The use of phosphate chemicals for internal boiler water treatment is more than 70 years old, and is currently used in about 65% of drum units worldwide. Coordinated pH-phosphate control was introduced in 1942 to protect boiler tubes from “caustic embrittlement” as well as the effects of condenser in-leakage of water hardness contaminants. The treatment was designed to preserve magnetite and provide protection against “caustic under-deposit” corrosion attack and hydrogen damage from “under-deposit acidic chloride” attack.
The use of coordinated treatment led to a number of failures believed, at the time, to be caustic gouging and, as a result, to the use of congruent phosphate treatment (CPT) with an operating range below the sodium-to-phosphate molar ratio curve of 2.6 (Figure 1-6). However, the move to CPT was also not without challenges. Many utilities experienced phosphate hideout with increasing load and pressure, (a decrease in phosphate and an increase in pH), and hideout return when the unit load decreased (an increase of phosphate occurring with a pH depression). The depression of pH on startup to below 7 or 8 is now known to exacerbate boiler tube failures, particularly those occurring by corrosion fatigue. More often, problems, notably acid phosphate corrosion (APC), arise as a result of the use of the mono- and di-sodium phosphate in a vain effort to “chase” hideout so as to maintain the control point within the CPT range. An
international survey conducted in 1994 showed that over 90% of drum units in the U.S. operating with congruent phosphate treatment experienced hideout, and more than 60% had corrosion consequences.
Figure 1-6
Schematic of Operating Ranges of Boiler Water on Equilibrium Phosphate Treatment (EPT), Congruent Phosphate Treatment (CPT) and Phosphate Treatment (PT). It should be noted that these are the old phosphate operating ranges, and are shown here for reference only. They shouldn’t be applied to fossil plants.
In the early 1990s, EPRI introduced a new guideline for phosphate treatment(2). These 1994 Guidelines introduced equilibrium phosphate treatment (EPT) and phosphate treatment (PT) after much EPRI and independent research into corrosion mechanisms (acid phosphate corrosion, APC), and after good operating experiences in Russia, Germany and on Great Lakes cooling waters. The ranges are shown in Figure 1-6, and although the Guidelines suggested that only trisodium phosphate and NaOH should be used, Figure 1-6 shows the lower bound of the EPT/PT regions at Na:PO4 molar ratio of 2.8. This was based on research conducted by Tremaine, which showed that maricite (the corrosion product of APC) would not form above higher molar ratios than 2.8 at boiler operating temperatures of 360°C (680°F)(33)
.
This guideline brought enormous success to the world of phosphate treatments in conventional fossil plants. It is not possible to form maricite (NaFePO4) with only trisodium phosphate additions. A more recent international survey in 2003 found that as an increasing number of operators had moved to EPT and PT, only about 40% of boilers now experience hideout and that less than 20% have evidence of acid phosphate corrosion.
Despite the two phosphate treatments, EPT and PT, successfully addressing acid phosphate corrosion, there have been an increasing number of operational uncertainties and some other corrosion problems:
• The improper use of low level phosphate treatments (such as EPT) have been associated with an increase of hydrogen damage failures in both conventional plants and HRSGs(34). There have been over 30 fully documented cases. This results from the wrong choice of boiler water control curves for chloride, which allow too high a level of chloride in the boiler water. Hydrogen damage has been increasing over the last 10 years. Basically operators did not make a clear distinction between the extremes of EPT and PT in their ability to “neutralize” contaminant.
• The name of EPT also caused lots of confusion because there isn’t really any “equilibrium”, and although EPRI published a road-map to derive the “equilibrium” level (minimum level that a boiler can maintain without hideout), very few organizations used this procedure. But many high pressure boilers continue to operate at about 0.2 ppm of phosphate or lower. • Most operators were not clear about the distinct differences between EPT and PT and thus
exactly where to operate. Figure 1-6 shows an artificial boundary between the two treatments at about 2.4 ppm phosphate.
• Most operators did not measure and/or know the percentage of mechanical carryover on their drum units.
• Many operators did not take into account the ammonia levels when operating at low phosphate levels or alternatively made the wrong connection. Similarly operators did not account for sodium associated with contaminant anions when these were present.
In summary, the 1994 Guidelines were very successful in addressing the main corrosion concern of acid phosphate corrosion, but the application of the guideline was often unsuccessful in addressing control and overall boiler corrosion protection (hydrogen damage).
These concerns have led to the need for a new control technology and operating philosophy for phosphate treatments, which could not simply be addressed by changing a few things in the EPT/PT envelope. The new guidelines now encompass a Phosphate Continuum (PC). The operating ranges are shown in normal and log plots in Figure 1-7.
(a)
(b)
Figure 1-7
The new PC treatment keeps the overriding criteria that drove the 1994 Guidelines, but also adds new features to address the concerns delineated above. This combination leads to a treatment which:
• Will be applicable to a wide range of boilers (pressures) and contaminant levels
• Has a wide range from low phosphate to around 10 ppm to assist in controlling hideout • Requires only the addition of TSP and NaOH to ensure that acid phosphate corrosion will not
occur and is thus encompassed by Na:PO4 = 3 and TSP + 1 ppm NaOH
• Covers the possibility of contaminant control at the low phosphate end by having a minimum of 0.2 ppm phosphate. This will prevent operations using < 0.2 ppm where there is
essentially no protection from contamination.
• Covers the high phosphate end for low pressure boilers and in cases where contaminants are likely/probable. (See Section 2.3 on selection and optimization of boiler water treatments.) • Continues the use of boiler water contaminant control curves (Section 4)
Phosphate continuum is a much better terminology as phosphate treatments are indeed a true continuum of treatments, as follows:
• Provides boiler protection from very low pressures (e.g. LP HRSG evaporator circuits at 60/70 psi) to very high conventional plants operating at drum pressures above 2600 psi (18 MPa) and to HP HRSG evaporator circuits.
• Is a continuum of phosphate treatments from 0.2 ppm to 10 ppm, with the minimum being to prevent excessively low phosphates being used which provide little resistance to contaminant ingress.
• Extends from the low phosphate levels (< 3 ppm) where the pH is moderated by ammonia, to the high phosphate levels, where ammonia has a smaller effect.
• The higher phosphate levels have a much greater ability to counteract contaminant ingress. • The opposite ends of the PC have much different abilities to partition sodium into steam
(Section 3).
Although PC is a continuum as described above, it also has to be recognized that there are two extremes of low (L) and high (H) phosphate levels. In Section 4 these two regimes have been called PC(L) and PC(H) with a very loose demarcation between the L and H variants at around 3 ppm phosphate. However, this demarcation is not marked on the PC schematic (Figure 1-7) to prevent organizations polarizing between the two treatments in the way that was done for EPT and PT. The emphasis must be on recognizing that there are these two extremes (and
intermediate conditions) and they will behave differently as indicated by the last set of bullets above. This very important point is discussed again extensively in the Rationale (Section 3) and in the Guidelines (Section 4).
In summary, the 1994 EPRI Phosphate Guidelines made a tremendous contribution to the industry. However, clearly operators/chemists have experienced numerous problems in developing reliable operating ranges, and there were some serious corrosion problems as
discussed in the next Section. Thus the time has come to make the previous phosphate treatments obsolescent. The introduction of PC has done that. It keeps the best features of the old, but also addresses the deficiencies of the old.
1.3.2 Introduction to Caustic Treatment and Operating Experiences Leading to