape, meaning that all the blowdown (including dosed phosphate/caustic) was flowing back into the feed water. This feed water was then being back fed to the high pressure circuit and also used as attemperator spray water for the superheaters!  

 

Below will give you an idea of just how high our conductivities were...

 

Accorting to VGB powertech guidelines, we should be operating within the following conductivity limits:

 

Intermediate pressure: 15 - 40 uS/cm

High pressure: <15 uS/cm

Feed water: <0.20 uS/cm (cation conductivity)

 

We were operating at:

 

Intermediate pressure: 140 - 160 uS/cm

High pressure: 40 - 50 uS/cm

Feed water: 1.2 - 1.8 uS/cm

 

VGB guidelines outline different action levels based on the operating value of you're main parameters (pH and conductivity). The recommended action that should be taken based on these operating values are...

 

Intermidiate pressure: Action level 2-3, loss of chemical control, possibly leading to short and long term damage, action should be taken to find and eliminate the cause within one day.

 

High pressure: Action level 2-3, loss of chemical control, possibly leading to short and long term damage, action should be taken to find and eliminate the cause within one day.

 

Feed water: Outside Action level 3, chemistry out of control connected with immediate damage, the unit should be shut down within 1 hour. 

 

We were operating for 3 days at these levels before the issue with the blowdown tank was discovered and manual blowdowns commenced to bring values back down into spec.

 

I guess the question is, and may be impossible to answer, is just how much damage is this likely to have caused to the boiler? Or rather how much scale is likely to have formed. No doubt this will justify extending next years tube sampling plan!

 

Maybe someone with more experience in this field may be able to chime in?

 

Thanks.

 

Edited by -G-, 24 November 2016 - 09:06 AM.

[Bobby Strain]

Unless you get leaking tubes before the planned shutdown.