Making Sense of Operating Guidelines for Utility Water Systems

Engineers should use validated guidelines to control the quality of industrial water systems and reduce microbiological fouling, corrosion, deposition, and boiler water carryover into the steam system.

The primary objective of water treatment is to maximize the safety, reliability, and efficiency of boilers, turbines, process applications, and heat exchangers. Validated guidelines are available to help users control the quality of the cooling and boiler water and reduce the risk of microbiological fouling, corrosion, and deposition, as well as carryover of boiler water into steam systems. Owners, operators, engineers, suppliers, and consultants can use these guidelines to assess the performance of the incumbent water treatment provider and the current water treatment programs and to develop a subset of specification limits and performance metrics for use in a competitive bidding process.

Boiler feedwater and boiler water consensus guideline document

The American Society of Mechanical Engineers (ASME) has updated their guideline, “Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Industrial and Institutional Boilers,” that defines key operating indicators (KOIs) for the boiler feedwater, boiler water, and steam purity (1). It’s difficult to create a consensus guideline that defines key performance indicators (KPIs) because there are very few real-time parameters that define the impact of deposits on heat transfer efficiency and the impact of corrosion on system reliability. The technical community considers these ASME consensus guidelines a defacto standard. Water treatment suppliers, the end user, and the equipment suppliers may select a narrower range for specific parameters; however, they rarely violate these consensus guidelines.

The document defines seven categories of operating metrics for boilers in industrial and institutional facilities based on the boiler design and the application. The first three categories are fuel-fired boilers in heavy industry (e.g., refining), electric power generation, and general manufacturing. The fourth category, industrial coil boilers, is for fuel-fired boilers in general manufacturing and comfort heating. The fifth category is for marine propulsion; the sixth category is electrode-type boilers for general manufacturing. The seventh category is industrial waste heat boilers that use heat from the process fluid to generate steam. Within each category, the document defines specification limits for feedwater and boiler water parameters as a function of steam pressure and duty cycle.

The document includes detailed information about the objectives of water treatment, including methods to control steam purity, corrosion, deposition, and boiler blowdown. For example, the discussion about steam purity provides information about the calculation method for the concentration of soluble contaminants in the water droplets in the steam, the steam purity specification limits from turbine manufacturers, and the calculations for sodium in the attemperation water (i.e., the water used to reduce the steam temperature). The risk of carryover of boiler water into steam that serves a turbine is so important that the authors have included the method to estimate specification limits for the concentration of silica in the boiler water (Appendix A of Ref. 1) and a sample of the calculations for the maximum specification limit for boiler water silica as a function of pressure and boiler design to meet a requirement of 0.020 mg/L of silica in saturated steam (1).

The document describes measurement techniques for the water chemistry parameters and highlights the importance of controlling each parameter. Table 1 shows the suggested water chemistry targets for feedwater in industrial watertube, fuel-fired boilers with superheaters (1). Notice that the authors have specified a wide range of pH for the boiler feedwater because the primary objective is to protect the materials of construction in the feedwater system from corrosion. This document also provides guidance for the minimum boiler water pH...