Re: working of hydrazine and trisodium phosphate

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Posted by Vern on December 10, 2001 at 19:43:31:

In Reply to: working of hydrazine and trisodium phosphate posted by maja on December 10, 2001 at 09:15:14:

: tri sodium phosphate and hydrazine are used for the removal of oxygen from boiler feed these chemical work. will they dirctly combine with oxygen or will produce some kind of protective layer on boiler metal surface.secondly if boler feed water is heated in a shell type feed water tank of copacity 5 tons and that is laying horizontly upto 212 f , 75 percent filled, will this heating will cause the dearation of water and upto what extant.will it be further needing to add chemicals for complete removal of oxygen.
: thanks and regard for sugesstions.

Tri sodium phosphate is not used for oxygen removal in boiler systems.
It is used for scale control. The phosphate combines with hardness
minerals such as calcium and magnesium and forms a precipitate which
can be removed by blowdown. This prevents deposit buildup in the boiler.
I'm not real familiar with hydrazine, but my book says it functions
preferentially as a metal passivator in the system. The reaction is slow
at 300degrees,it becomes very rapid by 392degrees. It begins to decompose
at 752degrees Fahrenheit and is 90% decomposed by 1112F, releasing ammonia
as a byproduct. Ammonia, in the presence of oxygen, is extremely corrosive
to yellow metals. Hydrazines reaction with oxygen does not
contribute any solids to the boiler water. Hydrazine is a known carcinogen
so take extra precautions when dealing with it.

As for your tank, if it is not an actual deaeration tank, water at 212F
can hold about one half of one ppm of oxygen. The water is not fully deaerated
at that point and will still need the addition of chemicals such as
hydrazine or sulfite to remove the oxygen and protect the boiler from oxygen
corrosion. As for what extent you would need to add chemicals, you need to
add enough chemicals to completely remove the oxygen and leave a residual
of a small amount. Sulfite typically requires a residual of 30-60 ppm as sulfite.
My book says hydrazine should be fed neat to the feedwater system to maintain a 0.05 -0.10
residual. Hope this helps. Vern

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