Reasons for the repeated disappearance of phosphate in boiler drum water

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Our facility is equipped with five steam drums, one of which is a heat exchanger steam drum. The heat transfer medium is catalyst; the pressure in the heat transfer system is 0.3 MPa and the temperature is 660°C. The pressure in the heat exchanger steam drum is 4 MPa, with a steam production rate of 70 t/h. The load remains stable over the long term, with little need for adjustments. The chemical added to the boiler was trisodium phosphate. In mid-June, the phosphates in the boiler water disappeared suddenly; adding this chemical only increased the conductivity, while the level of phosphates decreased. The conductivity rose to 900 μS/cm, with almost no phosphates present. After carrying out extensive wastewater discharge to replace the boiler water, by late June the phosphate levels returned to normal, ranging between 5 and 7, while the conductivity was between 200 and 300 μS/cm. By early July, the disappearance of phosphate occurred again, in the same manner as in June; adding chemicals only reduced the conductivity but did not affect the phosphate levels. The situation improved by late July, but a few days later, the phosphate disappeared once more. The conductivity is between 400 and 600 μS/cm. The phenolphthalein alkalinity of the boiler water is 0.1 mmol/L, while the methyl orange alkalinity is 0.23 mmol/L. The hardness of the drum water is less than 1 μmol/L. Unlike the previous two occasions, when the pH value of the drum water remained between 9 and 10, the pH value has continued to drop since phosphates disappeared in late July; it reached 8.3 yesterday, whereas the pH value of the boiler feed water is 8.2. The boiler feedwater hardness is <1 μmol/L; only one drum is experiencing problems, while the other four drums are functioning normally. Both the chemical dosing system and the feedwater system are shared. According to the information provided, the disappearance of phosphates occurs when the load on the plant is adjusted. However, in these past few months the load on the plant has not changed much, with only a slight variation of a few tons in the steam produced by the drum. What other factors could cause the disappearance of phosphates? Thank you!