This installment examines recommended monitoring parameters for the boilers in a steam generating system. Factors that influence boiler water treatment include boiler design, boiler/steam pressure, whether the steam is utilized strictly for process heating or for driving turbines, and issues related to condensate return and impurity ingress to the boiler water.
The Importance of Measuring pH
Power plant steam generators generally approach closed loop operation, because the steam only drives a turbine and is then returned in its entirety to the boiler. For the normally small amounts of water/steam that are lost, makeup comes from high-purity treatment systems, which normally ensures clean boiler water.
So, the primary monitoring parameter for utility boiler water is pH. The recommended range may vary slightly depending on the boiler type and operating pressure, but, in general, most guidelines call for a range somewhere between 9.2 and 9.8.
For the numerous units with water-cooled steam condensers, impurity ingress is always a possibility that may cause severe upsets in the high-temperature boiler environment. In these cases, pH becomes the most critical measurement. Virtually all industry guidelines recommend taking a unit off-line immediately if the pH drops to 8.0.
Tri-sodium phosphate (Na3PO4, TSP) in small dosages is still a common way to (temporarily) protect against contaminant ingress until the unit can be shut down. In some cases, small concentrations of caustic (NaOH) may be preferable over TSP.
Note: In the most common type of heat recovery steam generator (HRSG), the feed forward low-pressure (FFLP) design, solid alkalis cannot be utilized for low-pressure evaporator treatment. Rather, pH is controlled via condensate/feedwater ammonia treatment.
Recommended Analyses for High-Pressure Applications
For most high-pressure applications, the following analyses are recommended.
- Phosphate (for units treated with phosphate)
- Specific conductivity
- Conductivity after cation exchange (CACE)
The conductivity measurements, especially CACE combined with phosphate readings, can be valuable to ensure chemistry is balanced and minimizes corrosion potential. When collected and analyzed by a software program such as ChemTreat’s CTVista®+, this data can provide accurate analyses and trending of boiler water chemistry.
For industrial boilers, additional monitoring may be necessary. In many cases, makeup water treatment may only consist of sodium softening or softening and alkalinity removal. Furthermore, numerous industrial plants have complex steam and condensate return systems that may allow extra impurity ingress to the boilers. Often, the primary impurities are iron oxide corrosion products from the condensate return piping.
Treatment Programs for Boiler Water
A common boiler water treatment program for these systems includes phosphate and a polymer, the latter to keep particulates in solution.
If hardness ingress is not an issue, polymer-only programs may be preferable. In either case, determining polymer concentrations can be a challenging task. One method is to blend a trace amount of a fluorescent compound in with the product formulation. The very small quantity of the tracer compound does not interfere with chemical reactions but can be readily analyzed to provide a surrogate measurement of the chemical concentration.
Another possibility, which first gained traction in the cooling water industry a number of years ago, involves tagged polymers. A non-reactive molecule is added to the primary polymers to make them detectable by instrumentation. In cases where tagged polymers are applicable, monitoring is done directly and not by surrogate methods.
As you know, all systems are different. ChemTreat personnel have the knowledge and can supply the products and services to treat any boiler, from very large power boilers to small industrial units. Please contact us for more information. Like all other technologies, due diligence is necessary to determine the feasibility for utilizing methods. Always consult your equipment manuals and guides.
Additional details regarding power boiler sampling may be obtained from the “Technical Guidance Document – 2015 Revision: Instrumentation for monitoring and control of cycle chemistry for the steam-water circuits of fossil-fired and combined cycle power plants”; the International Association for the Properties of Water and Steam (www.iapws.org).