Sodium Hexametaphosphate to precipitate calcium in low-pressure boilers

Sodium Hexametaphosphate supplier

Sodium hexametaphosphate is a commonly used source of orthophosphate anion used to precipitate calcium in low-pressure boilers. With dispersants the calcium phosphate becomes a fluidized sludge capable of being blown down and out of the boiler before the calcium can form a deposit.

Example

The reaction rate is the first part of the equation and the residual the second part:

[ (0.72 x Ca + 1.08 x PO4/ Feedwater Cycles) ]
= ppm of 100% sodium hexametaphosphate in feedwater.

Again, spreadsheets can be made varying calcium in the feedwater, cycles, steam load, changing desired phosphate residuals, or holding the phosphate dosage constant.

Examining the spreadsheets reveals that if the feedwater calcium level is low, most of the product dosage goes to provide residual in the boiler water. If the calcium level starts to become significant, the reaction portion of the equation may become the major demand for the product. Obviously, control will be difficult with variations in feedwater calcium just as with variation with oxygen for sulfite control.

If the feedwater cycles are varied from 5 to 50 cycles, the dosage requirement changes by over a factor of 8, with most of the demand coming from the residual portion of the calculation. So even though residual may change significantly, if calcium is low and consistent, there is enough phosphate to satisfy the demand, and calcium carbonate scale should be avoided even with wild changes in blowdown rates.