From potato peels to phosphates: The evolution of boiler water treatmentA brief review of conventional utility boiler water treatmentIn the earl...

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From potato peels to phosphates: The evolution of boiler water treatmentA brief review of conventional utility boiler water treatmentIn the earl...
From potato peels to phosphates: The evolution of boiler water treatment
A brief review of conventional utility boiler water treatment
In the early days of power boiler development, makeup water treatment methods were not firmly established. Operators learned that the transport and accumulation of dissolved ions in boiler water, and especially the hardness ions, calcium and magnesium, would induce scaling. Rudimentary treatment methods to combat deposition had operators dumping potato peels or sawdust into the boiler, as some of the organic compounds in these natural materials, i.e., tannins and lignin, sequester hardness. Such methods were not very scientific. The development of synthetic ion exchange resins and IX systems for makeup water treatment greatly improved water quality, but internal boiler water treatment was still necessary to protect steam generators from corrosion and scale formation.

In the 1930s, tri-sodium phosphate (Na3PO4) emerged as a primary treatment chemical. For starters, TSP generates alkalinity to minimize general corrosion.

Na3PO4 + H2O ⇌ Na2HPO4 + NaOH (1)

Figure 1. A split tube sample showing hydrogen damage.2
Of equal importance, and especially back then when makeup water quality was “iffy”, is that TSP and the alkalinity it produces react with calcium, magnesium and silica to form soft sludges, which can be removed via boiler blowdown. Alkalinity also neutralizes the effects of other impurities such as chloride and sulfate, which can enter the boiler via condenser tube leaks and concentrate under tube deposits to generate acids such as HCl. The acids will initiate direct corrosion and propagate hydrogen damage, a very insidious mechanism that often requires extensive and costly repair.2

Guidelines for early power boilers called for phosphate concentrations in a range of perhaps up to 20-40 mg/L (mg/L is equivalent to the mg/kg units that appear in Table 1 below. The units are also essentially equivalent to parts-per-million.) However, as boilers increased in size, pressure and temperature; under-deposit caustic gouging (the opposite condition of hydrogen damage) became problematic due to the high alkalinity levels. As chemists began dealing with that issue, another complication arose. This involves the relationship of TSP solubility with temperature, whose effects are shown in Figure 2.


Figure 2. Solubility of TSP as a function of temperature.
The diagram illustrates that TSP solubility rises with temperature to approximately 300o F, but thereafter dramatically decreases. At the typical boiler water temperatures (600° F or greater) of utility steam generators, sodium phosphates are only slightly soluble, and most of the chemical precipitates on boiler internals. This phenomenon, known as hideout, leaves little phosphate in solution to combat impurity ingress. Furthermore, when boiler load is reduced, the phosphate redissolves. The cyclic nature of phosphate precipitation and dissolution can play havoc with chemistry control, and especially in this era of fast start combined cycle units that may regularly cycle on and off.

From this complexity, a variety of phosphate treatment programs evolved, with names such as coordinated phosphate, congruent phosphate and equilibrium phosphate. The first two included blends of TSP with disodium phosphate (Na2HPO4) and maybe even at times a small amount of monosodium phosphate (NaH2PO4). These compounds moderate alkalinity concentrations. However, researchers discovered that the phosphate deposition products from coordinated and congruent programs were often acidic in nature and could directly attack carbon steel. A more detailed discussion of this chemistry is available in Reference 1, but a key concept is that TSP is the only phosphate species now recommended for high-pressure units. The suggested PO4 control range for a 2500-psi conventional utility unit is 0.3-1.5 mg/kg to minimize hideout but still provide some initial protection against a condenser tube leak.1 Selection of TSP as the lone phosphate species minimizes the potential for acidic phosphate deposition. The preferred feed location for phosphate solutions is in the steam drum via a moderate diameter, e.g., 1-inch i.d., perforated pipe that extends along the length of the steam drum and below the water level.

Treatment nuances for multi-pressure HRSGs
When we examined feedwater treatment in Part 1, the focus was on triple-pressure, feed forward low-pressure (FFLP) HRSGs. As was noted, because the LP circuit in these units primarily serves as a feedwater heater for the IP and HP evaporators, solid alkalis cannot be utilized for LP evaporator treatment.

For this discussion, we will examine triple-pressure, stand-alone low-pressure (SALP) HRSGs, where solid alkalis can be utilized in all three evaporators, for this offers a comprehensive view of how phosphate guidelines vary per the separate conditions in each evaporator.

Figure 3. Flow schematic of a triple-pressure, stand-alone low-pressure (SALP) HRSG.
As compared to the FFLP HRSG schematic in Part 1, note that just the (small) amount of feedwater needed for LP steam production is routed to the LP evaporator. The bulk of the feedwater flows directly to the IP and HP evaporators. The SALP configuration allows more flexible chemistry control, albeit at perhaps slightly lower overall unit efficiency due to the reduction in feedwater heating as compared to the FFLP design. Table 1 outlines the evaporator chemistry target values or ranges at common evaporator pressures.

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