PHOSPHATE TREATMENTS

In developing phosphate treatment guidelines for HRSGs it was assumed that the HRSG would have and use duct burners. The risk of phosphate hideout and the potential for phosphate gouging are much higher when duct burners are in use. HRSGs that do not have duct burners may be operated with higher phosphate concentration or with congruent phosphate-pH control in the HP drum.

Both low-level and equilibrium phosphate treatments have been successfully used in HRSGs. Low-level phosphate treatments usually limit the range of phosphate between 2-6 ppm (mg/l) whereas equilibrium phosphate (EPT) typically maintains a lower phosphate concentration of less than 2 ppm (mg/l). Low-level phosphate treatments were designed for high-pressure fossil-fired boilers that experience or are susceptible to phosphate hideout. Hideout may be a precursor to phosphate gouging. Feedwater purity requirements for EPT-treated systems are comparable to those for AVT systems.

The most common cause of phosphate hideout for a HRSG is the operation of the duct burner. Depending on the capacity of the duct burner, different degrees of hideout are possible. Testing during duct burner operation is necessary to determine the appropriate chemistry range to use.

Using the drum pH, ammonia, and phosphate concentration, an estimate of the sodium:phosphate molar ratio can be developed from the following equation (Hull, et al.):

R = (1/P(){Kw/H+3P/D+2HPt/DK3+H2Pt/((DK3)K2) - H- AtKb/[(Kw/H)+Kb]}

Where:

R = Sodium to phosphate molar ratio

H = Hydrogen ion concentration = 10-pH At = Ammonia concentration in moles/l = (ppm NH3)/ (17,030.61)

Pt = Phosphate concentration in moles/l = (ppm PO4)/ (94,971.4)

K = Dissociation constant for water at 25oC = 1.008 x 10-14

w

K, = Dissociation constant for ammonia at 25oC = 1.7742 x 10-5

b

K1 = First dissociation constant for phosphoric acid at 25oC = 7.1121 x 10-3 K2 = Second dissociation constant for phosphoric acid at 25oC = 6.2373 x 10-8 K3 = Third dissociation constant for phosphoric acid at 25oC = 4.571 x 10-13 D = Ionization fraction = H3/(K1K2K3) + H2/(K2K3) + H/K3 + 1

To use the complete equation, the ammonia concentration in the HRSG must be determined by separate analysis. The above equation assumes that ammonia is used to control the feedwater pH. If high concentrations of amines are in use, the effect of the amine on the drum pH may affect the accuracy of the sodium:phosphate molar ratio. In these cases, the amine supplier should furnish the proper correction factors.

A separate equation can be used to calculate the amount of free sodium hydroxide:

ppm free NaOH = (R-3) x (ppm PO4) x 0.42139

The minimum sodium:phosphate molar ratio (R in the above equation) recommended is 2.8, with an upper limit of 3.0 +1 ppm sodium hydroxide per the equation. This should be achieved by the addition of trisodium phosphate only. Some may advise a minimum molar ratio of 3.0. Normally trisodium phosphate is the only solid alkali chemical required. The use of caustic as a treatment chemical is not recommended with a phosphate program; however, there may be circumstances where it is required.

There are a few cooling waters that will increase the pH of the drum water if they contaminate the condensate. If this occurs, blowdown should be increased to maintain boiler water chemistry within the prescribed limits. If drum water chemistry cannot be controlled in this manner, shutdown to avoid both HRSG and steam system damage should be considered.

The selection of the phosphate concentration in high pressure HRSG's must take into consideration carryover limits and potential for phosphate hideout. Phosphate treatment is not advised for HRSG drums operating above drum pressures of 2400 psig (16.6 MPa).

If the HP drum operates above1500 psig (10.3 MPa), the use of organic polymers as iron dispersants is not recommended due to increased concerns with thermal degradation. If iron corrosion products from process return condensate are a problem, other means should be used to remove the iron prior to its reintroduction into the HRSG. When considering the use of iron-dispersing polymers in HRSGs operating between 900-1500 psig (6.3-10.3 MPa), the risk of FAC from any acid by-products should be evaluated. At less than 900 psig (6.3 MPa) the use of polymers is a common practice, however, HRSGs operating with less than 1% blowdown get little benefit from iron-dispersing polymers. Preventing iron corrosion products from entering the HRSG is much more effective.

 
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