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Posted by DeltaCascade on August 15, 2002 at 23:30:12:

In Reply to: WATER TREATMENT posted by Steve on August 15, 2002 at 07:59:32:

: What effect does pH have when using a phosphate treatment??

First, boiler FEEDWATER pH must be alkaline, with pH of about 8.5, in the feedwater deaerator and associated piping and steam economizer, upstream of the steam drum, to avoid corrosion (and to facilitate chemical oxygen scavenger performance in the deaerator storage section).

Second, the alkalinity required in the boiler water (once feedwater has entered the steam drum and is mixed with boiler water), depends upon the
type of PO4 treatment program being used. This in turn will depend upon boiler pressure (and therefore temperature), and feed water quality available.

For an "old fashioned" precipitating PO4 program, alkalinity must be relatively high so that magnesium precipitates do not become, yes, "sticky", and stick to boiler tube walls. The Coordinated PO4/pH programs, for higher pressure boilers, require higher purity water and lower alkalinity and lower pH.

Remember that alkalinity in boiler feed water concentrates in the boiler due to evaporation of steam in the boiler. The relationship is not linear, however, in that carbonate alkalinity decomposes under heat to produce acidic CO2 gas (forming carbonic acid H2CO3 in downstream steam condensate piping, if any, and there usually is losts of it .. hence, amine treatment of steam condensate, but that is another topic). This leaves alkalinity, such as NaOH, behind.

Hope that helps. Good luck!

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