Coordinated phosphate

Boiler feed-water systems that use dernineralized or evaporated makeup or pure condensate may be protected from caustic attack through coordinated phosphate and pH control. Phosphate buffers the boiler water, reducing the chance of large pH changes due to the development of high caustic or acid concentrations. Excess caustic combines with disodium phosphate and forms trisodium phosphate. Sufficient disodium phosphate must be available to combine with all of the free caustic in order to form trisodium phosphate. 

Fig. 5. Coordinated phosphate—pH control avoids both acid and caustic corrosion. To convert psi to kPa, divide by 6.895.

If deposits are minimized, the areas where caustic can be concentrated is reduced. To minimize the iron deposition in 6.895-12.07 x 10 Pa boilers, specific polymers have been designed to disperse the iron and keep it in the bulk water. As with phosphate precipitation and chelant control programs, the use of these polymers with coordinated phosphate—pH treatment improves deposit control. 

The gauge pressure when using phosphates to reduce the residual hardness and when using a coordinated phosphate method for pH... 

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)... 

In practice, higher pressure boilers typically operate under AVT or coordinated phosphate (or similar) programs, which do not provide... 

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. 

Coordinated phosphate and phosphate-based derivations are designed for use with high-quality (demineralized or equivalent) FW in higher pressure WT boilers, for industrial, power, and cogeneration application. 

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. 

Coordinated phosphate control charts assume either that all contribution to pH level is derived from phosphate or that the buffering action of phosphate is sufficient to overcome the presence of other alkaline species, such as amines. Neither assumption is true. This may lead operators to conclude perhaps that a higher than anticipated bulk water pH level (caused by the presence of amine) should be rectified by the addition of MSP. This action may lower localized Na P04 ratios below 2.2 1.0, producing acid phosphate corrosion (phosphate wastage) and resulting in tube thinning and ultimately tube failure. 

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). 

Not detectable (ND) in these cases refers to free sodium or potassium hydroxide alkalinity. Some small variable amount of total alkalinity will be present and measurable with the assumed congruent or coordinated phosphate-pH control or volatile treatment employed at these high pressure ranges. 

Phosphate need not be used in boilers where hardness can be eliminated from the FW at all times. Its use is not recommended for boilers that may exhibit severe hideout (see the section on coordinated phosphate treatment). In these circumstances, sodium hydroxide is the preferred alkalizing agent and should be dosed to a maximum of 1.5x the sodium chloride content. 

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

The cobalt(III) ion has a small size-to-charge ratio and is therefore a strong Lewis acid which strongly activates coordinated phosphate ester... 

Fig.n.4 S urface complex oftetrahedrally coordinated phosphate (hatched) through two neighbouring, singly coordinated Fe-OH groups on the (101) face of goethite (Stanjek, unpubl.). 

Figure 6.57 Saccharide-based (thio)ureas that coordinate phosphate anions through hydrogen bonding even in water.

Cobalt-coordinated amide nucleophiles have also been observed to attack coordinated phosphate esters (equation 38),148 disulfides (Scheme 47)149 150 and nitriles (Scheme 48).151>152 Chelated amides can also be formed by intramolecular attack of cobalt hydroxides.153... 

Coordinated Phosphate Method. In this method of treatment, no free caustic is maintained in the boiler water. Fig. 10 shows the phosphate concentration versus die resulting pH when trisodium phosphate is dissolved in water. Recent laboratory tests show that the crystals which precipitate from a concentrated solution of trisodium phosphate at elevated temperatures contain disodium phosphate and that the supernatant liquid is rich in sodium hydroxide. The sodium hydroxide can destroy the magnetite protective film on boiler surfaces. To assure that no free caustic is present, a boiler-water phosphate concentration thal corresponds Lo a sodium-lo-phosphate mole-ratio of 2.6 is recommended above 1000 psi (68 atm), as... 

Fig. 10. Recommended phosphate content of boiler water for drum boilers, using coordinated phosphate treatment...

---