Boiler Feed Pump 1

[Milkboy]

Hey there Group

In boiler feed apps the metal of the mech seal often
get a red/brown coating on it.

Is this Ferrite?

Im looking for info on the source of the ferrite in these apps and how it can be dealt with (mag filters? and the like)

Is the ferrite suspended in the water or is it dissolved in and precipitates out and 'scales' onto metal work

Any advice / info or URLs appreciated

TIA

M

-
Milkboy

 

[JJPellin]

We see this quite a lot in our boiler circulating pumps. I apologize because I am not sure of the source. I am told that if the flow velocity is too low through the boiler tubes there is a particular mechanism that can eat up the tubes. I have heard terms such as Departure from Nucleate Boiling (DNB). But I have never researched the meaning. In one particular pump with lots of this rusty, pasty material, they had a tube failure in the boiler and ended up wanting to greatly increase the flow rate from the boiler circ pumps. We now run both of these pumps all the time. I am focusing on the pump and let others deal with the boiler. For the sake of the pump, we added a magnetic separator in the upper line from the seal to the cooler. We included a bypass so it can be cleaned on the run. We set up a PM to clean the separator twice per year. Our seal reliability has improved drastically. We are getting less of this material in the mag separator since they increased the recirculation rate. But, we still get enough to make it worth the time to clean twice per year.

Johnny Pellin

 

[metengr]

In boiler feed apps the metal of the mech seal often
get a red/brown coating on it.

Is this Ferrite?

No. It is probably iron hydroxide in the form of rust. There is iron transport going on (corrosion) in the boiler waterwall circuit and it should be investigated.

 

[metengr]

One other item to check on is - do you have any nonferrous (copper alloy) heat exchanger tubing in your boiler circuit? If so, the source of the red color could also be copper depositing out on the mech seal. It will also plate out in the waterwall tube circuit and eventually make its way to the steam turbine (high and intermediate pressure stages) rotating components.

 

[kenvlach]

Red-brown sludge is almost certainly rust as mentioned by metengr; usually written FeO(OH). The color is a good indicator of Fe⁺³. Copper immersion plating out on iron cannot build up to a pasty mess (unless the iron is really spongy).

Probable cause is high oxygen and maybe also low pH. Look for a Pourbaix diagram for iron-water in a corrosion or water treatment book. You want to maintain conditions where an Fe⁺²-containing phase (Fe(OH)₂ or Fe₃O₄) is stable in contact with Fe.

http://www.argentumsolutions.com/tutorials/thermexpert_tutorialpg4.html

http://www.corrosion-doctors.org/Thermo/ironE-pH.htm

There are too many variables to show everything on a single diagram for a given temperature, so you will find one type with the oxides given and another with hydroxides:

"The orange zone indicates where one could expect to see rust" -- (from corrosion-doctors link)

Marcel Pourbaix, the electrochemist/corrosion engineer & scientist:

http://www.geocities.com/neveyaakov/electro_science/pourbaix.html