Alkalinity vs. pH in Boilers
It is commonly accepted throughout the water treatment industry that hydrate alkalinity should be the key parameter in low-pressure boilers while pH be controlled in high pressure boilers, but is this justified?
One of the fundamental requirements in a successful water treatment program is to maintain pH high enough to minimize general corrosion of carbon steel while avoiding caustic attack (Figure 1). Depending upon the various metallurgy within systems, a pH of 9-12 is generally the target operating range. Most specifications for low-pressure boilers refer to hydrate (OH) alkalinity. In fact, the earliest water treatment for boilers was based upon accepting whatever feedwater was provided and adding an alkalizing agent to precipitate the undesirable materials for inhibition. Today it is considered to be a fundamental necessity that at a minimum, ion exchange is utilized for make-up water to remove calcium and magnesium that would otherwise contribute to scaling. It is consistent that as the working pressure of boilers increase, the tolerance of impurities in the feedwater decreases (Table 1). High-pressure boilers typically require the use of demineralized, RO or dealkalized make-up water to meet the demand for quality water. Demineralization removes virtually all minerals and dissolved solids.
Table 1: ASME Water Quality Guidelines for Boiler Feed Water
The main rationale for the difference between testing alkalinity or pH is that many species normally found in water systems also act to buffer the boiler pH. The most common pH buffer is phosphate, although carbonate, chelants and some polymers contribute to stabilizing pH. Understandably alkalinity is used as a key parameter in low-pressure boilers as pH may not be meaningful. As there are fewer to possibly no products available to provide any buffering in high-pressure boilers, pH is more representative of the alkalinity in the system. I would concur that as a general rule of thumb, alkalinity should be the key parameter in low-pressure boilers, and pH be controlled in high-pressure boilers.