Phosphate program

Where precipitating phosphate programs are used, residual calcium forms a sludge of the basic salt hydroxyapatite [Ca10(PO4)6(OH)2]. 

The primary chemical water treatment program employed to balance and control alkalinity and yet prevent the deposition of free sodium hydroxide includes a coordinated phosphate program and some derivations of this approach (such as congruent phosphate and equilibrium phosphate programs). 

Higher than desirable phosphate reserves when using a coordinated phosphate program (20-50 ppm phosphate rather than 5-10 ppm)... 

NOTE Where coordinated phosphate programs are employed, the entire rationale is designed to inhibit the presence of free caustic. As a consequence, the alkalinity buffering capacity is severely reduced and the tolerance for silica is likewise diminished. 

Coordinated phosphate programs These programs are formulated to avoid the formation of hydroxyl alkalinity (free caustic). They tend to be employed in higher pressure WT boilers and require very careful control. Variants include congruent control and equilibrium phosphate treatment. 

For precipitating phosphate programs it is necessary to ensure adequate BW alkalinity in addition to the phosphate reserve. Sodium hydroxide should be used. The alkalinity feed-rate is based on FW consumption and calculated as follows ... 

Chelant programs offer a number of benefits over phosphate programs, primarily ... 

SS/MA is very useful in cleaning boilers on-line that have been fouled as a result of out-of-control phosphate programs. It is a first-class polymer for operating phosphate programs with high FW hardness and voluminous amounts of sludge in the bulk BW. 

The chosen coordinates defining minimum/maximum pH levels and minimum/maximum phosphate (as ppm P04) led to the production of various coordinated phosphate program control charts for different pressure ratings (maximum phosphate solubility is a function of pressure). The area within the coordinates providies a simple control box for BW testing purposes. 

In practice, coordinated phosphate programs are held at a Na P04 ratio below 3.0 1.0 (normally around 2.8 or 2.85 1.0) because, at the higher ratio any high-temperature crystallization of TSP leads also to some DSP and NaOH. 

At later dates and as a result of specific water chemistry or new boiler design problems, various derivations of coordinated phosphate programs appeared. The basic philosophy relating to waterside control through the use of coordinated treatment programs still holds true today (albeit with modifications), and controlled-pH phosphate programs remain the principal choice for internal treatment of higher pressure boilers. 

Basic flaws in coordinated phosphate program control logic and other relevant issues include ... 

Because thermal gradients vary considerably within boilers, a typical BW sample, which is essentially representative of average internal bulk water conditions, is unlikely to provide sufficient valid information necessary for the critical assessment of conditions at boiler surfaces. This makes fine-tune control of coordinated phosphate programs within the areas they are most needed very difficult, if not impossible, especially because caustic and saline concentrations may be much higher under deposits than in the bulk water as a result of localized effects. 

NOTE The original rationale for using coordinated phosphate programs was to control under-deposit chemistry, not to minimize the presence of the deposits themselves. 

Coordinated Phosphate Program (Captive Alkalinity Program)... 

Typically, for boilers using coordinated phosphate programs, operating in the range of 600 to 1,200 psig. 

Congruent phosphate programs are modifications of coordinated phosphate programs that operate at a Na P04 ratio lower than the normal 2.8 or 2.85 1. Experience has demonstrated that (contrary to theory) diminished phosphate salt solubility (phosphate hideout) takes place under high-load conditions. This is in addition to the recognized problems of high-temperature crystallization of TSP at the higher ratio of 3 1 (with the formation of some DSP and free caustic). 

Traditional Coordinated and Congruent Phosphate Program control areas ... 

Figure 10.5 Coordinated, congruent, precision control, equilibrium and non-acid phosphate programs for higher-pressure WT boilers...

The equilibrium phosphate program is confined to those utility boilers operating under knife-edge control, where any changes in heat and mass transfer patterns at boiler surfaces and other high-heat flux areas may exert significant ripple effects throughout the boiler plant. 

Non-acid phosphate programs turn controlled-pH programs full circle by essentially using TSP as the only phosphate source and advocating that some free caustic (up to 1 ppm addition) is acceptable. 

Maintaining the correct BW Na P04 ratio using a congruent phosphate program may be achieved by mixing individual ingredients into a day... 

Hollander, Orin. Controlling the Chemistry of Coordinated Phosphate Programs in High-Pressure Boilers. Ultrapure Water, Tall Oaks Publishing, Inc., USA, September 1998. 

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